Electrical Archaeology- 1971 MFI T/E

[304065]

Ladies and Gentlemen:

Before I ask a few questions regarding some electrical troubleshooting I am reminded of Edgar Allan Poe's famous line, "Should you ever be drowned or hung, be sure and make note of your sensations."

With that out of the way, here's the situation: I have a 1971 T with an "E" motor that is having some electrical charging issues. I put my digital VOM on the battery terminals with the engine running at 3000 rpm and got 12.5 volts, suggesting either bad wiring, a bad alternator bulb, bad regulator or bad alternator. I checked the bulb and it's OK, verified continuity of the blue wire which runs through the bulb--terminal #11 of the 14-pin connector on the rear of the "console" -- and the wires leading into the shroud.

Now the fun starts: this car doesn't HAVE an external voltage regulator-- it has been removed from the "console" and replaced with an MSD 8910 tach adapter (which looks like the transformer from a doorbell). And yet, through the hole in the shroud run five wires: a large red (B+ from starter), a medium brown (D- from ground through hot-start relay), a smaller red (B+, to hot-start relay #51), a small blue (D+/61, field current from the warning light) and a medium black wire (DF, which runs to the "console" and presumably goes nowhere).

And now, at last, the QUESTIONS:

1. How can I test the voltage regulator without pulling the shroud? I suppose I could jump the blue wire (D+/61) to +12v which would energize the field and cause the alternator to go to maxium output, which would then be reduced by the regulator.

2. I suppose it's possible that the engine installers put an externally regulated alternator in the car, WITHOUT installing an external regulator. This would seem to explain the FIVE wires running through the hole in the shroud instead of the THREE required for an internally regulated alternator. Any way to check this, short of pulling the fan and looking at the back of the alternator?

Ok, see my related posts for the Story of the Smoking Dashboard, the mysterious MFI fuel solenoid/speed switch, and the Abandoned Wire Mystery. Not exactly Poe, I'll admit.

John

'88 Carrera 3.2
'71 E

[jkeyzer]

Does the bulb in the dash light up with the ignition on but the engine stopped? It should. In my case I also have to rev the engine once after it is started to "kick start" the alternator and get the light to go off. Not sure why.

It is possible your alternator was replaced with an internally regulated one, this has been done to my 72T. The installers left a bundle of wires in the console that go nowhere and are taped over.

If you check the battery voltage in the engine compartment on the fusebox do you see the same 12.5 or is it different (higher?).

Sorry I can't help more.

[911pcars]

Let's assume your charge light is ON with the key ON, engine OFF, then the light goes OFF after the engine starts. This indicates the alternator is charging but we don't know how much. If the light does not go OFF after starting the engine, the alternator and/or regulator is faulty.

To test for maximum output (or close to it), switch ON as many electrical accessories as possible (fan motors, headlights, rear window defroster, A/C fan/compressor, etc). Bring the engine rpm to about 2000 with a voltmeter attached to the battery. It should read about 14 + volts but not over 15. If you're only seeing 12-13 V or so (which you verified), this is battery voltage or whimpy alt. output and is not enough to charge/maintain the battery.

The alternator and/or regulator is not working correctly. Check all electrical connections (including grounds). Next step: Remove and have an electrical shop test it/repair it.

Sherwood Lee
http://members.rennlist.org/911pcars

[304065]

A point of clarification: With the ignition switch ON, the alternator light is OFF. Somehow, no voltage is getting to the bulb, through the blue wire (D+/61) to the field and back to ground. This would suggest that I am:

Not getting +12v to the red with black stripe wire that connects to one terminal of the bulb; or

There is an open circuit somewhere between the 14-pin plug, the alternator, and ground.

To check this, I turned the ignition ON and grounded pin #11 of the 14-pin connector (the male side)-- this should cause the bulb to light if it's good. But it didn't light, which suggests that I'm not getting +12v to the bulb.

I didn't note the rating on the bulb, don't know whether it's a 4-watt or a 2-watt, and there is no resistor in parallel with the contacts. But this shouldn't keep it from lighting up, it would just result in weak excitation current through the field on start-up, resulting in low output.

Thanks for all responses to date, hopefully this thread will help somebody else out in the future. Everything I know about early '70's 911 Electrical Systems, I learned here.

John

[Roland Kunz]

Hello

1.) The alternator light bulb is working as a resistor to preerrect the alternator. The ignition will run currency via the bulb as long the alternator doesn´t deliver higher voltage then the battery. If the charges are levels from both sides the bulb will have runing currency from both sides.
If you run curency from the backside ( T 14 connector ) you will only proof that the wire inside the loom is still isolatet as ther isn´t any ground on the other end.

Now one thing is that with the new Alternators the bulb was changed to generate a higher resistance as the preerectet field and the load at low revs are relatett to the curency gap.
You just need the new bulb holder with the correct bulb.

To avoid confusion some of the later versions even have a secound resistor paralelle to the bulb.

However the alternator is selferrecting ( to the correct side ) at a certain rev level and charges even without the external errection.

Testing a internal alternator/ regulator needs knowledge and is simplest done with a osciloscope analizer.

2.) when they updatet the old engine with a new alternator the wires are still within the loom and normaly noone is going to stripe them out. Just messure the resistance on the open connectors to find out if the wires are relatet to something.

Grüsse