platinummaker

Hi Guys,
In the process of removing center guage panel and console, all power to the car (72 coupe) went out. There were no pops, smoke, or smell of any short. Tried turning headlights on, but no joy there either.
I am assuming a fusable link of some kind. I believe there are a couple on this year C3, but have not gotten around to any sorting of the electrical and am not sure where to start. I’m thinking horn relay. Any coaching on the most efficient way of troubleshooting most appreciative. (Typed on bended knee)
Dave

derekderek

at starter and horn relay. ignition switch doesn't need ground, it is hot in and hot out. everything else does though.

Peterbuilt

As said above, one at the starter and one at the horn relay.
On the schematic, the one at the horn relay says 'FUSE' but it is a fusible link.

https://docs.google.com/viewer?a=v&p...GE5N2EzZGI2Mjc

Mooser

Main power goes from the positive of the batt. to the starter then through a fusible link to the horn relay, then splits off and goes many places but the important one is through another fuse link to the interior of the car and splits again to the ignition etc
Use a test light and check from the horn relay (+) to the engine (gnd) first, if it's no good there then it might be the fuse link at the starter and you'll need to check at the starter lug, if it is good at the horn relay, it probably took out the fusible link between it and the bulkhead fitting
M

Buccaneer

I'm assuming you disconnected the battery before you attempted to remove the panel, right? If not, you need to do that.

platinummaker

As an update, thanks to all for the generous info sent my way. As I looked at the horn relay, I noticed two 30 amp blade fuses emanating from said relay. Knowing that would have not been standard components in 72, I checked and they were both blown. After replacement all is thankfully back to normal. I will be diving into the wiring schematic to gain a better understanding. Thanks again for the help.
Dave

derekderek

I have no problem with replacing fusible links with real fuses. That was one of GM's less wonderful ideas...

SteveG75

Always disconnect the battery before working around anything electrical. Electrical shorts can easily cause fires.

kansas123

Glad to hear that Derek. Years ago when I lost my elec on my 72 due to a dead short, what the mechanic found was the blown fusible link between the horn relay and the firewall. He replaced it with what is in the picture and I feel more comfortable with it than I did before. Once in a while I go in search of the correct fusible link but they are hard to find......

derekderek

You want a fusible link? Drill a hole in each blade and solder it into the wires...

wadenelson

As an Electrical Engineer who has worked on autos a LOT I can explain the General's thinking on using fusible links.

When a fuse blows, what does the typical consumer do. Stop and figure out what caused it to blow? Correctly use their DMM to measure the amperage the circuit is trying to pull? Buy a clamp on meter or Fusebuddy to diagnose the CAUSE of the melted fuse?

Nope. they put another one in. And then another one. Often they'll put an even bigger one in, hoping and praying IT doesn't melt.

The whole purpose of fuses is to protect wiring from serious damage. Installing an oversize fuse can lead to major melting of a harness. Expensive to repair, PROPERLY.

Consumer doesn't know squat about fusible links, so GM engineers hid them inside wiring harnesses etc. When one blows due to a serious short it forced
the non-technical to have the car towed to the dealer, the short DIAGNOSED and FL repaired.

When I see and read about airline pilots resetting circuit breakers I cringe. It popped for a REASON. The airplane should immediately be grounded until the reason for a circuit breaker popping is determined, even if it turns out to be a "tired" circuit breaker. Unless they want to fly with that circuit de-activated for that flight.

When you buy a car and it's got old fuses on the floorboards, or a box of replacement ones in the glove box..... take the hint.

If you KNOW what you are doing and replace a FL with a fuse, that's great. I assume you simply briefly shorted something to ground while undressing the dash. But you COULD have a chafed power wire, and simply moving wires around rubbed it against something metal.

If it were me, instead of immediately replacing that 30A fuse with another 30A, I'd start with a 1 A, then a 5A, etc to confirm that you don't still have an ACTIVE short before plugging in another 30A. (With all lights and ignition off, etc)

The owners manual for my 1968 lists the locations, color, and ratings of the various fusible links for my car. And fusible link wire can be bought in bulk @ NAPA.

Greengear

The only reason to replace the fusible link would be a desire to keep everything 100% original. My 77 came with completly toasted wiring and two new harnesses. It was a pleasure to install the lectric limited brand harnesses. Since then I have added new circuits and fuses for fuel injection and msd ignition. I put the new fuses next to the fuse box. Next owner shouldnt have trouble finding them because the new fuses are in plain sight next to the fuse box. I also added a 250 amp breaker at the battery that is easy to flip when I want to cut power for repairs.
Modern cars have very large fuses in the fuse box that do the job of fusible links from older designs. Some newer cars have self reseting breakers as the main fuse.