Fog lights not getting power, What else to check? Options

[saigon71]

Everything goes bad eventually. Did you check your grounds? And the fuse?

I checked the fuse, but not the grounds...thanks.

Cleaned the grounds behind both headlights, no help.

[saigon71]

You are probably way ahead of me on this, but just to be sure...

The key has to be on and the headlights have to be on low beam.

I was hoping this was the "problem" but it wasn't. Fogs don't work on low beams.

[saigon71]

You are probably way ahead of me on this, but just to be sure...

The key has to be on and the headlights have to be on low beam.

Good points,you could check for voltage @ your fuseboard ,foglight relay pin #30 to at least isolate that as an issue then you know its further down the line,if that's ok it may be your switch but less likely.
Go well (IMG:style_emoticons/default/beerchug.gif) .

I've got 12V at fuse #10 and foglight relay pin #30.

[saigon71]

My fog lights don't work on either side. After trying different bulbs, I tested for power with a VOM - no power. Next, I swapped the fog light relay out with several others...still no power.

What are the next logical things to check? Do the fog light switches go bad?

Thanks.

Do your high beams work? If the one high beam that powers the fog light relay is burned out, the relay will never close and power the fogs.

Yes, high beams work.

[Porschef]

(IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/beer3.gif)

[Mark Henry]

Check the wire(s) to the lights for continuity.

[76-914]

I chased a problem similar to this. The relay and it's rubber socket had passed voltage/continuity tests a few times. When moving the relay harness out of the way ( I was working on another problem at the time and had given up on that circuit) I heard it click and found that circuit now working. When the fuse panel was reconnected it quit working. (IMG:style_emoticons/default/WTF.gif) I found that when that harness was in a "certain position" it did not function. The break was inside the rubber plug and when placed in that certain position it would break the connection. Otherwise it was good. Luck was with me that day. (IMG:style_emoticons/default/beerchug.gif)

[Spoke]

I've got 12V at fuse #10 and foglight relay pin #30.

12V at fuse #10 is good. #10 also powers the horn.

What voltage do you read at foglight relay pin 85? Should be 12V. This voltage comes from the foglight switch.

Once 12V at pin 85 is verified, check foglight relay pin 86. Pin 86 should be 0V with headlights on and on low beams. Pin 86 is connected to fuse #1/#2 which power the high beams.

Once 12V at pin 86 and 0V at pin 85 on foglight relay are verified, measure foglight pin 87. Should be 12V. Pin 87 powers the foglights.

Does the lamp on the foglight switch light up? Is the lamp on the foglight switch good?

[Spoke]

Here's a simplified schematic of the foglight circuit for a '74 914 as detailed in the Haynes shop manual.

There are 5 main components in the foglight circuit:

o Foglight switch
o Foglight relay
o Headlight dimmer relay
o High beam filament
o Headlight switch

To operate the foglights, the following conditions must be met:
o The headlight switch must be in the headlight position (2nd click outwards)
o The headlight dimmer relay must be on low beams.
o The foglight switch must be pulled out.

With conditions above met, current flow for the foglight relay coil goes through the headlight switch -->> foglight switch -->> foglight coil -->> headlight high beam filaments. The foglight relay coil shares 12V with the headlight high beam filaments. With standard headlights, the high beam filaments are much lower in resistance than the foglight relay coil thus most of the voltage drop is across the relay coil and the relay closes and foglights are on.

If the headlights are high impedance LEDs, it is not clear if the foglight relay coil will energize. The foglights can be made independent of the high beams by removing the wire from pin 86 of the foglight and attaching it to ground bypassing the high beam filaments.

[saigon71]

Fog lights are working again. I appreciate all the help. To summarize the problem,

I hereby charge myself with two counts of being an idiot:

Count #1: The original "problem" was due to the fact that I did indeed have the hi beams on and didn't realize the fogs only worked on low. (IMG:style_emoticons/default/chair.gif)

Count #2: Due to the "problem" discussed in count #1, I wanted to check the bulbs, so I popped the covers off. I left everything apart while troubleshooting. When the bulbs looked fine, I checked for voltage to the bulb and got nothing. I kept using the piece that holds the bulb for ground. The problem is that the bulb holder doesn't get grounded until the lens in installed. (IMG:style_emoticons/default/chair.gif)

I ended up busting a diode on a headlight motor during this ordeal so I added another item to fix while chasing a problem that didn't exist. (IMG:style_emoticons/default/chair.gif)