Electrics - ARGHHH!

[mean_in_green]

Hi,

My '82 SC has failed its MOT because the front and rear near side lights don't work, and neither do the number plate lights.

Bit of a mystery, as they worked perfectly when I checked them yesterday.

So, I checked the fuse board and sure enough the fuse for the side lights was blown. Replaced it, and it blew immediately when I turned the lights on to test.

Bear with me, as my electrical knowledge is limited...

I can understand that there may be a short in the side light circuit for the one side, but don't see why the number plate lights should also be inoperative?

Something seems to have happened that has simultaneously done the following:

1. Stopped front and rear side lights working on one side
2. Blown the bulb in the engine bay light under the bonnet
3. Stopped the number plate lights working

The bulbs seem okay.

Can a blown bulb cause a short?

Like I said: I checked all the bulbs yesterday and they were fine!

Ideas?

[mean_in_green]

Now discovered that the number plate lights have been spliced into the side light feed at some point in the past, thereby explaining why they're not working too.

The wiring at the light switch looks ok.

I guess I'll have to continuity test each section of the feed wire from the fuse box the lights?

[dshepp806]

Did this just happen? Or has it been working before?

One could remove all the pertinent bulbs,..replace the fuse (after doublechecking the rating)...engage the curcuit and go from there, incrementally adding each bulb element until you find the area causing the problem (i.e., "blowing the fuse") ....If WIRING is an issue, then you'll have break out the Bentley and get to work tracing wires,..particularly at the fuse box. She could be sharing an unauthorized circuit path, outrunning the path's current rating (fuse)..(DO CHECK IT"S VALUE FOR CORRECTNESS)

.....OR,...you've a hot wire that's grounding out the 12 VDC buss somehow. (this could be checked with an ohmeter (under no power),the + side copper to ground,..bulbs out of circuit...damned sure shouldn't be a continuity path.......

...various ways to go about it......definitely need to get the wiring right..sounds like you're on it!!

BTW,..when you "checked the bulbs yesterday", how was this done? Just a visual (ops check)? Or did you disassemble the lenns, remove/reinstall the bulbs? Jst curious.


Best of luck,

[mean_in_green]

Originally Posted by dshepp806

Did this just happen?

BTW,..when you "checked the bulbs yesterday", how was this done? Just a visual (ops check)? Or did you disassemble the lenns, remove/reinstall the bulbs? Jst curious.


Best of luck,

Thanks for your reply.

Yes - it did just happen, inspite of previously working fine.

Yesterday's check consisted of a walk round visual inspection prior to submitting the car for it's test - all was ok at that time, infact I nearly asked my fiancee to check them this morning but didn't as I 'knew' they were ok.

Your suggestion of how to approach the diagnosis is much appreciated, I've given up for today after four hours not really getting to the bottom of it.

Hoping fresh eyes will help in the morning!

Cheers for now.

[dshepp806]

Have you a nice meter, capable of measuring current?

Keep at it.....you'll find it. Make sure that fuse terminal is burnished and bent so that you've GREAT tension at the fuse contact point AND IS CENTERED NICELY WITHIN the block. Fuse should spin with resistance,..if not,..you've not got it right. No llose, moving fuse end caps..


BTW,..what do these fuel pumps typically pull for current? (nominally)???????????????


Best,

[mean_in_green]

Originally Posted by dshepp806

Have you a nice meter, capable of measuring current?

Keep at it.....you'll find it. Make sure that fuse terminal is burnished and bent so that you've GREAT tension at the fuse contact point AND IS CENTERED NICELY WITHIN the block. Fuse should spin with resistance,..if not,..you've not got it right. No llose, moving fuse end caps..


BTW,..what do these fuel pumps typically pull for current? (nominally)???????????????


Best,

Yes, I've borrowed a fancy meter from my brother. Will ensure the terminal is prepared as described, pull all the bulbs and start again in the morning I think.

Forgive me - I'm not with you on the fuel pumps thing?

[dshepp806]

Simply: just a suggestion to MEASURE the current draw of the fuel pump, with existing wiring and alternative wiring. (A quantification, if you will, of what's happening with the fuel pump current draw) One must be tuned in as to HOW this is properly done,..If not familiar with or have the understanding of "how it's done",..don't mess with this...better safe than sorry. I'm (still) curious as to what a nominal reading for fuel pump current draw is,..probably in the Porsche factory manual, which I don't have....can anyone chime in on this?


Best,

[Red88Carrera]

Don't forget to check the wiring to the engine compartment light. Mine was pinched under the light housing.

[mean_in_green]

Originally Posted by Red88Carrera

Don't forget to check the wiring to the engine compartment light. Mine was pinched under the light housing.

Thanks for the tip.

I fitted a new one the day before as it was missing when I bought the car, and did have a quick look to check I'd not missed something with it. I'll whip it off and check what I did again.

[mean_in_green]

RESOLVED!

More by good fortune than technical talent I feel, but resolved none the less.

More tedious checking with a meter revealed a short in the circuit to the engine bay light. Maybe that explains the absence of the light unit itself when the car came into my possession - the previous owner couldn't find the short. I've isolated it for the time being in order to get the car through its test anyway.

The number plate lights... have been spliced into the side light circuit behind the rear nearside light with a Scotchlock. It's the first time I've had an appreciation just how exposed to the elements the wiring is behind the rear light units - pretty poor design (dare I say it?!).

Replacement of the offending connector with a correctly soldered terminal covered in heatshrink has brought light back into my life, and hopefully a crisp new test certificate on Monday morning.

Thanks to all for your suggestions.

Simon

[OldTee]

Originally Posted by mean_in_green

RESOLVED!

More by good fortune than technical talent I feel, but resolved none the less.

congradulations. Now is the time to spring for a Bentley on your car and the 100 Projects book offered by our host. If you are going to keep a 25 year old car up and running reliably you need patience, information and help. You didn't give up, others might have. Happy driving.

[Red88Carrera]

Glad you got it! That's the 3rd time I've seen the same problem. The wires have to route through a clearance in the light housing base. If they don't, they'll get pinched between the housing and the deck lid.

[dshepp806]

HOME RUN!!!! YEAH!!!


Best,

[mean_in_green]

Thanks again for advice received everyone.

I bought the 100 Projects book before I bought the car, strangely enough. I get the feeling that the previous owner was reading the same material - each time I decide on an upgrade I find he's beaten me to it!

Whilst repairing the wiring, I was surprised how oxidised some of the copper core had become: quite dull brown in appearance.

Does this oxidisation impair the conductivity?