jwbeck17

Need your help with a confounding situation. For months I have been dealing with my aftermarket stereo - Porsche (Becker) CDR-220, shorting when the RPMs get over 4K. It looks like what is happening I a that the voltage is going over 14V at that point and the radio is cutting out (no sound, no lights or screen) until the RPMs dip down below 4k, and then all is normal.

Things I have checked:
- Used a multimeter to confirm spikes getting over 15v at throttle measured at radio connection and under the hood by me.
- removed alternator and tested at shop. All good.
- swapped out radio for matching unit. Same issue occurs.
- checked other radio connections and not seeing any abnormal issues

My suspicion is that the voltage limiter on the alternator is not functioning properly, but tests fine on the out-of-car alternator tester. It was not tested at a shop with the alternator in the car, so they did not test the revs.

I got the replacement radio from Stephen at SW Stereo, and his thought is that anything over 14V is causing the problem. I suspect it is the voltage limiter on the alternator, but maybe spikes to 15v is normal in the 89 and the radio type that was built later can't handle the voltage variance.

Before I but a replacement voltage limiter, just want to get your thoughts on the issue and see if that is the right next step.

Thanks!

Jason

Alan

The next issue is figuring out the high voltage situation - you are worring about entirely the wrong thing here - forget the radio - worry about the ECU's etc instead.

To be clear here 15v is not normal for any 928 - even 14v is not common - you have a serious issue!

From what you describe either:

A) The alternator regulator is actually bad
B) The battery is dropping out of the circuit when you see the voltage spiking

Without a battery connected to it the regulator can't regulate worth a damn...

If you've had the alternator/regulator tested A may be less likely (though you need to find a real alternator shop that can actually spin an alternator to be sure...)

So B is more likely here - esp. if the voltage issues are intermittent (sounds like they are)

Check all ground and positive primary connections:

0) Check all the direct battery connections (including secondary supplies @ +post)
1) Buy a new battery ground strap and install
2) Check the engine to chassis strap
3) Check the connections at the starter/ jump post & alternator

Check here means: disconnect the battery, disassemble, clean the terminals and reassemble tightly

I believe the radio issue is likely unrelated - but who knows - fixing this may solve that too. Its the least of your problems here though for now....

BTW whatever your meter tells you is probably not the whole story - the actual peak voltage is likely higher than the meter reads and very spikey under these conditions... really not good..

Alan

PS And I really do mean: just go out and buy a new battery ground strap - its a frequent problem area and removes one quite hard to test variable at ~low expense

Gary Knox

Jason,

The alternator had new brushes and voltage regulator installed in spring of 2013. They have about 2K miles, plus whatever you've driven since purchase. I bought the brushes/regulator from 928sRus. Had no radio issues in the 2K miles through early July of 2013.

Hope you can solve the issue.

Gary-

jwbeck17

Great detailed response as always, Alan. Thank you. Just a few additional thoughts...

Once I ruled out the radio I got concerned with what the voltage could be doing to the rest of the electronics. However, i would suspect that over time I would see other signs of problems. Only thing that i notice is the radio issue.

One thing I can't rule out either is that my multimeter could be malfunctioning as well. I'm the only one who has noted the issue.

Gary's right about the alternator. It seems to be in good shape, and the shops that I used have verified it. I personally removed the alternator for testing. The original shop I took it to didn't feel like they could do an appropriate job of testing (they only test in-car), so they took it to another Bosch specialist shop to test it and it passed and they noted the relative newness of the brushes. However, I'm not sure how they test the voltage regulator in that outside-the-car process. I was thinking of taking the car to my shop to test in the car to validate that I'm not crazy.

The "spike" is really not a sudden increase in voltage. It is a gradual increase that coordinates with the increase in RPMs. It can be repeated over and over by taking the RPMs over 4,000.

Great suggestion on cleaning the grounds, and that was something I was planning on doing anyway. Knowing these cars, it's just a good thing to do as a routine step. I'll check with Gary on the ground strap and will check it's quality as well. If it is in good shape and relatively new, then that probably is not the issue.

Even though it is a new part, my gut is telling me that this could be just a faulty voltage regulator. However, I just want to eliminate other possibilities before I rip out the alternator again and install an new regulator.

Thanks for your help and welcome additional checks and suggestions.

Alan

You don't understand me...

The spikes I am refering to are on top of the higher voltage you are seeing - these would be very short duration spikes that your multimeter will average to something much lower.

What does this mean - high voltage transients are what kill electronics - that your ECU's still work is great - but don't relay on them continuing to work if you have an alternator that doesn't regulate - for whatever reason...

Get another DMM or a fresh battery - find out what's going on - if you suspect you are hitting a meter indicated 15V you should be very very worried indeed. You just aren't concerned enough yet! I suspect if you saw the power supply waverform on an oscilloscope you be more worried...

Alan

jwbeck17

Cleaned up the ground points and the battery area today. They were pretty clean, though (Thanks, Gary). A new battery was installed this fall as well. I lost one of the screw ground posts during reinstall (slippery hands due to CorrosionX). So I can't test it tonight and will get a new bolt tomorrow. Fingers crossed this narrows down things.

mickster

Listen to Alan.

Go get or borrow a new meter. Harbor Freight, Home Depot, Lowe's, Walmart or Radio Shack will have one.

If the grounds are clean and the battery tight, regulator sounds like the culprit. Always my first guess on high voltage.

Questions: Is the battery holder installed and holding the battery in place? Any arc/burn spots on the top of the inside battery cover? Is there clearance from the top of the battery to the battery cover?

jwbeck17

Mickster - Thanks for the question: the battery compartment is clean. Battery bolted down. No arc signs or scoring. Clear space all around Very clean (Again, thanks Gary Knox).


Agreed with Alan and will get a new a Multimeter. I've completed the ground cleaning and tested with the old multimeter. At idle (cold) 14.21V, so that's concerning. ...and the radio still has the same issue.

The good news is that the idle RPMs are a bit lower now, probably related to the throttle cable clean & lube during the ground cleaning.

Once I get the new multimeter I will test again. If still bad. I will order a new voltage regulator from Roger and install.

Any other suggestions or any other things to look for?

Alan

Get the new battery ground strap.

BTW re: my comment on a "fresh battery" in my original post - I meant a fresh battery for your DMM - at low battery voltage it will be inaccurate.

Alan

jwbeck17

On top of it. Will do, Alan. Will order it along with a new voltage regulator to be safe.

Will take care of the multimeter shortly.

jwbeck17

Got a new and better multimeter. Here is what I found:

Engine off (warm): 12.37V at battery

Engine at idle (warm): 13.87V at battery

Engine at idle (warm): 14.36V at front battery post

Engine at 3000K: voltage starts to creep up

Engine at 4000K+: voltage fluctuates and reaches over 15V

My proposed next steps are:
- buy new ground straps and install
- buy new voltage regulator and install

Any other recommendations?

Alan

Measure the voltage across the battery ground strap when running - one probe on the battery -Ve lug and the other on the ground point bolt... what do you see? flex the strap around a bit while you measure?

Alan

jwbeck17

I'm getting a solid 14.04 at idle with probe on positive terminal and probe on ground screw at end of ground strap. Wiggling light or hard gets same result.

Alan

NO - what I said...

one probe on the battery -Ve (as in Ground) lug and the other on the ground point bolt

Alan

jwbeck17

Sorry, so you mean the negative post on the battery (battery connected end of ground strap) and one on the green handled ground screw at the end of the ground strap?

I apologize for my confusion as I am an electrical newbie here and mis understood.