Alternator question please help???

dbrisson · 05:11 AM

A few days ago I had the batt and ox light come on then go out after running at high rpms. Today the battery was dying on the way to work and finally died as I got to sears to buy a new battery.

If I disconnect either battery terminal while the car is running it shuts off immediately. I used to use this as an alternator test on other cars. Is it the same for the porsche?

Should I go the full alt route or just the regulator??? any ideas???

1982sc 911

RoninLB · 04-13-2007, 04:55 AM

set up a signature where you list what ride you have.

disconnecting the battery of a running engine sends high voltages surges into the system.

check operating voltage

304065 · 04-13-2007, 05:50 AM

I would go the full alternator route, because disconnecting the battery probably killed yours. Never, never disconnect the battery with the engine running in ANY car, because the alternator immediately spikes to full output, which can fry the diodes and burn out the regulator.

jmohn · jmohn's Garage

Total agreement with John.

Jerry M
'78 SC

mack6820 · 04-13-2007, 07:20 AM

most parts stores have the free alternator/battery check.

rick-l · 04-13-2007, 08:06 AM

Quote:

Originally posted by mack6820
most parts stores have the free alternator/battery check.

And when you've got the alternator out to take it to have it tested (little bit of work) take a look at the brushes and slip rings if it tests out good.

+1 on the no battery in the system test. In this CIS car (little electronics) you may have gotten away with no damage but a more electronics intense vehicle the best you could hope for is the protection circuits burn up.

rick-l · 04-13-2007, 08:12 AM

Quote:

Originally posted by john_cramer
I would go the full alternator route, because disconnecting the battery probably killed yours.

What would doing this hurt in the alternator?

dbrisson · 04-13-2007, 08:13 AM

I certainly think mine was dead before I disconnected the alternator.

As I was driving first the tach started acting erractic then shut off completely, then the radio shutoff.

I made it home under the new battery but it is low and I found a local supplier, hopefully they have the right part.

jmohn · jmohn's Garage

Quote:

Originally posted by rick-l
What would doing this hurt in the alternator?

The diodes.

Jerry M
'78 SC

rick-l · 04-13-2007, 10:04 AM

Quote:

Originally posted by jmohn
The diodes.

Jerry M
'78 SC

How?

Those are pretty big diodes. I can't believe they wouldn't be selected to handle the Peak Inverse Voltage of the windings. They aren't connected to anything so their power dissipation will be pretty low.

You would also think the VR would be sized to handle the OC voltage output.

jmohn · jmohn's Garage

Quote:

Originally posted by rick-l
How?

Those are pretty big diodes. I can't believe they wouldn't be selected to handle the Peak Inverse Voltage of the windings. They aren't connected to anything so their power dissipation will be pretty low.

You would also think the VR would be sized to handle the OC voltage output.

Apparently Bosch doesn't think it's a good idea to disconnect the battery -- this is from BOSCHUSA.COM:
Warning: Never disconnect a battery cable while the engine is running to “test” the alternator. Doing so can cause high voltage spikes that can damage the alternator as well as other electronics. (http://jobs.boschusa.com/AutoParts/FAQs/AlternatorsStarters/)

Jerry M
'78 SC

304065 · 04-13-2007, 10:54 AM

Uhh, except that the voltage spike that occurs when the battery comes out of series is WAY beyond the capacity of the diodes. That's what fries them.

But don't take my word for it, take the Factory's (Copyright 1965 by Porsche KG)

rick-l · 04-13-2007, 03:36 PM

Quote:

Originally posted by john_cramer
Uhh, except that the voltage spike that occurs when the battery comes out of series is WAY beyond the capacity of the diodes. That's what fries them.

But don't take my word for it, take the Factory's (Copyright 1965 by Porsche KG)

[list=1][*]I am NOT saying it is a good idea to disconnect the battery.[*]That wiring diagram contains a fairly strong statement that would remove all doubt...... if it is true.[*]In my rx-7 the battery cable broke while I was driving. The alternator did not vaporize.[*]From what I know about alternators I can't see how they would be damaged. The rest of the electrical system is another matter.[*]No way I'm going to try to prove it.[/list=1]Just because he took his battery cable off dosen't mean he needs a new alternator. More than likely all that is wrong is the brushes are worn or the Voltage regulator is cooked.

If he didn't fry any other electronics in the car (tach, speedo, lamda controller, radio etc.)he is ok with a fixed alternator.

dbrisson · 04-13-2007, 03:54 PM

The alt was clearly dead by itself before i did the impromptu test. When I made it to the sears it was starting to miss and had no electrical power the battyery was out. I put a new battery in to hopefully get me all the way back home, 70 miles. It did make the drive, now I am not sure if I had some charging or not but the battery was again low.

I found a good price on a Bosch unit $261 out the door, and installed it. Voltage output seems good and the car lights right up now. So $376 dollars later most of the electrical system is replaced.. Since I saw the lights come on a week or so before I knew something was up and expected and issue. At least now I know if the tach starts going bad there are batty or charging g...