exploder

I had a customer bring a car in today. He had a dead battery and jumped it off to drive it. After that he says that the car would crank and drive, but all of the lights on the dash were on. All of the gauges aside from the tach and alt. gauge do not work. I checked for codes and got no comm with all modules. I replaced battery with a new interstate and still no comm. I checked all grounds and still no comm. I need something else to try.

byronhunter

If car was fine before the dead battery, easy things, first just in case he hooked the jumper cables up wrong. Check every fuse. Dead battery and jumper cables should not cause the problem if done correctly.

exploder

I already checked all fuses in the engine bay and in the footwell.

dgrant3830

I'm guessing that your battery isn't up to full power yet. It needs to be all the way up before driving and expecting things to work normally.

Bill Curlee

Your problem is most likely one of two things:

- Your BCM is wet or was wet. Most likely caused by a clogged HVAC Air Box Drain. If that drain cloggs, the condensation backs up in the air box and drips on the BCM. If the carpet underlayment isnt wet, it may or may not have leaked. It doesnt take much moisture to cause those exact symptoms.

- There are communications wires inside the door wiring harnesses. The serial buss wires in that door wiring harness can short out because the wiring jacket pulls back on the wire. Open BOTH doors and pop out the rubber accordion tubes to expose the wiring harnesses. Fish the door wiring harnes connectors out of the door frame pocket and examine the connectors and look for that bare wire. Seal it up with liquid electrical tape and I bet you will be all set!!





If you shake the hell out of the accordion tube,,,I bet you the symptoms will change for the better! Recommend that you seal up the exposed wire with liquid electrical tape.

Please let me know how you made out.

BC

exploder

Ok so we are reasonably sure that the BCM is bad....called gm and the part is over a grand. Is there anywhere else I can get one?

dgrant3830

A BCM isn't that much money usually. Check with Gene Culley at gmpartshouse.com . He's always got the fairest prices around and the best customer service, on top of insuring you get the correct part. Just so you know up front, replacing the BCM must be done with a Tech2 to get all of the RPO codes into it and the security working otherwise the car will not start.
Bill Curlee is your best person to diagnose things. Listen to him.

Bill Curlee

WHAT DTCs are you seeing other than NO COMMS?

Did you try the door harness check????

Did you check ALL the BCM fuses and have you checked each BCM fuse to see that its getting 12 VDC??

Im trying to save you time and money. The BCM and PCM are NOT plug and play modules and must be programmed using the tech II/ There is an Emergency Sync procedure that you can use if you have to install a BCm on the road with out a TECH II:
-----------------------------------------------------------
BCM/PCM RELEARN PROCEDURE


You will need to conduct the PCM/BCM relearn process. This matches their handshake ID's and allows them to talk to each other.

Do this before you worry about VATS issues:

1. Turn on the key for 11 minutes.
2 Turn off the key for 30 seconds.
3. Repeat 1&2 two more times.
4. Turn on the key for 30 seconds.

The car should start and run with the new BCM. Good luck and report back on your progress.
Shirl
SD Racing Enterprises
Aguila, AZ
Coutersy of MrEracer


This will sync the PCM to the New BCM but all of your RPO options will not be retained. You will need a TECH II to program them into the new BCM.


If that doesnt work, Unplug all un necessary modules ie: Door Modules, Drivers seat Module, HVAC module, RFA Module, etc and see if the issue goes away The ONLY modules that you need to start the car is BCM and PCM.

If you isolate all those things, it will tell you if some other module is causing the issue:

BC

exploder

I have a tech 2 here at the shop and I know how to reprogram everything after replacement of the BCM or PCM. I have absolutely no codes only no communication through the DIC and the Tech 2. I tried messing with the door modules and I found no bare wires and wiggled the crap out of them and had no change. I cannot find a BCM for less than a grand anywhere...I am guessing it's because this car is a convertible. I did find that the battery had been leaking some acid down and it had landed on both the PCM and BCM.

Bill Curlee

What price did Gene Culley give you??

Do you have all the power and proper grounds for the BCM?

In my 98 it is FOUR fuses that supply the BCM:
Instrument Panel Electrical Center

BCM & IPC, Mini fuse# 25
BCM2 Minifuse #23
BCM A Mini Fuse #9
BCM /1 Mini Fuse# 13

Theres also an Data Link Connector fuse that should be checked

C/ALDL , Mini Fuse# 29

The grounds for the BCM are at:

G-104 (On the frame next to the battery)



& G-202 in the cabin



The ground for the BCM goes through a SPLICE PACK (SP-208). Its located on the engine compartment fire wall on the main wiring harness behind the battery. It look like this:


If it gets corroded, it looks like this!!!




There are TWO small grey connectors next to the BCM (to the left of it) There the STAR connections. The connectors join all the serial data wires to each module. The cap on each connector is a shorting buss that connects all the wires together.

On Star connector#1 the BCM and PCM serial data wires are on:

BCM = Pin "M" (Light Green wire)
PCM = Pin "B" (Dark Green wire)

You can read the serial wire to ground and if memory serves me correctly, you should see around 750 ohms.

For a test, you can disconnect the STAR connector cap on STAR Connector # 2 and see if it changes symptoms. It connects the left and right door control modules and seat control module and ALDL OBDII terminal to the main STAR buss. That will take those modules serial data line out of the equasion. It wont harm anything!

Make sure that the STAR shorting caps are clean dry, free of corossion and on tight.

Hope this helps

Bill C

Bill Curlee

If all the above things fail to help,,,,,you can take the BCM circuit board OUT of its case and see if there any damaged components on it.

I have seen boards that have been exposed to moisture in the pass foot well grow corrosion on the back of the board. That corrosion caused simular issues:

Heres a pic of the board. Be Careful,,,its ESD Sensitive!

exploder

Ok so I have installed a new BCM...programmed it with tech 2. I have also put a brand new ignition switch. I have also gone and got the corvette specific j tool for read star connectors. I still do not have the car fixed and I still have all of the same problems. Anybody got any more ideas.

exploder

Yeah I found the bcm in the footpanel. I didn't replace the tac module. Sorry for the confusion on that one.

Bill Curlee

Ok,,New BCM, power to modules is reported SAT. (you did check,,,correct. IPC and BCM??) Grounds to the modules need to be checked also.

Try this. Disconnect all the other modules. left and right door control modules, HVAC, RFA, SEAT Control Module, etc... You should only have the BCM, PCM and IPC on the serial buss. If another module is having an issue and interfearing with the IPC/BCM/PCM,, this will show that. The car should run fine with out all the other modules. You will get a LOT of NO COMMS messages.

BC

exploder

What I don't understand is this...I have done all of this and I can still read the PCM with the techII and everything is reporting fine...coolant/oil pressure/fuel etc. but the Instrument panel cluster maxes out the coolant hot, fuel goes to 0, oil pressure goes to 0 etc and this throws the car in to REP....GM dealerline says if you have these symptoms with the PCM reading and the IPC not it says replace IPC....I replaced the customers with mine and it does the same thing. I put it back in my car and it works fine. I can't get my head around all of this. I really need some more help. Another crazy thing is it drives fine while in REP...it doesn't hickup buck or anything.

huttler

Bill Curlee

Most REDUCED ENGINE POWER issues are a result of the Throttle By Wire system and componets. Is this a false REP issue or omething to do with the TAC/PCM and the other Throttle Position Componets???

Just thinking out loud.

exploder

I did remove the pcm completely and removed both the red and blue connectors. I checked for corrosion and found none. I also checked to make sure all of the pins were pushed in completely. I did not ohm each wire. I have not checked the coolant temp sensor for a malfunction or broken wire though. I will be doing that tommorow. I also have not checked the wires to the Drive by wire system but again will check that tommrow. I can say that all of those symptoms are exactly what this car is experiencing right now (good find by the way). I believe this REP is a false one as the car doesn't drive anything like a car that has gone into REP.

Bill Curlee

Remember,,,you can totaly un-plug th star 2 connector and easily isolate the star 2 stuff.

BC

exploder

I am going to unplug it again...I did that once, but I don't really remember what it did or did not do. So many ideas and things to try tommorow. Hey bill can I call you in Guam? Seriously...