RTE61

I've got pcm-0856 c h
and tcs no comm
and sdm u1040 c h
abs light and traction control lamps on cluster
dic says service vehicle soon


I found what they mean but what ground points should i
be checking or would it be faulty ebcm? unpluged ebcm turned
key and reset codes and no change all returned. So before i take off ecbm for repair just want to make sure it's not something else....thanks



update: checking grounds around car but if antbody knows which one would have the tcs and sdm in common it would sure help

73 LS-4

If you want to varify it's not something besides that ebcm, then the next step would be to varify that the ebcm is recieving all it's proper powers and grounds. Do a good visual check of the harness and connectors. And if you want to be real thorough, ohm out your wires between the ebcm and pcm to make sure they all have good wires running between them. If you find any of this faulty you should chase it first, repair and clear and recheck codes.


Pat Kunz

KCvetteowner

The Grounds involved are:

G103 Forward lamp harness, on LH frame rail, near brake pressure modulator valve (BPMV) In the engine compartment, look at the frame rail just behind the front driver-side wheel well.

G108 Forward lamp harness, below the battery tray on the frame rail.

BE SURE your battery is fully charged, when a battery starts going south you can get these errors. Sure you reset the codes on the DIC by holding the reset button until you hear a beep?? Need to reset all the codes under all the different systems.

RTE61

according to absfixers mine is unrepairable because of the no comm problem. I checked the grounds, fuses, connections but before i buy a new ecbm i still would like comformation because i also have air bags not working. so my ? is are the air bags not working because ecbm has no comm or ecbm has no comm because of air bag problem?

73 LS-4

Check the airbag sysem for codes and see what they say, I doubt they are related but you never know.


Pat Kunz

RTE61

air bag code is U1040 Loss of Communications with TCS so i guess my air bag problem is because of tcs problem
and pcm code P0856 - Traction Control input signal is because of tcs too

Bill Dearborn

Code P0856 indicates the PCM isn't receiving a good requested torque signal. That means the EBCM, the wiring between the EBCM or the PCM could have a fault. With the no com on the TCS that is a good indication the problem may be in the EBCM or its voltage supply. Battery and battery ground connections, the ABS fuse, wiring between the underhood electrical center and the EBCM can all affect that along with the EBCM ground which is located at G108. G103 is the ground for the BPMV and pump motor circuit which goes through the relay in the EBCM. This isn't an issue with the relay so G103 is not the culprit.

As for the U1040H code is the air bag warning light lit? The U code indicates the restraint system has missed some State of Health messages which are periodically sent out on the serial bus by all modules. Since you got the code that indicates the SDM is talking to the serial bus. You don't show any other codes for the SDM so I suspect you don't have a problem there unless the air bag light is on. The codes for the SDM are:
B0016, B0017, or B0018
B0022, B0024, or B0026
B0051
B0053
B0090
B0091
B1001

Here is the diagnostic for P0856. This shows up as a 5 column table in the service manual but the way the forum interprets the printing it will show up as a single column table.
DTC C1277 or P0856
Circuit Description

The EBCM and the PCM simultaneously control the traction control. The PCM reduces the amount of torque supplied to the drive wheels by retarding spark timing and selectively turning off fuel injectors. The EBCM actively applies the brakes to the front wheels in order to reduce torque.
The EBCM sends a requested torque message via a pulse width modulated (PWM) signal to the PCM. The duty cycle of the signal is used to determine how much engine torque the EBCM is requesting the PCM to deliver. Normal values are between 10 and 90 percent duty cycle. The signal should be at 90 percent when traction control is not active and at lower values during traction control activations. The PCM supplies a pull up voltage of 5 volts that the EBCM switches to ground to create the signal.
The PCM sends a delivered torque message via a pulse width modulated (PWM) signal to the EBCM. The duty cycle of the signal is used to determine how much engine torque the PCM is delivering. Normal values are between 10 and 90 percent duty cycle. The signal should be at low values (around 10 percent) at idle and higher values under driving conditions. The EBCM supplies a pull up voltage of 12 volts that the PCM switches to ground to create the signal.
When certain PCM DTCs are set, the PCM will not be able to perform the torque reduction portion of traction control. A serial data message is sent to the EBCM indicating that traction control is not allowed.
Conditions for Running the DTC

The engine is running.
Conditions for Setting the DTC

C1277
The PCM diagnoses the requested torque signal circuit and sends a serial data message to the EBCM indicating a fault is present.
P0856
One of the following conditions exists:
•
The PCM detects that requested torque signal is out of the valid range.


•
The PCM does not receive the requested torque signal.

Action Taken When the DTC Sets

If equipped, the following actions occur:
•
The EBCM disables the TCS/VSES for the duration of the ignition cycle.


•
The PCM will store conditions which were present when the DTC set as Fail Records data only.


•
The Traction Control and Active Handling indicator turns ON.


•
The DIC displays the following messages:


-
Service Traction System


-
Service Active Handling


•
The ABS remains functional.

Conditions for Clearing the DTC

•
The condition for the DTC is no longer present (the DTC is not current) and you used the scan tool Clear DTC function.


•
The condition for the DTC is no longer present (the DTC is not current) and you used the On-Board Diagnostics Clear DTC function.


•
The EBCM automatically clears the history DTC when a current DTC is not detected in 100 consecutive drive cycles.


•
The PCM automatically clears the history DTC when a current DTC is not detected in 40 consecutive warm-up cycles.

Diagnostic Aids

The following conditions can cause this concern:
•
An open in the requested torque circuit


•
An short to ground or voltage in the requested torque circuit


•
A wiring problem, terminal corrosion, or poor connection in the requested torque circuit


•
A communication frequency problem


•
A communication duty cycle problem


•
The PCM is not receiving information from the EBCM


•
Loose or corroded EBCM ground or PCM ground


A DTC P1571 may set along with several other PCM DTCs if the key is held in the CRANK position while the engine is running. The starter lockout function of the PCM is enabled several seconds after the engine is running and prevents the starter from engaging while the engine is running. This will cause a partial loss of power to some components and systems.
Test Description

The numbers below refer to the step numbers on the diagnostic table.

3. Clear the DTC in order to verify that the fault is present.

4. Measure the requested torque signal in order to determine if the signal has a valid duty cycle.

5. Measure the requested torque signal in order to determine if the signal has a valid frequency.

11. This vehicle is equipped with a PCM which uses an electrically erasable programmable read only memory (EEPROM). When replacing the PCM, the replacement PCM must be programmed.




















































































































































































































































Bill

RTE61

Thanks that gives me plenty to look at. Not sure how far I can go on the list unless I can find a scanner.

Bill Dearborn

You have all the scanner you need right on the dash board. Just use the DIC to read the codes. Better than any autoparts store scanner you can buy.

Bill

99MALLETT396

My car is having the same problem as yours, did you figuer out a way to fix it? Please let me know thanks.

RTE61

Just to let everyone know the results. I sent my ecbm to modulemaster they could not fix for 150.00 but sold me a rebuilt one for 200.00 + tax and shipping = 227.00. Hook it up and cleared codes and all is back to normal. Except for my code for crank system relearn still looking for a scanner that will allow me to do that. Would rather spend the money for the scanner than to pay for the stealership to relearn it. Thanks for everyone input on this matter.