Alright, I'm stumped... help a brotha' out...

[TWork]

Okay, I've got a alternator/red light on dash board issue I'm a bit stumped by. I've thoroughly search the archives on the issue.

Some time ago, I got stranded. Car wouldn't turn over. All indications was a dead battery. Called AAA, jump started just fine, said that the alternator wasn't charging -- I believe the reading was like ~11.2 or so. No red light on the dash, no pre-warning.

Got the car home, charged the battery, replaced the alternator. Car ran fine. Decided to replace plug wires and coil and at the same time removed the smog pump. Ultimately decided to replace the battery with a new Optima also. Put in a new ground wire from battery to body of the car at the same time. Now, I've got a low red light on at idle and its full on over 3k rpms. Belt seems fine, I don't hear any slipping and it feels appropriately tight. Battery reads ~12.5 v and when the car is idling it reads the same. So, it would appear that the alternator isn't charging.

I know its possible that I've had two bad alternators, but I think its probably more likely that I've got something else wrong. Any ideas here? Both alternators were the updated alts with the internal regulator.

Thanks,

T.

[304065]

Was this a new, tested good alternator? If so, then the most likely case is that you hooked up the wires to the wrong terminals on the alternator. If you hooked the D+/61 to the D- terminal, or it's somehow pinched or grounded, it could cause the symptoms you describe.

More likely: Bad voltage regulator.

The critical fact you mention is this: there's a difference in warning lamp intensity between low rpms and high.

Here's how I would approach this- disconnect the 14-pin engine harness connector, locate MALE pin #11 (first find female pin #11 on the engine side of the harness and locate the male pin that matches). Connect a jumper from male pin #11 to ground. Switch the igntion on. The warning lamp should glow brightly. Now remove the jumper and connect the harness again. With the ignition on but engine not running, the warning lamp should glow brightly.

What you've done is checked the continuity of the D+/61 "blue wire" circuit. This supplies +12v power to the alternator field through the voltage regulator. Current flows out of the battery through the ignition switch into the bulb holder through the filament into the bulb holder into the blue wire through the 14 pin connector and into the D+61 terminal on the voltage regulator- where it bumps up against the VR's diodes, so it can't flow into the stator, but it can flow into the rotor and into ground. It induces a current in the rotor which creates the field, which induces a current in the stator, which current is rectified by the VR's diodes, which flows back into the blue wire circuit, equalizing the voltage on both sides of the bulb which goes out.

If the alternator isn't charging with the lamp on, it means current is either flowing to ground prematurely before it hits the rotor and engergizes it, or the VR's not sending that current into the rotor.

Good luck! Don't burn the car or yourself up! If you remove the alternator to check it, please disconnect the battery first, you don't want an 880 amp arc welder back there.

[acapella8]

I posted this question in another thread, but didn't get an answer yet. Searched and couldn't quite find the answer.

On a 1978 911 SC: Do the brake lights and the Hazard Lights share the same system?

To clarify - my hazard lights work, but my brake lights do not.

If they share the same system then I know my bulbs are good, but something else is at fault correct?

Thanks!

Tom

[TWork]

Thanks for the input John. I'll definitely give it a shot and see what it turns up. Just as fyi, the "new" alternator was a rebuilt unit that I picked up from Parts Heaven. So, I assumed it to be good, but didn't verify myself before installing.

[ZOA NOM]

John Cramer, you are GOD! That was about as clear and complete a description of a potentially complex circuit as I have ever read on this board. This is what makes this community so invaluable. Outstanding.

[Joe Bob]

With the battery FULLY charged....
Battery should read 12.5 volts at the terminals, if not bad battery.
At idle, 12.5-13.0
2500 rpms, 13.5-14.5....anything more your gonna cook the battery
most likely the regulator is tits up.

Anything less the alt is not putting out enuff juice...could be loose belt, slipping belt, break in the D+, failing alt, bad regulator...

DO NOT charge a dead or low battery with the engine alternator...it is designed to MAINTAIN a charge not bring it back from the dead....use a 110v charger.