Fog lights inoperative -- 911SC

[david05111]

Hey Gents-

The fog lights on my '80 SC aren't working. The car has been sitting for about 25 years, and I'm not sure when the last time they worked was.

The first thing I did after turning them on and them not working was to check for power at the light itself. I figured the bulbs might be bad, but there's no power at the light.

After I determined that, I checked for the relay and discovered, to my surprise, that there was no relay. So I grabbed the one from my fuel pump and plugged it in to test again. No change.

The next thing I checked was the fuse. It was in rough shape, but it wasn't bad. I replaced it just the same and checked for power at the fuse itself. No power.

So far:

- Lights don't work
- No power at lights
- No power at fuse



In my mind, that leaves the headlight switch and the actual fog light switch, as well as maybe a ground issue. Am I on the right track?

I should note two more things:

1) Not that long ago, I replaced the turn signal switch on the column and had to access the headlight switch and beehive associated with it. I didn't see anything amiss at the time.

2) There is no backlight on the fog light switch when I pull it out. Up until now, I've assumed that the bulb is just bad, seeing as the car has sat for so long.


What's my best course of action at this point? Should I test for power at the fog switch itself? Should I be checking for power at the relay port? Is there a test I should perform at the headlight beehive itself?

I could use some guidance. Thanks

[smadsen]

You know the fogs only work with the ignition on and the headlights on low beam.

[david05111]

Originally Posted by smadsen

You know the fogs only work with the ignition on and the headlights on low beam.

Yeah I should have mentioned that's how I've been testing them. That's my mistake.

[steven c]

If they are aftermarket like mine are there is no telling how they are wired. Here are some diagrams to get you going.

[david05111]

Originally Posted by steven c

If they are aftermarket like mine are there is no telling how they are wired. Here are some diagrams to get you going.

They're factory fogs. These diagrams are great though. I'll report back. Going to re-check to make sure there isn't power at the fuse. Since it's first in line, there should be power there at the very least.

[david05111]

I pulled the electrical schematic out of the Bentley manual, which looks similar to this.

I checked for continuity between port 87 on the relay and fuse 1, and it has it.

I also checked for power at port 30 on the relay and it has 12 volts.

For some reason, power isn't getting from port 30 to port 87, through the fuse, and out to the lights.

To confirm, I jumped ports 30 and 87 and got power at the lights. So the issue is definitely upstream somewhere.

[steven c]

Originally Posted by david05111

I pulled the electrical schematic out of the Bentley manual, which looks similar to this.

I checked for continuity between port 87 on the relay and fuse 1, and it has it.

I also checked for power at port 30 on the relay and it has 12 volts.

For some reason, power isn't getting from port 30 to port 87, through the fuse, and out to the lights.

To confirm, I jumped ports 30 and 87 and got power at the lights. So the issue is definitely upstream somewhere.

You are correct. That would be the low beam power to the switch and then the switch to the relay. First apply 12v to the relay switch input and see if the relay works and the lights come on. Then go find the issue.

[david05111]

Still trying to resolve this. I’ve been studying the diagrams both here and in the manual and am just a little flummoxed.

I’m desperately trying to avoid pulling the fog light switch. After reading that I have to pull it out the frunk and remove the blower, etc. that sounds like a nightmare.

I’m going to confirm the relay is good, though I’m pretty sure it is.

I have a few questions

1). Where does the switch get the power for the illuminator in the switch itself? The one in the switch that’s supposed to come on when you pull it. Does that power come from the relay back to the switch or does it come from another source, like the headlight switch? Mine doesn’t light up, which indicates either a power issue or a blown bulb…can’t tell which.

2). How does the headlight switch lock out the fogs when it’s not on? Does the headlight switch interact with the fog light relay or the fog light switch, and does it do so directly?


What I’m trying to isolate is any possible problem except the fog light switch. If there’s some sort of issue in the wiring getting to the fog light switch, that might be easier to deal with given what a pain in the ass it is to get the switch out of the back of the frunk and that you can’t get a new OE switch if it is bad.

[steven c]

David,

It seems from search that some fog light switches light dimly all the time and then brighter when pulled on. Mine does not, so all power comes from the low headlights for fog lights. The headlight switch locks out fogs by being off or on high headlights. I'm not sure how the rear fog is powered thru the switch?

You can check another way up front if you want. Check the wire to relay pole 85(the trigger) and try turning on low beam headlights and the fog light switch, it should have 12v. The car will need to be running I have found on my car. If no power to 85, then they will never light up.

Good luck.

[david05111]

Originally Posted by steven c

David,

It seems from search that some fog light switches light dimly all the time and then brighter when pulled on. Mine does not, so all power comes from the low headlights for fog lights. The headlight switch locks out fogs by being off or on high headlights. I'm not sure how the rear fog is powered thru the switch?

You can check another way up front if you want. Check the wire to relay pole 85(the trigger) and try turning on low beam headlights and the fog light switch, it should have 12v. The car will need to be running I have found on my car. If no power to 85, then they will never light up.

Good luck.

So power is routed from the low beams to the fog light switch somehow? Is that some sort of split that comes off the line? Or is a line that comes out of the headlight switch that is powered at the same moment the line to the low beams is relayed out of the switch?

Just trying to figure out where to go if there isn't power at 85. The implication would be that the fog light switch isn't sending power to 85 correctly. But that means I'm going to have to check the source of the power to the fog light switch at it's origin point, rather than checking it at the actual fog light switch (since I don't really want to remove the fog light switch at the moment).

So I don't know where to check for that origin power, whether it should be checked at the headlight switch, the fuse box for the low beam lights, at some sort of split off the low beam wiring post-fuse box, etc.

[steven c]

David,

I do not have a complete car wiring diagram but they are available out there, Google is your friend. My car is not factory fogs so I do not know how low beam power gets to the switch from the factory but this wiring is pretty simple and if you have to access the switch, color coded wires should make sense. There are some small(18 gauge?) grey wires running around the fuse box for this type of use. Just take your time, run the tests and if you want to jump past accessing the switch, find a complete car wiring diagram. Good luck, you will get it.

[steven c]