Norcal

This is a great FORUM with a lot of experience...

I have been trying to solve this for months and would appreciate help from someone who has experience with this strange problem. Since I have tried all the things from the manual and this Forum I think it may be a unique problem.

I've owned the 2001 GC 4.7L CA emission since new. I did have the transmission rebuilt a couple months before this issue came up.

I think something connected to the TCM is pulling up the signal voltage through the TPS - only when there is a cold start, or if it is below 50 degrees even with a warm engine.

Symptom
Cold start – revs to 1,700 (75 degrees ambient temp) or to 2,200 RPM 56 degrees ambient temp) for 2-3 seconds then to about 1,000 RPM. With colder temps , the car is extremely rough running and surging until 30% throttle or more and warm engine. Check engine light comes on P0123 - TPS High in < 10 seconds every time.


After engine is warmed to temp, code resets with key on and never comes on until the next cold start (unless it is below 45-50 degrees then it continues to fail even with the engine warmed up).



I have driven over 180 miles with no check engine light including multiple restarts. Then the engine starts and revs to about 1,500 RPM and quickly settles down to 1,000 then 700 RPM

Diagnostics so far:

Checked TPS
5.1V in and good ground to PCM and chassis Signal wire backprobed - key on .85v within spec of .3-.9v operates to 3.7V with steady increase of throttle body - engine off.
Key on, TPS connector disconnected there is a static voltage coming from the PCM/TPS of 5.7V which goes down with the TCM is disconnected.



Upon engine start, signal never goes beyond 1.2V at idle yet still triggers P0123
Replaced TPS (Borg Warner) - no change.
Took off throttle body and cleaned. It was not dirty.
Inspected all hose connections with engine running and sprayed carb cleaner listening for changes in RPM with engine running to check for intake leaks but none found.


Ohmed signal and ground wires to PCM/ECM and no resistance or change with wiggling wire bundles
Checked signal wire branch to connector to TCM and it is 0 ohms.
Checked signal wire to all other wires in the bundle and there are no shorts. Visual inspection looks good.


Replaced Clock Spring thinking it could have pulled the signal voltage hi via a bad trace in the PCM


Since it is cold temp related, I unplugged and checked other sensors that transmit temp info. I checked each for 5V input and ground and replaced the following: MAP sensor, Engine coolant sensor, intake air temp sensor


Replaced PCM –No change
Disconnected TCM and the problem went away. And key-on voltage on TPS voltage went down to .65 V.
Replaced TCM – no change with used unit. Same problem.




I wonder what other sensors that might be affected by temp that might connect with the TCM that would pull the TPS signal hi.

dave1123

The only other one I can think of it the fluid temp sensor in the trans and seeing you've just had trans work done, this might be the problem. It's a thermistor on the valve body and comes with the solenoid pack. It connects directly to the TCM and doesn't allow engagement of the O/D between the temp ranges of below 32*F and above 240*F. There is a resistance value per temp chart, but IDK where to find it. It's probably the same as the IAT.

Norcal

Thanks for the idea, I figure the combo of TCM and temperatures must lead to something like that. It that accessible from underneath? I can't find the wiring diagram for the TCM which would help with knowing all the inputs.

Is there any description of which signals interact or affect others?

dave1123

There's a plug on the side of the transmission for all the wiring. I'll see if I can find a pin-out for you. It's inside the pan on the bottom of the valve body and is immersed in the fluid.

dave1123

Norcal

Thanks so much for the diagram. I wonder if a good scanner can tell me the values of the temp signal at the TCM to avoid taking off the pan to look at the actual thermistor.
I will try to check for the resistance at the TCM plug but it would be best to see the operational voltage.

dave1123

IDK if this helps, but I've discovered that most sensors are powered up all the time with the key on, and the grounds go thru drivers in the PCM or TCM. I became aware if this when checking injectors. They are hot and they are grounded in the PCM to operate at the right time. Likewise, the ignition coils are ungrounded to fire the spark.

Norcal

Thanks for the response and wiring diagram, Dave 1123. I see your from SYR area, I am from ROC! Now Norcal. Rather than drop the pan to test or look at the other connectors, today, I unplugged the TCM and it still failed P0123 when cold! Previously that did not fail. I thought it would be a hi voltage leak from the TCM into the TPS signal.

I also tried something else - I unplugged C1, C2 and C3 on the PCM and the TCM - to see if there was a short to the signal wire, gnd or 5V input with the connector to the TPS off. The 5V input on the connector showed 12V! I unplugged all the relays and fuses in the box near the battery and only unplugging the Ignition fuse caused it to go away. Thant makes sense. Once I put everything back in it was 5.1 V again. So, is that normal ops?

Also - with all connectors on except the TPS connector, key on, the signal back voltage is 5.6v coming from the PCM or TCM. It seems weird that their would be a 5.6V coming back to the TPS from the signal wire. Once plugged into the TPS though, it shows .85 V and goes to 4V as I move the throttle to full open - both within spec.

When I start the car cold the TPS signal wire never shows more than 1.5 V or so with no spikes as it revs to 1,500 - 2,000 and settles down to 1,000 then 700 RPM within 7 seconds, and then it triggers the P0123 again.

I think I need to see a good wiring diagram including all the things that connect to the PCM and TCM. Any description of what signals affect the cause of a TPS hi code would be helpful.

At this point, if I bring it to a shop and they have to go through each step, it will many $100s just to get where I am at. I want to figure this out - with the help of the forum :-)

dave1123

I bought the FSM for my WJ from pacificcoastmanuals.com and learned a huge amount about the electronics. The major problem is they show all the external circuits but not the module's internal circuitry. They DO have a lot of diagnostic procedures and circuit checks though. The 2 pages I sent you are from the 42re manual that was a separate publication of part of the FSM.

Norcal

Thanks! I decided to buy the manual to get the whole picture and am reviewing it. The issue shows up in the warm up mode - described below:

Given that once warmed up after resetting the CEL it works fine, I'm thinking the cranksaft and camshaft sensors are not the problem. I already replaced the MAP, Intake and coolant temp sensors and they have never generated a code.


The PCM determines the proper ignition timing
according to input received from the crankshaft position
sensor.
ENGINE WARM-UP MODE
This is an Open Loop mode. During engine warmup,
the PCM receives inputs from:
† Battery voltage
† Crankshaft position sensor
† Engine coolant temperature sensor
† Intake manifold air temperature sensor
† Manifold absolute pressure (MAP) sensor
† Throttle position sensor (TPS)
† Camshaft position sensor signal
† Park/neutral switch (gear indicator signal—auto.
trans. only)
† Air conditioning select signal (if equipped)
† Air conditioning request signal (if equipped)
Based on these inputs the following occurs:
† Voltage is applied to the fuel injectors with the
ASD relay via the PCM. The PCM will then control
the injection sequence and injector pulse width by
turning the ground circuit to each individual injector
on and off.
† The PCM adjusts engine idle speed through the
idle air control (IAC) motor and adjusts ignition timing.
† The PCM operates the A/C compressor clutch
through the clutch relay. This is done if A/C has been
selected by the vehicle operator and requested by the
A/C thermostat.
† When engine has reached operating temperature,
the PCM will begin monitoring O2S sensor
input. The system will then leave the warm-up mode
and go into closed loop operation.
.................................................. .........

IDLE MODE - it works fine after cold start, warm to operating temp and CEL reset - UNLESS its below 40 degrees out.

When the engine is at operating temperature, this
is a Closed Loop mode. At idle speed, the PCM
receives inputs from:
† Air conditioning select signal (if equipped)
† Air conditioning request signal (if equipped)
† Battery voltage
† Crankshaft position sensor
† Engine coolant temperature sensor
† Intake manifold air temperature sensor
† Manifold absolute pressure (MAP) sensor
† Throttle position sensor (TPS)
† Camshaft position sensor signal
† Battery voltage
† Park/neutral switch (gear indicator signal—auto.
trans. only)
† Oxygen sensors
Based on these inputs, the following occurs:
† Voltage is applied to the fuel injectors with the
ASD relay via the PCM. The PCM will then control
injection sequence and injector pulse width by turning
the ground circuit to each individual injector on
and off.
† The PCM monitors the O2S sensor input and
adjusts air-fuel ratio by varying injector pulse width.
It also adjusts engine idle speed through the idle air
control (IAC) motor.
† The PCM adjusts ignition timing by increasing
and decreasing spark advance.
† The PCM operates the A/C compressor clutch
through the clutch relay. This happens if A/C has
been selected by the vehicle operator and requested
by the A/C thermostat.