Guys
It’s been a while since I posted to this forum. Anyway, I’m back and need some help in diagnosing an alternator charging problem. Here are the particulars: The car is my faithful for many years, 105,000 mile, 1972 911T with original S.E.V. Motorola alternator and matching voltage regulator.
Recently, the alternator charging light began to stay on, notwithstanding engine speed. The light will glow red at idle, and gets progressively dimmer as engine speed increases. However, the light never goes out entirely. Here are some photos showing the relative brightness of the light at idle (950 RPM), 2000 RPM, and 3000 RPM, with no electrical load.
The first thing I did was to check voltage across the terminals of the single Optima battery that I installed about 7 years ago. With the engine off and the car not driven for a week, voltage is 12.87. Next, I started the car and measured voltage across the battery terminals at the engine speeds shown in the chart below. I took two sets of measurements – one with all electrical accessories off, and the other with the high beams, stop lights and fan motor on. These are the results.
950 RPM -- 13.2 volts no load-- 12.9 volts with load
2000 RPM -- 15.3 volts no load-- 13.5 volts with load
3000 RPM -- 15.2 volts no load-- 13.6 volts with load
The supplement to the workshop manual indicates that the alternator output should be 14 volts, +/- .5 volt. So, it is clear that the alternator is under spec at idle which clearly explains the red light. At no load, it’s over spec and at load, it is within spec, albeit on the low side.
As you can see from the pictures above, the alternator light on the dash glows red at 13.2, 15.3 and 15.2 volts. As one would expect, it also glows red at 12.9 volts. However, it also glows red, however dimly, at 13.5 and 13.6 volts. If I drive the car at a steady 2800 RPM in daylight, I can see the red light glowing. If I switch on the headlights at that speed, the light dims dramatically, and almost looks normal.
One other piece of information. For the last 7 years, the battery has been left on a Battery Tender charger all the time, except when driving, of course. For the first couple of years, I disconnected the battery ground strap, but I guess I got lazy and for at least the last 5 years or so, I have left the battery connected while on the charger.
I checked the connections at the battery and everything is tight and clean. I also removed the electrical panel from the engine compartment, removed the voltage regulator, cleaned all connections, and re-installed.
Everything was fine and none of these actions altered the symptoms. And yes, the alternator is grounded to the engine case.
Here are my questions:
1) From the above voltage readings, do you think the problem is in the alternator, the regulator, or both?
2) Why does the charging light glow at 15.2 volts? I though that it only glowed if the voltage from the battery exceeded the charging voltage?
3) If the alternator is toast, I suppose my only option is to find some one to rebuild it, since no one I have been able to find carries a replacement for the 55 amp, 770 watt Motorola and Bosch units from these early 911s. Any suggestions on shops that can do this work? I do not want to install the more powerful, internally regulated, and 9.3 mm longer alternator that requires a new housing.
4) Is it worth trying a new regulator only? Is see that our host sells a unit at a reasonable price.
Any other thoughts and suggestions would be greatly appreciated.