Dead 964 After Jump Start

[mgthompson]

A not so mechanically inclined friend jump started his '89 964, in the dark, and managed to get the jumper cables reversed. He says that it is absolutely dead now. Nothing electrical works.

I have decades of BMW experience, but little with Porsche. I am hoping that there is a fusable link or some kind of main fuse that is blown. The owner can provide more information if necessary. I will see the car next week and would like to know what to look for. Any guidance would be helpful.

[ischmitz]

ouch..... depending on what gave he might have fried some vital components such as the DME, the alternator diode pack, the central informer, the alarm module and/or other electronic modules required for the car to run. Most electronic modules have some rudimentary form of protection but still suffer damage when reverse voltage is applied.

It really all depends on how strong his battery was and how well it "pushed" against the incoming jump-start cable voltage. How long were the jumper cables in place until he noticed his blunder? Did the other engine idle or was it reving pretty good. What kind of jumper cables - pretty beefy or regular?

Check fuses and let us know what happens when you turn the ignition to ACC and to ON. You should see some instrument lights and such. If you are lucky it took out the fuses for modules on Terminal 15.

Ingo

Ingo

[mgthompson]

The battery was absolutely dead, and still is. The 964 never started so the electronics may be a mess. The bad news is that all of the fuses look OK.

I will see the car next week. We will try a jump start again and see what happens.

Are there any schematics for the 964 online?

Thanks for the suggestions.

[mgthompson]

It turns out that the inexpensive jumper cables melted and disconnected. That probably limited the amount of damage.

Bought a battery maintainer, but the original battery would not charge. We installed a new battery and it started right up. The garage filled with clouds of smoke, but after a few minutes of running it cleared. It has not been run in about a year so I guess that it has some oil leaks.

Went for a short ride. Runs great, everything seemed OK, but the ABS light came on. The ABS fuse was blown. Replaced it, but the light is still on. ABS control unit fried? Is it safe to drive without ABS as long as the driver is aware that it is not working?

The battery light is off when the ignition is on and on when the ignition is off. The battery voltage stays at 12.1V with the engine running. Just a fried voltage regulator? I would be surprised if the diodes were fried because they are designed to work with reverse voltage.

There is a constant 3A draw on the battery with the ignition off. Pulled all of the fuses and relays and saw no difference. This probably supports the fried voltage regulator theory.

It sounds like there is an exhaust leak on one cylinder under acceleration. Just a gasket?

Thanks for your help.

[ischmitz]

Hey, good news after all. I agree that the voltage regular is damaged and that is what most likely causes the 3 Amp draw. I don't think the diodes will deal with reversed voltage. Also, once you pull the alternator check the engine to alternator harness carefully. There might be a short, it is very comon. The other item could be that filtering capacitor in the engine electrics box. It is an electrolytic capacitor and they do not like reverse voltage at all. Depending on how much it saw it could have blown up. You can simply disconnect it. It is not really required.

Not sure about the ABS unit. You need the hammer or PST2 to check what made it unhappy. Which fuse was blown?

And the exhaust leak is a gasket or rusted heat exchanger. Don't worry about that for now.

[1990C4S]

ischmitz: Is there any 964 diagnostic laptop software side from Duramateric? I noticed you have a few posts on this topic....

[ischmitz]

The scanners are falling into three groups:

1. on the lowest end of the scale is the laptop solution that was featured on Rennlist. Do a search for "scantool" or something similar. Some guys made a simple interface (level converter) and wrote a scanning software. It is free and you can built it yourself. It does very basic fault code reading and actual value reading. Not sure where the project stands right now.

2. This is where the Durametric and maybe some others come in. These solutions have lots functionality of the factory tools and they are reasonably priced for the hobby mechanic.

3. Factory scanners such as Bosch Hammer (up to early 993), PST2 (up to Cayenne, 997) and PIWIS (all but Panamera), PWIS-2 (just released). The older tools can be found used. The more recent versions command hefty price tags.

That's a short summary - let me know if you need more specific information.

Cheers,
Ingo

[1990C4S]

Thank-you. I will do some investigation.

I like 'free'.

[mgthompson]

Originally Posted by ischmitz

Not sure about the ABS unit. You need the hammer or PST2 to check what made it unhappy. Which fuse was blown?

It was fuse #16.

[ischmitz]

Fuse 16 is the main Terminal 15 fuse for the ABS control unit. If the fuse continues to blow you possibly have a damaged ABS control unit or Valve unit. If the fuse only blew once then this was caused by internal protective devices.

A control unit ususally has a Zener diode and a regular diode in reverse direction present between the supply voltage and GND. The regular diode acts as a crowbar becomes conductive when you reverse polarity of the supply. The Zener diode becomes conductive in an overvoltage situation. Both are to protect the sensitive electronic components. Depending on how long the condition was applied either one of the diodes can fuse and become a conductor.

So if the fuse continues to blow when you replace it one has to assume the ABS control unit is damaged. If it only blew once during the event the protection worked as designed. Replace the fuse, reset the fault codes and be happy.

Ingo

[mgthompson]

Thanks for the info. The fuse did not blow when it was replaced.

[ischmitz]

All the better - so the ABS control unit seems to be fine and the fuse only blew because of the reverse polarity. Ingo