Charging system mystery.. HELP PLEASE!

[supersignman]

I have a 1986 Porsche 911 3.2 and I had the clutch replaced at a local Porsche mechanic last week. On the way home from his shop I stopped at a restaurant and the car wouldn't crank back up but I could hear the solenoid clicking. I got some guys to help me push it off and it fired right up. The battery was reading 10V so I replaced it and everything was fine for a couple of days until I checked the voltage and it was 11.3V. I surmised the alternator was bad so I followed Wayne Dempsey's diagnostic instructions. If I turn the key to the on position the alternator indicator lamp does not light up. Supposedly that means the bulb has burned out, the regulator has failed, the DF wire is disconnected or the alternator has failed. The shop that did the work on my car was closed this week for the 4th so I decided to go ahead and put in a new alternator myself. I'm still not getting a charge and the alternator indicator light still doesn't light up. What else should I look for? Is it possible the shop left something disconnected when they dropped the engine for the clutch change?
Thanks for your help!

[timmy2]

Check the transmission to body ground strap is tight on both ends.

[supersignman]

Thanks timmy2, I checked the transmission to ground strap and it's tight and I could tell where someone had cleaned the ends to make sure there was good contact. Is that the only ground strap from the engine to the body?

[timmy2]

Well, there is the alternator case to engine case bolt ground wire or strap as well.

[timmy2]

Battery ground to body too.

[supersignman]

Yes, I made sure the ground wire to the engine case was good when I changed the alternator and the battery to chassis ground strap is good also because the car cranks fine and everything works as normal except the battery light doesn't light up when the key is turned to the on position and the alternator doesn't charge the battery. Who knows, maybe I got a bad alternator as a replacement too?

[john walker's workshop]

Check for corrosion on the 14 pin plug engine to electrical panel. Blue alternator exciter wire and starter wire go through it.p

[spoke]

Originally Posted by supersignman

Yes, I made sure the ground wire to the engine case was good when I changed the alternator and the battery to chassis ground strap is good also because the car cranks fine and everything works as normal except the battery light doesn't light up when the key is turned to the on position and the alternator doesn't charge the battery. Who knows, maybe I got a bad alternator as a replacement too?

The GEN light or whatever it's called in the 911 goes between the battery voltage and a voltage generated by the alternator which is used as feedback to the voltage regulator which in turn drives the armature to start the charging process. No light = no charging. Thus when you turn the key to on, the light goes on and when the car is started the light goes off as the alternator starts up.

This light also provides the bootstrap current for the armature to start up the alternator. In other words, no light at startup, no alternator working.

Check the bulb to make sure it is working. Then check the voltage of both bulb socket pins. You should have 12V on one side and likely 0V on the other.

[supersignman]

Thank y'all very much for taking the time to help me on this. This is my first 911 and I absolutely love everything about it. My only regret is that I didn't get one 20 years ago!

I checked the 14 pin engine plug and it looked fine but i went ahead and sprayed some electrical contact cleaner on it and plugged/unplugged several times. Still no change.

I then pulled the gauge and checked the bulb itself and it is fine. When the bulb in and the key in the on position both the blue/red wire (going to the bulb) and the red/black have +12V to ground but no light. When the key is off with the bulb in, neither wire has any voltage but here is the strange thing... if I remove the bulb while the key is off I then get +12V to ground from the blue/red wire and 0V from the red/black wire. I checked it several times to make sure my meter is operating correctly and I keep getting the same thing.

[supersignman]

Just to make sure I'm diagnosing this correctly, the alternator should be putting out something over 13V, correct? After I put in the new alternator I recharged the battery to 12.7 volts before cranking the car. With a volt meter monitoring the battery while cranking it drops to approx 12V and then stabilizes back at 12.7V after the car has started so the voltage is exactly the same before and after. Every alternator I've had any experience with charges at at least 13.5V but I wanted to make sure that's the case here. I also revved the engine but no change in voltage.

[pampadori]

Since all of this started when the transmission was removed to replace the clutch, I would consider looking at the ground strap there with more scrutiny.
Remove it and check the resistance across it.
And since it is removed now, replace it if it's not been replaced in the last 10 yrs. They are a common failure on our vehicles.

Test it's function but using a jumper cables as a makeshift ground strap. Attach one end to fan housing and the other to the chassis metal. Check for charge voltage to see if it went up

[supersignman]

OK. I clamped one end of some jumper cables to the magnesium fan housing and the other end of it directly to the negative post of the battery. Prior to starting the car i was reading 12.5V. During cranking it dropped to around 11V and then while running it was reading 12.35V. Still no battery light coming on and no charge from alternator. I'm wondering if that blue "DF" wire could have possibly gotten cut during the installation of the engine? Is there any way to check that without removing the alternator again?

[supersignman]

Also, I am correct in assuming the voltage regulator is built into the alternator on a 1986 model?

[supersignman]

I ordered the replacement alternator from Pelican, part # AL-401-X-INT

[midnight911]

i'd also suspect the alternator but it might be just a worn voltage regulator in the alternator. easy to replace. yes, VR is in the alternator, held with just two screws, pops out, pops in. more economical than replacing the entire alternator.

[midnight911]

https://photos.app.goo.gl/OlmAa3VZ1XvAbfcb2

[spoke]

Originally Posted by supersignman

Thank y'all very much for taking the time to help me on this. This is my first 911 and I absolutely love everything about it. My only regret is that I didn't get one 20 years ago!

I checked the 14 pin engine plug and it looked fine but i went ahead and sprayed some electrical contact cleaner on it and plugged/unplugged several times. Still no change.

I then pulled the gauge and checked the bulb itself and it is fine. When the bulb in and the key in the on position both the blue/red wire (going to the bulb) and the red/black have +12V to ground but no light. When the key is off with the bulb in, neither wire has any voltage but here is the strange thing... if I remove the bulb while the key is off I then get +12V to ground from the blue/red wire and 0V from the red/black wire. I checked it several times to make sure my meter is operating correctly and I keep getting the same thing.

With the bulb out, I would have expected the voltage on the red/black wire to be 12V and the blue/red to be zero but you found just the opposite. This is according to my 911 turbo model 86 schematic.

[supersignman]

OK, thanks for the pics midnight911. I'd assume the new alternator has a good voltage regulator but maybe not. I don't know what else to do. It seems like a pretty remote possibility that I'd have two bad alternators that are doing the exact same thing. I just hate to take it back to the shop that worked on it for them to throw another alternator at it and then find out it's a wire that's not connected somewhere.

[supersignman]

Yes, spoke. I just double checked it. With the key off and the bulb out the blue/red wire (on the back of the bulb) to ground reads +12V and the red/black wire to ground reads 0V.

[spoke]

There is a connector with the alternator wires on it but I'm not sure where this connector is located. The D+ pin should be 0V when the engine is not running. This is the blue/red wire.