1972 Wiring Harness to Fuseblock Connection Problem

[Larry M.]

A no-electrical power problem on our 1972 has me stumped.

I have no power to the ignition, headlights, brake lights, radio, interior lights, warning buzzers, and maybe a few other items I haven't noticed yet. The car is basically dead as it sits.

Unplugging the wiring harness from the bulkhead connector on the firewall, and using a test light on the wire terminations that mate with the bulkhead connector, I've confirmed that I have power at the battery gauge wire, the horn relay connection, and the main black wire from the starter which (I believe) should power all the inoperative items I mentioned at the top.

Under the dash, using the test light on the fuse block terminals, I find no power.

Thinking I may have a problem with the black wire terminal not sufficiently contacting its bulkhead mate, I first bent the terminal slightly one way, but had no positive results, and then slightly bent it the other way, still with no positive results.

On the fuse block, I can see no way to probe what might be behind it.

This problem occurred inexplicably. On whatever day I last drove the car, it operated fine. The next time I went to drive it, it was dead.

It seems to me that I need to concentrate on the main wire connection, but other than what I've done so far, I don't know what else to try.

Do any of you have suggestions?

Thanks!

Larry

[Edward J.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Larry, the main power source is the ignition switch, everything leaves the switch when turned on to power fuse block and fusible links at the starter area. try starting at the switch under the column, maybe one of the wire connections has heated up enough to open the connection.

[Larry M.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Ed,

Thank you. I readily admit that I struggle following and understanding the chassis service manual wiring diagram as well as one I have from Dr. Rebuild (easier than the GM to read), but as I follow the 'path' from the battery to the starter, it appears that power is teed off from the starter, leading to the horn relay and to the bulkhead connector, which then would provide power to everything on the passenger compartment side of the firewall. Does the fact that the headlights, brake lights and courtesy lights are inop indicate a problem with something other than the ignition switch? Those normally all function without the ignition turned on. Am I misunderstanding that?

Larry

[Edward J.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Larry, I was studying the diagram, from what I see there is a red wire leaving ignition switch(12R) heading to fuse block. try and start by wiggling the harness at ignition switch and see if anything happens? what I was looking at the feed wire that powers headlights and followed it as I see it starts and finishes. try it and see what you think.

[Mark E.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

"... I have power at the battery gauge wire, the horn relay connection, and the main black wire from the starter which (I believe) should power all the inoperative items I mentioned at the top. Under the dash, using the test light on the fuse block terminals, I find no power."

Larry,

You're on the right track. A wholesale failure points to the bulkhead connector or a fusible link. The black wire you checked from the starter to the engine harness connector at the bulkhead is for the battery gauge. It's not a power source, so that's not the problem.

The major power source for most if not all the stuff that's not working is a 12R wire on the front harness connector that goes to the junction block on the horn relay. This lead has a fusible link near this block. If the 12R wire has power at the junction block (it's a different color at the block because of the fusible link), check for power at the front harness connector. Then progressively check for power going downstream: the dash harness side of the bulkhead connection, its connection at the light switch, its connection at the ignition switch.

Since more than the ignition is affected, I suspect the fusible link, bulkhead connection or maybe the light switch connection.

[Larry M.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Ed and Mark,

Thanks for the clarification and the suggestions. I'll check them out, although it might not be for a little while. Have a lot of other items on my burners right now.

I will give any updates.

Larry

[Larry M.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Mark,

You absolutely nailed it; thanks!

When I touched my test light to the harness connector for the 12R wire, it lit so dimly that I almost didn't see it. I looked at the fusible link connections at the horn relay, and saw that they had both been replaced sometime long ago by a previous owner, and the repair looked to be in very poor condition.

I replaced both links, checked with the test light at the harness connector and had a strong glow. Plugged it back in, and everything works.

I know so little about electrical circuits I thought I was in serious trouble; thanks so much for your help!

Larry

[Mark E.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Larry,

I'm glad it's sorted out.

My award for "best electrical diagnosis without being there" goes to Rich M. in this thread:
https://www.forums.ncrs.org/showthre...ght=Electrical

[Larry M.]

Re: 1972 Wiring Harness to Fuseblock Connection Problem

Mark,

I read Rich's diagnosis. Agreed; that's a very impressive display of skill.

As a side benefit of my fusible link repair last weekend, my battery gauge, which had chronically (like, forever) read slightly to the discharge side of center and had been a continual source of irk, now reads dead center.

Larry