VersaTrak: Rear wheels not engaging?

I have a 2003 Silhouette/versatrak system. My question is: How can you tell if it is working? I suspected that the rear wheels were not engaging when the front tires were spinning in some mud and nothing was happening with the rear tires. (there was Not an AWD disabled lite) I went home and jacked up the front wheels with roller jacks (in case the car did move a little) and started the car and put it in drive. Slowly let my foot off the brake and just the front tires freely spun (as you would expect them to do when lifted about two inches above the ground) but no movement at all in the car. The rear tires (still on the cement) did not push the vehicle at all. Also, at that point, the "AWD Disabled" light flashed on. I applied brake and stopped the front wheels from spinning, and the AWD warning light went out. Tried it again with same results. I slowly accellerated the front wheels to 30-40 MPH....nothing. Front tires just freely turned. Tried it in reverse...nothing (AWD Disabled" lite on each time until the front wheels stop) Lowered the car.. Did a test drive...and it drives fine, no AWD Disabled lite, no "rear end moan" no clanking, nothing. It just seems like the rear differential is not getting any torque applied from the PTO Shaft as GM calls it. Or more like what a front wheel drive only silhouette would drive like I suppose?

Do the rear wheels need to be moving before the PTO shaft applies power/torque to the rear differential? Shouldn't there be enough torque to the rear differential to move the vehicle on the rollers at least a little bit?

The manual says: "All power is directed to the front wheels as long as no speed difference is sensed. If one of the front wheels begins to slip, the rear-mounted gerotor units pressurize fluid to engage the clutches that redirect torque to one—or both—rear wheels, enabling the rear wheels to provide traction that allows the vehicle to keep moving forward despite muddy, snowy, or slippery road surfaces. Hydraulic pressure forces a disc-shaped piston to engage the seven-plate clutch and apply torque through the halfshaft to each rear wheel. If the rear wheel is also on a slick surface, it will match the speed of the front wheels, reducing hydraulic pressure and disengaging the halfshaft and wheel. Torque from the propeller shaft is then re-routed to the other rear wheel. The propeller shaft uses a rear sliding constant velocity joint, which allows the driveshaft to vary in length to compensate for rocking motions of the transverse engine and transaxle."

SO: Is there some failure that would not apply torque to the rear wheels when the front wheels are spinning?
Thx

Kevin

I suspect a failure in any of the wheel speed sensors (built in the wheel hub/bearing units ) coud inhibit AWD....but would disable ABS also..... so the ABS light would glow...

A scan can see if the bcm/awd control unit is commanding the transfer case to engage if so the issue is in the rear, if not usually a code would be set.

Thanks LMP and Maxwedge.
The ABS light is NOT on. Thus, I think the wheel speed sensors are O.K.

So maybe Maxwedge's thought on the BCM not commanding the rear transfer case to engage? There are no 'fault lights' of any kind, but maybe a fault of that type would not trigger or initiate any type of panel light?

My only other thought might be....
Does there need to be some, (maybe just a tiny amount) rear wheel speed sensed, before the rear differential is commanded to engage?

Thx

The All Wheel Drive (AWD) system provides On-Demand all wheel drive, distributing variable torque/power to the rear wheels via individual axle shaft assemblies. On-Demand drive is provided to each of the rear wheels only when slippage is detected at the front wheels. As long as there is no slippage at the front wheels, there is no front-to-rear speed differential and no need for rear wheel drive torque. In the event there is front-to-rear wheel speed differential/slippage, a rotational speed difference between the gerotor pump components (rotor and housing) occurs. In those instances, the rotor draws fluid from the sump and through the internal passages of the differential carrier, sending pressurized fluid to a piston (actuating the specific rear wheel clutch pack). In the event of a spare wheel (of smaller diameter) is used on any of the four positions, the wheel rotational speed difference is detected by the wheel speed sensors of the ABS system. The powertrain module directs the differential inlet valve to close and block oil flow to the gerotor pumps. The inlet valve also monitors the sump oil for an “overtemperature”condition. If differential oil temperature exceeds 110°C (230°F), the valve will close and block oil flow to the gerotor pumps. In both spare wheel usage and overtemperature conditions, an activated inlet valve will illuminate the control panel AWD Disable indicator.
View the list of major components that make up the AWD system below.

AWD Disable indicator


The AWD Disable indicator is located in the instrument panel cluster. This lamp is used to inform the driver that the AWD system has been disabled and no torque will be applied to the rear wheels during a slip condition. The AWD Disable indicator is controlled by the powertrain control module via a class 2 message.

Differential Clutch Pump Actuator Check Valve


The differential clutch pump actuator check valve controls the oil flow to the gerotor pumps. Without fluid pressure the pistons cannot apply the clutchpacks for rear wheel engagement. The actuator check valve will open upon engine startup and remain open unless commanded closed by the powertrain control module. The actuator check valve also monitors the sump oil for an overtemperature condition. If differential oil temperature exceeds 110°C (230°F), the valve will close and block oil flow to the gerotor pumps.

Powertrain Control Module


The powertrain control module monitors the data from the ABS controller and Rear Drive Module (RDM) for proper operating conditions. If inappropriate conditions are present the PCM commands the differential clutch


A little info for ya!

Thank you GMTech1!

Certainly could be the rear drive module it would seem. Any way to check that?
Do you know if a fault in the RDM would give any indication or warning/fault light on the panel? Such as AWD disabled, or ABS inop or ????

THx

KP

Well....Lifed the rear and front wheels off the ground.
Definitely the rear differential is not commanding power to the either of the rear wheels. The drive shaft from the transmission to the rear differential is turning when in drive (Front tires are turning too of course) but the rear wheels are slowing rotating, and I can stop them with my hands. (either side or both sides) When I stop the rear wheels, the ABS malfuntion light comes on. But then I let go, and they start to turn slowly, the light goes out. WTH could that be???

THx

KP

see post # 3.

OK..update #3.
I verified that the signal from BCM IS being sent to rear differential. And the problem of the rear wheels are not engaging is still the case.
I am assuming then that either the "rotor is NOT drawing fluid from the sump and through the internal passages of the differential carrier, sending pressurized fluid to a piston (actuating the specific rear wheel clutch pack)", or the differential inlet valve is stuck closed???...or
Maybe the "differential clutch pump actuator check valve" is stuck? (Can that happen?)
Do any of those events happen with these "VersaTrac" systems? Is this a common malidy?

Thanks.

KP

I'm still struggling with the rear wheels folks.
Next question: Do any of you know how to check or test the clutch pump check valve? Do any of you have the wiring schematic so I can check to see it a voltage signal is being sent to the Clutch Pump Check valve. (IE: There is an electrical harness going to th rear differencial and it is a 3-wire plug) What voltages should be sent on which wire to tell the Clutch Pump Check Valve to open and allow fluid to engage the gearator clutch packs?..


Also, does anyone have the Service Manual or Publication describing the "All Wheel Drive (AWD) system function tests"

Thx
KP

I'm still struggling with the rear wheels folks.
Next question: Do any of you know how to check or test the clutch pump check valve? Do any of you have the wiring schematic so I can check to see it a voltage signal is being sent to the Clutch Pump Check valve. (IE: There is an electrical harness going to th rear differencial and it is a 3-wire plug) What voltages should be sent on which wire to tell the Clutch Pump Check Valve to open and allow fluid to engage the gearator clutch packs?..


Also, does anyone have the Service Manual or Publication describing the "All Wheel Drive (AWD) system function tests"

Thx
KP

The signal to the check valve is sent by the PCM. It is a pulse width modulated signal, so its measured in duty cycle, not voltage. The signal will vary from 49-90% on start up, drop to 16-28% after 3 seconds, then back to 49-90% every 30 sec after that.

As Max said earlier, a scan tool and a meter capable of reading duty cycle is needed to properly diagnose this system.

Ask and you shall receive (AWD System Functional Inspection)!

All-Wheel Drive (AWD) System Functional Inspection


Functional Inspection 1

Functional Inspection 1 is a mechanical system inspection for both the left and right side gerotor pumps, pistons, and clutch packs and will assist in determining if:
^ The clutch pump check valve is in the "open" position.
^ The left and right side gerotor pumps have the ability to "pull" fluid from the sump and send pressurized fluid to the left and right side clutch pack pistons.
^ The left and right side pistons will engage the specific side clutch packs.
^ There is a mechanical connection between the rear differential ring and pinion, the propeller shaft, the transfer case, and the transmission assembly.

1. Raise the vehicle on a hoist. Lift the vehicle in a manner that will allow all four wheels to rotate during testing.
2. Start the engine.
3. Move the gear shift lever to the NEUTRAL position.
4. Rotate, by hand, a rear wheel in a forward direction. If the All-Wheel Drive (AWD) system mechanical components are functioning properly, the front wheels will rotate at the same speed.
5. Rotate, by hand, the opposite side rear wheel. If the All-Wheel Drive (AWD) system mechanical components are functioning properly, the front wheels will rotate at the same speed. Failure of the front wheels to rotate during this test may indicate a "closed" clutch pump check valve, malfunctioning gerotor pump, piston, clutch pack, or valves internal to the unit.

Functional Inspection 2

Functional Inspection 2 is a mechanical system inspection for both the left and right side gerotor pumps, pistons, and clutch packs and will assist in determining if:
^ The clutch pump check valve is in the "open" position.
^ The left and right side gerotor pumps have the ability to "pull" fluid from the sump and send a variable and progressive rate of pressurized fluid to the left and right side clutch pack pistons.
^ The left and right side pistons will engage the specific side clutch packs at a progressive rate.
^ There is a mechanical connection between the rear differential ring and pinion, the propeller shaft, the transfer case, and the transmission assembly.

1. Raise the vehicle on a hoist. Lift the vehicle in a manner that will allow all four wheels to rotate during testing.
2. Start the engine.
3. The gear shift lever should be in the PARK position.
4. Rotate, by hand and in a forward direction, a rear wheel at a slow rate of speed. Rotation of the wheel should be possible.
5. Rotate, by hand and in a forward direction, the opposite side rear wheel at a slow rate of speed. Rotation of the wheel should be possible.
6. Rotate, by hand and in a forward direction, a rear wheel at a fast rate of speed. A properly operating pump, piston, and clutch pack will activate and create a progressive increase in wheel rotational resistance.
7. Rotate, by hand and in a forward direction, the opposite side rear wheel at a fast rate of speed.
A properly operating pump, piston, and clutch pack will activate and create a progressive increase in wheel rotational resistance. Failure to feel a progressive rate of resistance may indicate a "closed" clutch pump check valve, malfunctioning gerotor pump, piston, clutch pack, or valves internal to the unit.

Functional Inspection 3

Functional Inspection 3 is a mechanical system inspection for both the left and right side gerotor pumps, pistons, and clutch packs and will assist in determining if:
^ The clutch pump check valve is in the "open" position.
^ The left and right side gerotor pumps have the ability to "pull" fluid from the sump and send a variable and progressive rate of pressurized fluid to the left and right side clutch pack pistons.
^ The left and right side pistons will engage the specific side clutch packs at a progressive rate.
^ The rear differential has the ability to provide rear wheel torque and override the resistance created by the parking brake assembly.
^ There is a mechanical connection between the rear differential ring and pinion, the propeller shaft, the transfer case, and the transmission assembly.

1. Raise the vehicle on a hoist. Lift the vehicle in a manner that will allow all four wheels to rotate during testing.
2. Start the engine.
3. Move the gear shift lever to the DRIVE position.
4. Accelerate the vehicle lightly until all four wheels are rotating at an equal slow speed.
5. Depress the parking brake slightly, gradually increasing the parking brake retention to create resistance in the rear wheels. The rear wheels should stop spinning. The front wheels should remain spinning.

Important: Do not accelerate the vehicle with the parking brake applied for a period longer than 5-7 seconds.

6. Lightly depress the accelerator pedal, increasing engine RPMs in order to determine if the rear wheels can provide enough torque to "override" the resistance created by the parking brake assembly. Failure of the rear wheels to rotate during this test may indicate a "closed" clutch pump check valve, malfunctioning gerotor pump, piston, clutch pack, or valves internal to the unit.