Fog Lights, Wiring Mod for Full-Time, Early 928

Fog Lights, Wiring Mod for Full-Time, Early 928

The great fog light caper... -or-
How to get them working the way I wanted.

The goal of this procedure is to make the fog lights illuminate whenever the switch is pressed in and the ignition switch is in the "on" position.

This procedure has only been tested/completed on my 1982 US spec 928, if you own another year, or a Euro/ROW car it may or may not work. You have been warned! If you don't have the factory schematics some of this will not make much sense because I'm going to refer to wiring points by the codes in the factory book. If you don't own a volt/ohm meter Go down to the local Radio Shack and invest $15 in one. You will need it to make sure you have the correct wires.


As designed by Porsche, the fog lights light up only when
A) The parking or headlights are on, and
B) The fog light switch is on, and
C) The High Beams are not.
See schematics (current flow diagrams) for type 928 USA model 82 parts III (3) and IV (4) in volume VI (6) of the factory manual set.

Let's start with the fog lights themselves in part IV:
The high side of both lights goes to fuse #1 (16amp) then into relay XII (12). The other side of the relay contacts goes to line 30 which is the B+ (+12V battery rail) circuit. This portion requires no changes, but pointing it out helps locate both the fuse and the relay for discussion. :-) Look at terminal 85 on relay 12. Found it? Now follow the unmarked wire onto the section 3 diagram and over to just below combo relay XIX-XX. Note where it connects to fuse 24, that feeds the left side headlight high beam circuit.

Got it? OK if you want your fog lights to continue to work when the high beam headlights are on, this circuit has to be changed. If you don't care, or want them to shut off when your high beams are on skip this section and proceed to the switch mods. in section 2.

The way it works from the factory is like this:
As long as the left high beam is not powered up by the combo relay, the light bulb filament has a very low resistance to ground. This means that for all practical purposes the connection at fuse 24 is at ground potential. When the relay turns your low beams off and your high beams on, this point rises to the B+ value. (B+ is roughly +12V to +13.8V in a car electrical system. For convenience's sake I'm just going to call it B+ from here on...)

This wire has to be moved from the current connection at fuse 24 to a full time ground connection. The procedure is as follows:

1) Disconnect your battery ground to kill all the car electrics. The easy way to do this is just lift the ground strap in the bottom of your tool storage area. PHOTO The wing nut or nut is removed and the strap lifted and allowed to hang in the air.

2) Open up your fuse/relay panel and remove both the two screws holding the lower wood panel part in place and the two 10mm bolts holding the upper portion of the fuse panel in place. Remove relay XII (12) the fog light control relay, then: Locate point 85 on the relay board. You can use the diagram on the removed relay to see which pin is #85.

3) Disconnect your OXY sensor wire and move it out of the way. Gently lift the relay panel off the two rubber grommets that the lower pins are in, and rotate the top of the panel towards you as seen in PHOTO.

4) Most of the wires on the back side of the relay board are all the same shade of Grey. :-( The wire from point 85 is about 6" long and goes to the fuse 24 high side contact, where it is crimped in with another larger Grey wire to the fuse clip. I used a small sharp set of cutters to remove the end of the wire from the fuse 24 connection. PHOTO (I also used a meter to verify I had the correct wire first... that's the red test lead in the bottom of the picture.)

5) Once I had it loose from the fuse 24 connection I spliced in about 8" of wire and a crimp lug on the end. PHOTO Keeping this new wire clear, reinstall the relay board back in place. Tie the new extended spliced wire to one of the 10mm bolts as you put the whole thing back together. PHOTO Reverse this whole procedure until you have everything back in place and your battery ground strap reconnected.

Now test the change. At this point the only change should be that the fog lights no longer turn off when your headlights are on high beam. Work OK? Cool! Now on to section 2:


If you thought section one was fun, boy are you going to love this.

OK back to part IV (4) of the schematic: Locate the fog light switch again and note the wire connected to point 30 on the switch. This is where your +12V comes into the circuit for the fog lights. As built by Porsche B+ is applied to this wire when your main light switch is on. Closing the fog light switch applies B+ to terminal "S" (That's an S not a five) on the switch. Follow the wire up to relay 12 and now you know where the B+ signal to switch your fog lights comes from. Remember that after the section 1 mod (above) terminal 85 on relay 12 is now at ground always. Follow the wire from terminal 30 on the fog light switch back onto part 3 of the diagrams. See where it ends up on terminal 58 of the main light switch? Well we are going to move it as well...

1) Disconnect your battery again (See section 1.)

2) I removed my steering wheel and pulled my whole dash pod apart to do this part. You *MAY* be able to do it by just removing the main and fog light switches from the front. PHOTO You have to get both the switches out and off the plug in connectors. PHOTO

3) Locate point 58 on the back of your main lighting switch. It should have two Grey wires on it. The smaller one was the one which runs to the fog light switch (point 30) in my car. The factory diagrams show it as the larger wire... Go figure. Use your meter to verify which is which. Carefully cut it from the back of the main light switch connector at point 58. I had to unwrap both wire harnesses to locate and verify I had the correct wire, but if your comfortable, you can just cut it free. PHOTO

4) I then wrapped the loose grey wire around the loose harness to take up the extra. PHOTO

5) I then spliced into the main B+ feed wire from the ignition switch into the main light switch. It is connected to point X on the main light switch. (The diagram says this is a Black W/Red stripe wire, but mine was Black W/yellow stripe. PHOTO When I was done it looked like this: PHOTO

6) I then taped the whole harness back up again. PHOTO At this time I reconnected the switches and reconnected the battery long enough to test everything. Then disconnected the battery again and started putting it all back together. PHOTO

7)The whole dash then went back together, the steering wheel went back on and the results: PHOTO

Larry Ware
82 auto Moosgrunmetallic (Moss Green Metallic)
928OC charter member
Florida plate: "V8 SHARK"

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