To fuse or not to fuse that is the question

[jackportia]

As always confusing myself with options. If I install the recommended headlight relay kit which is also supposed to protect the gauges do I still need to install a fuse at the switch itself? 1986 Carrera.

[RSTarga]

Separate items I believe.

[timmy2]

Relay kit protects the hi/low stalk contacts.
Fuse protects the wiring to gauge lights.

[dan88911]

Is this what you looking for jackportia?
http://www.pelicanparts.com/911/911tech_articles.htm

[dan88911]

Here is another thread.
Porsche Forums - Pelican Parts Forums

[hcoles]

As a general rule I put in a fuse if the wire is not protected by a fuse in the circuit. I think there are places in the stock wiring where the wire itself becomes a fuse - not a good idea because it can be hard to find the open if the wire burns open. Of course a fire is possible. There are a number of threads that show what others have done when putting in relays for the high and low beams.

[Koizumi]

Originally Posted by timmy2

Relay kit protects the hi/low stalk contacts.
Fuse protects the wiring to gauge lights.

Yep. Fuse ‘em both and don’t look back.

[sugarwood]

Is it OK to fuse all 3 wires going into 58a for dash light fuse at headlight switch?

[T77911S]

you can never put in too many fuses in but you can put them in the wrong place and you can put in fuses that are too big, IE, fuses need to be as close to the power source as possible.
remember, fuses protect the wiring, not the device or load. fuses are sized for the wire size/ current capability of the wire.
porshe did it all wrong with the head lights.
the fuses are AFTER the switch and dimmer switch.

[ChrisHoover]

Fuses isolate circuits so if a load fails it doesn’t take everything with it. Conductors should be sized with enough ampacity plus some overhead for the conductor’s load. Its interesting that newer cars fuse the battery (fuseable link) but older cars do not. Batteries can explode if they aren’t current limited. Fuses thus guard against that possibility as well.

[dan88911]

Sorry about earlier post.
Lets see if I got right this time.
Dash lighting fuse installation

[dan88911]

Sugarwood, posted the other I tried to post.

[jackportia]

Porsche electrical repairs/upgrades gives me the willies. "Excellence was expected" is something I thought was a given instead of a cliche when it comes to Porsche electrical wiring. It is appreciated that there are members on the forum who are willing to take the time to give back to an appreciative member their relative experience. Thanks to everyone for their thoughtful and helpful response.

[ChrisHoover]

Not for nothing but I spent a good part of the last 3 days studying the 1966 911/912 wiring diagram. It comes nowhere close to modern complex low voltage best practices. Some of the problems have been cited on this site many times. Things like switches carrying too much current, lack of relays, and lack of fuses on some circuits. There are a lot of folks restoring to factory original. I’m not going to do that. The whole wiring harness will be scrapped and redesigned.

[Rawknees'Turbo]

Originally Posted by ChrisHoover

Not for nothing but I spent a good part of the last 3 days studying the 1966 911/912 wiring diagram. It comes nowhere close to modern complex low voltage best practices. Some of the problems have been cited on this site many times. Things like switches carrying too much current, lack of relays, and lack of fuses on some circuits. There are a lot of folks restoring to factory original. I’m not going to do that. The whole wiring harness will be scrapped and redesigned.

You should check out the electronics screwups associated with the entire Turbo run - plenty of laughs to be had.

And the amazing thing is that the 911 electronics worst practices remained the norm throughout several decades (at least through 1989), even while the 944 side of the factory modernized. And come to think of it, the electronics shart show is still strong with the pre-1990, 911 parts production (aka - official, Porsche branded, "might work . . . for a while", relays that look like a six year old soldered the half-baked components together). Super lame on Porsche's part.

[ChrisHoover]

Got started this week drawing a ladder diagram of an imagined improvement on the harness. There are choices to make and I want to map it all out and see if the choices are correct.

I am calculating the loads on the switches and relays. It is difficult finding complete electrical specs on the various parts. How much current does a Bosch/Tyco relay draw? If it were IDEC or P&B I could look up every spec. For some reason for “auto” relays we dont need that much info.

Today I stared at the butchery that is my 4 to 6 cylinder, points to MSD, conversion engine bay. I took lots of pictures and noted everything that should change. The goal is a pristine bay worthy of Luftgekult.

[timmy2]

Bosch 12 volt relay coil draws about 150-200 ma.

[Teutonics]

Originally Posted by T77911S

...porshe did it all wrong with the head lights. the fuses are AFTER the switch and dimmer switch.

Bumping this thread up as I found it after doing all of the headlight relay install searches and realizing that if you tie in to your relays with the wires from the TOP of the fuses, it appears you no longer have fused protection on your headlight switch circuit. Of course there's not much current draw after relay installation, but that doesn't help much if you have a wiring abrasion or other short to ground.

From all the wiring diagrams and pictures posted in other threads, I haven't seen where anyone is fusing the switch circuit (in addition to power circuit) to the relays (unless they're tying into the wiring at the bottom of the fuse).

This thread seems to validate my concern... or am I over-thinking this?

[timmy2]

You never had fuse protection to the switch. Only the headlight wires coming off the bottom.

[Teutonics]

Originally Posted by timmy2

You never had fuse protection to the switch. Only the headlight wires coming off the bottom.

Ahhhh... yeah. I was over thinking it. The best solution would be to add a fuse coming off of the battery.