1972 total electrical failure. Please help!

[Timothy I.]

[Mike T.]

Re: 1972 total electrical failure. Please help!

Tim - It's been a while since I had my 69 Roadster but am wondering if the C3 Firewall Harness Connections are the same as a Midyear.
If so, on the drivers side firewall (engine compartment side) there should be two harness connectors, typicaly a white/opaque plastic connector that is rectangular in shape. There should be two of them side by side. These actually plug into the forward face of the fuse block you see on the inside of the car. It's not uncommon for one or both of those connectors to get a bit corroded and when I hear about power outages, that's the first place to check. Depress the locking plastic prongs to let the harness connectors to pull free and check for corrosion. Lot's of other places to check too but the firewall connectors are a quick and easy place to check. Pardon the so-so pic I just took but that's what you should see on your firewall for the harness connectors.IMG_2239.jpg

[Mark E.]

Re: 1972 total electrical failure. Please help!

Start at the battery and check for voltage as you go "downstream": solenoid, horn relay, alternator...

Then check for issues where the voltage stops. Since you saw smoke and some damage near the horn relay, my guess is one or more of the fusible links in the harness near that area blew. There's also one or two fusible links between the starter solenoid and harness connection on the cowl. This happened for a reason so check for issues (loose or corroded connections, damaged wiring, bad repairs, shorts,...) after the fuse links are checked/replaced.

Also check if an accessory (like a blower motor) is drawing too much current.

[Bill B.]

Re: 1972 total electrical failure. Please help!

There is a fusible link that originates at the Starter top terminal that consists of a 14 gauge dark brown wire and another fusible link that consists of a 20 gauge orange wire. The dark brown one should be located near the starter, the second one attached to the horn relay. These fusible links are designed to open and break the circuit like a fuse if there was an overload before its routing to the fuse panel. Sometimes, these fusible links are subject to corrosion, which weakens the wire, thus causes more heating of the wire in the corroded area.

Because you had your blower motor and headlights on together, it may have just been too much for the link, if it was subject to corrosion.

You do want to check to make sure you don't have a wiring short somewhere between the starter's Batt terminal and the horn relay, and the horn relay to the fuse panel.

I'd give an educated guess that one or both fusible links had opened, which would cause the complete loss of power as you observed.

Let us know what you find.

[Bill B.]

Re: 1972 total electrical failure. Please help!

Here is a pic of the wiring diagram for the 1972 Corvette. Follow that 12R wire from the Solenoid terminal to the Horn Relay. That is the path where the two fusible links are located. Check all fusible links for opens. Each link should have essentially nearly zero ohms of resistance72 Corvette WD.jpg

[Philip A.]

Re: 1972 total electrical failure. Please help!

Do u have a test light? If so check for voltage on each side of the fusible link by the starter. Ill bet you'll have voltage on one side and not the other. If so you have found the culprit

[Timothy I.]

Re: 1972 total electrical failure. Please help!

Many thanks to all who replied. Lots of helpful advice. I'm not much on wiring, and most of my exerience has been with earlier cars so I wasn't aware of fusible links. I'm pretty sure that the fusible link off the horn relay blew, so I'll replace that first, since it's on top. If that gets the car running, I'll put the car on the lift, and check the wiring off the starter solenoid. If not, I guess I'll have to raise the car. I'll let you know how I make out. Thanks again!

[Pat H.]

[Timothy I.]

Re: 1972 total electrical failure. Please help!

That's great information! Thanks, Pat. I suspect that might be my problem.

[David M.]

Re: 1972 total electrical failure. Please help!

While your at it check all grounds.
Start with the neg battery terminal to the fame. Positive cable down to the starter. Clean them. Then spray terminal protectant on them once reinstalled.

The engine should have a ground strap from the block to the passenger engine mount. Clean both ends. Replace it if its haed/cooked same for the battery cables. Cables & wires can visually look good but be shot under the insulation...ohm them out. High resistance = shot.

Any vehicle 10 years and older will experience deteriorating grounds. Especially the older cars which were assembled without weather tight connectors seen on modern vehicles.
As the grounds and other connections age they increase in resistance until failure. During this high state of resistance everything electrical has to work harder to maintain normal operation, power windows, WW motor, starter, alternator, VR... et al.

Bottom line make sure all major & minor electrical connections are clean and the wires are sound. Ohm out. Consider this routine maintenance on our collector cars.














in question.

The firewall bulk head connector is another place for hidden corrosion.

[David M.]

Re: 1972 total electrical failure. Please help!

Man I don't know what happened to my thread #10 above with the large spacing. I see a spelling error well. Why cant I get into my own thread to correct it?

While your at it check all grounds.
Start with the neg battery terminal to the fame. Positive cable down to the starter. Clean them. Then spray terminal protectant on them once reinstalled.

The engine should have a ground strap from the block to the passenger engine mount. Clean both ends. Replace it if its hard/cooked. Same for the battery cables. Cables & wires can visually look good but be shot under the insulation...ohm them out. High resistance = shot.

Any vehicle 10 years and older will experience deteriorating grounds. Especially the older cars which were assembled without weather tight connectors as seen on modern vehicles.
As the grounds and other connections age they increase in resistance until failure. During this high state of resistance everything electrical has to work harder to maintain normal operation, power windows, WW motor, starter, alternator, VR... et al.

Bottom line make sure all major & minor electrical connections are clean and the wires are sound. If in doubt Ohm them out.
Consider ground cleaning as routine maintenance with our collector cars.

The firewall bulk head connector is another place for hidden corrosion.