Need some help wiring up a relay to some fogs Options

[Jameel]

Finally getting the front end of my '71 looking finished. I mounted up these Cibie and Hella lights and got them wired up today. All went well during testing, but on a drive I blew a fuse, then outright melted another one. Came home to rethink.

I have wired up the J West H4 relays on several 911's, but I have not done it on a 914. I had assumed when wiring this that the round relay in the fuse panel under the dash would be sufficient. It's not apparently!! So, couple questions.

1. I'm running four 55W halogen bulbs, and that works out to 4.58 amps per bulbs, so less than 20 amps total. The Hella bulbs came with a 30 amp relay. Is that relay beefy enough to handle the four 55W bulbs at 20A?

2. If I do wire up this relay, where do I put it? How do I handle the round relay in the fuse panel under the dash?

3. If I switch out all the bulbs for LED's, will the round fuse be all I need?

[Jameel]

I was looking closer at the fuse and whadya know, the metal is intact. What causes the ends to get roasted like this without the wire breaking?

[Mark Henry]

Relay it near your fog lights you leave the existing wire/relay alone, it plugs into the new Bosch relay and it's now just a switch with no big load. You run a heavier wire to the new relay, that's now the power for the lights. I'd mount it near the lights, make an extension wire to run from one of the the old fog light +wire to trigger the new relay, You can do this without cutting any of the stock wires.

[Jameel]

Relay it near your fog lights you leave the existing wire/relay alone, it plugs into the new Bosch relay and it's now just a switch with no big load. You run a heavier wire to the new relay, that's now the power for the lights. I'd mount it near the lights, make an extension wire to run from one of the the old fog light +wire to trigger the new relay, You can do this without cutting any of the stock wires.

Thanks Mark. Sounds like I'm going to have to run a new wire up the tunnel to the relay. Ugh. This is so easy on the 911 because the battery is right there. Is there a way to do this using the existing harness?

[Spoke]

I was looking closer at the fuse and whadya know, the metal is intact. What causes the ends to get roasted like this without the wire breaking?

You're showing an 8 amp fuse but said earlier you expect the current to be 20A. Did you try a 20A fuse?

Notice the fuse plastic melted on the ends. This means there is quite a bit of resistance between the fuse ends and the metal tabs of the fuse panel. Have you cleaned the fuse ends and panel metal tabs? I recently did this on my 914 using a dremel and wire wheel. Had to be careful not to bend any tabs or break any fuses.


When I say quite a bit of resistance I mean like 0.1 ohm.

Power = I * I * R. Say you have 8amps and 0.1 ohm, that comes out to 8 x 8 x 0.1 = 6.4W! That amount of power dissipation will definitely melt the plastic without blowing the fuse.

If you can't figure out the existing fuse arrangement, Mark's suggestion is a good solution.

[Jameel]

Great info. Thanks! I have not yet taken a close look at the panel. And for all I know the PO had the wrong fuse in there.

[windforfun]

I was looking closer at the fuse and whadya know, the metal is intact. What causes the ends to get roasted like this without the wire breaking?

Heat due to a voltage drop due to contact resistance. Current melts the metal, but not the plastic.

[Jameel]

Took a closer look at the panel. I don't have an owner's manual, so I'm a little unsure of what fuse does what, and what size it's supposed to be. The cover plate has an icon of fog lights on the 3rd fuse from the right, but my melted fuse was the 4th one over. Weird. I think the PO wired things up wrong. There is a inline fuse going to the switch too, definitely not Porsche stuff. If anyone has a fuse box diagram for a '71 I'd love to see it. Looks like I've got some sorting to do to get this right.