Fuse 18 Blowing

[maxman]

Hey guys. I recently noticed that my clock and interior lights stooped working in my '82 Euro. I checked the manual and it looks like that stuff is driven by fuse 18, which is supposed to be 5 amps.

I checked, and sure enough the old fuse was blown. I could not see what amp rating the old fuse was as it broke in half as it took it out, but it was black in color. Anybody know how many amps a black fuse is?

Anyway, I tried twice to replace with a 5 amp fuse, and it blew within seconds. I then tried with an 8 amp and it was the same story.

Any ideas?

Thanks!

~Max

[equality72521]

A black one is a fuse that got too hot for too long. You have a short or ground issue. Gonna have to start at the beginning with a meter and start working you way through the circuit until you find the problem.

[Mothy]

From memory the light under the front hood is on the same circuit. Mine suddenly developed a short for no explainable reason a couple of years ago - the cable from the light has a spade connector about 6" from the light (to allow you to remove the hood).

Try disconnecting that and measure resistance between the wire to the light and earth. If it has zero resistance = short, some resistance = bulb OK, open circuit (infinite resistance) = blown bulb.

If its a short, leave wire disconnected and replace the fuse - problem should not re-occur. You then have to work out why the light is shorting - but you can do that without urgency.

If hood light checks out OK, then move on to other devices powered by circuit - interior lights can be popped out to isolate them one at a time.

There are circuit diagrams to follow on this site - Tech Info Centre - very helpful.

Best of luck

Tim

[86 911 Targa]

Originally Posted by maxman

Hey guys. I recently noticed that my clock and interior lights stooped working in my '82 Euro. I checked the manual and it looks like that stuff is driven by fuse 18, which is supposed to be 5 amps.

I checked, and sure enough the old fuse was blown. I could not see what amp rating the old fuse was as it broke in half as it took it out, but it was black in color. Anybody know how many amps a black fuse is?

Anyway, I tried twice to replace with a 5 amp fuse, and it blew within seconds. I then tried with an 8 amp and it was the same story.

Any ideas?

Thanks!

~Max

All of the above suggestions are good.

Yellow=5A
White=8A
Red = 16A
Blue = 25A

Black fuse ????

Here is the circuit for our '86.

It may/may not close to your model/year.

I fused our dash lights with a 3A fuse
next to the headlight switch.
Along with the footwell blowers (10A),
the front condenser blower (8A),
the evaporator relay (10A),
and added headlight relays.

You may pm me.

Gerry

[maxman]

Turns out that there was a 2 Ohm (6 Amp) short in the right hand interior light. The wire was pinched by the last guy who put it in and it just now started to short.

I love it when it's easy.

[NOLAsc]

Got the same thing going on with my '82. Fuse 18 blows as soon as I connect the ground to the battery. I noticed it when my clock stopped working.

Bentley says fuse 18 powers interior lights, clock, glove compartment light, luggage compartment light. They left out one other thing, that my owner's manual included: lambda system.

Maybe that's contributed to my hard cold-start issues. Dying in coughs and spits.

Not looking forward to tracking down this short.

[Mothy]

Just take it slowly and methodically, isolating parts and using a multimeter to search for shorts to ground.

[Bob Kontak]

Originally Posted by NOLAsc

Maybe that's contributed to my hard cold-start issues. Dying in coughs and spits.

The O2 relay under the passenger seat can be a problem for running but by itself will not usually blow the fuse if bad. However, you have no lambda system and I think the frequency valve dies if that fuse is gone. Get crackin' - it will be a simple fix - just isolate the parts. For example - Unplug the O2 relay and see if the fuse stay intact. Pull the lights out, etc.

82 electrical system in it's entirety can be found in the tech info center.

[NOLAsc]

Thanks y'all. I left the car in the garage at work last night and can't wait to get back to it today.

BTW Bob, thanks for all the patient and thorough explanations over on scott.k's thread, http://forums.pelicanparts.com/porsche-911-technical-forum/587238-911-sc-starting-issue.html -- particularly using the dwell meter for mixture adjustment.

-- Shawn

[T77911S]

i prefer test lights for electrical TSing, but thats just my peferance for simplicity sake. (i also use to work at an auto electric shop).

remove the fuse, connect the test light in place of the fuse. the light will be on as long as there is a short to ground. this will keep you from wasting fuses. dont forget, any light that is on, such as the trunk light, will also make the test light come on, so bulbs need to be removed to prevent them from giving you a false indication. also, dont forget to check the door switches.
you can also use a test light "backwords". IE, connect the "ground" clip to 12v and probe for what is shorted to ground, with the fuse removed.

[crustychief]

I am glad you got it sorted out. I am also a fan of the easy ones!

[whiteSC]

Shawn,
+1 on T77911S's method for chasing down the short without eating through fuses. Another less elegant approach could immediately determine if the O2 part of the circuit is shorted.
From the diagram, fuse 18 has two wires on its load side - a red one that feeds only the O2 relay and a black one the feeds the lights and clock. If you can disconnect the black wire at F18, put in a fuse and it blows, then you know that there's a short in the red wire for the O2. Yes you're possibly out one more fuse, but if holds, you can see if your cold start improves with the O2 relay re-energized.
Then it's onto troubleshooting the non critical side of the circuit. It shouldn't be too hard using T77s method and the lights should be easy enough to unplug one at a time.
Good luck, Jeff

[steely]

check the glove box light too - they have their hot notoriously close to the chassis.

[Bob Kontak]

Originally Posted by NOLAsc

BTW Bob, thanks for all the patient and thorough explanations over on scott.k's thread.......................

Quite welcome, Shawn. Thanks for mentioning. It is a fun hobby/obsession. I like learning from the CIS guys who are deep into it here.

[NOLAsc]

Originally Posted by whiteSC

...determine if the O2 part of the circuit is shorted.
From the diagram, fuse 18 has two wires on its load side - a red one that feeds only the O2 relay and a black one the feeds the lights and clock. If you can disconnect the black wire at F18, put in a fuse and it blows, then you know that there's a short in the red wire for the O2. Yes you're possibly out one more fuse, but if holds, you can see if your cold start improves with the O2 relay re-energized.

Jeff, that was a great suggestion. I had been looking at the tech forum diagrams, but when I went back to Bentley, I immediately saw what you were seeing. I removed the black wire and lost the voltage across the fuse terminals, plugged in a new fuse with only the red connected to the bottom, and it didn't blow.

Didn't clear up my cold start issue, but helped rule out this as a question (although I should throw a new relay on the O2 unit just to see). Good to get the car home though.

More trouble shooting to come -- on cold start and the short.

T77, I see a new test light in my future. Thanks for the suggestions.

Shawn

[NOLAsc]

I replaced the relay on the O2 unit under the passenger seat this morning and... got my car back. Fired right up. Drove great. No pops. No snorts. No coughs. Big freaking grin.

Now, besides needing to find the short to the cabin lights and clock, I need to fix the fuel mixture. [Edit: without the lamda circuit powered, and with the car refusing to start...] the only way I could get the car started and home from work was to do the great no-no: richen the mixture with the 3-mm allen wrench.

Thanks for the help everyone.

[T77911S]

the fuse does not blow after replacing the relay?

if started the first time, what happened the second time? did the fuse blow? was the motor hot or cold when it would not start?

[NOLAsc]

Originally Posted by T77911S

the fuse does not blow after replacing the relay?

if started the first time, what happened the second time? did the fuse blow? was the motor hot or cold when it would not start?

Cold start was the issue. Warm start was fine -- no backfire. Has continued to start well this morning.

Fuse did not blow. Jeff (whiteSC) pointed out that two wires come out of fuse 18: red to the lamda circuit; black to the lights. Once I unhooked the black wire, I no longer read voltage across the broken fuse. The short is in that circuit; not in the O2 circuit.

I could do without lights in the cabin. I had already disabled the one in the trunk (I leave it open sometimes in the garage). Never tried the glove box light. But I really miss the clock. I'm looking forward to finding and fixing the short.

Shawn

[Bob Kontak]

Originally Posted by NOLAsc

Fuse did not blow.

Cool. Interesting info. Good job White SC.