C MUTT8145

Hey guys, after swapping an LS1/T56 into my '87 GTA, I have developed some intermittent electrical gremlins:

• The turn signal and high beam indicators always remain dimly lit.
  
• The fuel and volt gauges read zero unless the cluster back lights are turned on (The back lights don't actually illuminate the panel, I just mean when I turn the dimmer switch all the way up/turn on the headlights).
  
• The oil pressure gauge and water temp gauges also read slightly low unless the cluster back lights are turned on.
  
• When either turn signal/the hazards are turned on, all gauges "twitch" in time with the turn signal indicators, the radio's led clock temporarily goes dark, and the rear defroster relay/indicator light will click/flash, respectively.
  

Every once in a while everything will function fine, but more often than not I experience the aforementioned issues. I'm convinced I have a bad ground somewhere, as I have checked the cluster itself with a multi-meter and found no glaring issues. I have read all the threads I could find regarding similar issues, all of which essentially much came to the same conclusion of there being a poor ground somewhere.


My main question, though, is where does the dash harness ground to the chassis? I gutted the interior early in this build to install a rollcage, but decided to take a different route and reinstalled the interior a few months ago. During this time, the dash harness was left in place, albeit laying on the floor. The OEM engine harness (305 TPI/T5) and ECU were removed from the car. Besides a few connections to the C100/VSS buffer box, as well as the +12v feed from the alternator to the OEM fuse block, my engine harness and under-dash harness are not connected. My engine harness is perfectly grounded via its own custom wiring, but I am unsure if the dash harness was supposed to tie into the OEM ECU harness/grounds somewhere, or if they have their own grounds that I simply overlooked or forgot to connect when reassembling the interior.


I would greatly appreciate any advice on how I should go about finding this bad ground, or any info on how the dash harness grounds out to the chassis. Thank you for your time!

John in RI

iTrader: (167)

Don't know that this will help, but:

Check the C207 dash harness-to-ECM harness plugs under the pass side of the dash. There might be a few BLACK and a few BLACK/WHITE wires at those plugs that *might* need to be grounded.

You can see the plugs I'm taking about as well as each wires ID here: https://www.thirdgen.org/forums/elec...cm-wiring.html

I don't know if grounding the BLACK & BLACK/WHITE wires will help,.... but if those circuits were getting ground from the ECM harness - and that harness is gone - then grounding them again *MIGHT* help.

QwkTrip

iTrader: (22)

On my 89 Firebird there are two ground wires that attach to the metal rail support for the dash right next to the steering column. Look for two black wires with ring terminals.

And there is a third ground wire that went out the passenger side kick panel to the engine bay. That one tripped me up for a while because I accidentally got rid of that wire, so I traced it back to the c207 connector and then extended it again to the rail for the dash support.

I now have blue painter's tape that you see when you take off the dash panels, and written on it is the word "2 GROUNDS ---->™ with the arrow pointing to where the ring terminals are. And then I do similar thing on the passenger side where I connected the third ground, "1 GROUND ---->" with the arrow pointing to the ring terminal.

IROCZman15

iTrader: (25)

woah, interesting! glad i read this. I just eliminated my C207 connector beyond the inside connector. so the wiring still goes to the passenger dash area, but nothing goes through the firewall/fender area. you think i should be mindful to find the ground wires) in the C207)? do you remember what didnt work when you had that particular ground off/disconnected? I haven't taken any ring terminals for grounds off under the dash, but the c207 is certainly disconnected.

QwkTrip

iTrader: (22)

Ya, I was testing my bypass wiring of the PASSkey module and noticed the device had no ground. Eventually figured out it was common ground for the datalink connector, PASSkey module, and the speedometer buffer box.

I think I eventually ended up restoring two wires back to the c207. One was the ground, and the other I used as a power source for my Dakota Digital SGI5 speedometer signal converter.

QwkTrip

iTrader: (22)

By the way, I discovered all this (and more) with a multimeter before ever putting battery to the car. You can jack things up royal by testing wiring with a battery power source. I re-wired probably 70% of my car (the number keeps getting larger as the story is told over the years ) without a single error or problem when battery was connected the first time. That doesn't mean I didn't make errors. I just caught all the errors before ever putting power to anything.

Here's the trick... I did all my wiring checks and troubleshooting using a multimeter in the "continuity check" mode without ever hooking up battery power to the car. Totally safe and won't damage anything if you make a mistake. For example, if I wanted to check a fuse or a circuit then I would connect one lead from the multimeter to the battery side, and the other to whatever I was testing and check for continuity. Same goes for ground checks and testing for short circuits. This is how I checked out the ignition switch, entire fuse box, found bad grounds, made sure of no short to grounds, and made sure my wires went where I thought they went.

The only time that method confused me was when testing the door light switch for interior lighting. That circuit back-fed through another circuit (brake pedal?) and confused the heck out of me for a while. Never did figure it out until I got hold of a proper GM service manual and studied a few wiring diagrams.

IROCZman15

iTrader: (25)

I was dong some wiring today and pulled the black/white stripe wire out of the C207 connector and mounted it to the metal dash support metal bar so thats grounded. I remembered this thread as I was doing it, thanks fellas!!

I also removed the pink/black wire from the C207 and plan to use that to trigger my Holley sniper efi, since that pink/black wire is hot in start and hot in run. right?

otherwise, I don't need anything else from that C207 connector anymore????? perhaps my purple wire if I want to retain my manual toggle switch for my TCC and brake pedal switch involvement??? but this thread did come to mind when I was dealing with that black/white ground wire at the 207 !

QwkTrip

iTrader: (22)

Easy test, turn the key and find out. Do a continuity test from battery cable to the wire; turn the key and find out which positions there is continuity.