PAGING ALAN!! Need insight. Chassis Positive??
05-12-2011, 11:45 AM
#31
Rennlist Member
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.
"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.
If you focus only on the voltage difference that you are reading between exhaust (and presumably the engine block), then it really is pretty simple.
There is a big fat wire between the engine block and chassis-- under the car, on the passenger side.
And the exhaust is connected to the engine block, but not connected (electrically) to anything else.
So if you are measuring a voltage between the exhaust and the chassis, then that big fat ground wire is not connected.
It can't be-- you cannot have any significant voltage between two points that are connected by a big fat cable. Period, done, end of discussion.
You can't fix this with a keyboard. You can isolate it further by poking around under the car with the voltmeter, but it has got to be that engine strap-- either the stranded wire itself has turned to green powder, or the connections are not clean and tight, causing a lot of electrical resistance.
A missing/bad engine ground-strap will cause all sorts of other problems, since it is also the return path for some high-current items (alternator, starter), and the engine electronics are grounded to the block while other circuits are grounded to chassis. It is a critical piece.
05-12-2011, 12:23 PM
#32
Rennlist Member
Thanks Jim -- what I was getting at is if the ground strap from the battery to chassis is disconnected (or if it just broke) then your extra wire or DVM would be the only ground path. So whatever is drawing power, e.g. dome light, etc would cause a voltage potential that would cause the spark and the meter reading.
05-12-2011, 12:27 PM
#33
Nordschleife Master
First off,
Ensure that your front of engine harness has not dropped down and is rubbing on the AC bracket!
After that, disconnect the battery and measure how many amps load there is on the battery. There will always be some. If it pops the fuse right away then you have greater than a 10amp load which is HUGE and your battery would be going dead pretty quick (read overnight).
Ensure that your front of engine harness has not dropped down and is rubbing on the AC bracket!
After that, disconnect the battery and measure how many amps load there is on the battery. There will always be some. If it pops the fuse right away then you have greater than a 10amp load which is HUGE and your battery would be going dead pretty quick (read overnight).
05-12-2011, 12:43 PM
#34
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Edit... I just reread your 1st post ... You took your reading from the battery to your exhaust ? If you did that then check the strap on the battery to ground
Last edited by sharknoob; 05-12-2011 at 01:15 PM.
05-12-2011, 01:00 PM
#35
Rennlist Member
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There are several ground straps between the chassis and engine - the big one just helps take care of heavy loads, mostly the starter. Disconnecting/breaking/failure of the big ground strap does NOT disconnect the engine from the chassis.
05-12-2011, 01:09 PM
#36
Rennlist Member
Reversed Batt polarity
Rob...just throwing this gorilla on the table.
You stated not removing the batt and charging it in situ when required.
Is it at all possible the charger leads were reversed and the batt then was reverse charged?
Can you try different good battery?
You stated not removing the batt and charging it in situ when required.
Is it at all possible the charger leads were reversed and the batt then was reverse charged?
Can you try different good battery?
05-12-2011, 01:10 PM
#37
Rennlist Member
I'm guessing the battery ground stap is at fault here. He never stated having voltage between the exhaust and the chassis. He only stated having voltage between the exhaust and the neg battery terminal, and as stated above. The engine to chassis has redundant ground paths. There is only a single battery to chassis path inthe negative battery cable.
05-12-2011, 01:12 PM
#38
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I had thought of that (reversed battery polarity) earlier, but didn't mention it because I don't think that the car would run if the battery voltage is reversed - solid state devices (ECUs, for example) really, really don't like reverse polarity.
05-12-2011, 01:21 PM
#40
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"I'm guessing the battery ground stap is at fault here. He never stated having voltage between the exhaust and the chassis. He only stated having voltage between the exhaust and the neg battery terminal, and as stated above. The engine to chassis has redundant ground paths. There is only a single battery to chassis path inthe negative battery cable."
Agreed. If you disconnect the negative battery cable only, then check for voltage you will have positive voltage on any chassis/engine ground in relation to the negative post on the battery. There are quite a few constantly-hot devices on a 928, and the return voltage for each of those is just sitting on the chassis waiting to get back to the battery ground.
If the battery ground strap is connected and you get positive voltage from the chassis (including the exhaust) to the battery negative post, it seems likely that there is a high-resistance connection somewhere between the battery post and the chassis. It is possible to have a high resistance connection that will "burn through" and make a good connection when you put a low-resistance high-load on the system - such as the starter.
Agreed. If you disconnect the negative battery cable only, then check for voltage you will have positive voltage on any chassis/engine ground in relation to the negative post on the battery. There are quite a few constantly-hot devices on a 928, and the return voltage for each of those is just sitting on the chassis waiting to get back to the battery ground.
If the battery ground strap is connected and you get positive voltage from the chassis (including the exhaust) to the battery negative post, it seems likely that there is a high-resistance connection somewhere between the battery post and the chassis. It is possible to have a high resistance connection that will "burn through" and make a good connection when you put a low-resistance high-load on the system - such as the starter.
05-12-2011, 01:53 PM
#41
Electron Wrangler
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Well from everthing you have said so far I'm with Dave.
It sounds likely you have an intermittent battery ground strap. For the car to ever run it has to be occasionally working. For you to measure the chassis/exhaust positive wrt the battery negative terminal the only single point failure is the battery ground strap. Since these are well know to fail in this exact manner - I'd suspect that.
Simple tests:
When you measure the chassis/exhaust as positive wrt battery negative terminal - what voltage do you measure between battery negative terminal and the Ground point at which the battery ground stap is bolted (behind the tool panel/tray)?
if there is ~12v here you are measuring voltage across what should be huge conductor (which means it isn't!)
Measure exhaust to battery negative while manipulating the battery ground strap - pull/twist tug - what happens to the voltage?
if it moves about wildly thats proof too...
Perhaps even simpler is to just connect one end of one lead (say ground) of a booster cable to the ground point bolt and the other end to the battery negative post - how do things measure then?
In the end I think you will find you need to replace the battery ground strap.
Alan
It sounds likely you have an intermittent battery ground strap. For the car to ever run it has to be occasionally working. For you to measure the chassis/exhaust positive wrt the battery negative terminal the only single point failure is the battery ground strap. Since these are well know to fail in this exact manner - I'd suspect that.
Simple tests:
When you measure the chassis/exhaust as positive wrt battery negative terminal - what voltage do you measure between battery negative terminal and the Ground point at which the battery ground stap is bolted (behind the tool panel/tray)?
if there is ~12v here you are measuring voltage across what should be huge conductor (which means it isn't!)
Measure exhaust to battery negative while manipulating the battery ground strap - pull/twist tug - what happens to the voltage?
if it moves about wildly thats proof too...
Perhaps even simpler is to just connect one end of one lead (say ground) of a booster cable to the ground point bolt and the other end to the battery negative post - how do things measure then?
In the end I think you will find you need to replace the battery ground strap.
Alan
05-12-2011, 06:34 PM
#42
Rennlist Member
Thread Starter
Dave is right, and I'm an idiot.
Thanks Jim -- what I was getting at is if the ground strap from the battery to chassis is disconnected (or if it just broke) then your extra wire or DVM would be the only ground path. So whatever is drawing power, e.g. dome light, etc would cause a voltage potential that would cause the spark and the meter reading.
Now I'm embarrased, as well as still chasing the fuelling issue that started all this.
Thanks to everyone for their input however, and I promise to turn on my brain before I start a help thread again......
05-12-2011, 08:13 PM
#43
Rennlist Member
Sillier things have happened many times over. At least it sounds like nothing has been fried by reverse polarity. I once sold an older English Pos earth car to a guy. A week or so later he calls me up and says its not running well, idling rough etc. What have you done? Nothing. Sure? Yes....well not much. What? Swapped it to Negative earth so I could put a modern radio in it. Oh, is that all. Well, seeing as the coil is polarity sensitive, you might want to swap its low tension leads.....
My 928 failed to turn over at all once. Not in a rush, so I opened the hood, got the dvm out, started looking fo 12V where it ought to be, working out if I needed to get underneath etc....spent half an hour on it before I even thought to look at the gear selector.....
jp 83 Euro S AT 53k
My 928 failed to turn over at all once. Not in a rush, so I opened the hood, got the dvm out, started looking fo 12V where it ought to be, working out if I needed to get underneath etc....spent half an hour on it before I even thought to look at the gear selector.....
jp 83 Euro S AT 53k
05-12-2011, 11:28 PM
#45
Rennlist Member
Glad to help Rob, no question is a dumb question if the answer saves you grief. 
I think there was a thread here a while back, "dumb things I did in the garage" or something like that. If you can find that and read through it might make you feel better.
I think there was a thread here a while back, "dumb things I did in the garage" or something like that. If you can find that and read through it might make you feel better.
