Fuse blows after shutting car off!?!

[chgostout]

This one is perplexing ... but I have faith that you guys can help me figure this out!

My '80 vette just developed a problem of blowing the clock/courtesy fuse ... AFTER I turn the car off !?!

There were no changes I made to anything I'm aware of, that could have brought this situation about.

This 20 amp circuit has the clock, lighter, courtesty lights, anti-theft alarm, glove box, dome light, power antenna, horn relay, and power door locks on it.

After I change the fuse I'm able to work everything as always, and drive the car normally, ... and it's only AFTER I shut the car off that the fuse blows. It doesn't go upon turning the car off, but sometime later. How much later I don't know. I just come back to the car ... and the fuse has blown!

I've looked at the physical wiring at the fuse box and on the firewall and see nothing out of the ordinary.

Is there anything any of you know or could suggest, ... short of my running down each wire in the circuit?

I am not setting the anti- theft alarm after turning the car off, so I wouldn't think there would be any more reason to suspect that than anything else.

I wouldn't think there'd be any relation between the car heating up and the fuse blowing on the car cooling down, as with some ground fault that happens when the metal contact contracts due to cooling - because I replace the fuse while the car is cool or cold - and the circuit is good!

I've even tried to blow the fuse by putting a full load on it ( using the cig lighter while actuating the power door locks, power antenna, etc. - and I cant' get the fuse to blow that way!

Any ideas?

 

[mapman]

Is the Courtesy Light and Dome Light delay operating properly?

That is do the dome/courtesy lights come on when you open the door, and do they remain on for 30 seconds after both doors are closed?

 

[chgostout]

Yes, Mapman, when the fuse is working, the delay of the courtesy and dome lights works properly.

 

[mapman]

Does the fuse ever NOT blow?

What does the wiring going into the doors look like? Seems like a short to ground and the doors might be a suitable spot for the wiring harness to wear.

I also have a 1980, but it is in too many pieces to see if the lights work! LOL

May be time to go wire by wire. The 1980 Corvette Shop Manual has pretty good schematics of the wiring diagram and descriptions of the circuits and commector locations (page 8A-31. I also have the wiring diagrams from NCRS, they are pretty clearly reproduced.

Looks like you can isolate the dome light and the spare tire light at connector C124 located under the Left Hand side of the instrument panel. The other lights, clock, lighter, and vanity mirror on this circuit all seem to run to a common splice (S260).

 

[chgostout]

Since this problem began, the fuse has blown after every use.

The wiring going into the doors looks ok. I could test that by opening & closing the doors multiple times to see if the fuse'll blow. I could also jump out of the car, instead of using the door
after turning off the engine, ... to see, if that might make some strange difference.

If these tests don't prove anything, and no one else has any EASY answers, I guess I'll resign myself to the time consuming job of starting to run everything down

Thanks mapman, for your help & suggestions. Good Luck with your '80.

 

[sac001]

I also have an 80, and I'm thinkng that something may be shorting out when the Courtesy lights go off.

Remember, when you shut off the ignition, the courtesy lights inside & under the license plate come on for about 30 seconds. After 30 seconds they shut off, I believe as a result of a relay switching off. If that relay is bad, and shorts out, instead of shutting off, that may be the problem.

The only other thing going on is the Power Antenna. To test the theory, put a meter across the fuse, make sure the Antenna is down, then shut off the ignition. Wait until the Courtesy light shut off, and see if that is when the fuse blows. Also watch the meter to see if you are getting an amperage spike when they shut down.

If its not the Courtesy Lights / Relay, you are going to have to start checking wiring, opening & closing doors etc to try to pinpoint what makes the fuse blow.

Good luck,
Steve

 

[chgostout]

Thanks Steve. You've offered some logical hypotheses, but I've already tested your theories - to no avail.

Somehow, this problem is related to running the car up to at least normal operating temperature, and then shutting it off. Sometime after that, the fuse blows - tho not right away, as I've tested that! So, somehow, as the car cools down, it goes !?!

When it is cool or cold, and hasn't run for awhile (meaning at least a day or more), I can change the fuse, have everything work, and while it's all working, I've opened and closed the doors, had the courtesy lights go off & on their own, run the power antenna up & down several times, use the lighter at the same time, etc., and the fuse WON'T BLOW!

After getting it warmed up, and turning it off, everything still works, as I've gotten all the things on the circuit to operate, RIGHT AFTER DRIVING THE CAR ... It's sometime after that, I'll come back to the car, to drive it again - And the fuse is blown!!!

Now what sense does that make?

 

[sac001]

Take a look at the wiring for the Power Antenna and the Underside Courtesy lights. These wires run awefully close to the exhaust system, and its possible the heat is causing something to happen, although I'm not sure what that might be.

Does this fuse also control the center gauge lights? I'm not sure what else is on this circuit, but I think you need to be looking at wires near heat sources.

Good luck,
Steve

 

[chgostout]

I think I've found the culprit!

Several months ago I had the car in a vette specialty shop getting some things fixed. One of those was having new door locks installed and hooked up to the anti theft system. This system is the only thing on this circuit that is "powered up" with the key off, and it because of this that the fuse mysteriously blew after i'd shut the car off & got out and locked it. It was probably unlocking it with the key sometime later, that blew the fuse. Because of these 'symptoms', my brother, who's in Portland, and been an A.S.A.E. mechanic for 30 yrs., suggested disconnecting the anti theft system, as a first check. I should've listened to him, not just because he's m'brutha ... but because he's been doin' this for over 30 YEARS!

So, upon going out to the garage today to test some new theories, I went to unlock the driver's door ... and found the actual doorlock was loose!! I hadn't detected this before.
A couple weeks ago, I was goin' around the car doin' one of my periodic lubrication routines and remembered reading that GM recommended WD-40 for the locks. My locks had been stiff & hard to turn with the key, ever since getting the car back from the shop. So, with the pinpoint tube in the cap, I gave the locks a few shots, pushing the tube into the lock from the outside.

This lubrication was what started me & us on this journey into never-never land!

Why, I don't know, ... & I don't take blame for doin' anything wrong ... I did just what GM recommended! And if the WD-40 contributed to the intermittent shorting of the lock wiring, and also helped loosen the lock from the door, ... that doesn't seem right either!

I'm gonna bring the car back to the shop and have them re-do the doorlock & switch, and hopefully, ... get it right this time!

The biggest help in identifying the correct connector under the dash, to disconnect the anti theft, was the corvette assembly manual - NOT the shop manual. The assembly manual showed the connector and the number and colors of wires in it, so it could be found and identified quickly. Finding & disconnecting this was alot less time, and foolin' around, than taking off the door panel & disconnecting it inside the door! When I bring it back to the shop ... I'll let them fool around with all of that!

So for now, I don't have a working anti theft system, but my radio (power antenna), & cigarette lighter (I smoke), (among the other things on that circuit) : WORK AGAIN!

I can live without the anti theft for awhile, as the car's in my garage most of the time anyway, but I couldn't drive without the stereo or lighter!