The odometer on my 1980 928S (Euro model) has been broken since I purchased it about a year ago. This weekend I finally decided to take out the instrument panel and see if the problem was related to a broken gear (it was). I put everything back together and connected the negative terminal to the battery only to see sparks flying.

Figuring I screwed something up, I took the dash pod apart again and unhooked anything that looked electrical. Hooked up the battery. Still sparks.

I removed all the fuses and re-connected the battery. Sparks like it's the 4th of July. I was sure this would have at least helped me isolate the cause but no dice.

I disconnected the starter. You guessed it - battery still sparks.

I have eyeballed every wire I can see in the dash pod area and nothing looks frayed or damaged. I did have the battery on a new trickle charger recently but I was able to connect the battery without any problems before I started messing with the odometer.

I have little experience in electrical work and my usual group of enthusiastic mates don't want anything to do with wires. Before I call in a pro (and spend cash), I'd really like to see if I can figure this out myself. Are there any obvious things I should be looking at?

 

I had a disaster with the hazard switch when I was installing LED lighting in the pod. The hazard switch is unfused, for obvious reasons and internally shorted, causing severe wiring harness issues. Unplug your hazard switch and see if you still have the short and be sure that nothing is touching the pod on the switch harness connector.

 

That’s very good information for the OP. It would also be easy to switch on a turn signal while doing work in the pod; they stay on, steady light not flashing, if the switch is activated, but I’m not sure if they could with the fuses and relays pulled.

 

@jschiller Thanks for responding. I did disconnect all the pod switches as part of my initial investigation but I thought I will do it again just to make sure. Firstly, I assume by hazard switch, you are referring to the bit in the first attachment.

Attached is also the wiring, disconnected. I can't see any obvious areas where they are touching each other inappropriately



Even after completely removing the switch so nothing can be touching anything, I still get the short.

 

I didn't pull the relays, only the fuses. Would the relays make a difference in this situation?

 

Note the pod is a ground point sliding the switches in /out with the battery connected can cause a short
Always disconnect the battery when working on electrical systems in a 928

Note the odo reset and fog lamp switch will fit to either of those pod connectors make sure to label the connectors

 

I don’t know, but you could remove them to see if the issue resolves and then reinstall one by one to try to isolate the problem.

But all I was suggesting is that perhaps the turn signal switch is activated right now and perhaps causing your short. It would be easy to do while fiddling with pod removal / install, but it’s also easy to check.

 

William - put a 10A fuse in between the battery + post and the battery cable - does it blow? If not - connect the battery fully and see what happens. Under normal conditions there may be some sparks when you initially connect the battery - particularly if the battery lug connectors are not fully clean. If you are now very tentatively touching the connections together you will make the sparks worse.

Maybe its a real problem... but it may not be.

Alan

 

What Alan said
when I connect the ground strap I expect to see some sparks fly till the wingnut is tight./

 

I was really hoping it was as easy as this, but alas, it appears to be a real problem. I used an 8A and 16A fuse and both blew. In retrospect, I have been connecting and disconnecting the battery for a while now as I have been trickle charging the battery. There weren't any sparks before.

 

The turn signal box is disconnected. I've tried to disconnect everything practical. There are one or two smaller connections (like the light dimmer and odo reset button) that are still connected but I haven't been messing with those so it seems unlikely that they are the problem.

 

Have you put a multimeter on your battery to get a voltage reading? It should be 12.5-12.8V. BTW, You should be wearing protective eyewear when working around with this issue.

I suppose it may be worth the time to just have the battery tested with the proper battery diagnostic equipment; even though I have never heard of this sort of issue being attributed to the battery, yours was a part of this via the trickle charger. Just a thought though.

 

Did you pull all the relays as suggested earlier?

If not, do so now and proceed.

 

Unplug your ignition switch and see what happens.

 

The ignition was disconnected earlier in the process. Made no difference.