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Hello all, apologies if thits isn't posted properly, this is my first post here. I'm chasing an issue with a 1998 corvette I just bought and need some advice. The car runs and drives great, however there is an intermittent issue while the key is on, whether driving, idling, or sitting in accessory or run position. First the DIC will display "Service vehicle soon" sometimes accompanied with the "service column lock" message, then everything goes out, the HVAC head lights, the active handling lights, and all interior lights go out, the traction control button stops working, the hatch button and fog light switch stop working, and if the car isnt running the radio shuts off. The security light comes on, and then everything works again for a few seconds until the issue repeats over and over. It seems to me like the BCM keeps going in and out, but I can't figure out why. It isn't a security problem because it will still start. I recently went through a lot of the grounds and cleaned them up thinking that would help, but the issue hasn't gone away. Not sure if it is a related issue or a separate issue, but if you shut the car off you have to wait a few minutes for it to start because it will just click. There are quite a few codes, most modules having a U1064. If anybody has seen this issue or has any leads on what to check it would be much appreciated
All codes present: {PCM- P0801, P1341, P1626} {BCM- B2527 (Horn relay, the horn relay is constantly energized and thus unplugged)} {Instrument panel cluster- U1064, U1160, U1176} {Heating and air conditioning - B0363, B0446, U1064, U1160, B0361, B0363, B0365, B0367, B0441} {Left Door control module- U1064} {Radio- U1064} {RFA- C2115, U1064} {Right Door Control module- B2253, B2283, B2285, B2265, U1064, U1096} {Seat Control module- B0851, B0856, U1064, U1160}
Sounds like an ignition switch issue when different components suddenly don’t work…if you know how to use a multimeter when all this is happening you can check battery voltage with key on and then check voltage on a few different fuses…try fuse 18 (HVAC) in IP fuseblock…when you compare battery voltage to voltage at fuse 18 they should be only a 0.1 to 0.2 voltage difference.
Ok awesome I will try that, I saw in one of your other comments to check voltage drop from battery to fuse 21 I think after the ignition switch so i was going to try that later. It's strange because the hvac display will still light up and show temp its just the lights for the buttons that go out as well as the interior lights and I can hear the relays clicking on and off on the IPC lile the bcm is just shutting down repeatedly. I will check voltages in a bit and see what I have.
Well, I checked all of the fuses to the BCM and modules in the IPC, they're all getting battery voltage and there is minimal voltage drop from the battery to the fuses after the ignition switch. I pulled the BCM out and inspected it, no water damage and everything looks good, tightened down the screw that holds the board to the metal casing and tightened the ground wire that goes from the casing to the column lock module and I thought I fixed my issue. No column lock message and no BCM cutting in and out the whole ride home (50 or so minutes, it was wigging out earlier just pulling into the shop after 5 minutes) I get close to home and hit the window switch and the lights go out again and it's back to doing its thing so maybe I have a lead on that. I checked the serial bus earlier from the DLC thinking maybe I'm having a bus issue and it was a 7 volt square wave that seemed to come and go, not sure if the wave is supposed to be constant but it was definitely there. Not really sure what to do from here though.
You may try to disconnect the star connector ( 4 wires) temporarly in order to see if the serial dataline to these modules are corrupting the serial com bus .
If these wires are partially broken due to opening/closing of doors a partially broken serial bus wire or connectors in doors could disturb the serial communications
Ok I will try that next, I tried wiggling the door accordians and everything and nothing has changed. Would it have serial DTCs if the bus was getting corrupted or can it just happen without a code?
If any of these three serial lines , mainly the doors are causing serial communication problems they will dissappear when this star connector are disconnected (doors/window/seats are not working when this star connector are disconnected .
I think only DTC U1255 will be set when communication is totally is missing from any of the modules a 1998 car
Normally if the data bus is shorted to power or ground you will have either a U1300 or 1301 and the car will either start and stall or not start…yes, you’ll see a 0-7 volt square wave and I set my time base in MICRO seconds…is the signal just disappearing or going to 12 volts or ground…I would remove all 3 BCM connectors and check power and grounds…I would make sure those power and ground terminals aren’t spread apart and if you can do a “pin drag” or a terminal tension test I would do that…if you can verify all BCM powers and grounds are good it looks like you need a new BCM…I’ll post the BCM pin out below…just concentrate on battery and ignition feeds and the grounds…the BCM C2 connector is the top one, C1 is middle and C3 is bottom.
Alright so I checked all of the power and grounds to the bcm, everything looks good. Voltage drop across ground was minimal and power was alternator voltage and never went away. I found that removing the courtesy lamp relay (42 I believe) causes the bcm to stop shutting off, so until I figure this out I guess I will leave that relay out. So at this point I'm not sure if it's the bcm or something to do with the courtesy lamp circuit causing it to drop off.
This is your relay…BTW, fuse 29 CL/ALDL powers pin 16 of the DLC (where your scan tool hooks up to)…what are you seeing on the scope ??…is the signal dropping out or you seeing 12 volts or ground on the waveform ??
I don't want to lead this troubleshooting in a different direction, but I believe that the problem is a Ignition switch problem or a problem with the relays the Ignition switch powers up, my 2 cents.
Seems like the signal is just dropping out like the modules were talking, it goes from 7v square wave to flat and keeps doing that in little bits, I will pull It in and get a picture of the signal later but it didn't seem like it was shorted or anything and there wasn't 12 Volts.
I don't want to lead this troubleshooting in a different direction, but I believe that the problem is a Ignition switch problem or a problem with the relays the Ignition switch powers up, my 2 cents.
I'm not doubting or arguing with you, but wouldn't I see a voltage drop on the ignition wire going to the bcm if it was an ignition switch problem? From the battery terminal to the ignition feed from the bcm there's minimal drop, when the bcm cuts out it still has source voltage. I can check it again just to be sure though.
Seems like the signal is just dropping out like the modules were talking, it goes from 7v square wave to flat and keeps doing that in little bits, I will pull It in and get a picture of the signal later but it didn't seem like it was shorted or anything and there wasn't 12 Volts.
With battery disconnected measure the resistance from pin 2 to pin 4 (chassis ground)…that ground is G200 which is also the same ground for the left instrument panel courtesy lamp if you refer to that wiring diagram…in either case you should a very HIGH resistance…I believe I saw 8.8 K ohms (8,800 ohms) on my 2001…what do you have there ??..I also see that G203 is involved in that courtesy light circuit and is one of your BCM grounds…normally if a module like the BCM loses a ground it can back feed power onto that data wire and if the module is just bad it can back feed power or ground onto the data wire…and this is what I was taught.
So it might not be the BCM not is cutting out but rather that it is maybe loosing ground connection. I am going to follow this thread as I hope you determine what the final resolution is. Good luck
I've checked ground and power to the bcm while it was acting up, it's constant source voltage on all the battery lines and the ignition switch line even when the bcm cuts out. The most voltage drop I saw going back to the battery negative post from each of the grounds was .07v so I'm pretty confident power and grounds are good, I will check the bus again today with the scope and check resistance from bus power to ground at the DLC and report back. If the waveform is good and power and grounds are good one would assume it would point to a bad BCM?
I'm not doubting or arguing with you, but wouldn't I see a voltage drop on the ignition wire going to the bcm if it was an ignition switch problem? From the battery terminal to the ignition feed from the bcm there's minimal drop, when the bcm cuts out it still has source voltage. I can check it again just to be sure though.
Ignition switch contacts more often than not yield intermittent failures. So if you are taking voltage measurements on the load side of the ignition switch when you have a hard failure, that is one thing. If you taking voltage measurements when the system is static and nothing is malfunctioning, that tells you nothing related to the failure. If you do decide to take a look at it, just replace it with a quality replacement. I'm not a parts changer, but if you have to remove and open it up, you might as well change out the 25 year old ignition switch
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That said, your specific issues appear to be triggered via the serial buss. You have the due dilegence checks out of the way, now it's time to start isolating.
C5 BCM issues
