First off, I have a 1979 with 46,500 miles. I was told that the ampmeter needle on older 928s run at low numbers. I have a new battery. No real problems. At idle the needle hovers right under 12. With the lights and the A/C on (which I never do, since it blows warm), the needle drops down to below 10.

I never really noticed before since everything seemed to be working fine. When I start the engine the red light on the ampmeter comes on. It still turns over though. Then turns off.

Two times today, when I dropped it into gear, the radio cut off for a second and the warning light came on. Another time, the radio cut off when I flipped on the lights. When I turn off the car, everything seems to reset itself.

The next weird thing that happened is the warning light wouldn't reset itself. Despite the fact that no other dash warning lights came on. Actually, it would reset while stopped and at idle, then it would come back on when I accellerated and would not reset, until the next stop.

I drove it straight to the Porsche repair place that has done most of the work on the car. (not the dealership) They hooked it up to their battery tester and told me I needed to replace the alternator. He didn't check the tension of the alternator belt or for any blown fuses.

The pervious owner gave me an alternator. So, I have one in a box. The Porsche place said to bring it in tomorrow and they would drop it in for me.

BTW, I am not a "check book" mechanic. I will do some smaller jobs myself, like brakes, lights, fuses. However, I do not have a garage or a lot of tools yet. So, I have to rely on the kindness of one grumpy friend, who complains that 928s are trouble brewing and to sell before it eats me alive. This weekend, I am going to attend a local PCA event, in the hopes of meeting someone here who appreciates our cars and is willing to lend a hand. In the mean time...

Two questions:

- Replacing seems like the right thing to do since they would be checking the tension among other things, right? OR is it an easy enough job for me to wait and see if I can get a hand from another Porsche owner. The PCA club only has two other 928 owners. Both '84s. I e-mailed one guy directly and got no response.

- How much should I expect to pay? Labor rate is $75 bucks and hour.

Thanks in advance for any advice,

 

It is hard to tell with what you've got.

The 928's sensors do funny things with low voltage problems.

Your red volt meter light should come on before the engine starts up. This light basicly tells you if the alt. is chargeing, or not.

Also, it could be a ground problem. The 928 has about a million, and does funny things when the grounds are dirty.

The 928's voltage sensor is also often inaccuret.

To figure out your problem, first you need to figure out if you have a burnt out idoit light. If the red butten won't make it go away, it could be serious.

Next, go somewhere and get your self a cheap multi meter. Check the voltage at the frount jumper post. Check the battery voltage. Dissconect the black jumper strap, and check it's resistance.

Next, clean all the grounds you can find. You should do this anyways. There are like 6 major ones, plus a million minor ones. Clean the battery, of course. Clean the alt. connecters. And of course, check the belt tension of course.

I'm curious what he's found wrong with the alt. The only thing I could think of that he could diagnose without checking the alt. belt tension is if it was giveing out ac ripples. Hum...

Good luck.

If you need a wireing diagram from the WSM, PM me, and I might beable to hook you up.

 

Viribus, I'm pretty sure that the voltage light is supposed to light w the key in the ON position. It's part of the circuit to feed the excitor terminal of the alt and goes out when the alt starts to charge causing the voltage on the excitor terminal to rise and the voltage on both sides of the lamp to be the same. Not 100% confident but fairly sure.

that said, James, I agree w Viribus. Get a cheap multimeter and make some measurements. Also clean those grounds. When I had problems w my '82, I was getting 'false' OXS warnings. Cleaning connections cleared the problem. In the process of working on this, I put together an alternator troubleshooting procedure that's up at:

<a href="http://www.928oc.org/tip/tip692.htm" target="_blank">Nichols tips..alternator troubleshooting</a>

You can do the testing w/o removing the alternator from the car. Start w the simple stuff first and measure and clean. You may in fact need an alternator but do these cheap tests first!

Jim

 

James: Was visiting in-laws near UH last month, going to Big Isl next week.

"Check book mechanic" -- first time hearing that, maybe that is what I am. If you do brakes, which I don't, isn't the alt kind of plug and play?? Agree you should check readings, but if you have one in the box and you trust the mechanic, I think I would just switch'm. Let us know what readings you get.

 

How come cars run differently away from the mechanic? Today the car ran strong...

I postponed the alternator switch at the mechanics, until I run though the steps given from those who replied.

thanks,

 

If the low voltage light is illuminated, even with the key in the off position, one or more of the diodes within the alternator are blown. Time for a rebuild or replacement.

Labour to replace the alternator should be around 1-1.5 hours.

Make sure they install the cooling shroud off the old one.

 

Wally, can you explain something to me?

I thought the red light was on the circut used to excite the alternator. I thought when you turn the key on, it light is suppost to come on, meaning the battery is exciteing the alternator, but the unit isn't chargeing the battery. When you start the engine, the red light goes out, saying that the alternator is exciteing itself now, and chargeing the battery.

And of course, when the key is off, all lights are off, right?

Did I somehow miss understand this system?

Thanks.

 

My alternator went 5 days ago, so I think. I was playing with my gauges and now.... no charging at all. It was fine before. I think that it might be the exciter in the cluster. Is there a way to check it with out pulling the alternator? Better yet, is there a way to bypass it with a simple relay??

 

James,
Similar situation here.
For the longest of times I had low readings 12v and under.
If I didn't trickle charge overnight every 3 days
the brain would reset at a light or stop sign because the voltage had dropped below whatever minimum needed simply because the brakelights and, or, turn signals finally ate up the required minimum voltage.
Long story but cannot rely on a local rebuild.
Went the auto parts store route: 1st alt. had a bad diode, second alt. was a Marshall with a different housing, pully, and shaft diameter., 3rd one worked perfectly. :sidebar: about 1 1/2 years ago I had bought a spare alt from/thru Jim Moorehouse. I wanted to use dissicant bags and Vacuum seal the alt. for storage.
I cleaned it up and took it to an Auto zone for function test before storage, the guy behind the counter gave it to a kid who I believe fried it in place -shoulda' seen the sparks fly -I thought he was welding - " sorry sir, you have a bad alt"

Anyhow, after I finally installed the last rebuild -don't forget to put a good trickle charge on the Batt. or you 'can' smoke a fresh alt on start up- all of my intermittant caution and warning lights dissappeared, electrics, windows, locks, radio, all performed better since I purchased the sHARk.
You will be pleased!
Along similar lines, my red alt warning came on, voltage dropped, brakes, turn signals ect. got flaky again and the brain is resetting.
Fear not! This I believe is due to not being able to re-install the protective housing and cool air tube -different alt housing-. We had some severe rainstorms - man the oars -and heavy duty sandstorms then HOT weather which probably contributed to it's untimely death.

My still unanswered question is:
Why does the red alt. low voltage light stay on after the key is removed from ignition?

In other news, a rebuilt alt. from an auto parts store is under $130.00 and comes with a lifetime warranty.
When and if you remove the alt take flicks of overall area, mount position, bolt locations, ect.
You can take good digital flicks by using a light colored sheet or towel, the digitals usually have a program that let you modify light, contrast...so that even a black picture can usually be saved. Knowing where everything goes is half the battle. You need to build some ramps -not much room under there- which makes the work very easy. Clean while you are there.
Finally, our 944 Turbo Spy in Oklahoma suggested what I've been thinking all along.
AN AC/DELCO REPLACEMENT! Some Mdification of the mount 'may'be necessary and I won't be done in time for your needs but I'll post when done.
We are not alone!

 

James.
Check out the More electrical glitches ...Film at 11 post/replies.
Seems like good news, and there is a wealth of information on sheep -nervous disorders-, goat -roping-, and prescribed footwear when interacting on a social level with domesticated -more or less- barnyard/feedlot "friends"...
I did notice a huge information gap concerning tree stumps and cattle, but, I'm sure with our diverse membership someone will enlighten those of us less versed on that topic...:0!

 

Red battery light stays on due to feedback thru a bad diode in the alternator.

 

Is there a way to bypass the "triger" on the dash cluster?? I just want to know if the problem is in the cluster BEFORE I pull out the alternator. My main light in the center turns on, but not the one by the battery indicator.

 

ViribusUnits,

Check your Profile for a private message that is waiting for you.

James

 

Unhook the D+ wire going to the back of the alt. and see if the light goes off....if it does then the alt. or the regulator pack in the back of the unit is no good.

 

Fixer,
Thanks,
I guess I wasn't paying attention to Dennis -as we all should- I noted the problem as being in the alt. as per his and your suggestion but, dauhhh, somehow forgot the beautiful simple purpose of the noble diode.
Thanks.