Intermittent TCS NO COMM
05-22-2011, 01:04 PM
#1
Drifting
Thread Starter
Intermittent TCS NO COMM
Symptoms: On first fire, the EBCM doesn't come online, dash lights up, DTC says NO COMM. After about 10 minutes of driving, it'll come back on magically and report a C1277. After that it works fine for the rest of the day (verified by getting into ABS/traction control and having it activate).
A few years ago I had the EBCM go out, and I took forever to replace it (had lots going on). Once I did that, it was intermittent with codes saying the BPMV was bad (thing wouldn't always cycle). I replaced that over the winter and the system was working fine.
I did have one incident when putting in the ipod adapter into the aux jack on the head unit where moving the climate control box the TCS went out. Once everything was put back together and the battery cable pulled/replaced this were back to normal.
Is there a ground (or something else) shared between the TCS/HVAC that could be questionable? When the EBCM first went out the HVAC module went out as well, but somehow I was able to get that online and working again, but not the EBCM. If it helps, the display on the HVAC module are now very dim.
I've previously checked the grounds behind the driver's side headlight, and also by the battery, I've been through the serial wires from the star connector (or whatever they call it) to the EBCM. Is it worth building a jumper to connect the EBCM and the PCM together and turn off all the other modules to see if its still intermittent?
Oh, car is an early 01Z.
A few years ago I had the EBCM go out, and I took forever to replace it (had lots going on). Once I did that, it was intermittent with codes saying the BPMV was bad (thing wouldn't always cycle). I replaced that over the winter and the system was working fine.
I did have one incident when putting in the ipod adapter into the aux jack on the head unit where moving the climate control box the TCS went out. Once everything was put back together and the battery cable pulled/replaced this were back to normal.
Is there a ground (or something else) shared between the TCS/HVAC that could be questionable? When the EBCM first went out the HVAC module went out as well, but somehow I was able to get that online and working again, but not the EBCM. If it helps, the display on the HVAC module are now very dim.
I've previously checked the grounds behind the driver's side headlight, and also by the battery, I've been through the serial wires from the star connector (or whatever they call it) to the EBCM. Is it worth building a jumper to connect the EBCM and the PCM together and turn off all the other modules to see if its still intermittent?
Oh, car is an early 01Z.
05-22-2011, 04:54 PM
#2
Tech Contributor
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Location: Anthony TX
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St. Jude Donor '08
Document ID# 684616
2002 Chevrolet Corvette
--------------------------------------------------------------------------------
DTC C1277 or P1571
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.
P1571
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.
Clear the DTC in order to verify that the fault is present.
Measure the requested torque signal in order to determine if the signal has a valid duty cycle.
Measure the requested torque signal in order to determine if the signal has a valid frequency.
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.
Step
Action
Values
Yes
No
Schematic Reference: ABS Schematics
Connector End View Reference: ABS Connector End Views or Powertrain Control Module (PCM) Connector End Views in Engine Controls-5.7 L
1
Did you perform the ABS Diagnostic System Check?
--
Go to Step 2
Go to Diagnostic System Check - ABS
2
Inspect the EBCM ground and PCM ground, making sure each ground is clean and torqued to the proper specification. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 3
3
Install a scan tool.
Turn ON the ignition, with the engine OFF.
Use the scan tool in order to clear the DTCs in both the EBCM and PCM.
Turn OFF the ignition.
Start the engine.
Does the DTC reset as a current DTC?
--
Go to Step 4
Go to Testing for Intermittent and Poor Connections in Wiring Systems
4
Turn OFF the ignition.
Disconnect the EBCM harness connector.
Install the J 39700 universal breakout box using the J 39700-300 cable adapter to the EBCM harness connector and the EBCM connector.
Start the engine.
Measure the DC duty cycle between the requested torque signal circuit and a good ground.
Is the duty cycle within the specified range?
5-95%
Go to Step 5
Go to Step 6
5
Measure the DC Hz between the requested torque signal circuit and a good ground.
Does the frequency measure within the specified range?
121-134 Hz
Go to Step 8
Go to Step 6
6
Turn OFF the ignition.
Disconnect the J 39700-300 cable adapter from the EBCM connector.
Important
Disconnecting the EBCM connector and turning ON the ignition could cause other modules to set loss of communication DTCs (Uxxxx). Once the EBCM is reconnected, the EBCM may set DTC C1298.
Turn ON the ignition, with the engine OFF.
Measure the voltage from the requested torque signal circuit to a good ground.
Does the voltage measure within the specified range?
4-6 V
Go to Step 10
Go to Step 7
7
Turn OFF the ignition.
Disconnect the powertrain control module (PCM) harness connector.
Test the requested torque signal circuit for the following conditions:
A short to voltage
A short to ground
Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 10
8
Turn OFF the ignition.
Disconnect the powertrain control module (PCM) harness connector.
Test the requested torque signal circuit for the following conditions:
An open
A high resistance
Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 9
9
Inspect for poor connections the harness connector of the PCM. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 11
10
Inspect for poor connections the harness connector of the EBCM. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 12
11
Important
The replacement PCM must be programmed.
Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement in Engine Controls - 5.7 L.
Did you complete the repair?
--
Go to Step 13
--
12
Replace the EBCM. Refer to Electronic Brake Control Module (EBCM) Replacement .
Did you complete the repair?
--
Go to Step 13
--
13
Use the scan tool in order to clear the DTCs.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC reset?
--
Go to Step 2
System OK
--------------------------------------------------------------------------------
Document ID# 684616
2002 Chevrolet Corvette
2002 Chevrolet Corvette
--------------------------------------------------------------------------------
DTC C1277 or P1571
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.
P1571
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.
Clear the DTC in order to verify that the fault is present.
Measure the requested torque signal in order to determine if the signal has a valid duty cycle.
Measure the requested torque signal in order to determine if the signal has a valid frequency.
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.
Step
Action
Values
Yes
No
Schematic Reference: ABS Schematics
Connector End View Reference: ABS Connector End Views or Powertrain Control Module (PCM) Connector End Views in Engine Controls-5.7 L
1
Did you perform the ABS Diagnostic System Check?
--
Go to Step 2
Go to Diagnostic System Check - ABS
2
Inspect the EBCM ground and PCM ground, making sure each ground is clean and torqued to the proper specification. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 3
3
Install a scan tool.
Turn ON the ignition, with the engine OFF.
Use the scan tool in order to clear the DTCs in both the EBCM and PCM.
Turn OFF the ignition.
Start the engine.
Does the DTC reset as a current DTC?
--
Go to Step 4
Go to Testing for Intermittent and Poor Connections in Wiring Systems
4
Turn OFF the ignition.
Disconnect the EBCM harness connector.
Install the J 39700 universal breakout box using the J 39700-300 cable adapter to the EBCM harness connector and the EBCM connector.
Start the engine.
Measure the DC duty cycle between the requested torque signal circuit and a good ground.
Is the duty cycle within the specified range?
5-95%
Go to Step 5
Go to Step 6
5
Measure the DC Hz between the requested torque signal circuit and a good ground.
Does the frequency measure within the specified range?
121-134 Hz
Go to Step 8
Go to Step 6
6
Turn OFF the ignition.
Disconnect the J 39700-300 cable adapter from the EBCM connector.
Important
Disconnecting the EBCM connector and turning ON the ignition could cause other modules to set loss of communication DTCs (Uxxxx). Once the EBCM is reconnected, the EBCM may set DTC C1298.
Turn ON the ignition, with the engine OFF.
Measure the voltage from the requested torque signal circuit to a good ground.
Does the voltage measure within the specified range?
4-6 V
Go to Step 10
Go to Step 7
7
Turn OFF the ignition.
Disconnect the powertrain control module (PCM) harness connector.
Test the requested torque signal circuit for the following conditions:
A short to voltage
A short to ground
Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 10
8
Turn OFF the ignition.
Disconnect the powertrain control module (PCM) harness connector.
Test the requested torque signal circuit for the following conditions:
An open
A high resistance
Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 9
9
Inspect for poor connections the harness connector of the PCM. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 11
10
Inspect for poor connections the harness connector of the EBCM. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 13
Go to Step 12
11
Important
The replacement PCM must be programmed.
Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement in Engine Controls - 5.7 L.
Did you complete the repair?
--
Go to Step 13
--
12
Replace the EBCM. Refer to Electronic Brake Control Module (EBCM) Replacement .
Did you complete the repair?
--
Go to Step 13
--
13
Use the scan tool in order to clear the DTCs.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC reset?
--
Go to Step 2
System OK
--------------------------------------------------------------------------------
Document ID# 684616
2002 Chevrolet Corvette
05-23-2011, 09:29 PM
#3
Drifting
Thread Starter
I do think I should figure out the intermittent no comm first, right? Once thats done then I can look to the C1277.
So what does the HVAC and EBCM have in common? The grounds aren't common, so I'm baffled here.
So what does the HVAC and EBCM have in common? The grounds aren't common, so I'm baffled here.
05-28-2011, 12:24 AM
#4
Drifting
Thread Starter
Turns out its a loose EBCM connector. The safety broke, and I put a small zip tie in there, I guess it allowed enough movement for things to come and go. I'll have to trim down a big zip tie to fit and get it in there.
05-28-2011, 11:02 AM
#5
Tech Contributor
Member Since: Dec 1999
Location: Anthony TX
Posts: 32,736
Received 2,180 Likes
on
1,583 Posts
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Its amazing how the simple things screw us ut as much as the big things! 
Good job!
BC
Good job!BC
