Offroad Trailblazers and Envoys

Author Archives: Trail X

Factory Spring Part #’s and Associated Lift


The table below works by looking up your stock front spring part number or tag color on the top line first. This can be found by looking at your stock spring tag, or by looking for the associated RPO code in the glove box (see below for RPO > Spring # conversion).
  • 6## = Left front spring
  • 7## = Right front spring
## = the letters below:
  • 15125881 – FH, HM, HU, FZ, JC, CC
  • 15125882 – HN, HW
  • 15125883 – HP, HX, CB
  • 15125884 – FL, HY, JL, HC
  • 15125885 – FM, HD
  • 15125886 – HT, JA, JR, JT, HF, JC
  • 15125887 – RL, RX
  • 15125888 – RM
  • 15125889 – HK, HL
I think these particular springs were not used in all years. However, we do know that the I6 2wd received mainly lavendar and dark green springs. When you add in the 4×4, the SWB I6 trucks received mainly Grey and pink springs. The LWB 2WD vehicles also received Grey and Pink mainly. Finally, LWB 4x4s mainly received light blue and orange. Hopefully that lets you know approximately where your vehicle would fall (even if you don’t have a spring in the provided part number range). Once you know the spring on your current vehicle, look up or down the table to see which spring could give you the desired lift (listed on the left). KEEP IN MIND, THE NUMBER LISTED ON THE CHART IS SPRING LIFT, NOT WHEEL LIFT. My lower control arm is 15.5″ from the fulcrum to the ball joint centroid, and 10.5″ from the fulcrum to the strut mount. So in theory, if you are supposed to get, say 20mm of lift based on the chart, divide that by 10.5, then multiply it by 15.5. You get 29.5mm of actual wheel lift. (Or to make it simple, just multiply the chart value by 1.47 to get estimated wheel lift.) Going to spring # 15125889 will give the most lift possible. It is the stiffest possible spring for the TB. For someone with a stock Lavender spring, they can attain 1.39″ of strut lift by going to the 89 spring. Using the conversion above, this gives 2.04″ of wheel lift. tb-springs[1] All the calculations were easier to do in metric, so I only converted the bottom row to inches for your reference. The conversion from mm to in is to divide by 25.4. Consider this your disclaimer. Use this information at your own risk. A higher spring number WILL produce a stiffer, rougher ride. If you lift your vehicle too far, you could break something. Always make sure you triple check your calculations first.

Inner & Outer Tie Rod Replacement

Tie rods connect the rack and pinion to the steering knuckles.  They essentially transfer the lateral motion of the steering ram to the tires.  They go through a lot of abuse, especially off road.  Every bump, steering input, and braking motion put stress on the components.

Simple depiction of the inner and outer tie rods.

Tie rods are generally made up of two components.  An inner tie-rod, and an outer tie-rod.  The inner tie-rod is an in-line ball joint, the outer has another ball joint, but it is a right-angle ball joint.  The threads that connect the inner tie rod to the outer are how the toe alignment is adjusted. There are two different thread sizes available.  14mm and 16mm.  To know which you have, fit an open-end 15mm wrench over the threads.  If it passes over the threads, you have 14mm, if it doesn’t fit, it’s 16mm. The biggest wear issue is when the ball joints begin to get loose.  This leads to a lot of play in the steering wheel and poor steering input.


Estimated time: 1 hour per side Tools:
  • Adjustable Wrench (or a very complete wrench set)
  • Pittman Arm Puller
  • Inner Tie Rod Tool
Soak the accessible threads with PB blaster a couple days ahead of time. Begin by jacking up the wheel that needs tie-rod replacement.  I prefer jacking up the vehicle directly under the lower control arm.  This keeps the suspension in a very similar position to the on-road position and makes tie-rod replacement easier.  I also put a jack stand under the frame as a safety measure. Take the wheel off. Take the outer tie rod nut off.  The nut is a 21mm (or 22mm if aftermarket).  The top stud crown is a 10mm.  It can help keep the stud from rotating… but it probably won’t.  If it does rotate, you probably should have replaced the tie rod months ago. PICT3719 Once the nut is off, loosen the jam-nut towards the inside of the inner tie rod (not pictured). Next, use a Pittman arm puller to pop the outer tie rod out of the knuckle. PICT3720 The outer tie rod should now be easy to unthread from the inner tie rod. Remove the clamps on the inside and outside of the rubber boot that covers the steering ram and inner tie rod.  I used a skinny flat-head screwdriver and pried on the ‘mushroom’ portion of the clamp. PICT3721 Push the white cover sleeve off of the ball joint.  Cast it aside once you get the tie rod off – it probably won’t fit the new tie rod, and it doesn’t really serve a purpose. Below is a video taken by “The Roadie”.  This exhibits severe inner ball joint wear. To remove the inner ball joint, I used a special tool from Harbor Freight. This will require some modification to fit our over-sized tie rods.  Below you can see how much I had to grind away with a die-grinder.  I had to grind off maybe 1/8″ from the bore of the inner shoulder and 1/4″ or so from the inside of the C clip (see red arrows).  With a little modification, this tool works great. PICT3731 Use the tool on the tie rod.  This will take a lot of torque to break, there is thread locker on the inside threads. PICT3723 Once the inner tie rod is out, it’s time to put the new one back in.  Don’t forget the high-strength thread locker. PICT3724 Carefully thread it on by hand.  You do not want to mess up these threads. Torque it in with the tool (if this comes out, you lose steering control). (This picture shows exactly where I used the stock jack to lift the LCA off the ground.) PICT3726 Place the jam-nut on the new tie rod.  If you’re going to compare old jam-nut location to new in order to get your alignment close, measure to the small shoulder-nub with R or L on it.  Do not use the threads as your reference, as they can start at different points and be different lengths. PICT3725 With the jam-nut at the proper location, put the boot over the outer tie rod.  Begin threading the outer into the inner.  You may want to apply anti-seize to this area.  Thread it down to the jam-nut. PICT3727 Put the outer ball joint into the knuckle and tighten down the castle nut.  Don’t forget the cotter pin (if your aftermarket tie-rod has one). PICT3728 Adjust the alignment, tighten down the jam-nut. Then move the boot back into place.  I used pipe-clamps to secure the boot back in place. PICT3729 Put the wheel back on, and drive the vehicle around the block to check for any funny steering issues. You should get a professional alignment done after making a change like this.

Suspension Lift Installation

This article is brought to you by BDS Suspensions.

Front Installation

1.Park the vehicle and a clean, flat surface and block the rear wheels for safety. 2. Raise the front of the vehicle and support the frame with jack stands behind the lower control arms. 3. Remove the front wheels. 4. Disconnect the sway bar from the frame (2 bolts per side). Allow the sway bar to hang down. Retain sway bar mounting bolts. suspension-fig-1 5. Under the hood, locate the driver’s and passenger’s top strut mounting nuts (Fig. 2). Remove the two nuts hold each strut to the frame. CAUTION: DO NOT remove the center strut nut. It is under extreme pressure. suspension-fig-2 Perform the following steps one side at a time, starting with the passenger’s side. 6. Remove the nut that mounts the strut to the lower control arm (Fig. 3). Retain nut. 7. Using a pitman arm puller or equivalent, remove the strut from the tapered mounting stud. Note: If a puller is not available, taper can be freed by striking with a hammer while prying out on the strut mount. (Fig. 4) suspension-fig-3 suspension-fig-4 8. Remove the strut from the vehicle. 9. Mark the relationships between the top strut plate, coil spring, coil spring mount and strut (Fig. 5). Also mark what side of the strut faces out. The mark on the top plate will need to be transferred to the new top plate provided to ensure the strut is reassemble correctly for proper alignment during installation. suspension-fig-5

Note: A high-quality wall mounted coil spring compressor (Branick or equivalent) is recommended for the installation. A rod style compressor is sufficient, but use extreme caution and inspect all components for signs of wear and/or fatigue before each use.

10. Compress the spring enough to relieve pressure from the strut rod nut. Remove the nut and remove the top washer, strut plate, rubber/plastic isolator and lower washer. 11. Transfer the mark on the OE top strut plate to the new one. The new strut plate has a third mounting hole in it that will face the outside of the vehicle. Install the OE isolator on the new strut plate by lining up the alignment marks. 12. Install one of the provided stem washers (117300007) and stem bushing (SB14BK) with sleeve (87-1). The protruding sleeve should be opposite of the stem washer. (Fig. 6a / 6b) suspension-fig-6a suspension-fig-6b Install the new strut plate/isolator (02171) on the strut and compress the spring enough t 13. o install a second bushing and stem washer. Retain the assembly with the OE nut and tighten until the bushings deform. Note: The bushings should deform to the point when the center crush sleeve bottoms out. 14. Release the spring compressor. 15. Install the modified strut assembly in the vehicle by aligning the studs in the top plate with the original mounting holes. The third hole in the plate will align with an existing hole in the frame. Fasten the strut to the frame with 10mm nuts and 3/8” USS washers on the two studs (Fig. 7). Leave hardware loose. suspension-fig-7 16. Install a 10mm x 30mm bolt, nut and 3/8” USS washers in the third hole (Bolt pack #528). Leave hardware loose. 17. Install the strut on the lower control arm tapered mounting stud and fasten with the OE nut. Snug nut but do not tighten. Final torque will be done with weight of the vehicle on the suspension. 18. Torque the upper 10mm hardware to 35 ft-lbs. 19. Repeat procedure on the driver’s side. 20. With both sides complete, reattach the sway bar to the frame with the OE hardware. Torque bolts to 40 ft-lbs. 21. Install the wheels and lower the vehicle to the ground. 22. Bounce the front of the vehicle to settle the suspension. Torque the lower strut mount nuts to 80 ft-lbs. 23. Check all hardware for proper torque. If installing a #121201 “front only” kit there will be left over hardware provided with the kit. 24. Check hardware after 500 miles.  A complete front end alignment is necessary.

Rear Installation

25.  Block the front wheels for safety. 26. Disconnect the rear track bar from the driver’s side of the axle. Retain hardware. (Fig. 8 ) 27. Raise the rear of the vehicle and support the frame with jack stands in front of the lower control arms. 28. Remove the wheels. 29. Support the rear axle with a hydraulic jack. 30. Remove the OE rear shocks. Retain shock hardware. (Fig. 8 ) suspension-fig-8 Perform the follow steps on one side at a time, starting with the passenger’s side. 31. Disconnect the upper control arm from the axle. Retain hardware. (Fig. 8 ) 32. Lower the axle and remove the coil spring and upper rubber isolator. (Fig. 9) suspension-fig-9 33. Apply loctite to the threads of three 5/16” x 1” bolts (Bolt pack #528) and thread them into the provided coil spring spacer so that ¼” of thread is still showing. (Fig. 10) suspension-fig-10 34. Install the coil spring spacer (02172) in the vehicle by aligning the 5/16” bolt heads with the slots in the frame coil mount. Once the bolt heads are in the slots, turn the spacer to lock it in place. Note: The spacer will be loose in the mount. The bolts are only there to hold the spacer in position while installing the coil spring. 35. Install the OE rubber isolator and coil spring in the vehicle. Take care not to over-extend any brake lines. 36. Raise the axle to compress the coil spring enough to reattach the upper control arm to the axle. Fasten the control arm with the original hardware. Leave hardware loose. 37. Repeat procedure on the driver’s side of the vehicle. 38. With both sides complete, install the new BDS shocks with the provided bushings, sleeves and OE hardware. Torque hardware to 55 ft-lbs. 39. Install the wheels and lower the vehicle to the ground. 40. Bounce the rear of the vehicle to settle the suspension. Install the track bar in the axle mount with the OE hardware. Torque to 125 ft-lbs. 41. Torque the upper control arm bolts to 95 ft-lbs. 42. Check all hardware for proper torque. 43. Check hardware after 500 miles. Source for this article:

Front Wheel Hub / Bearing Replacement

This article covers how to diagnose and replace a front wheel bearing on a TrailBlazer (or Envoy). Wheel bearings are a complex and fairly expensive item for a TrailBlazer.  They serve to support the vertical weight of the vehicle, transfer lateral turning forces from the wheel to the body, transfer the power from the front CV shaft to the wheel, house the ABS sensor, and still allow the wheel to rotate. Below you can see the complex inner workings of this assembly.  There are actually two tapered roller bearings inside a wheel hub in order to accommodate the different forces on the wheel.  Between them is the wheel speed sensor that sends signals to the ABS.
A cutaway and section view of a Timken wheel bearing.

A cutaway and section view of a Timken wheel bearing.

When a wheel bearing fails, it generally does so slowly.  The seals are generally the first thing to deteriorate, which allows water and other foreign particles to enter the bearing space.  This can lead to accelerated wear or pits in the bearing races.  The result is a grinding noise.  Eventually the slight fouling of the races leads to wear of the rollers.  In time, this gives the hub some freedom to move around.


To diagnose a wheel bearing, you will generally hear a grinding noise from the front end of the vehicle.  As it gets worse, this grinding can be felt in the floorboards.  The grinding may be worse at certain speeds.  Mine have been worse at around 35 mph, and 70 mph with barely any noise at about 53 mph. The next step is to determine which side the bad bearing is.  Conventional wisdom says to note if the grinding goes away in a turn.  The wheel with less weight when the noise goes away is the bad bearing.  However, I have not found this to work for me.  The best method of identifying the bad bearing is to lift the wheel off the ground and spin it by hand.  Listen carefully for any sort of light grinding sound that times with the rotation of the wheel.  I have used that method without fail, although the bearing must get some wear before the method works. When selecting a new bearing, I do not recommend buying one of the cheap ones off eBay.  I’ve purchased mine off Rock-Auto.  When it comes to the wheels and the safety of your vehicle, don’t trust it to eBay.


Estimated time: 2 hours 1. Remove axle nut.  Requires 35mm deep socket (or possibly 36mm if you have aftermarket CV shafts). wb1 2. Jack up vehicle so the wheel with the bad bearing is off the ground.  Properly support the frame with jack stands. wb2 3. Remove wheel. wb3 4. Remove 10mm screws that hold on brake cable.  Remove the plastic V clip that holds the wheel speed sensor wire to the bracket. wb4 5. Remove the two 18mm screws that hole the brake caliper on.  This will take a lot of force to break, you may need a breaker bar.  I use a pipe over my ratchet. wb5wb6 6. Remove the brake calipers from the brake disk.  Depending on the wear on your brakes, you may need to use a large C clamp to compress the brake pistons in order to remove it. Hang the brake caliper from the upper control arm with wire or a coat hanger. wb7 7. Remove the wheel spacer if you have one (I find it easier to use an impact wrench here), and remove the brake disk.  My wheel spacer had seized to the hub’s inner bore.  If that happens to you, you’ll have to be crafty and find a way to pry on the back of the brake disk. wb8 8. Remove the pinch bolt and nut from the upper control arm.  This requires two 15mm wrenches. wb10 9. Remove the upper ball joint from the upper control arm.  Lay the knuckle down so it’s hanging off the lower ball joint.  Be careful not to over-extend the CV shaft boots.  If you have to, tap the wheel-end of the CV shaft with a hammer to unseat it from the wheel bearing. 10. Remove the three 18mm screws on the back side of the knuckle that hold on the wheel bearing.  This is tricky because the knuckle moves.  Position your body to hold the knuckle, or have someone help you. If your wheel bearing doesn’t come out easily, thread a screw back in and tap it with a hammer.  It should pop out. Remove all of the wire clamps from the wheel speed sensor wire and remove it from the plug that’s mounted to the frame.  The plug has a gray locking key that you need to remove first. wb11 11. Pull the brake shield off the bearing.  Mine was corroded on pretty good.  Be careful not to bend the brake shield too much; you don’t want it rubbing on your brake disk. wb12 12. Assemble new bearing into knuckle.  Remember the brake shield’s prior position.  The ABS wire should snake through the hole in the shield. Be careful not to cross-thread one of the 3 hub bearing screws.  They are 18mm and need to be torqued to 105 Nm (77 lb ft).  Again, this may require some assistance, but I was able to manage it myself. wb13 13. Reattach the upper ball joint and reattach the pinch bolt and 15mm nut.  Tighten the nut to 40 Nm (30 lb ft). wb14 14.  Put on the brake disk and (if applicable) wheel spacer. wb15 15. Reattach the brake calipers.  Two 18mm screws torqued to 180 Nm (133 lb ft). wb16 16. Reattach the brake line bracket with the two screws, 10mm. wb17 17. Run the wheel speed sensor wire back to the plug.  Don’t forget the plug lock. wb18 18. Reattach the wheel, drop the vehicle to the ground, and torque the 35mm axle nut to 140 Nm (103 lb ft) wb19 19. Take the vehicle for a slow drive around the block (try to hit a few bumps even) just to make sure everything sounds right before hitting any high speeds.

Transfer Case Operation: NVG 226 (NP8)

The New Venture Gear model NVG 226 transfer case is the dual speed, automatic, active transfer case in the Chevrolet TrailBlazer and GMC Envoy.  It provides five modes of operation:
  • 2 HI
  • Auto 4WD
  • 4 HI
  • 4 LO
  • Neutral.
General Operation: The Auto 4WD position allows the capability of an active transfer case, which provides the benefits of on-demand torque biasing wet clutch and easy vehicle tuning through software calibrations. The software calibrations show more features such as flexible adapt ready position and clutch preload torque levels. The technology allows for vehicle speed dependent clutch torque levels to enhance the performance of the system. For example, the system is calibrated to provide 0-5 lb ft of clutch torque during low speed, low engine torque operation, and predetermined higher torque for 20mph and greater. This prevents crow-hop and binding at low speeds and provides higher torque biases at higher vehicle speeds, to enhance stability. The NVG 226 requires no clutch shimming. the transfer case control module controls for the wear of the clutch and different clutch torque levels. The software learns adapt ready positions, which are for the correct clutch torque. The learned adapt ready positions vary as the unit wears over its life.
The power flow from the transmission to the prop-shafts when in 2WD.

The power flow from the transmission to the prop-shafts when in 2WD.

When the NVG 226 is in the 2 HI mode, the power flows from the transmission to the input shaft gear (1). The input shaft gear is connected to the rear output shaft (5) by the high/low range collar (2). The range collar inner teeth, high speed, are engaged with the input shaft gear (1) high speed position teeth. At the same time the range collar is slip splined to the rear output shaft (5). The rear output shaft delivers the power flow to the rear propshaft (6). The position of the control actuator lever shaft (8) allows no clutch engagement. The shift detent lever (7), which moves the shift rail and shift fork (10), is in the high speed position on the control actuator shaft (8).
  4 HI and A4WD
Showing the power flow from the transmission to the prop shafts when in 4HI and A4WD.

Showing the power flow from the transmission to the prop shafts when in 4HI and A4WD.

In the 4 HI mode, the power flow to the rear propshaft is the same as in the 2 HI mode. To deliver power flow to the front propshaft during the 4 HI position, the transfer control module commands the encoder motor to apply the clutch to a calibrated torque. the encoder motor turns the control actuator lever shaft (8). A brake in the encoder motor holds the control actuator shaft in the full clutch position. The control actuator lever shaft (8) is cam designed and the cam action moves the clutch lever (4). The clutch lever (4) pivots on the control lever picot studs and moves towards the clutch apply plate, to engage the clutch. As more pressure is applied to the clutch apply plate, the clutch disks are compressed. Using inner clutch disks, which are engaged with the clutch hub (3), and the outer clutch disks, which are engaged with the clutch housing, the power flow is delivered to the clutch housing. The clutch hub (3) is splined to the rear output shaft (5), and the clutch housing rotates on a needle bearing on the rear output shaft (5). The chain drive sprocket is splined to the clutch housing. The power flows from the drive sprocket, through the chain, to the chain driven sprocket. The driven sprocket is splined to the front output shaft (9). The power flow is delivered to the front propshaft through the front output shaft (9). During the Auto 4WD mode, the power flow is the same as it is in the 4 HI mode. Except during the A4WD mode, the encoder motor rotates the control actuator shaft lever to the learned adapt ready positions. Rotating the control actuator to the carious positions changes the clutch torque level. When a differential of front propshaft and rear propshaft speed is recognized, the transfer case control modules commands for more, or less clutch torque.
  4 LO
The power flow through the transfer case when in 4WD and low range.

The power flow through the transfer case when in 4WD and low range.

When shifting the transfer case to the 4 LO mode, it commands the encoder motor to turn the control actuator lever shaft (8) to move the shift detent lever (7), and to apply the clutch. The shift detent lever (7) moves the shift rail and the spring dampened shift fork (10). The shift fork (10) moves the high/low range collar (2) on the rear output shaft (5) splines toward the rear of the transfer case. The range collar (2) inner teeth, high speed, disengage from the input shaft gear (1) high speed teeth. The range collar (2) outer teeth, low speed, then engage in the planetary carrier teeth. The power flow is now from the input shaft (1) planetary teeth to the planetary gears in the carrier. Rotating the planetary gears, which are engaged in the annulus gear, the carrier rotates. The carrier, that is engaged to the range collar, then drives the rear output shaft. Therefore, providing a 2.69:1 reduction to the speed of the rear output shaft. The power flow to the front propshaft is the same as it is in the 4 HI mode. A neutral position is obtained when the range collar is not engaged to the input shaft gear or the planetary carrier. Neutral position is used for towing the vehicle.

4WD Mode Selector

The 4WD Selector Knob   The NVG 226 transfer case features a rotary 4 mode shift control switch located on the instrument panel.  When the ignition key is in the RUN position, the transfer case shift control module monitors the transfer case shift control switch to determine if the driver desires a new mode/range position. At a single turn of the transfer case shift control switch, the lamp of the new desired position will begin flashing to inform the driver that the transfer case shift control module has received the request for a new mode/range position. The lamp will continue to flash until all shifting criteria has been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position will remain ON constantly. During normal driving situations, the transfer case can operate in the Auto 4WD mode. In the Auto 4WD mode, the transfer case shift control module monitors rear wheel slip speed, based on the inputs from both the front and rear propshaft speed sensors. When the vehicle experiences a rear wheel slip condition, the transfer case shift control module sends a pulse width modulated (PWM) signal to an electronic motor, which is the transfer case encoder motor. This motor rotates the transfer case control actuator lever shaft, applying a clutch pack. This clutch pack is designed to deliver a variable amount of torque, normally delivered to the rear wheels, and transfers it to the front wheels. Torque is ramped up to the front wheels until the front propshaft speed sensor matches that of the rear propshaft speed sensor. Torque is ramped down to the front wheels. The process would repeat if rear wheel slip is detected again. The NVG 226 transfer case has the added feature of also providing the driver with 3 manual mode/range positions:
  • 4HI 4-Wheel Drive high range
  • 2HI 2-Wheel Drive high range
  • 4LO 4-Wheel Drive low range
The driver may choose to select any of these mode/range positions while driving the vehicle. However, the transfer case will not allow a shift into or out of 4LO unless the following criteria has been met:
  • The engine is running.
  • The automatic transmission is in Neutral.
  • The vehicle speed is less than 5 km/’h (3 mph).
This transfer case also has a Neutral position. A shift to the Neutral position allows the vehicle to be towed without rotating the transmission output shaft. Neutral position may be obtained only if the following criteria has been met:
  • The engine is running.
  • The automatic transmission is in Neutral.
  • The vehicle speed is less than 5 km/h (3 mph).
  • The transfer case is in 2HI mode.
Once these conditions have been met, turn the rotary switch clockwise past the last position and hold for 10 seconds.  When the system completes the switch to neutral, the red neutral lamp will appear.

Transfer Case Control Module (TCCM)

  The Transfer Case Control Module (TCCM)   The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information. When the Vehicle is in the AWD mode, the transfer case shift control module monitors the speed of the front and rear propshafts in order to detect wheel slippage. When wheel slippage is detected, the module applies a clutch pack contained in side the transfer case. This clutch pack is used to lock-in and apply the front propshaft, transferring torque to the front wheels. The clutch pack is applied by a motor/encoder assembly. When slip is no longer detected by the transfer case shift control module, the clutch is no longer applied.

Transfer Case Motor/Encoder

  The Transfer Case Encoder Motor The transfer case Motor/Encoder consists of a permanent magnet (PM) DC motor and gear reduction assembly. It is located on the left hand side (drivers side) of the transfer case. When activated it turns the sector shaft of the transfer case (clockwise or counter clockwise) to shift the transfer case. The Motor/Encoder is controlled with a pulse width modulated (PWM) signal by the transfer case shift control module. This circuit consists of a driver on both the Motor A and Motor B circuits. The encoder motor is bi-directional to allow the motor to shift the transfer case from 2HI or 4HI to NEUTRAL and 4LO positions.

Transfer Case Encoder

The encoder is mounted to the transfer case motor/encoder assembly and is replaced as an assembly. The encoder converts the sector shaft position (representing a mode or range) into an electrical signal input to the transfer case shift control module. The module can detect what position the transfer case is in by monitoring the voltage returned on the encoder signal circuit. This voltage translates into AUTO, 2H, 4H. NEUTRAL. and 4L or in transition between gears.

Transfer Case Motor Lock

The transfer case motor lock is used to provide a 2H, 4H, and 4L lock-up feature. When the lock circuit is energized, the transfer case encoder motor is allowed to turn. When the transfer case is placed 2H, 4H, or 4L the motor lock circuit is de-energized and the lock is applied. This assures that the transfer case remains in the current gear position until a new gear position is requested. When AUTO is selected the motor lock remains applied until an adaptive mode (torque is applied to the front propshaft) is required. During an adaptive mode the motor lock circuit is energized and the motor lock is released, enabling the encoder motor to turn and apply or release torque at the front propshaft.

Transfer Case Speed Sensors

There are three speed sensors on the automatic transfer case (ATC), two on the rear output shaft and one on the front output shaft. Each speed sensor is a permanent magnet (PM) generator. The PM generator produces a pulsing AC voltage. The AC voltage level and number of pulses increases as speed increases.

Vehicle Speed Sensor

One of the two on the rear output shaft is the vehicle speed sensor (V SS) input to the powertrain control module (PCM). The PCM sends this information to the transfer case shift control module via the Class 2 Serial Data bus.

Rear Propshaft Speed Sensor

The transfer case shift control module converts the pulsating AC voltage from the rear transfer case speed sensor to a rear propshaft speed in RPM to be used for calculations. The rear propshaft speed can be displayed with a scan tool.

Front Propshaft Speed Sensor

The transfer case shift control module converts the pulsating AC voltage from the font transfer case speed sensor to front propshaft speed in RPM to be used for calculations, and to monitor the difference between the front and rear sensor speed. It is also used in the AUTO (Adapt) mode of operation to determine the amount of slip and the percent of torque to apply to the front axle. The front propshaft speed can be displayed with a scan tool.

Service 4WD Indicator Lamp

The SERVICE 4WD indicator is an integral part of the cluster and cannot be serviced separately.  This lamp is used to inform the driver of the vehicle of  a transfer case system malfunction. The SERVICE 4WD indicator is controlled by the TCCM via Class 2.  The SERVICE 4WD lamp will only illuminate if the TCCM can electronically verify there is a malfunction.  This means that a mechanical failure of the front disconnect would not be identified by a SERVICE 4WD indicator.