Quartermaster 505

Headscratcher/call for help – yet another C5 electrical mystery - absolutely deeply grateful for anything at all that anyone can suggest.

1999 C5 FRC
Gen IV LS3 running original PCM
416 Stroker
BTR4 cam
823 Heads, trunnions
HPTuners tuned (speed density tune – MAF is removed)
Mcleod twin disc clutch
Manual transmission
Interior elec all stock hardware

To date, car has behaved normally/well with regard to elec issues

Recent work performed:
- R&R trans/diff/tq tube
- install trans bracing/mounts etc in rear
- install Viking coilovers
- install AMT Vulban motor mounts
- R&R headers – add ceramic coat – speed engineering
- handfab mid pipe, ceramic and heat wrap under tunnel etc

Following work detailed above, the car drove well/properly for a few days, behaved completely normal in all regards.

This past Friday afternoon, the car started, drove a short distance. Stopped car. Idles well. Give power, lift off throttle. Car dies. IPC reports NO COMM with all modules. I disco’d the battery a few times and got it to restart a couple times but it died as soon as I lifted off the throttle after giving it power the first time after a start.

Now, the car simply will not start no matter what we do – detail below on troubleshooting steps we’ve taken; car is at my friend’s shop in Arlington TX and he’s a veteran wrench with many years behind him – I’m not a pro but I’ve got about six years of experience debugging nightmare C5 electrical issues from a previous project and I consider myself pretty experienced, so this is baffling the **** out of me – here’s what we’ve tried so far:

- verified all grounds are cleaned/reinstalled, free of corrosion etc. Verified the rear chassis ground in particular. They are all good.
- replaced battery with new known good batt
- verified charging system behaving at spec
- removed, cleaned, reinstalled main fuse box terminals/nut (was clean already)
- went through both fuse boxes – all good
- replaced Ignition switch as this is a known source of bad behavior – new part is clean and good
- unplugged & replugged BCM connectors, checked clean & dry
- checked TAC & ECU harnesses are clean & dry, good connection
- unplugged BCM star combs, clean & dry
- unplugged BCM Star 2 comb to eliminate those modules, same behavior
- unplugged both BCM Star combs – wired up a manual bypass in which only the IPC-BCM-PCM are live. Same failure behavior
- verified ground to all BCM Star connector pins
- verified door serial connectors/harnesses are clean/fully seated and no shorts (wire insulation is not pulled back or contacting)
- verified power at DLC (OBDII connector where we connect to read the databus)


Current behavior:
Insert key – turn to acc or to run position – fuel pump enables but does not stay on to prime as is usually heard – IPC boots up but the time taken to boot up is much slower than usual (about 5-8 seconds the dash is dark, then it boots). IPC boots, needles sweep gauges. DIC displays all failures it can display; tires, low fuel, reduced power, etc. In diag mode, DIC displays NO COMM for all modules aboard the vehicle.

IPC displays a correct voltage on the batt needle, however coolant temp needle is pegged at max redline. Engine is off, of course.

When we turn key to START position, starter engages and cranks, turns the motor, however no fuel or spark appears to be firing, no start.

At this point the only logical conclusion we are landing on is a PCM that has died – we don’t believe BCM to have died as we can hear the relays in the BCM closing when powered, fuses all good, lights all work etc – but given the serial data config of the C5 our assumption is that if one module in line fails, it can lead to a no comm scenario from all modules, potentially.

Theoretically the IPC, BCM and PCM are all suspects at this time (dead module), but without a spare of each its difficult to rule one out.

We are at our wits end and I’m about to just whip out a credit card and hit up Summit for a Holley Terminator X kit, and just rip all this GM **** out so that I never have to deal with a C5 electrical nightmare ever again.

If anyone out there has anything we haven’t tried, any advice to offer, please do share – I am willing to pay cash if someone experienced out there is up to help troubleshoot either onsite in DFW area or via call. I basically did not sleep this weekend…

mmartinez

I would suspect that either the BCM or PCM has failed causing the no comms condition. The obvious solution is to send both modules to a diagnostic and repair service, a quick internet search will reveal several business that specialize in repair of automotive electronics modules.

lucky131969

I'll PM you some thoughts. Do not send out the PCM or BCM just yet.

C5 Diag

Sounds like a serial data bus issue especially with No Comms to EVERY module…cleaning the buss bars on the Star Connector really doesn’t accomplish that much…does your wrencher use an oscilloscope ??….the single wire data line produces a 0-7 volt square wave when using a scope…with a DVOM probing each data line wire combs removed see if each wire shows about .3-.4 volts or so and the voltage should be fluxuating…if you see 12 volts or near ground on that data wire that’s no good…if you jumper the BCM to the PCM with both Star Connector buss bars or “combs” removed the car should start and the issue is the BCM or PCM…lot of our members have issues with the driver door module connector under the rubber boot between the door and A pillar…shake the boot a few times and see what happens.


https://www.youtube.com/live/f_NEEdg...SZCBZuaxNwtLef

C5 Diag

What are your thoughts on this concern ???…always good to share information on the Forum !!

Quartermaster 505

Thank you sir yes we did shake both doors, checked the connectors, verified the wire insulation isn't pulled back etc as can happen

We did remove both star combs and jumped just BCM IPC PCM still same no start/no comms condition

Did not do an oscilloscope check... let me see if we have that equip handy

C5 Diag

A quick test with your scan tool is remove both combs and jump the purple data wire to the BCM and see if your scan tool can now communicate and then try jumping to the PCM….see which is causing the issue.

oelarse

Here is the drawing of your DLC and the serial wiring .
Try to jump a wire from A to M (BCM) or A to B (PCM) and check if your scanner are able to communicate with any of these modules .
The serial signals looks like the last images with 7 volt pulses if the serial signals are OK

mmartinez

We always mention the serial data buss, my question is which module actually generates the pulses the BCM or PCM?

C5 Diag

All of the modules generate “pulses” between each other…a video below !!…CAN and Class 2 discussed !!

mmartinez

Thanks C5 Diag, having been a electronics technician I like knowing how exactly things work.

C5 Diag

You can find a lot of great videos on CAN and serial data bus communication on YouTube…if you don’t understand how it works you can’t diagnose it…you definitely don’t learn this stuff by turning wrenches !!

lucky131969