Headlights flashing when fog lights are on

c101guy

Registered User
I have a problem with my headlights flashing on-off when I have my fog lights on. If I leave the fog lights off, the headlights seem to work fine (I stopped using the fog lights several months ago and have not had any issues, but I want to fix whatever is not right).

Is this a headlight switch problem (on the dash) or a foglight switch problem (on the console)? How can I best diagnose this?

Any insight on where the parts may be available for a 90 SC?

Thanks
 
As I understand it the wire feeding the fog lamps is too thin a gauge and causes issues with the headlamp switch. I would search the posts on here. I believe the fix is to replace the fog lamp harness with some heavier gauge wire and then replace the headlamp switch.
 
Actually, the proper fix is in three parts:

1) Add a relay wiring harness for both the low and high beam headlights.

2) Add a relay wiring harness for the fog lights.

3) Replace the headlamp switch.

Consider #4, replace the MFS while you're at it.

The main problem is that ALL the current for EVERYTHING is drawn through both the headlight switch AND the MFS. Wiring is smaller than it should be, contacts are smaller than they should be, and it fails due to being undersized for the load.

RwP
 
Replace the headlight switch. Then wire the fogs through a relay that bypasses the headlight switch. That's all you need. Fogs get their power throught the headlight switch. If you bypass it you win two ways. The headlight switch won't be overloaded and you can use the fogs with the headlights off.
Alan
 
Replace the headlight switch. Then wire the fogs through a relay that bypasses the headlight switch. That's all you need. Fogs get their power throught the headlight switch. If you bypass it you win two ways. The headlight switch won't be overloaded and you can use the fogs with the headlights off.
Alan

That ALMOST fixes it. But since the low and high beams go through the switch ALSO, I'd highly recommend relaying THOSE also.

Unless you want the same problem to possibly return later, but without the fogs on.

Also, the connector on the back of the headlight switch is suspect at this time.

Not to mention the wiring at the MFS, plus the MFS itself.

RwP
 
That ALMOST fixes it. But since the low and high beams go through the switch ALSO, I'd highly recommend relaying THOSE also.

Unless you want the same problem to possibly return later, but without the fogs on.

Also, the connector on the back of the headlight switch is suspect at this time.

Not to mention the wiring at the MFS, plus the MFS itself.

RwP

I agree

The connector on the back of my headlight switch was totally melted. Wiring in relays doesn't take that long and is not that expensive. I got an increase of almost 2 volts to my lights after wiring in the relays and going to shorter bigger gauge wire.

Sean
 
Thanks everyone, I really appreciate the input.
I'll have to work on the wiring aspect (I'm far from an electrician), but I was wondering if anyone could lend insight on how to remove the headlight switch - I know from past experiences with pulling apart dashes on other vehicles, it usually results in a lot of broken plastic and a lot of cursing - any tricks?
 
There are only 2 screws where the gauges are. Just be careful pulling it off. I've had mine off so many times I can't count, it's not a huge issue.

I usually work it off from the top around the left across the bottom.

Use a little screwdriver to pop the headlight knob off. There's the little tab on the bottom of the knob.

I am far from an electrician myself but rewiring the lighting is not difficult. There are many posts describing how.

The nice thing is you can wire it so both highs and lows can come on at the same time giving you lots of light for late night, middle of nowhere driving.

Sean
 
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