Massive electrical problem C4
Thread Starter
Drifting
Joined: Sep 2009
Posts: 1,758
Likes: 1,501
From: Willis, tx
Massive electrical problem C4
91 roadster six speed, 180k miles, rebuilt engine at 170k. The car runs perfect until the other day it died in the driveway. I got it restarted and into my shop. It will hardly start, sputters and dies. Now get this, key off, step on break and the LCD lights up for a moment when I let the brake off. Key on, LCD lights up as normal, but step on the brake and it starts dimming and is out in about three seconds. Let the brake off LCD returns to normal. Turn the lights on step on the brake and the LCD doesn't dim. Now also no tail, brake lights, or rear side markers. I can get it to run (rough), stepping on the brake will kill it. I suspect a massive short but don't know where to start. I have looked at the wiring (nothing obvious), and cleaned the hot wire distribution post behind the battery. Open to any suggestions.
Melting Slicks
Joined: May 2014
Posts: 2,888
Likes: 590
From: New Port Richey FL
91 roadster six speed, 180k miles, rebuilt engine at 170k. The car runs perfect until the other day it died in the driveway. I got it restarted and into my shop. It will hardly start, sputters and dies. Now get this, key off, step on break and the LCD lights up for a moment when I let the brake off. Key on, LCD lights up as normal, but step on the brake and it starts dimming and is out in about three seconds. Let the brake off LCD returns to normal. Turn the lights on step on the brake and the LCD doesn't dim. Now also no tail, brake lights, or rear side markers. I can get it to run (rough), stepping on the brake will kill it. I suspect a massive short but don't know where to start. I have looked at the wiring (nothing obvious), and cleaned the hot wire distribution post behind the battery. Open to any suggestions.
Last edited by s carter; May 14, 2017 at 06:16 PM.
Thread Starter
Drifting
Joined: Sep 2009
Posts: 1,758
Likes: 1,501
From: Willis, tx
S Carter, I took your advice and began looking for bad grounds, started tracking the tail lights from back to front. I came to a gang plug where the harness goes into the right rear interior compartment (where the top folds into). What a corroded piece of crap. While I was getting the carpet away for a look see two wires broke out of the plug and a couple more are just hanging on by a thread. It is covered with green corrosion. Since the plug is unusable I will have to plug each wire in separately. With that plug undone the car won't run because of the fuel pump wire is in the plug but the LCD works correctly now.
If I could give some advice, you C4 owners it would be advisable to look at that plug and clean any corrosion from it. I don't know why it corrodes being in the interior of the car but mine sure did.
If I could give some advice, you C4 owners it would be advisable to look at that plug and clean any corrosion from it. I don't know why it corrodes being in the interior of the car but mine sure did.
Team Owner
Joined: Aug 2008
Posts: 32,968
Likes: 6,464
From: Rochester NY
St. Jude Donor '09 thru '26
Regardless of whether or not that solves ALL your problems, it sounds like you found something that was badly in need of repair. Just remember to check it again when you hit about 360k miles! 
Melting Slicks
Joined: May 2014
Posts: 2,888
Likes: 590
From: New Port Richey FL
S Carter, I took your advice and began looking for bad grounds, started tracking the tail lights from back to front. I came to a gang plug where the harness goes into the right rear interior compartment (where the top folds into). What a corroded piece of crap. While I was getting the carpet away for a look see two wires broke out of the plug and a couple more are just hanging on by a thread. It is covered with green corrosion. Since the plug is unusable I will have to plug each wire in separately. With that plug undone the car won't run because of the fuel pump wire is in the plug but the LCD works correctly now.
If I could give some advice, you C4 owners it would be advisable to look at that plug and clean any corrosion from it. I don't know why it corrodes being in the interior of the car but mine sure did.
If I could give some advice, you C4 owners it would be advisable to look at that plug and clean any corrosion from it. I don't know why it corrodes being in the interior of the car but mine sure did.
Well I Commend you on finding That Nasty Plug and hope it's repair goes well, and fixes all. But I Can't get totally past that you may have a problem with one of the many wires and plugs you needed to touch in doing the rebuild awhile ago. I would look at some of those Connection, I would at least give a good look at the Engine to Body Ground and the connections at the Starter. Somewhere you are getting Back Feeds Looking for Grounds and that is why some things dim and other things work.
Last edited by s carter; May 14, 2017 at 09:22 PM.
Thread Starter
Drifting
Joined: Sep 2009
Posts: 1,758
Likes: 1,501
From: Willis, tx
Thanks, I'll do a thorough check on plugs and grounds. Here is something that may help someone sometime. About a year ago my battery would run down if the car sat for a couple of days. Since I drive the car every day it was not a big concern. One night I walked by the car in the dark and noticed my tail lights and interior lights were glowing. It was so slight that it wasn't visible in the daytime. This condition was very intermittent and I never figured it out. Now I believe this corroded plug or some other one is feeding power into a ground.
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,214
Likes: 1,173
From: Southlake, TX
I came to a gang plug where the harness goes into the right rear interior compartment (where the top folds into). What a corroded piece of crap.
If I could give some advice, you C4 owners it would be advisable to look at that plug and clean any corrosion from it. I don't know why it corrodes being in the interior of the car but mine sure did.
If I could give some advice, you C4 owners it would be advisable to look at that plug and clean any corrosion from it. I don't know why it corrodes being in the interior of the car but mine sure did.
Do you (or anyone else??) know where that block is located on a 91 coupe? You mentioned that your block was located in the valley where the top went down, but any idea of where it is on the coupe? I'll check mine.
Thanks!
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