Testing Alternator?

TerryBPP · 08-28-2003, 04:25 AM

Where is the best location to test the output of the alternator? Its reading 12.3V at the terminal at idle and 12.9V at 2000rpm. I was wonder if its possible to ge a Voltage reading prior to it reaching the battery. By the Way, I bought a Optima Red Top and I love it.

ruf-porsche · ruf-porsche's Garage

Try testing it with most of the accessories on to really load the system. Terminal location is a good location or the hot wire to the starter is another. I don't think that the extra length from the alternator to the battery will cause that much of a voltage drop.

Rot 911 · 08-28-2003, 05:12 AM

12.9volts at 2000 rpm is too low. Should be more like 13.5-14volts

TerryBPP · 08-28-2003, 05:39 AM

I figured that since the voltage increased 1V with the rpm that means the alternator is working reguardless of on the numbers . Right?

73911guy · 08-29-2003, 06:28 AM

The voltage regulator is designed to reduce (ie eliminate) the fluctuations you noted. Sounds like yours is bad.

Jim

'73 911

Otto · 08-29-2003, 10:35 AM

Does this car have an external regulator or internal?

EdT82SC · 08-29-2003, 01:27 PM

Terry, with the voltage that low your alternator will need to be replaced. I recently replaced mine, and the voltage now sticks between 13.6 (at idle), and 13.9 (at 3000 rpm). Your alternator is just very sick, and not totally dead (yet). Before I replaced the alternator I saw similar readings to yours. Fortunately, this is one of the easier DIY projects you'll do on your car. I've read on this board that if you have an external voltage regulator like your car does that now is a good time to upgrade to an alternator with a built in voltage regulator, though I did not need to worry about that. Good luck!

Otto · 08-29-2003, 02:53 PM

Sorry, I did not see that you have a 76 911. That car came with an external regulator. The way to tell if you need a regulator or alt., is to jumper with a wire the two spade lugs that are in line after you have removed the regulator. When you remove the regulator, there will be three holes where the regulator plugged in. Jump the two that are line, start the car and measure the voltage @ 2000 rpm. If the voltage is 13.4 to 13.9 replace the regulator, if not replace the alt.....

RoninLB · 08-29-2003, 11:14 PM

I wonder if an internal regulator can be full fielded?
... Ron

Lorenfb · 08-30-2003, 05:45 AM

You can't full field an internal voltage regulator type alt. You can check
the voltage at the fuses in the engine compartment or at the external
regulator where it's jumped to full feild it. On the internal regulator alt.,
you can check the voltage at the alt. light in the inst. unit at the wire
going to the alt. (#61).

The main test for any alt. is that the alt. light must be "on" with the key
in "run" and the engine not running. Once the engine starts, the light
must go "off" and not glow. This BASICALLY checks the alt. that it got
"excited" and "started".

When the alt. is out, make sure that the alt. battery wire has a soldered
connection at the connector lug. Porsche SCs and early 3.2s were just
crimped which causes problems like you describe. Also, the alt. wire at
the starter can cause at problem and must be solder at the connector lug.
This is the case also for the alt. wire at the battery.

Good Luck
Loren
'88 3.2

RoninLB · 08-30-2003, 07:00 AM

Quote:

Originally posted by Lorenfb

You can't full field an internal voltage regulator type alt.

mystery solved.

Thanks