Last night the dash lights on my 1972 Coronet quit working. Everything else works, guages, directionals, etc, just the instrumanet cluster lights, radio lights and shift indicator light (column shift) aren't working.

Of course the first thing I checked was the fuse and it was blown, I replaced it and it blew again. Anybody ever have this problem? Any idea where to start diagnosing it?

No dash lights symptom is more often than not a failed rheostat [dimmer] portion of the headlight switch. The diagnosis (and for some, the permanent "fix") is to jumper across the connections at said dimmer switch.

The problem with that cookie-cutter solution is that it addresses the typical fail to an OPEN circuit. You clearly have a dead short to ground, rather than an open.

Therefore, what I would do were I in your shoes would be to DISCONNECT the wires into/out of the dimmer before jumping across them. If the dead short is in the dimmer... problem solved (either permanently, or until you replace the switch). If you still have the dead short (blowing fuses) after taking the dimmer out of it, revert to standard short-chasing techniques.

Just one man's opinion and educated guess. There will be others, I'm sure.

Make sure it is grounded properly.

My dad had the same problem in his 71 satellite back in the 80's. His solution was to pull out a cigarette lighter and read the gauges by the light of the flame!

Do you have a multi-meter? If not I suggest getting one, even a cheapie one, makes finding the problem easier than trying to use a 12v test light.

definitelly you have a short LOL

I bet you lost also parking lights... is the same fuse what everything you told plus parking lights.

So you will have to check socket by socket on exterior lights, and also glove box.

BTW, Cluster lights have an extra fuse, just to the output of dimmer to all the dimmer controled net ( cluster, radio dial, ashtray, accesories lights and console or column shifter dial ), but I don't think probably existant short is related to dimmer net since that fuse SHOULD blow first, unless dimmer fuse is overrated. Is one of the top fuses, and is separated from the buss bars, with an orange wire on back.

All the exterior lighting is working.

Is it possible that a burn't out bulb is causing the short?

I have seen a burnt bulb create a ded short but they were the tail light bulrs.Wire inside broke and hit the grounded socket.Ive seen it twice in 40 years.Like suggested you have a dead short so your grounds should be good.Like suggested pull the switch and disconnect the wires.See if the fuse blows then.Rocky

In case it matters, my dome light still works off the headlight switch.

It is interesting that your dome light still works, to the extent that it might be interesting that your cigarette lighter still works - they are electrical, and in the same car. However, they are not part of the same circuit with which you are having the problem.

I side with Rocky in that it has happened in the history of mankind that a bulb's filament has broken in such a manner as to short to ground with very little resistance/load. This happens in large bulbs with beefy filaments, and in a frequency to be well under statistical significance. A burned out bulb, especially in something as small as a dash lamp, is going to be an OPEN.

What is causing your fuse to blow is almost certainly a special type of "short" circuit - one that is a DEAD SHORT to ground, with no load or "work" being done by the electrons on their way to ground. The circuit is very simple, with very little going on, and a component notorious for failure along the way.

Admittedly, some of these points might be subtle, and not immediately apparent to someone who has not spent decades working on cars. Other concepts, however, should be as basic as they come - what happens when a bulb burns out, what makes the fuse in one circuit blow repeatedly, and the nature of a dead short...

...especially when the one posing the questions is a trained engineer who sees fit to belittle the unwashed masses, both in threads on the General Board, and his sig line [edit: Yes, I see you just now changed it]. Imagine us being foolish enough to think a car should be equipped with a dipstick for the transmission.

I was planning to bite my tongue, and just offer polite help, until I read your followup questions.

Perhaps your dash lights are out because of the presence of a "caveman stick" in your car somewhere...

Your killing me....

Why would you assume a trained engineer would understand the age old concept of "a short to ground".
Surely a simple Renaissance Short must not exist in his car... for this is truely the work of gremlin's.

I think I'll go check my trans with a Caveman Stick... while I still have one.


Duffman, the "Multimeter" is your freind

First of I'm an ME not an EE. I know a little about electrical, I had too to get my degree, but not enough to do a failure analysis. So your comments are out of line.

I understand that I have a short, I'm just trying to find help in determining what component(s) could cause it so I can minimize the amount of work need to find the problem. A lot of times there are known, common failure modes and that is what I'm looking for.

BTW the dome light switch is at the end of the rheostat for the dash lights, and since the dash lights stay illuminated while the dome light is on I thought the fact that it still worked might be relevent. Forgive me if I'm not 100% familiar with the design of a headlight switch in a 37 yo car.

Since the cheapest and easiest place to start is the headlight switch I'll begin there. Thanks to all who responded with useful information.

I realize that you aren't reading this.

But others can still see what a meanie I am. That final straw of helping you with the definition of "short" is real fightin' words.

Bad Steve.

BTW Engineers spend most of there careers specializing in a certain topic, and they strive to become masters of that topic. Very rarely is an engineer a "jack of all traits".

As for the dipstick subject, in that thread I was defending the engineer(s) who removed it because none of us knew the rational behind the decision. I was giving reasons why it might of been removed when a bunch of you continued to insist that all engineers are idiots. Once sombody else jumped in to help support my position you all went silent. I guess if you can't gang up on somebody it's no fun huh?

"jack of all traits". The term is Jack of all trades. No we just got tired of you not listening to those of us that keep up with our own vehicles.