Hi hope someone can help me.

I have a 1995 coupe vette around 80k miles that is not starting. I’m using a GM MDI and tech2win and I get a history CCM DTC 41:PCM loss of Comm”. This DTC is only in history. I don’t get any current DTCs which I assume means it DTC happened in the past but not registering currently.

However using tech2Win I get the following behaviors

When in the PCM menus:
I get intermittent drops in connections in tech2Win
- If I go to read sensor data some times it works then connection drops and works again.
- If I read the VIN I get dropped connections then eventually gives me the VIN number
- If I try to turn off the Malfunction indicator lamp (service engine light) it only says off a few seconds then turns on by itself.

When on the CCM menus
Everything works fine with not intermittent connections problems on tech2Win
- If I read sensor data it works
- If I read the VIN it works
- If I turn off indicator lamps they stay off until I turn them back on

So this leads me to believe there is an intermittent PCM loss of comm that it threw a DTC in the past but currently still have the problem but not severe enough to throw a new DTC

I’m using the FSM and said to check for faulty PCM connections and unwanted voltages on serial data line. Power and grounds are OK however I’m getting intermittent voltages on terminal 9 of the OBDII connector (serial data line). I get a fluctuating voltage from 4-5v. I isolated all the modules that need to be connected to the CCM serial data line (PCM, air bag, TCM, HVAC). So it seams that the CCM itself is generating this voltage because the voltages on the serial data line cease only when I disconnect the CCM from the harness.

I read on the FSM that the CCM is the serial data line master and that the CCM is awake even if there is no key in the ignition.

If there is no key in the ignition should there be a fluctuating 4-5 v on the CCM serial data line? Is this part of its function of being awake or is the CCM creating this noise that is preventing the PCM to communicate with the CCM? I’m trying to determine if I have a bad PCM or CCM.

 

If you could get your hands on a Tech1 I'd try that and see if it reflects the same communication errors. Most everyone I know if it's an older OBD1 car they grab a Tech1 in preference to using a Tec2 for most tasks.

A communication problem I'd think I'd prefer "technology of the times".

 

^ i'd agree.

In the past i've had a history code 41 and could start the vehicle. However if it is current, the vehicle will not start. Something seems out of place...

FWIW, i'm pretty sure my H41 code was generated when datalogging with datamaster. Not sure if it was an intermittent connection or the program itself, but it hasn't come back since the last datalog, nor did I have it before datalogging.

Did you try clearing codes from the DIC?

 

Any signs of battery being drained or rough idle?

 

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I don't have access to a tech1... I could not find one on eBay. I thought about getting a tech2 or the MDI which emulates tech2. I decided in getting an MDI clone. I thought that the tech2 (or emulator) had all the functionality of the tech1.

 

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I have also tried to connect TTS datamaster . In the past I had been successful but this time I have not been able to connect successfully. i get a lot of dropped connections.
I jumpered the OBD according to FSM to see DTCs on the instrument panel but I only got CCM DTC12.... Which means everything OK.

I also checked the DTCs with the tech2win:
-on the PCM i don't get any current DTCs. if i check for history ones i get dropped connections from tech2win and PCM like i mentioned before then eventually it goes through and tell me there are no history codes.
-I don't get any current DTCs on the CCM. only the history DTC41 that i mentioned on the first post. I was able to clear the history DTC codes successfully and no new codes have appeared.

 

I have had some problems for a while and had not been able to figure them out until recently new problems came up and now car does not start.

Earlier Symptoms

• Engine would not start after rain if I hit rain puddles while driving. i had to wait for a while for it to start (for years I only used car when there was no rain to avoid getting stranded)
• while on highway driving if i let go of accelerator it seems like the transmission downshifted and if I accelerated again then car seems to have trouble getting back to the faster gear. it seemed to needed higher RPM to get back to the original gear and when it did the car jolted so it was not a smooth shift.

Recently within a week all these additional symptoms came up one after the other:

• Engine hard to start. I had to keep pumping accelerator where before i could turn engine on without touching accelerator.
• when it finally started the Engine had a rough idle where before it ran smooth. it worked like this for a few days.
• then one time while driving home a SYS indication came out in the instrument panel.
• next day the engine would not start (only crank)
• now Engine hard to crank. (does not crank continuously). i could hear the starter solenoid engaged with the engine but engine would not turn easily. like if engine too hard to turn.

I suspected the battery and even thought the guy at auto store said it was OK i decided to change it anyway. After the battery replacement it cranks better than when had old battery but only slightly better. still seems like starter struggles to keep engine cranking. i also replaced both positive and negative cable connectors to battery post since the stock ones had corrosion hidden under the plastic cover. Still not starting just a weak crank. I can crank maybe 2-3 times then i just hear the starter solenoid engage but starter cant turn engine.

At this point is when i started tracing the history DTC41 like i mentioned on my first post.

 

Check out this thread for lots of information on DTC 41:

https://www.corvetteforum.com/forums...code-41-a.html

 

The history DTC that i mentioned I got from the CCM:
CCM DTC 41 PCM loss of communication

Not the one you suggested from the link:
PCM DTC 41 Ignition control circuit

Are they related?

 

I guess I messed that up. I thought that thread was about how to fix a DTC 41 communications fault. I have seen a thread on that topic that went into it in depth, which was what I was trying to post.

Maybe somebody else who knows the correct thread can post it.