learpilot2

When I bought my 928, the voltage on my dash read 14 volts. I started getting fluxuations, and i had it replaced with a re-manufactured bosh.

When I got it back, the dash voltage read only a constant 12 volts instead of 14 volts and worked for almost 2 years. It just recently started making intermitant drops to 10 volts at idle, and the car quits. I had the battery checked at autozone, and their tester unit indicated the battery was good, and the alternators voltage regulator was bad. I took it in for another alternator replacement.

I had another bosh reman alternator installed. When I got it back, at idle, it barely read 12 volts on the dash. When I turned on the A/C at idle, the volts drop to 11 volts. When I then turn on the headlights in addition to the A/C, the volts drop to 10 volts (red line) when the car engine is at idle. When I drive the car with RPM above idle, the voltage rises back up to 12 volts. This does not seem normal.

Does this mean the voltage regulator on the new alternator is bad? It was installed by a mechanic that does not often work on porsches. Seem like an easy job to R&R an alternator.

Any ideas? Is there anything else I should check on the car. Seems odd in 2 years, 3 alternators installed all behave differently, and not in a good way.

Thanks
Rick

FredR

Rick,

I have on idea what they change on rebuilt alternators but the regulator/brushes and bearings would be my expectation and a polish of the commutator type of thing.

At start up the battery voltage will drop -possibly down to about 10 volts on full current flow and then at tickover will probably settle at something les than 12 volts. Best place to measure the voltage is the hot post.

Once the motor is running it should show about 13.5 volts on the dash just above tickover. If not then i would suspect soemthing not right but different models etc...

Regards

Fred

dr bob

Rick--

For real troubleshooting, the dash voltmeter is generally not the best tool. Use a real voltmeter and check at the battery terminals, battery positive to ground with load like headlights but engine not running (checks the ground cable...), then at the jump post under the hood, and finally at the positive connections at the CE panel. Do these both engine-off and engine-running, and you'll get a good idea what the voltage is and where it's dropping due to resistance.

I chased similar symptoms for a couple years. My 'excellent' electrical diagnostic skills caused me to replace the alternator, make a new heavier-gauge battery positive cable, re-clean all grounds and connections. Ultimately the early advice from the group to change out the battery ground strap was the correct solution.

Both 928 International (Mark/Tom) and 928srus (Roger et al) have sourced a nice 2ga ground strap, and added the insulating sleeve. Good places to get one if you don't want to do that stuff yourself. 928 Specialists, Carl, and others may have one also.

In the meanwhile, there's a list of annual electrical maintenance items available on the 928 Specialists website at www.928gt.com in the Wally's World section. It includes cleaning all the ground connections. The voltmeter grounds with all the other dash instruments at 2 points IIRC. One over the central electrics panel and the other near/above the driver's feet.

MainePorsche

+1 on what dr bob says.
You may be surprised at the difference between the dash voltimeter and the reading of say the hot post.
Clean ALL grounds. Look at New Visitor thread at top of Forum page for ground locations.
As car idles, and gets hot, alternator output degrades due to regulator. If ventilated properly, and moving, it should be OK.
How old is ground strap in the rear off the battery ? If you don't know, or it's old, that would be first culprit I think. It may 'look', but it may not be good.