Backwards Battery!!!

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01 May 2011 09:41 #4642 by wrentham636
Got my 928 for a good price due to a "starting problem". I never saw this before but the main cause was a battery which had failed internally and reversed voltage. That's right, voltmeter connected pos to pos showed -12.4 volts. Now the really bad part. Before I found the battery issue, I thought I had just a failed alternator so I disconnected it and tried starting the car. I'm amazed considering what I know now but the engine turned over (in reverse possibly) and all of the electrics as well but engine would not start.

Now that I have a good battery in the car I'm wondering if I have a major issue with the ECU? symtoms at this point are, no spark at plugs (have not checked all), fuel pump is ok when i jumper xx relay. I'm checking grounds today. Again, my main fear is that the reverse volts fried my ECU.

Sparkless in Massachusetts

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01 May 2011 11:37 #4647 by 928mac
Replied by 928mac on topic Backwards Battery!!!

Got my 928 for a good price due to a "starting problem". I never saw this before but the main cause was a battery which had failed internally and reversed voltage. That's right, voltmeter connected pos to pos showed -12.4 volts. Now the really bad part. Before I found the battery issue, I thought I had just a failed alternator so I disconnected it and tried starting the car. I'm amazed considering what I know now but the engine turned over (in reverse possibly) and all of the electrics as well but engine would not start.

Now that I have a good battery in the car I'm wondering if I have a major issue with the ECU? symtoms at this point are, no spark at plugs (have not checked all), fuel pump is ok when i jumper xx relay. I'm checking grounds today. Again, my main fear is that the reverse volts fried my ECU.

Sparkless in Massachusetts


Hey Sparkless, What do we call you? I'm Brad.

To try and help answer your questions;
The starter motor will spin backwards but the bendix drive (gear) will not engage. (unless porsche has something I have never seen, ILL check my starter in a few hours to dbb check.

Connecting the car up in reverse polarity more then likely will fry a few things but you may have gotten lucky so we will need to do some step by step checks.

First off (because I dont know you or your skill level)
what do you mean by "voltmeter connected pos to pos showed -12.4 volts."
do you mean red meter lead to pos+ on the battery and black neg- lead to the ground?

Make sure that the meter reading is correct and that we are not chasing a ghost.

Never remove something like an alt. from the circuit or remove a bat cable while the engine is running unless you have lots of money to replace electronic parts!

It is also possible that with the Alt. disconnected that one of the wires feeds the coils, but I do not have a schematic in front of me and I am not familiar with that circuit.
So hook it back up

Once it is hooked up and the battery is fully charged (correctly charged)

recheck for spark and let us know.

BTW you may want to document this into your profile thread for others to see,
and post pictures of checks with meter so we can see the readings and connections.
Post pictures of any connections or test that you done or had off.


Good luck and TTYL today.


Brad

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01 May 2011 13:07 #4648 by wrentham636
Replied by wrentham636 on topic Backwards Battery!!!
Hey Sparkless, What do we call you? I'm Brad. Thanks Brad, BobG works.

To try and help answer your questions;
The starter motor will spin backwards but the bendix drive (gear) will not engage. (unless porsche has something I have never seen, ILL check my starter in a few hours to dbb check. I know for sure that I got the engine to turn over when I had connected to the bad battery. Electronic background here and I know how reversed dc voltage should play with something like the starter motor different from ac voltage.

Connecting the car up in reverse polarity more then likely will fry a few things but you may have gotten lucky so we will need to do some step by step checks.

First off (because I dont know you or your skill level)
what do you mean by "voltmeter connected pos to pos showed -12.4 volts."
do you mean red meter lead to pos+ on the battery and black neg- lead to the ground? Took the battery to local AutoZone and while I know they're not the technical braintrust we look for, I assisted with a connection of a voltmeter with test leads touching red to positive terminal, black to negative, reading on meter was -12.4v dc. We also tried their snazzy battery tester and again with leads connected properly, the tester swore that we had the leads swapped. They said that they had seen batteries with this issue but usually with a very low 6v dc +/- voltage, never a full 12.4v charge.

Make sure that the meter reading is correct and that we are not chasing a ghost.

Never remove something like an alt. from the circuit or remove a bat cable while the engine is running unless you have lots of money to replace electronic parts! Unfortunately never had the chance, have not seen this engine run yet. I know, scarey but I know the seller and he had started the car over the winter with no issues. New clutch, timing belt etc so I know it was fairly well taken care of.

It is also possible that with the Alt. disconnected that one of the wires feeds the coils, but I do not have a schematic in front of me and I am not familiar with that circuit.
So hook it back up I did replace alternator and the new one is connected. Someone had said that car will not fire without alternator connected.

Once it is hooked up and the battery is fully charged (correctly charged)

recheck for spark and let us know.Good point, have not charged new battery since install. 11.9v currently. I'll put battery tender on and charge fully before retrying.

Also, in the interest of full disclosure, this car had a mouse/chipmunk problem over the winter as well. I have spliced (one at a time) and shrink tubed at least 15 connections most of which were in the bundles plugged into the CE.

One other thing that makes me worry about the cars brain is initially (again with bad battery installed) I could hear that electronic squeal from the ECU telling me that highv was getting to it. Now with correct battery I no longer hear that sound at all. :(


BTW you may want to document this into your profile thread for others to see,
and post pictures of checks with meter so we can see the readings and connections.
Post pictures of any connections or test that you done or had off. I will update as you say. Pics etc... Thanks Brad!

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01 May 2011 15:06 #4652 by 928mac
Replied by 928mac on topic Backwards Battery!!!
Right on Bob, hopefully it will fire up for you.

I have my starter off and the gear only turns one direction and slips in the other direction so that means it must have been cranking in the correct direction which is good and there should be no damage from cranking backwards, as it cant.

I would double check the connections to the Alt to make sure that a wire could not have been switched.

here are some manuals to down load

cannell.co.uk/Manuals.htm

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01 May 2011 16:57 #4653 by wrentham636
Replied by wrentham636 on topic Backwards Battery!!!
Recognize this relay? It is in the starter relay position XIV but doesn't match up with the relay listed here, www.928gt.com/t-8586fuse.aspx

I think I'm in the market for some relays... Need to eliminate suspects.

Attached files [img]media/attachments/4792=563-photo (11).jpg[/img]

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01 May 2011 18:15 #4654 by 928mac
Replied by 928mac on topic Backwards Battery!!!
I bought some spares from Roger.

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02 May 2011 08:07 #4664 by martinss
Replied by martinss on topic Backwards Battery!!!

Recognize this relay? It is in the starter relay position XIV but doesn't match up with the relay listed here, www.928gt.com/t-8586fuse.aspx

I think I'm in the market for some relays... Need to eliminate suspects.


A Google search shows it to be an obsolete Bosch relay for BMWs from the 70's and 80's. Some references do say it can be used on Mercedes and Porsche, but I don't know if that is true.

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