headlights not working... 91 GP - need help!

[jmgtp]

My headlights don't work. I verified that I do not have power at the 20amp headlight circuit breaker. Tail lights don't work either.

I ran a test wire from the + battery to the circuit breaker and the headlamps come on with the switch. The fusible link is blown but I don't know where it is. Anyone know?

Thanks,
John

[jmgtp]

Okay, I found and replaced the fusible link and they worked.

Started it up, lights on/off and everything seemed great. So I start to drive, all lights are off, and I come to a stop and see some smoke from the hood. Sure enough it blew out the fusible link that I just replaced.

The switch appears good. I had the help of my father, an electrical engineer, to look at the schematic in the GM service manual. The switch seems to test good.

Anyone every have a problem like this? It must have a short somewhere but I have no idea where.

[jmgtp]

I still need some help with this if anyone has any insight.

I have now replaced the fusible link and the headlight switch. The lights worked just fine with the engine off and on. However, it seems that as soon as I start moving, it blows the fusible link. The link blowing may coincide with me pressing the brake pedal - as that is the last thing I did before moving. Or perhaps something is intermittently grounding out and the motion of the car was enough to ground it. OR it could be pure conincidence. The brake lights have to be on a different circuit as they continue to work normally even after the fusible link blows (the one that runs the headlights).

So, after the above happened, I replaced the link again and this time it blew right when I hooked up the battery - so this was with the ignition and headlights both off.

No doubt that I have a short somewhere in the headlight circuit. If it is blowing the link rather than tripping the circuit breaker or tail light fuse does that pretty much guarantee that the short is upstream of the circuit breaker and fuse? If so that makes things more simple.

My next step, one that I'm not looking forward to, is replacing the power wire for the head/tail lights comepletely. This means running a new wire from the + post all the way to the underdash area to feed the headlight circuit breaker and taillight fuse and disconnecting the old wiring at those same points - I will be incorporating a fusible link at the + post - just the same as the factory harness.

Anyone have any other ideas or experiences? Of all the things that can and do go wrong on an 18yr old car non functioning headlights are keeping me from driving!

[55trucker]

John....where are you finding this fusible link?.......I'm not aware of GM using them at all (at least going back as far as this bodyline) and I can't find one described in my GM manual for the cars lighting system (it seems odd to have both a fusible link and a breaker on the same system...one makes the other redundent). You mentioned that your taillights are going out with the headlights?...The two circuits are independent of each other besides passing thru the headlight switch together. The taillights are on the same circuit as the Park lights. As for the short in the system, has the headlamp system been upset or altered recently to possibly cause this? The headlight circuit breaker is a 20amp unit located in the electrical component center under the instrument panel, just to the left of the heater box. Outside of a physical short across a live to ground, might want to start looking at relays that may have shorted. Try starting with the mini quad relay, its under the instrument panel as well, just above the park brake, ...go thru the removable items in the system and see if that finds the problem.

Steve

[jmgtp]

Hey, thanks for the reply.

The fusible links are connected directly the the + jump post. The one that keeps blowing is the 14 gauge and as far as I can tell the only thing it powers is the head/tail lights. THe brake lights are an independent circuit so they still work when I hit the pedal, as do turn signals. I'm going to check the relay, I assumed it was near the circuit breaker which is under the passenger dash but it sounds like it is located near the park brake. I have the GM manuals/schematics and the help of an electrical engineer (my father) to interpret them. He thinks the issue is a short to ground upstream of the circuit breakers/fuse.

[55trucker]

Silly me!..my bad....Yeah....didn't dawn on me the two links on the starter solenoid... what does seem interesting is that you stated the one you replaced (red) is a 14 gauge? the inline (hot at all times) is a 20amp circuit breaker does your fusible link have a lower blow rating than the circuit breaker trips at? The link is the last line of defense for the entire circuit. When I've got all of my headlamps (minus the fogs) on I'm drawing 15 amps, the link is a 14 gauge... how many amps is it rated for...30+amps? Odd indeed.

Steve

[jmgtp]

Problem resolved, I have working lights again.

The short was in the wire between the jump post and the circuit breaker. If you think about it, the link is necessary because if there is a short before the circuit breaker then there would be no protection at all and it would cook the wire until it burned. Instead, with the link it just burns the link up and you save your car from a meltdown.

Long story short, I abandoned the original power wire from the + post to the circuit breaker/tail fuse and replaced it with new wire and that has resolved the issue.

There are two small gauge fusible links, the smaller of the two powers the head and taillights and the larger of the two seems to feed most of the interior accessories (power locks, windows, roof, etc) and maybe the brake lights/turn signals.

[55trucker]

Now I understand why your fusible links are not at the starter where they would normally be, in another recent topic post you stated that your battery is now in the trunk and you've moved the links to the jumper post on the shock tower. That had me somewhat puzzled why they would be in that location. Glad you can now *see* at night.

[jmgtp]

Well the headlight link and the other small gauge one are actually connected directly to the + post from the factory. The post has a small cover that comes off the underside and there is another stud/nut there that secures the links. I never touched the starter one since the original cable that went to the starter also went to the jump post so they are already connected - I just pulled the 'bolt' out the the original + cable and insulated it. The new + cable coming from the trunk is tied right to the post.

Going on a few days problem free!