928 at last

Or confirmation that I'm screwed.
Fairly long story in related thread....here..
%*&$*&^%# !!! Thoughts (& Help) Requested
It seems I have a (to me) very strange problem.
With the relays out for the ignition, EZK and LH, and the fuse for the fuel pumps also removed, a battery ground touching my exhaust tips resulted in what appears to be a short.
Confirmed with digital volt meter. Actually I'm getting a slightly higher reading from the negative terminal to the exhaust tips than I get from the positive to negative on the battery itself.
The truly strange thing is that theres been no smoke, burning, minor or major explosions or other nastiness.
But it appears that somehow, the exhaust is being (quite) positively charged.
Any help here would be deeply appreciated. I don't want to proceed further until I'm very sure I'm not going to screw anything else up or further damage what may already have been done in.
TIA

jcorenman

Rob,

I'm no Alan, but have a look at the ground strap from engine to chassis (passenger side, under the car).

Disconnect the battery, then take the ground cable out completely, clean both ends and clean the block and chassis where it connects, and check the cable itself for corrosion, etc., then reassemble.

The exhaust connects to the block which connects to the chassis, so there should never be any voltage measuring from exhaust to chassis.

Good hunting!

928 at last

Thanks!,
But,
I've been under the car at least twelve times in the last three years, and I've cleaned and checked every ground point I can find along the way. And I've used the search function to find some of them.
The main engine to chassis on the passenger side I check everytime I'm in there, so I'm pretty sure all is well in that department.

Man, this is frustrating....

I had though I was just dealing with a fuel delivery issue.....

Imo000

What does the big +12V, from the battery to the front looks like?

jcorenman

OK, then crawl back down there with the voltmeter (set to DC volts). Stick the black probe on the chassis (a cleaned-up swaybar mounting bolt should do nicely), make sure you have a good clean connection. That point is called "ground". Now start poking the red probe at various components: Engine block first, should read zero. Then exhaust, then tailpipe-- which is where you read a voltage, right? Should be zero-- anything connected to the chassis should read zero volts, when compared to "ground".

Report back here, geeks are standing by...

Imo000

You can also disconnect the first thin red wire at the battery and see if the power is still at the exhaust tip, then the second little wire and then the big one. At some point the +12V at the tip should dissapear.

dprantl

This is very strange... I am wondering how something hasn't welded itself and broken the ground path to +12...

Dan
'91 928GT S/C 475hp/460lb.ft

mickster

+1

Mrmerlin

I am not really sure what your measuring and why, what is the meter set to measure?
is there a battery hold down installed?

on a long shot check that the wire harness hasnt worn through on the front swaybar and that the starter connections are not touching the oilpan.

also check the positive cable along it whole route from the bat box to starter and also the ABS connection and or see if it is shorting near the ABS /power steering reservoir

neilh

With the ground strap in place and the +ve connector disconnected!
Put your meter on its lowest OHM range and measure from the exhaust to the grounding point under the back panel ( next to the wiper motor) and then from exhaust to the -ve terminal on the battery.
All should read either zero or close to.
Remember, the exhaust is supported in rubber straps, so if the ground at the engine end is broken you could get strange results - i'm guessing the current in that 12v you see is so low it does not make a problem.

928 at last

Everything looked ok last time in, but I'm obviously going to have to trace everything now. And, of course, work is really busy right now so this could take some doing......
I'll have to start tracing (hopefully tomorrow) and see what happens.
Additional problem is of course that I'm at home and not at the shop with the hoist, and with the car lowered, access to anything underneath is a PITA....
Might be up at the starter, but with the recent TT job, could be any number of things that were disturbed on the way there..

Mrmerlin

did you put the battery in backwards and connect the pos cable to the neg terminal?

At any rate dont connect the battery to the car till you figure this out,
in fact i would put all of the relays back remove the jumpers and put back in the fuel pump fuse,
and any other things you may have disconnected

928 at last

That's the problem, I'm afraid,
"That point called ground" is apparently now 12.89 volts positive.
I'm well aware that the engine and all chassis points should be "-" or "ground" or "earth", but in any event should not be carrying a positive charge.
Which, unfortunately for me, is exactly my problem. While the exhaust is indeed rubber mounted at the rear, it is most definitely bolted to the engine block, which by default must also be positively charged. And that in turn is connected to the chassis, etc., etc,
Thus, checking positive to positive as you suggest will only result in a zero reading in theory, however, the polarity is the issue.

the flyin' scotsman

Did you have the starter off Rob?

What was the car doing when the issue started?

tveltman

He described a serious arc. I suppose its possible that if he wasn't familiar with arcing in general that "serious" could really mean "minor", but I have to trust that if he says it's big then there must be some serious current capacity. If I was a betting man, I would say that there is an intermittent short to the block from one of the +12V wires. If it makes a poor contact, but will still short, then the reference point "ground" is actually not zero, but instead some variable number. This could produce variable pumping ability of the pump(s). Maybe you just got lucky the other day and the short was insulated away from the chassis, and you saw 55 psi. Short comes back, pump can't run at full power, low fuel pressure. Bad ground reference will also play havoc with your LH and EZK, so it is unsurprising that Bill might have diagnosed LH failure.

Just my 0.02. Try doing as suggested above and disconnect the +12 V cables one at a time until the problem disappears. You should also try keeping them disconnected individually (i.e. only one cable disconnected at once) in case there is some weird combination of shorts, etc.