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1975 DeVille headlights

5K views 11 replies 3 participants last post by  75sled 
#1 ·
Hey y'all,

I have a problem with my headlights on my 75 coupe. I was servicing the alternator the other day, and shorted the hot side of the battery to the engine block by accident. Now my headlights won't come on. All of the tail lights and parking lights come on, but no head lights. The bulbs are all brand new. Discovered that the switch supposedly has a self-reseting circuit breaker but it hasn't reset in 5 days. Anyone have any suggestions?

They are twilight/sentinel, but i have also tried manually turning them on and still nothing.
 
#3 ·
Thanks for the insight. Now a new, yet, possibly related problem. The battery has died between Saturday and yesterday. Obviously a short somewhere, but it seems so unlikely to be related to what happened. There were two wired connected to the back of the alternator. Those two wires were reconnected. I don't see how shorting one of those wires to ground could cause another short elsewhere. Seems to me like it would cause an open, the most likely case being in a fuse. I haven't had time to research the battery problem but i suspect it is related to the light problem in some way. AAAAHHHHHHHH! I hate electrical problems.
 
#4 ·
Verify that you have 12 volts at the fat wire on the back of the alternator. The thickest wire that attaches to the stud. If no 12 volts then the alternator will not power up to charge the battery. That voltage comes from a wire at the starter, so my fusible link theory could affect both problems.
 
#5 ·
I will give that a shot. The thing that confuses me is that the battery died while sitting. Because I cranked it to move it out of the shop. Four days later when I went to move it agian...nothing. Thanks for the tip, I will sure try that.

BTW, I was thinking, should there be some sort of insulating washer on that connection at the back of the alternator? It just dawned on me that that wire is hot, and if it weren't insulated from the body of the alternator, then it would be shorting the battery to ground.
 
#6 ·
Well I finally got a round-tuit. My alternator is charging the battery. Once i jump it off and run it for a little bit it will crank the car for hours to come. However, letting it sit over night it will be dead a gain. So i know the alternator is working. Now for that fusible link. I only found one wire going to the starter. The big positve "cable" and there wasn't another wire that ran along with it. It was the only wire going down there.

I checked the votage at the light switch and it has 12 volts. I don't have a diagram yet to know if the voltage is in the right place, but I suspect that it is. This has really baffled me. It doesn't make any since as to why my head lights won't come on and my battery is dieing overnight.

It is killing me because i haven't driven the old beast in a while and i just got my cooling system repaired with a new radiator and I WANT TO DRIVE HER!!!
 
#7 ·
Hi,
On the light switch you should have 12v on the yellow wire, this is from the battery via a fusible link that should come from the starter connection. There should be 3 fusible links, yellow, black/red and purple. When you turn the headlights on you should get 12v on the light blue wire on the light switch, this goes to the floor mounted dimmer switch. From there it becomes tan for dipped and light green for main beam. If the circuit breaker in the light switch has failed I don't think any lights work. Try disconnecting the alternator overnight and see if the battery stays good. You may have damaged an internal part of the alternator by shorting it out. Let me know as I have the factory wiring diagram to help with fault tracing.
Regards
Alan.
 
#8 ·
Well, I have good news and bad news. The good news is, i found the short. Apparently when i was servicing the alternator, I dropped the insulating washer that goes on the post where the battery connects to it on the back. I replaced it with a plastic washer and now my battery isn't going dead anymore.

The bad news is, I still don't have head lights. I will check what you mentioned Alan about the voltages on the light switch. That sounds like a good place to start. So what you are saying is the fusible link runs between the switch and the starter? Do you know where exactly the link is located in the wire?
 
#9 ·
Hi 75sled,
The fusible link is located at the beginning of the wire and is soldered in series with the wire. It looks like an extension to the wire. They should be at the starter motor end of the wire. On the terminal of the starter motor with the battery connection should also be the three fusible links. They may be difficult to see as I think they run in a sleeve to protect from the exhaust manifold heat. I must admit I can't see a connection between shorting the alternator and no lights, but we will see when you find what's wrong. Do the side (parking) lights work or is it only headlights not working?
 
#10 ·
All lights work except the four head lights. It really is queer that those two situations are linked. The only thing that I can come up with is because the hot side of the battery was basically tied to ground, when i went to start it with the twilight sentinel set to come on, it just over amped the wire. Bizarre, but i guess we will see. I hope I can check that this evening.

So do you have to solder the fusible links back in or to they plug in or what?
 
#12 ·
Well I finally got the schematic. I had to have it blown up to a 2ft by 3ft page just to read it. Turns out, what happened with the alternator was most likely coincidental to the lights not working. After tracing wires for hours, i finally found the problem inside one of the main plugs that is mid-harness. Luckily it was only about 6 to 8 inches from the light switch and not in the fire wall or up next to it. Anyway, the hot wire to the headlights from the switch (which is light blue) had burnt the connectors that are inside of this plactic plug. Not very badly just enough to keep it from conducting. I jumped 12 volts to one side of the main plug and got nothing. I jumped it immediately to the other side and VIOLA I had lights. I simply pulled the wire out of the connector and butt spliced it together. WOO HOO!!! One project down, God only knows how many to go, to include the exhaust system that fell off yesterday. Oh well, I knew these things would happen with a 33 year old car.
 
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