Howdy Forum,

My 78 Pace Car is doing some wonky stuff. The hazard/brake fuse is blowing constantly so apparently I have a short somewhere... I'm trying to isolate the location of the short, but can't find the various wiring harnesses. Does anyone know the locations of any or all of these harnesses? Specifically, the rear light harness & the front light harness?

Thanks!

 

No easy task ahead but the AIM (assembly instruction manual) shows the routing of the harness assemblies. The rear harness runs up along the rear and enters from the driver's side. It probably runs from the driver's console up under the rear harness with a grommet entering the rear by the rear bulkhead above the wheel well. If you have an automatic, there is a backup relay that is part of the circuit while four-speeds mount the relay switch on the transmission.

The front also runs up under the nose (see it with hood up) and enters from the driver's side. In the engine compartment its tucked up with the vacuum hoses. But that's the easy part. You probably see that there is a hazard flasher that plugs into the fuse box. Your vehicle has the switch (steering column), fuse and flasher, both located in/on the fuse box.

The best wiring schematic is the 1978 Electrical Troubleshooting Manual and is 14 x 11...not easy to scan but Corvette manual vendors always have these available. Wilcox offers a PDF version of the AIM as well which shows the harness routing. I've attached the related circuit below but it got saved upside down...oops!

Hope this helps!

 

The rear lamp harness connects to the cabin harness above the driver footwell on the outboard side. There are a BUNCH of connectors in that area they they're typically stuffed wherever the last person in there could find room That connector has the greatest number of connection spaces (twelve I believe but there are a couple open slots). You have to remove the trim panel below the steering wheel and the A/C duct to get to this.

The front lamp harness connects to the rear of the fuse box on the engine firewall below the brake booster. There's a single bolt (5/16" I think) in the very center that secures it--you must have a universal joint for your socket set to get to it. That's a two-part connector--the half on the outboard side is for the front lamp harness, the inboard side is for the engine harness. DO NOT remove the "yucky" grease you find inside that connection! It's special dielectric grease and it's essentially eternal. If you do anything add some additional--you can get it at any auto parts store.

I'm 99.9% certain that the car will be completely dead electrically with the front lamp harness disconnected (even if you separate the connector and leave the engine harness connected) as the front lamp harness powers the fuse box most or all of the fuse box. Simply disconnecting it won't help with a search for a short. Since your problem is with the stop/tail circuit the front lamp harness shouldn't have anything to do with it.

 

For what it's worth, my 78 (also a Pace Car, go figure) had an issue with the tail lights blowing fuses. Turned out, it was the horn circuit. One of the horns was so seized up that it would blow the fuse when tapped. Rebuilding the horns fixed the problem.

However, this issue predated my time with the car - so the car had been worked on by a shop (have receipts). They ended up pulling a fair bit of the horn mechanics out of the steering column AND pulled the horn relay off to "fix" the problem.

Not very likely, but there's a chance that you've got something similar going on with your horn circuit, combined with an always blowing horn that instantly cooks the fuse. If the horn relay is attached, detaching it may help.

I agree with the SwampeastMike about the rat's nest by the driver's feet, too. It's very easy for a wire to get pinched, particularly if the steering column or pedal box has ever been dropped.

 

My '78 also popped fuses because of the alarm horn that was rusted , but as I remember it affected the interior lights.