another alternator/voltage regulator question

[POinNJ]

The short story is 68 911 with 70t engine car is still being put back together. Shop put engine in last year but alternator light was on faintly. Charge at battery(new Interstate last year) was never above 11.98 (battery had been drained somewhat while I was testing other circutis, lights, horn etc) no matter what RPM the engine was running at.
I removed the alternator, and checked it according to the 101 project book. It appeared to have open windings, so I ordered a rebuilt replacement. I installed the replacement, reconnected everything. I turned the ignition key on and the alternator light was nice and bright red, I started the car and the light went off. I measured the voltage at the battery and for the first time, it was above 12 volts, slightly at idle. I shut the car off and went looking for my wife so she could increase the RPMs and I could check the battery voltage again. Now I'm back to the same symptoms as before, alternator light on faintly and no voltage above 11.98 despite engine rpm.

Additional points:
1) The voltage regulator is original.
2) I didn't find a ground strap from the alternator to the engine block, so I made one from 12 gauge wire from one alternator stud to the block.
3) I checked for the transmission to body grounding strap and it is good and tight.
4) I bought a replacement voltage regulator based on this part #STANDARD MOTOR PRODUCTS Part # VR124 which is listed as a replacement for a 70T alternator. I have tried this VR both prior to installing the new alternator and after the new alternator. No change in voltage, however the alternator light does not come on at all(with ignition switch on or while running)

Is is possible that the original VR is faulty and the new one is incorrect?

[POinNJ]

I investigated further and am still baffled:

With the ignition off: D+(blue wire) from VR to back of gauge=no voltage
With the ignition off: Blue/Yellow on the other side of the gauge bulb= no voltage
So far so good.

With the ignition on with old VR: D+(blue wire)= approx 4 volts, 12 volts from blue/yellow with the bulb on dim.

Same scenario with new VR: D+(blue wire)=.07 volts, 12 volts from blue/yellow with no lighting of the bulb.

Testing at VRs at D+(blue wire)
Old VR=6.5 volts with ignition on.
New VR= 12 volts with ignition on.

Jumped D+ and DF as described here at the VR connector: http://www.pelicanparts.com/techarticles/914_alternator_troubleshoot/914_alternator_troubleshoot.htm
Light went out, voltage at battery 11.78.

Put 12 volts to DF on both VRs as listed in the article with D+ and D- connected through the connector and Alt light is on without ignition on, with ignition on light is out.

New alternator installed yesterday and appeard to operate correctly the first start up then back to this. Alternator was tested prior to shipping on 6/5/09 with test results included in package.

Please any insight would be great!

[POinNJ]

Anyone? Where are the electricall gurus?

[304065]

Ryan,

The search feature can help you find dozens of threads describing your exact problem, so if somebody doesn't answer right away, you can still make progress.

First of all, if the battery only measures 11.98v, it needs to be replaced. Put it on the battery charger for THREE DAYS nonstop, if it doesn't measure 12.6 volts or better, it's junk.

Second: 1968 911s had a Bosch 35A alternator and Bosch voltage regulator. 1970 911Ts used a Motorola 55A alternator. You are always supposed to use the same make of regulator and alternator according to the factory. So, get yourself a Motorola regulator, or something aftermarket that is adjustable like a Transpo, so you can set the output voltage yourself. You are clearly handy enough to use a voltmeter so this should be simple The problem with mixing brands is that the "set-point" for DF voltage is different based on the input voltage. Probably not a HUGE deal but a good idea if this is intended to be a long term fix.

I would NOT mess around with full fielding the alternator, you will probably just fry something.

You don't need to measure the voltage levels on the various components, that's making it too hard. Since you know the bulb works and you know you have +12V on the blue/yellow side of the bulb holder on the igntion switch, simply do the following:

1) Disconnect the blue wire at the bulb holder. Using a jumper, measure the resistance in the blue wire from the bulb holder terminal back to the D+/61 terminal on the voltage regulator. Should be close to zero ohms, anything more is a high-resistance connection somewhere.

2) Measure resistance from the blue wire at the alternator (without it connected) back to the VR. That should also be zero.

3) For a final check, with the ignition on, ground the blue wire that connects to the alternator D+/61 to ground. The bulb should glow brightly. DO NOT LET THE B+ TERMINAL TOUCH GROUND IF THE ALTERNATOR IS OUT OF THE CAR, YOU CAN KILL YOURSELF OR BURN THE CAR DOWN.

Now you know you have continuity in the blue wire. After that it's either a bad VR or the alternator. Since the alt is new, I would suspect the VR.

Report back here with your results and I'll give you more advice. Good luck!

[POinNJ]

John, thanks for the reply. I try to exhaust the search function as much as possible, before posting, but I couldn't quite find what I was looking for. I'm away for a few days, but will post with my results when I return home.

One question, am I correct in assuming with the ignition on, there should not be any voltage at the D+ at the gauge? How about voltage from the alternator D+ to the D+ input on the VR?

Thanks, Ryan

[POinNJ]

Problem Solved!! . It was a two part problem. The first and major problem was that the PO had two splices to lengthen the harness on the DF (black) line and there was a break. I figured this out while checking continuity like John suggested. The second problem was the VR (original). The replacement non adjustable replacement worked fine once the DF and the rest of the harness were cleaned up and fixed properly. Thanks for the insight John!