Fog light troubleshooting

[kev01]

Good day fellow forum members,

I need a bit of help with figuring out the fog light wiring on my 76 ROW 930.
Below is a pic of the fusebox.

When I switch on Park lights & Fog lights – headlights come on but no foglights. Interior foglight switch bulb is on. Seems that the white wire to fuse 7 has continuity to pin 86 and powers up the headlight when the foglight switch is activated
I then removed the relay to test and switch park lights & fogs on – Power at pin 85 but now no power to fuse 7 on the white wire
I connected pin 30 to 87 and foglights power up so the rest of the wiring looks fine.
I am using the red relay and as far as I know, pin 85 is supposed to be earth, while pin 86 should be power from the switch. I tested the relay in this manner and it worked fine before installing in the car. After installing and testing the relay again it no longer works so obviously suffered some damage.
Any wisdom on how the system works and what may be wrong?

[timmy2]

Try spreading the pins of the relay with a razor blade.
May not be making good contact in the socket.

[kev01]

Thanks Dennis.
Had a look and does not seem to be the pins.
Below is a diagram I found. Mine is the ROW so the ** wiring. What does not make sense to me is 85 is supposed to be ground but is connected to the live of fuse 8 on the diagram.

[kev01]

Anyone with a view on what could be the issue?

[ClickClickBoom]

Grounds are a common bugaboo on many system failures. In the aircraft world nav light failure often would show 12/24v at the positive, but no lights. When checking the ground it would often show infinite resistance at the ground to airframe connection. When checking the voltage check the resistance from the bulb socket to a known ground. Foglights are usually infrequently used and as such are a prime candidate for corrosion issues, being on the front of the car close to the ground and water is a recipe.

[timmy2]

in your photo it looks like a grey wire on top of terminal 2 is disconnected?
Also it seems as though the hi beam and low beam outputs are reversed looking at the photo?
on the bottom of the fuses, 5/6 should be yellow and yellow/green, 7/8 should be blue and blue/white.
The Grey wire from the fog lamp relay should be to the bottom of fuse 2 to work the European way.

[kev01]

Thanks. You are correct about the high and low beam wiring swopped. I will get to that next. I move the grey wire on fuse 2 to the botoom so it now supplies the fog switch with power as originally intended. Am I reading the wiring diagram correct? Pin 85 goes to fuse 7 instead of being an earth as other relays are wired?

[timmy2]

For the euro version, yes fuse 7.
Swap the hi and lo wiring and the fog circuit won’t light up your low beams.

[kev01]

Thanks to FrankM for his post on the following thread
https://forums.pelicanparts.com/porsche-911-technical-forum/1056774-troubleshooting-headlight-relay.html
My wiring now is in line with the comments below.
"the brown wire, pins 86 of each relay holder should have continuity to ground (measure at any ground point)
Pins85 (yellow & white wire) should have NO continuity to ground when the lights are switched off "
Still not working but I am getting closer...next to check the relay.

[LIRS6]

My issue/resolution - fog lights not working, lights on and fog light switch pulled. Checked for power at the fuse box. No power to the fogs. Swapped fog light relay with another relay - still no power. Thought it was likely the switch, but before going down that route, I swapped in yet another relay - power! ...but still no lights. Removed glass on fog lights and checked bulbs - both dead. Bulbs ordered. Tried to buy "black" 5 pt relays - nowhere to be found in stock. What's the deal with that, why are they seemingly unavailable anywhere (unless you want to spend $45)?

Jason

[automobile]

Was at fog lights on a '79 Euro 930 & installing headlight relays , it seems that while US cars have the fog relay 85 terminal going to ground , the Euro has 85 grounding through the main / high beam filaments in the headlight bulbs , guess not enough current to actually make that filament light up. Then when you switch on headlights & move from dip / low beam to high / main beam , the +12v now at 85 "cancels" the +12v at 86 & hence the fog relay switches off , as do the fog lights

[Robey5]

@kev01 - thank you for this thread and for your note of resolution. I was able to track down a quirky issue with my fogs, and was able to get my problem sorted (blowing fuses) and we’re all back in business now. In sum, make sure your grounds are clean and solid when tracking down electrical issues, people!