Alternator Not Charging '77 911s - Whats This?

[Avanti]

Alternator not charging. Trying to identify the solid-state looking box in the photo below the voltage regulator. The plug on this box has no power at any of its three wires and it connects to the voltage regulator. Tried full-fielding the alternator but no output. I don't see this box in the diagram in my Bentley manual but that covers 78-83. What is it and what are the wires from on the plug that goes to it? (brown, black and blue)

[timmy2]

The small box is the radio noise suppressor. Known to fail.
Just unplug it in both locations and move the black plug of the wiring harness with the 3 blue/black/brown colored wires, up to the bottom of the black box (the voltage regulator) where the suppressor is plugged in.
Blue is D+, black is DF, brown is D- if you look at a schematic. (Also conveniently marked on the black plug)

[Avanti]

Thanks Dennis,

There is no power at the plug (blue, black, brown wire) All fuses are good, however, looking at the diagram it appears to be unfused. Charging system light working.

Could anything else besides a broken wire cause the blue D+ to not have power with the key on?

Can anyone confirm is the charging system wiring diagram for a 78-81 SC is the same as a 77?

THX!

[timmy2]

It is the same wiring as for the '78. Is the alternator original or has someone installed an internal VR type and just left the old wiring in place?

This link to my Dropbox with the '77 schematic/current flow diagram ought to help you.
https://www.dropbox.com/sh/jxno8o6qae3roun/AADW1qBcRc53LoEG0TPbRwFva?dl=0

[mysocal911]

Originally Posted by Avanti

Could anything else besides a broken wire cause the blue D+ to not have power with the key on?

Can anyone confirm is the charging system wiring diagram for a 78-81 SC is the same as a 77?

THX!

1. The charging light powers the D+ to 'start' the alt.
2. They are the same. The '82/'83 used an internal reg.

[Avanti]

Originally Posted by timmy2

It is the same wiring as for the '78. Is the alternator original or has someone installed an internal VR type and just left the old wiring in place?

This link to my Dropbox with the '77 schematic/current flow diagram ought to help you.
https://www.dropbox.com/sh/jxno8o6qae3roun/AADW1qBcRc53LoEG0TPbRwFva?dl=0

Thanks for the diagram.

I haven't seen the alternator yet but I doubt its been converted to an IR unit because the plug on the regulator is not bypassed.

Looking at the diagram, the D+ wire gets its current (unfused) from the ignition switch to the charging light bulb circuit. Then it goes to the plug at the regulator.

Right now there is no power at the D+ wire on the plug at the regulator. When the engine is running the charging lamp is on so that tells me the circuit is good to that point (charging lamp).

Unless I am missing something, my problem is in the wiring between the charging lamp and the regulator.

BTW, this car has not run in some time and I got some info from the previous owner. He connected the battery cables backwards at some time when the car was in storage. I understand something could have been fried, however, I would suspect it would be something in front like the charging lamp bulb and not after this point.

Anyway, will do continuity tests on the D+ and other wires this weekend but any thoughts from members who have been here before are appreciated. Would also like to confirm for sure if there are any fuses in the charging circuit. THX!

[timmy2]

Charging light on with engine running above 1500 rpm means it isn't charging. The light means there is an imbalance.
Pin 11 in the 14 pin engine harness connector at the rear of the fuse panel should have continuity to the dash lamp and to the blue wire in the 3 way VR plug, and then to the alternator D+ terminal.

Reversing the battery cables will cook the alternator in a hurry. There are no fuses in the charging circuit.

[Avanti]

Tim,

Thank you for the info on the pins. Will save me time running the continuity tests.