928 Tech Tips

I'm on my second alternator since BeaveRun. The first rebuilt ($220 local auto parts store) self-destructed after about 15 minutes. The replacement doesn't start charging until I rev it over 4000 rpm. Turn the key on and it shows about 12V, start it and it shows about 11v, rev it to 4000 and it starts charging, 13+ volts and stable until I shut it off again. Fiddled with the connections in the pod and on the alt, all seems OK, no change in behavior. Wierd, definately RPM related, not time or temp. Anyone seen this before? Regulator problem?

Also, has anyone swapped a GM alternator in?

Bill
'84 s 5 sp. blu3 (still needing work)

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Bill,

This is a very common problem. The exciter circuit on the 928 runs through the instrument cluster and the alternator light. There is a back up resistor to insure the circuit does not open up and fry the regulator. Turn the ignition switch on and check the alternator light. It should be on. If it is not then you need to pull the pod and change the bulb. While you are in there might as well change most of the other light bulbs as well. It is easier to pull the pod with the steering wheel off. Be sire to set the wheels pointing straight when pulling the steering wheel. Of course while you are in there it is always a good time to contact the pod man and get that cracked pod repaired.

Dan the Pod Guy.
Portia's Parts

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An alternator converts rotary energy into electrical energy by moving a magnetic field (the armature or rotor) thru the coils in the housing. The magnetic field MUST be present for the alternator to generate electricity.

Once the alternator starts charging, it supplies its own field current.
Since there is now voltage on both sides of the light/resistor combo, the light goes off. Once the alternator starts charging, the light/resistor combo has no effect.

Wally Plumley
928 Specialists

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Wally:

Would this be why all my idiot lights stay on when I start the car until the engine revs to about 1,800 rpm?

TIA,

Dave

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Probably not.

Your problem sounds more like an ignition switch problem or bad connections in the pod. Start by reaching up under and making sure that the connections on the back of the switch are tight - wriggle the plug and push it on tightly.

Wally Plumley
928 Specialists

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ok,
so what does it mean if i have the opposite problem. my alternator on the gt cuts out over 4000 rpm. voltmeter (and measured voltage) drops to below
12. doesn't matter if the car is moving or not.

tom falkenberg
89 s4 shaaark

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My guess would be that the brushes are getting bounced off of the commutator.

Wally Plumley
928 Specialists

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Tom,

High speed cutout is normally a slipping belt. Look for polished pulley, melted belt, leaking antifreeze or ps fluid or oil or whatever.

After you know it ain't that then go after the voltage regulator. After that go after a new alternator cause the unreplacable part of the windings are
toast. I don't think it is the windings because they are normally temperature and load dependant when they first start to blink out and they go out intermittently to start.

Jay Kempf
25 year old alternator still going.....

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Wally and David,

Wally is correct on this. However, the lights will stay on when the alternator bulb is burnt out. I had this same problem on an 84. Reving the engine will kick the alternator in and cause the rest of the lights for the warning system to resync and act normal.

On the very early cars - 78 or so the warning lights on the dash will sometimes not go off after the car starts. This is a different problem and is generally intermittent and rare.

Replace the alternator light bulb and you will be a happy camper.

Dan the Pod Guy

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>How critical is the indicator bulb to this excitation threshold? In that these bulbs are difficult to find, and that Radioshack apparently has some
>alternative ones available, I'm curious to find out whether the use of a non-standard replacement lamp might/would cause a problem by altering the
>threshold of the excitation circuit. Would you happen to know the total resistance of this resistor/lamp combination?

I would expect it to be fairly critical - enough that I would use the correct bulb. Both we and Mark have the bulbs.

No, I don't know the total. Remember that checking the resistance of a light bulb is not a trivial task - the resistance varies a lot from cold to
hot.

>One might consider an LED replacement for most of these Pod Bulbs because of their difficulty of access. Would you anticipate any of the other "idiot
>lights" being a functional part of the actual control-system of any of the other systems that they monitor?

I am not aware of any other case where the bulb is anything other than a light source.

Wally Plumley
928 Specialists

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Dan Brindle wrote:
>The alternator does not start charging when I first start the engine until I rev it up to over 2500 RPMs. After that all is ok. I have changed alternators - not for this reason but because I had an extra rebuilt on hanging around.

>I thought it was a burnt out light on the instrument cluster but all of the bad bulbs have been replaced. Before I dig deeper into the wiring diagrams anyone seen this one before.

This magnetic field in the rotor is initially created by the excitation current that is supplied thru the charging warning light and the parallel resistor. If the excitation current is too weak (bad resistor, bad bulb, bad connection), but present, the alternator will START charging only at higher-than-normal RPM levels. If the excitation current is not present at all, the alternator will never charge. Sounds as if you need to replace the 68 Ohm resistor on the voltmeter or clean the bad connections in the excitation wire (probably at the connector at the jump start terminal).

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Wally,

Thanks for the help.

After a few years of engineering school I was pretty clear how an alternator converts mechanical energy to electrical energy. I was also amazed to learn the conversion is in 98% efficiency range. Pretty good as mechanical things go. Now if we could just get a 928 motor up to half that there would be no competition left at the track. :)

You pointed me in the right direction but left out a couple of critical steps. I had been assuming all along the charging issue was related to a burnt out alternator light. I had put off changing the bulb until I pull the windshield and pod. What I did not know is where the alternator light is located. I made the assumption it was one of the small twist off bulbs on the cluster. When I found a burnt out one I assumed I had located the problem.

On further inspection at your prompting I took another look at the alternator light located at the bottom of the gauge and sure enough it was not lit. How can this be? I replaced all of the bad bulbs. Pulling the cluster down and looking at it I discovered there was no bulb aligning with the alternator light on the cluster. Further investigation found the alternator light is built into the cluster and soldered into a special housing and screwed into the gauge itself. What Porsche engineer thought that one up?? I guess this is why the 928 does not have Toyota reliability built in. It must be something about unions wanting to get mechanics working.

After spending about 20 minutes taking the cluster apart I managed to remove the bulb and housing and tested it only to find it was good!
Bummer!

Getting out the trusty ohm meter I traced things down. The problem was not with the bulb but the connection to the gauge. Cleaning and tightening up the screw holding the resistor completed the circuit and fixed the problem.

Dan the Pod Guy
Portia's Parts