high beams blow fuse on 78 620

[Rubberman]

I haven't had time to get in to it yet, but wanted to see if anyone maybe had a quick fix on here before I started. This started about a week ago on my 78 620, everytime I hit the high beams it blows the fuse. Anyone have this problem before.

[moparvwfreak]

bad relay maybe???? oh god why does this sound like what i am dealing with in a round about way?

[Bleach]

Higher powered H4 bulbs?

 

That happened to my 1978... oh wait, does that year have a relay? I think Doug said they got rid of the relay for a couple years. The fuses would always be hot on mine.

 

for a quick fix, you can not use your high beams. I had to resort to that. I never did get around to running my own headlight relays for high and low beams.

[Rubberman]

I'll check for relays. The lights are standard lights, nothing special, if thats what your saying Bleach. I have just been running with the low beams, but I'm gettin older and the eyes aren't as good as they used to be at night. I'll have to wait until tommorrow to tear into her, working today and tonight.

[Bleach]

well, I'll be working on a projector lense conversion for my 280ZX this year. (hopefully) maybe you can do the same on your truck to give you better visability.

Although the BMW headlights have one built in. :D

[datzenmike]

The relay moved from out under the hood, maybe the rad support?, inside the cab in '75 and '76. I seem to remember on the steering column behind the gauges? could be wrong. In '77 it moved back out onto the right inner fender for that year. The '78 and '79 do not have a relay and hi or low are selected by the dimmer switch on the column.

 

Up behind the glove box are two harness plugs, a 12 pin and a 10 pin. Unplug the 10 pin plug and turn headlights on low beam. If the fuse does not blow, the problem, possibly a wire rubbing against ground, is forward in the engine compartment. If the fuse does still blow, the problem is in the cab and you have eliminated everything forward.

 

Under hood, try to visually inspect the engine side harness looking for recent damage. The low beam hot supply wire is a Red/Black stripe.

 

In cab, to test the Red/Black stripe wire going back to the glove box connector for a short to ground, un-plug the 10 pin connector to the steering column. Connect a test lamp to a 12 volt source source and touch the Red/Black wire. If it lights you have a ground in that wire. Prey that you don't because it is in the main harness behind the dash. (it's unlikely)

 

If the wire checks out good and the fuse still blows, the problem is likely in the steering column dimmer switch. It can't be the light switch because it supplies the same power for hi and low beams, and it's the dimmer that selects where it goes.

 

This should narrow it down at least.

 

mike

Edited January 7, 2009 by datzenmike

[Rubberman]

Thanks Mike

[datzenmike]

No problem, it's sort of like detective work.

[Orange620]

I asked the same question a week ago. Bandit240 posted the 78 wiring diagram showing the relay on the right inner fender.

 

http://forum.ratsun.net/showthread.php?t=8733

[datzenmike]

Unfortunately, it's wrong for a '78 but right for a '77. Plug 13 on the '77 diagram is listed as for the voltage regulator. '78s did not have an external voltage regulator on the inner fender. The last year for the voltage regulator and the headlight relay on the inner fender WAS '77.

Edited January 7, 2009 by datzenmike

[Rubberman]

Hey Mike, I did your test on unplugging the 10 prong connector and the fuse did not blow or get hot. So I take it that I probably have a ground issue under the hood. I have a service manual for a 79 620, I would think everything should be the same on my 78. It does not show any relays for the headlights. I gotta go to work shortly so I had to quit messing with her. I gave a quick look under the hood and nothing stood out ground wise.

[datzenmike]

'78-'79 should be very close for headlight wiring. At least it's out front where you can get at it.