just installed new alternator - still no good

[tight as a bull's ass]

Well, the (new) rebuilt alternator is in the car with new diodes, new brushes, new bearings, etc....Fan belt is good, no slipping.

the charge light is now very bright at idle ( 950 RPM)...when I step on it (2000 RPMs), the light goes completely out and the headlights actually get brighter.

before I put the new alternator in, the charge light was dim at idle and remained dim at high speeds.

Checked all grounds, look good, battery is good also.

any ideas? It's a 1970 911T

Thanks

[Britwrench]

Check the charge rate with a voltmeter across the battery terminals. Or go to an auto-electric shop and get the charge rate checked with a load tester.

The light can come on at idle..maybe your idle speed is too low or the tacho is reading low (fairly common).

[LR993TT]

voltage regulator?

[304065]

Emmanuel:

Ask yourself: Why is the charge light glowing? It is because current is flowing out of your battery, into the ignition switch, out of the switch and into the warning light, then out of the warning light through the blue wire back to the engine compartment, through the voltage regulator, back to the 14 pin connector on the rear of the console, then through the cable to the engine, to the D+/61 terminal on your alternator, then through the alternator field windings to ground.

The purpose of this current is to provide "excitation" to the alternator field- current running through the field windings in the rotor (the moving part) turns them into an electromagnet, which generates current in the stator (the non-moving part) which then runs through a bunch of diodes, charging the battery. That current also runs BACK through the warning light circuit: when the voltage generated by the alternator is approximately equal to the voltage in the battery, the light goes out.

If that's not happening at low speeds, either the belt is slipping, or one of the diodes in the alternator is bad, or there's still a loose connection somewhere.

If that seems confusing (it was to me the first couple of hundred times) here's an EXCELLENT web page that's also specific to an externally regulated Motorola alternator:

http://web.raex.com/~volks/schem/mot_alt.htm

Check also this post by the Early S Man:

http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=30833&highlight=alternator+idiot+light

Good luck! Electrical problems are a pain, but eventually the entire system becomes demystified (and probably REPLACED.)

JFC

p.s. Anyone have a '71 starboard sight headlight/foglight/turn signal/horn wiring harness they'd be willing to part with?

[avendlerdp]

It goes out at 2000 rpm? Drive the car for a while and see if it gets better. My 70 has always shows me a alt light at low rpms until the battery gets charged up from the drain of starting the car. It takes about 3 minutes of driving and then the light stays off. My charging system is in great shape but if I run the lights, wipers, radio, etc, the light will somtimes glow at idle. Sounds like you are charging for sure though. You may have a bad battery connection and that is making things a bit slow to "volt" (not a real use for the word!) up. Rock on, Alex

[tight as a bull's ass]

Thanks for the useful information. As an engineer, I can appreciate the scientific approach to solving problems.

However, sometimes you get impatient and replace stuff until problem gets fixed. (doesnt always work)

Update:

Replaced voltage regulator with new one. (had installed rebuilt alternator earlier in the month)

The light still stays on at idle, goes out at 2K or higher RPMS.
Checked all grounds and made sure I cleaned the connections.

Took the car to get it checked at Sears, battery was good and car was charging at higher RPMS but not at idle...

This is gonna drive me nuts.



Thanks for reading.

[stormcrow]

I remember reading an article about the alternator light coming on with a new alternator and to solve the problem I believe a resister was installed across the bulb to solve the problem.

Let me see if I can find the info.

Steve

[304065]

Steve,

Don't bother: you are referring to the Porsche service bulletin that explains the update to an internally regulated alternator from 1982 onward. The issue is that to drive the field circuit in the internally regulated alternator so that it will charge properly, you need to install a 91 ohm 5w resistor in PARALLEL with the existing bulb, to increase the current in the "Exciter" circuit that connects to the D+/61 terminal.

The reason you use a resistor in parallel and not a bigger bulb is that the bigger bulb from the cars with the later style oil pressure gauge, with the two warning lights closer to the center of the gauge, doesn't fit the socket of the older cars.

Les: I think you have a busted diode in the trio, that's supposed to drive the field current, so that you arent' getting sufficient "self-exciting" effect. Sometimes alternator rebuilds don't go right, would be my guess. Try it with a NEW alternator and see if that doesn't solve your problem. . . this is a $200 gamble.

[jk911]

I totaly sympathise with you "" tight as a bull's ass ""

I have the exact same problem new batery, new voltage reg , rebuilt alternator tested 2 X (fine), checked wiring , alstalled 5W 68 Ohm resistor, cleaned and checked whole path of D+/61 and NOTHING

I still can't drive my car at night, it just doesn't charge at idle !

I dropped it off at the auto electrician this morning cause I give up !

Porsches are unique cars with unique problems ! ?????????????