RDS928S

Thanks Wally will check.
The owner's manual states low voltage readings could be due to loose v-belt tension?
Any thoughts?

Thanks
Rich

WallyP

Absolutely - even (or perhaps especially) with the polyrib belt on the later cars. The timing belt is tightened to 5 on the 32-valve cars using the Porsche gauge - the alternator belt is tightened to 9!!

kuwona

Thanks again Wally and Garth. Okay – as you can see, I haven’t had much time to work on the Porsche lately. I’m in the middle of a thousand things. Today I got in there with the volt meter and discovered… I’m dumb. I guess I need a bunch of things explained in order to hunt down the problem. Anybody have time for electricity 101?

1 – Is a “bus” the same as a “fuse” or maybe a “fuse socket”? I think I understand what "bus" means in this context, but i still wouldn't know one if I saw it.

2 – When I check a connection at a fuse, should I touch the top or bottom contact?

3 – Should I be looking for 12 volts between ground and each of the points on my list, or between the two points directly (as if i was just checking the continuity along a wire)?

4 – Should the engine be running for all these checks, or is the switch in the "run" position good enough?

5 – The list I saw on the other post (from Wally) is for an 82. Is it different on my 86.5?

I feel pretty clueless. Maybe somebody could say something like, “put the black probe from your volt meter on the top contact of fuse 15 and the red probe on the green wire coming off the flux capacitor. Turn on the engine and see if you get 12 volts DC?”

Thanks again. I'm learning slowly but slowly.

GW

the flyin' scotsman

GW...........an old professor once said to me; ignorace can be fixed, stupid is forever. At least you're asking questions not blindly destroying the car.

1 - a 'bus' from an electrical perspective is typically a high current wire or copper bar used in high power applications. In the 928 the battery cables can be considered buses as an example.

2 - If the fuse is not blown it wont make a difference; if it is blown you'll only see 12v on one side.

3 - If your checking for voltage attached the black probe to the cars unpainted chassis for a ground and use the red probe to check for power. Have the DVM set on DC volts.

4 - Just switch the key on, perform the tests then start the car and re perform the same; then you'll know they're good in both conditions.

5 - Basic electrical tests are good for all modesl/ cars for that matter. Download the fuse/relay panel info for your car year from the 928 specialists site.

Last question.........see #3

Keep asking the questions.............good luck!

kuwona

Here's some new info. I just jumped a wire between the positive post and pin 1 at the 14 pin connector. I expected this to bypass the exciter circuit and let my battery start charging. It didn't have any effect. The battery continues to drain with the engine running.

dr bob

The intent of the exciter circuit is to provide something more than the rotor's residual magnetism to get the charging started. There's a diode bank (2 diodes, actually) between the output of the windings and the excitation terminal, so that generated votage from the alternator itself will take over as soon as possible. Without that little kick-start, the alternator will eventually make current, it just takes more RPM's to make voltage when the rotor's magnetic field is relatively weak.

So from this, one can deduce that the alternator will never charge if one of those excitation diodes goes bad, if any of the primamry diodes goes bad, if the brushes go bad, or if a rotor winding is damaged by long bouts of overexcitation, trying to charge a battery that won't charge well.

Each of the diodes in the main bridge as well as the excitation pair can be tested with a DMM. The excitation pair testing requires that the brushes not contact the rotor, but otherwise it's pretty straightforward testing in place.

kuwona

Thanks Bob. I did take the alternator to the alternator shop and the guys tested it. They saw that it charged well with upto almost 150 amps. But just to be sure, they took it apart, cleaned and inspected its bits and pieces and gave it back (all for free and with a smile by the way - Boston Auto Electric gets 2 thumbs up). They would have noticed a bad diode, right?

dr bob

I would hope so. No diodes equals no charge, eh? (sorry...)

So it's back to basics. With the alternator installed, key on but engine not started, you should read a few volts at the excitation terminal on the alternator. This is the current flowing through the bulb and the 68 ohm resistor in parallel. No voltage at all? Work your way back to the 14-pin connector, keeping in mind that the current is small in that circuit, so even a mildly corroded connection will keep the exciter from getting excited. Cars missing that plastic rain shield over the connector and the jump-start fitting are subject to a lot of corrosion damage to those parts. About a $6 replacement part, crime to be without it, etc.

At least your test guys have eliminated the Big Worry part for you. It's a bad connection someplace.

WallyP

If you apply 12 vdc directly to the proper (very important word there!) pin on the proper side of the 14-pole connector and the alternator still doesn't charge, the most likely failure is in the wire harness across the front of the engine. We have seen those wires crumble from heat and corrosion...

kuwona

Okay - thannks again guys. I'll go and get the alternator back out again to access the small terminal. If I jump that to 12VDC+ and the battery starts charging, I know I have to replace the wire, right? I assume the small terminal on the alternator is connected by a single wire to #1 on the 14 pin connector. Is that right? You've been really helpful so far. Thanks.

kuwona

Update -

I just took the alternator out and attached a length of wire to each of the terminals (I think they are called "+" and "D+". I reinstalled the alternator with those wires hanging below the car so I could access them. Voltage accross those wires is as follows...

Engine off = 12.45 VDC
Ignition on = 0.0XX VDC (some minimal voltage)
Engine running = 12.22 VDC

I didn't bother to rev the engine high, because it is my understanding that even at idle the voltage should be higher than the battery voltage. I crossed the wires and took voltage from them to ground. Again, the voltage went down instead of up when I started the engine. My alternator light comes on when I cross the wires.

For some reason this surprised me.

Any thoughts?

kuwona

Next...

I now have the electrical part of my ignition switch out. Is there a way to test it?

Thanks