Quick Alternator Question...

[djmcmath]

I just replaced my voltage regulator (symptoms: voltages fluctuating wildly from 13.5-15.5 while driving). On re-install, the alternator light wouldn't go out. I pulled it back out, took it to a local DC electrical shop for a quick $5 test before I shipped it out for a rebuild ... and it tested good. He said it generated 14.2V real solid, no problems.

So I put it back in the car (long painful hassle, but I'm getting really good at it now) and started troubleshooting. With the car off, the voltage reads 12.5-13.0ish, depending on what loads I have running. That's gotta be just battery voltage. With the car on, it reads 14.6V. Adding loads (lights, radio, etc.) lowers the voltage slightly, but not significantly -- which would lead me to believe the alternator and VR are both working correctly. Changing RPM does not alter the voltage, which is rock-solid steady now.

But the light is still lit, and I don't know why. My best guess is that I have the wrong wattage bulb, but wouldn't that cause problems with charging? And wouldn't the bulb wattage be associated with the alternator rather than the VR, which is the only new part? So my alt/VR gives every indication of working correctly, but the warning light won't go out. Help?

TIA,

Dan

[djmcmath]

Hey, I'm an idiot. Can I persuade a moderator to move this to the 911 Tech forum? Z-Man, isn't that twice in one week? TNX.

Dan

[Z-man]

Need to know what car you are having this problem with so i can move it to the appropriate tech forum, where you will get a better response.
-Z.

[island911]

Yeah "Alternator Question" I figured this was a Kerry bash.



Hey, have you check the battery?

[Z-man]

Moving.

Although I'd much rather discuss the virtues of alternator voltage differences than deal with all the political mud-slinging going on here...
-Z-man.

[jason2guy]

when i did mine, upon reinstall i wired it incorrectly and the light wouldnt go out. check your connections

[jpnovak]

Teh alt. light works by comparing the voltage output of the alt. to that of the battery. If the battery has a higher voltage - the light comes on. If the alt. has a higher voltage, the light goes out.

I am assuming your Carrera has an internal regulator. It sounds to me that the blue? wire from the D+/61 termanal on the regulator has a bad contact and is not reading the same voltage as the B+ output from the alt.

[djmcmath]

Z-Man, you're too darned fast! The first three posts happened in the span of 3 short minutes. Wow! Thanks for the help, Jamie and Jason.

I'm not above admitting that I may have wired the thing incorrectly, though I hate the idea of opening everything back up to correct it. Tiny blue wire goes to the "61" contact (which is D+), the collection of 3 red wires goes to the big B+ contact, and the two brown wires go to the remaining contact (unlabeled, but I think it's DF), right? Yes, the VR is internal; brand shiny new from Pelican, says "Valeo" on it. Cleaned and lubed both of those connections before installation, too.

I did clean the connections with steel wool and electrical lube before reconnecting them, so the loose connection is perhaps less likely than an end-to-end connectivity problem. Is there a way to test that without pulling the @#$%ing alternator back out of the car? Heck, once I've pulled it out, how to I test that, short of running a long piece of wire from one end to the other to check the resistance? And further, if the blue D+/61 wire wasn't connecting fully, wouldn't that keep the alternator from "booting" (computer term, don't know the alternator term) correctly? I thought that the little blue wire connection forward to the warning light was the boot-strap for the field excitation?

Still , but less so now, because I'm at least in the right forum.

Dan

[djmcmath]

Bump -- Did I get the wiring correct? Should I just panic and take it to a shop?

TIA,

Dan

[jpnovak]

If you only have three wires they are hard to screw up. yes, REd to B+, Blue to D+/61 and Brn to D- (gnd).

If you have a good V output at the battery I think your alt is working fine. Next step is to measure the Voltage on the alt. lamp itself. This is the end of the blue wire. If the Voltage does not match the alt output (which should be higher than the battery) then you have an issue with the D+ section of the wiring. Next check the same voltage on the blue wire at the 14 pin connector. just pop the cover off it while its plugged in.

If you have the correct voltage on the blue wire then the bulb may be suspect. Have you changed it recently?

[djmcmath]

Thanks Jamie -- I did in fact wire it as you say, except there were two brown wires. I don't know if that makes a difference.

I haven't changed the bulb recently. I can check the voltage on the blue wire at the 14-pin connector as soon as I get home. I'm a little confused about the other spot I should be checking the blue wire voltage? "This is the end of the blue wire" -- but where does that end come out?

Thanks again -- the alternator still doesn't make sense to me, though I'm definitely gaining ground.

Dan

[djmcmath]

Sorry to take so long to update; my home internet connection is down. Ok, checked the blue wire at the 14-pin connector -- reads 14.68V, just like everywhere else on the bus.

I tried to check the voltage at the bulb last night, but couldn't figure out where to put the leads. I'm sure it's something obvious, and I'll feel stupid when someone points it out, but I couldn't get it. Do I need to yank the whole parking brake / alternator light assy out of the dash so I can get at the wires on the back?

Thanks again for the help,

Dan

[jpnovak]

Dan, The two brown wires are fine. They are just additional grounds to parts of the fuel injection system. They are connected to the alt as it should be grounded through an additional braided lead to the case.

You can simply pull the gauge forward from the dash. If the rubber is old it may be difficult. it just pulls out.

Locate the bulb for the alt light. It sould have two contacts, a blue wire and a red/black wire. Just measure voltage to the blue wire and gnd. If the voltage reads he same 14.68, then your charging system is working properly and there is some other problem. Not sure what that could be. there is a diode in that system which may be bad. I do not know the location of that diode, nor its specs to test.

[304065]

Hey Dan

There are three current paths for the alternator output voltage.

1) Through the B+ wire (FAT red) to the starter;

2) Through the B+ wires (thinner red) to the 14-pin connector; and

3) Through the blue wire (pin #11) back to the bulb.

So you measured the output voltage and got 14+. This means your alternator and regulator are working properly.

But your light is still on. The current path through the light is as follows:

Out of battery to ignition switch
from ignition switch through red wire with black stripe to bulb holder
through bulb in holder (mounted in oil pressure gauge)
from other bulb holder contact through blue wire back to electrical console
through blue wire to voltage regulator

Now, when you turn the key on with the engine off, current flows OUT of the battery through that path, through the blue wire and into the voltage regulator. This current provides the initial magnetic field so the alternator can begin to charge. Once charging begins, the output voltage is fed back INTO the blue wire, where it balances the current flow coming from the battery. Since the electrical pressure (voltage) is the same on both sides of the bulb filament, no current flows in the circuit, and the bulb goes out.

Now, if the alt is charging, and current is still flowing from battery to the alternator, that tells you that you either have a defective voltage regulator (unlikely) or that the resistance in the "blue wire" circuit is high enough that the output voltage, WHEN IT REACHES THE BULB, is lower than the battery voltage.

Answer: clean all the connection in the blue wire circuit.

Then remove the oil pressure gauge CAREFULLY, start the car, then once it's running, remove the blue wire from the bulb holder, and measure the voltage between the blue wire and ground. It should be the SAME as the battery voltage. If not, you still have a high-resistance connection somewhere.

Be CAREFUL with the wires behind the dash. The only reason I know anything about Porsche electrical systems is I pulled out my oil pressure gauge and started a fire. Now I know all about how to make an original-looking wire harness for behind the dash!

Good luck!

[jpnovak]

John, Thanks for expanding on the information.

[1fastredsc]

That's odd, my valeo never goes over 13.8.

[Lorenfb]

"That's odd, my valeo never goes over 13.8."

You've probably got a few voltage drops from the rear to front, some heavy loads,
or a regulator which has a low setting. The Paris Rhone (Valeo) alt.s usually put
out about 14.7, especially in a 911SC.

[djmcmath]

Originally posted by john_cramer
through bulb in holder (mounted in oil pressure gauge)

John, that is some excellent data. I almost feel like I understand the thing now. But hang on -- the nature of my question suddenly changed a whole lot. I thought the alternator light was the one next to the parking brake light, near the stereo. It's a circle with a dashed circle around it. I was also under the impression that the parking brake and alternator lights were somehow linked, so when the alternator light is lit, so is the parking brake warning light, and cycling the parking brake doesn't help.

Have I totally troubleshot the wrong problem? Aargh!!!


Dan

[304065]

It's the thing with the battery on it.

[djmcmath]

Um, yeah, belay my last, I'm an idiot. Now to find my shop manual so I can figure out what that other light is. (sigh) Sorry to bother everybody, and thanks for the awesome troubleshooting guide and the idiot check, John. (Sir, I'm afraid to report that the idiot check has returned positive...)


Dan