1974 911 2.7 Alternator Not Charging

[2147cs]

Help Please! The Battery was replaced a year ago and tested last week all is good...was down to 25%.... trickled charged the battery to 12.6V. Alternator lite comes on with key on and goes out and stays off when at idle. Across the battery with car running 12.6V Rev it up to 3000 rpms 12.8V. I replaced the Marchel red plate Alternator with a rebuilt (2 of them actually) wired correctly, I replaced the Mech. VR with newer style Solid state. @ the bulb (while running) the blue wire is showing 14.4V on the red/black wire 12.6V...the bulb is very dimly lit not bright at all......what am I missing?

[Targalid]

I had similar symptoms which were ultimately solved by cleaning all the connections passed by the blue wire. It passes through the 14 pin connector, so clean the connections there. It passes through the noise suppressor and then goes to the voltage regulator, at least on a 76. Clean each connection and check the alternator light. I also had a voltage meter connected to the cigarette lighter so I could monitor voltage output. With an assistant in the driver seat to monitor the condition of the alternator light (on or off), voltage at the cig lighter (when engine is running), moving wires and testing connectors, you will find the problem.

[grizzfan]

Sorry to highjack your post, but I have a similar problem in that the alternator light on the dash doesn't seem to go out but flickers most of the time while driving. I can leave the car in the garage for extended periods and it starts right up. Am I missing something or do you recommend the fixes you outlined?

Thanks,

Tom

[dfhtrhjn]

I had some problems with the alternator in my 74s. Rather than mess with it a bunch I just replaced with a slightly more modern one (from valeo), and it charges way better then the old one ever did. Our host carries it, part number 911-603-120-06-M262

[2147cs]

Can anyone verify the Voltage at the Alternator Bulb while engine is running?
Red/Black wire side 12.8V and on the blue wire 14.4V's.... thanks.

[clarkd]

14.4 volts on the blue wire indicates the red plate style alt is charging. If you only have 12.8 volts at the battery with the engine running, then most likely you don't have a good connection from the B+ on the alternator to the battery. Check/clean the connection at the starter for the red wire comming from the alt.

[2147cs]

Thanks I'll check it out tomorrow and let you know.

Thanks again

[2147cs]

Took some measurements today. I disconnected the battery
both Pos/Neg:

1. 2.7m ohm between Pos/Neg 2 disconnected @ the cables
2. Alt B+ and Grnd connected to Alt .78k ohm
3. Alt B+ disconnected from Alt to grnd Open
4. Alt B+ disconnected to Battery Pos .5 ohm
5. Alt B+ disconnected to Battery Neg. Grnd open

Disconnected Brown and Blk wires from Alt no change
same results as above.

Another bad Alternator? Any thoughts?

[rgmrgm]

I had a like charging problem with my 78sc and fixed it by removing the radio noise suppressor that was installed on the voltage regulator

[2147cs]

I removed that early in my troubleshooting. I searched this forum for charging, blue wire issues and have read everything I could and could not find someone with a similar issue........ Not ready to give up yet.

[timmy2]

Have you tried changing the bulb? Maybe it has a high resistance internally?

Looking for something simple to try.

[manbridge 74]

Look deep in this thread.....

Alternator Issues

[2147cs]

I've measured across the Alt bulb socket with the bulb in place 12 ohms pulled the bulb and measure across the bulb only..... 12 ohms. Does anyone have a P/N for this bulb 12V 2W no base?

I've read the thread you pointed to a number of times.....was there a section of this thread that you could point me to? I have the old style Marchel Alt with external VR.

If someone could ohm out a Marchel B+ to Case outside the car (standalone) I'm reading .77 k ohms which is darn near a short and that does not seam correct... maybe off base here.... but..... the question lingers.

Thanks for your help.

[timmy2]

The resistance to ground and the resistance between the pos and neg cables will vary as you rotate the alternator.
If it doesn't change you should have your alternator tested at a shop.