Troubleshooting Headlights (Relay Kit Problem)

[dlbracey]

I just picked up my first 911 today (89 targa), which is very, very exciting and I drove it from Connecticut to Virginia without any problems or so I thought. When I pulled into the driveway I noticed that my headlights weren't on even though the switch was pulled into the on position and the running lights were on. Neither headlight in low beam or high beam were coming on. So I checked the fuse panel, noticed that there were 4 fuses involved and unlikely that would be the problem, but then I noticed that a relay had been installed which appears to be the Sucro relay that was sold on Rennlist a few years ago.

I can't find a good installation thread or instructions on this kit, but it appears that an in line fuse was installed between the relay and the positive battery connection. This 20 amp fuse is fried to the point that it melted the fuse holder. I'm guessing this is the problem, but my question is: should I remove the fuse completely or should I soldier in a new inline fuse holder and a new fuse? If a new fuse, what amp fuse should be used? See Pics:





And one of the car...



Thanks for your help! Dan

[porwolf]

Looks like the fuse holder got burned due to poor fuse contacts. I would replace the fuse holder and use a 20 Amp fuse which should be good for two 55 Watt headlight bulbs. Higher wattage bulbs need bigger fuse. In that case, like with 100 Watt bulbs you also may want to make sure that all the wiring between the battery, relays and even to the headlights is appropriate. Factory wiring is only designed for 55 Watt bulbs. If the in-line fuse blows again you need to check for a short in the headlight wiring between the battery and the bulbs.

[SiberianDVM]

What porwolf said.

I just installed the JWest relay version in my '77 this past weekend, as part of an over all clean-up of the wiring. I used a "heavy duty" in line fuse holder (12 ga wire) in the wire to the battery, but I used a 30 amp fuse.

[timmy2]

Test that the coils in the relays are not fried.
Otherwise proceed as others have indicated .

[dlbracey]

Thanks everyone. I was able to clean this up last night and everything seems to be back in order. I did find one thread that mentioned it was not a good idea to have an in line fuse in this location as it creates a single point of failure for all 4 headlights as opposed to the factory setup of one fuse per light, this seemed to make sense to me so I opted to not put a fuse in that location I guess at the risk of frying the relay. Shouldn't a short in the headlight wiring trip one of the 4 original fuses with this setup?

[86 911 Targa]

For our M/Y 86.

Yours should be close.

Good luck,

pm me as needed.

Gerry

[Jonny H]

Three things wrong with that melted fuse holder:

1) That type usually claim to be rated 20A. That's the holder rating which means you cannot use a 20A fuse. It would have to be a lot smaller, say 15A for the fuse to blow without melting the holder first. Personally, I would not use a cheap inline holder for my headlamp feed. They are only crappy spade connectors if you look inside

2) When the high beams are on, the dips are on as well. The standard fuses are 4 x 8A so that is 32A rating.

3) The fuse is only there because there is a risk of the wire shorting somewhere between the battery, the relay wiring and the wiring to the fuse panel.

A properly engineered solution negates this risk by not having any additional wiring, integrating the headlamp relays onto the panel itself. Did I mention we make such a product?

[dlbracey]

Thanks Jonny, I've removed the inline fuse completely for now. Your solution looks great, but I'm not quite at that point with this car.

[Tremelune]

The 20A fuse is there to protect the length of wire from the battery to the fusebox and is a Good Thing. The fusebox is there to protect each headlight wire when they are split off from the larger wire.

The wire coming off the battery is not long, so if you're confident that it won't rub something and short, you can probably get away without fusing that run. Each headlight wire should be individually fused (the easiest way is to run them through the factory fuses).

Some background:

Fuses protect the wire, not the device, and as such should be matched to the gauge/capacity of the wire. If you fuse a 12awg wire with a 20A fuse, and then splice that 12awg into four 16awg wires, the 16awg wires are no longer protected, because 20A of power can cause them to heat up to the point where they catch fire. Whenever you reduce the gauge of a wire run, you must also reduce the rating of fuse that protects that run of wiring.

Bad website, excellent information:

FUSES

As an aside, inline ATC spade fuses are likely to be superior than the ceramic fuses used by Porsche decades ago.