91 SE, Intermittent headlamps and fog lamps after warmup

[robharrisva]

After a period of driving, I experience an intermittent problem with the headlamps and fog lamps going off and on. The voltage on the electrical system does not appear to change during the outage.

Anyone have a similar experience? Thanks.

[kilroypr]

Well I do have but in my case is when I use High beam for a long time. I had a friend that had exactly the same problem you have and his case was a short on the cabling inside the dash(UGLY)

[robharrisva]

Well after all this time, I have finally found that the problem only seems to occur when using the fog lamps in conjunction with the headlamps. With the fog lamps turned off, the headlamps stay on.

Now I need to look further into why there is no adverse reading on the electrical system while the problem is occurring. It is as though the fog lamps are somehow wired in series (vs. in parallel) with the headlamps.

[richtazz]

It's most likely the fog lamp relay is going bad or the dimmer switch. The relay turns the fog lights on when the low beams are on, and off when the hi-beams are on. When the relay goes bad, they can back feed power into the headlamp switch. Since the dimmer switch re-routes power thru the relay to turn the fogs on or off depending on position, a bad dimmer will cause the same symptoms.

[robharrisva]

Thanks a million for the info RichTazz. I have been dealing with this for a few years now. If I successfully resolve it, I'll be sure to come back and share my findings. I think I'll start with the relay since it seems to be more readily accessible.