1972 911T Alternator Charging Issue

[T Bird]

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.

[Gogar]

A dim or lightly glowing light isn't (necessarily) a problem. My 72 exhibits the same behavior. If it glows as brightly as the handbrake/oil light, then maybe you've got a problem.

If it does bother you, you might try cleaning all of the spade connectors and contacts involved with the Alt, Reg, and the light, and the 14-pin connector in the engine bay, and it may get better.


However, your voltage is a bit high. If you like, you CAN open up the cover of your SEV regulator and bend the tab ever so slightly to adjust the voltage. Check out this thread from when I had voltage issues. Different regulator, but mechanically identical to your SEV.

Edit: right about post #13:

Name this VR !!!!!!!!!!!

[Christien]

Subscribing - sorry, nothing to add, but I've got the exact same problem. (except my electrical panel doesn't look anywhere near as nice as that one!)

fwiw, I'm also tracking down an ignition-related stumbling problem, and I can tell you that replacing the CD box has no effect on the alt. light.

[frogger]

If this was happening to me, I'd pull out the alternator and have it tested. I wonder if there are any bad diodes in your alternator.

[Gogar]

I just wanted to add that you could also remove your alt and check out the brushes. If they're worn down too far, It's possible that a new set would help out. It's just two more screws, once you get the alt out.

[T Bird]

Gogar
I replaced the alternator brushes about 7,000 miles ago, when I pulled the alternator to do the paint job on the fan and fan housing. While I haven't pulled the alternator since then, I am quite certain that all the connections on the alternator are tight and clean, since that's how I left them. I also cleaned the connections on the regulator this weekend, as well as all of the multi-pin connectors to the electric panel. None of that had any effect. The only connection I did not check was the B+ connection at the starter (from the alternator). If that gets corroded, it can interfere with the charge from the alternator to the battery. However, since I am showing over 15 volts at the battery, I presume that the B+ connection is OK. At least, it was about 1,500 miles ago when I cleaned it and relaced the rubber boot over the connection.

I don't want to ignore this, as I have not experienced a glowing light before. Prior to this situation, the alternator light would go out and stay out about a second after firing up. The voltage readings are mostly out of spec. At this point, my assumption is that I fried a diode or otherwise damaged the alternator by leaving the battery connected while on a charger. Looks like I will need to pull the alternator and have it tested, as frogger suggested.

Does anyone have any recommendations for a shop to test/rebuild this alternator?

[304065]

John,

The fact that your Optima is 7 years old does raise a question for me. I don't have any experience with them that old, but know that a lead-acid would be dead well before that time. That said, hammer it on the 50 amp setting for a few minutes(with it out of the car, please) then stop charging, let it rest for a bit, and measure the open-circuit voltage. Optimas often won't take a charge unless you do this.

So your testing suggests that it's NOT the regulator, and the fact that you've got fairly new brushes in there is encouraging also. Did you clean the corrosion off the D+61, DF and D- in all the important locations?

* Back of alternator
* Pins #11, 12 and 13 in the 14-pin connector, both the female (engine) side and the male (console) side)
* Ditto where the wires enter the chassis through the second 14-pin under the console. This is an unseen haven for moisture and debris, precursor to the crumbly green stuff.
* Bulb holder contacts at rear of oil pressure gauge.

It doesn't happen very often, but remember that there should be TWO paths back to the battery for current flowing out of B+ -- one through the fat red wire to the starter and then through the welding cable to the battery, and the other through the D+/61 (well, there's also a B+ through the engine harness for the defroster, but that's not your problem). The path through the starter is big conductors with lots of surface area and TWO junctions before the battery clamp. The other has SIX before you get to the battery.

That's a long-winded way of saying I think you have a high resistance in the blue wire circuit somewhere. Easy enough to measure from the battery to the D+ on the back of the alternator. I can get my alternator out in 15 minutes, don't forget to disconnect the battery.

Good luck!

[jess p]

a good and fully charged battery should measure 13.02-13.06 V . With the
engine running you would expect a range of 14.2-14.8 V. I suspect your
voltage regulator is at fault. Voltage is too high, but then even though
Optima batteries are great 7 years old is about the general life.




76 911S
81 911SC
66 912 V8
65 912
71 914
H2 DRAG BIKE FAST-BY-GAST

[T Bird]

Everything I've read indicates that a battery showing 12.6 volts is 100% charged - 12.4 volts is 75% charged - 12.2 volts is 50%- and 12.0 volts is 25% charged. At 12.87 volts after a week's rest, the battery appears to be fully charged. Admittedly, this reading is not under load.

My Optima is a yellow top deep cycle marine battery and has lived virtually its entire life (while the car is not being driven) connected to a Battery Tender. I don't think the problem is in the battery. As I understand it, the red alternator light is supposed to come on when the battery voltage exceeds the voltage output of the alternator. Since I'm measuring voltages in excess of 15 volts at the battery while the engine is on, the light should not be on.

John - I'm not sure I followed your explanation. How do I measure resistance from the battery back to the D+ connector?

[304065]

John: There's more than one way for voltage to get from the B+ terminal on the back of the alternator to the B+ terminal on the battery. One way is through a fat red wire from the B+, through a rubber grommet just forward of the engine oil thermostat, to a big ring terminal on the starter, and then through the large black wire from the starter, through the tunnel and up to the Battery.

The other way goes through the warning lamp. Out of the alternator "trio" diodes, into the blue wire, through the 14 pin connector on the back of the electrical console, where a short blue wire branches off to connect to the D+/61 terminal on the VR, then back into the 14-pin connector into the car's main wire harness, then forward to the bulb holder for the warning lamp in the oil pressure gauge. The other side of the bulb holder is connected to a red wire with black stripe that goes to the ignition switch.

Your understanding of the operation is correct: when the ignition is turned on with the engine not turning, the battery voltage (from B+ to ignition switch to bulb holder) is higher than the voltage at D+/61 (which is zero).

When the voltage regulator senses that voltage on D+/61 is less than the "set-point" or desired alternator output voltage, it sends whatever it's getting on D+/61 to the field through the DF wire. So the battery voltage flows out of the battery, into the igntion switch, through the bulb, back to the VR, into DF, into one brush, through the alternator rotor, out through the other brush and to ground. Because current is flowing in this circuit, the bulb lights up.

Now, when current's flowing in the field with the engine turning, a current is induced in the stator which gets rectified by the "trio" diodes and fed OUT of the alternator at D+/61. Let's say it's about 14V for discussion sake. This is enough voltage to "balance" the voltage flowing from the battery and the light goes out. (The potential difference of 1.5V isn't enough to make the bulb glow bright enough that you can see it.)

Now, even though you may be seeing 14V at the battery, that could be due to the fact that the alternator output is reaching the battery via the starter cable route, not via the blue wire. This is most often caused by a high resistance in the blue wire circuit that's dropping the voltage.

How to measure? Well, I would dig out the volt-ohm meter, set it to the 1K scale and measure each segment from the biggest to the smallest. Disconnect the blue wire from the bulb holder, put one probe in there, put the other probe on MALE pin #11 on the electrical console. (The pin numbers are in very small numbers on the FEMALE side of the connector). Shouldn't be more than a few ohms of resistance: 16 gauge copper has .0473 ohms per foot, and I figure there's about ten feet of wire in there, so about half an ohm for the wire, and there are TWO connections in that run, one from the car to the first 14-pin, then the second to the next 14-pin.) If you get a high resistance that means something's amiss. Next, measure from the ring terminal on the back of the alternator to the FEMALE pin #11 in the 14-pin connector. High resistance? And so on.

[T Bird]

John
Thank you for that very thoughtful explanation. I'll get to work with my DVOM and let you know what I find.

[T Bird]

John & folks
I did a little testing with the Fluke 88. I disconnected the blue wire at the bulb in the oil press/temp gauge and at the votage regulator. I then measured resistance in the blue wire from the bulb connector to the voltage regulator connector. Resistance is .6 ohm. I believe that indicates normal resistance for a run of wire this length.

I cleaned the blue wire and red/black wire connectors at the bulb, as well as the bulb holder and bulb with isopropol alcohol. These connections were clean and shinny to start with, but since I had access to them, I went ahead and gave them a good cleaning anyway.

I fired up the car and the same symptoms present -- moderate glow at bulb at idle; faint glow at 2K RPM and 3K RPM.

I next took voltage readings at the bulb connectors themselves, as follows:

engine off/key on: blue wire = .99 volts; red/blk wire =12.8 volts; red light glowing brightly
1000 RPM: blue wire = 12.6 volts; red/blk wire = 13.6 volts; red light glowing moderately
2000 RPM: blue wire = 14.7 volts; red/blk wire = 14.6 volts; red light glowing dimly
3000 RPM: blue wire = 14.95 volts; red/blk wire = 14.65 volts; red light glowing dimly

These voltage readings at the blue wire bulb connector suggest to me that the blue wire is not the problem. Moreover, the D+ output of the alternator is a bit high I think, but it looks like that aspect of the alternator's performance is not the problem. Further, from the 15 volt readings I took at the battery terminals as indicated in my first post, it looks like the B+ output of the alternator, while again high, doesn't suggest a problem with the B+ side of the alternator. Stated more simply, this alternator is making DC in abundance at both D+ and B+.

I'm a bit perplexed why the red light remains on at 2000 & 3000 RPM, since voltage from the blue wire is greater than the voltage from the battery at these engine speeds. Any ideas?

I'm suspecting now that my problem may be in the voltage regulator. The B+ output of the alternator is out of spec low at idle and out of spec high at 2000 and 3000 RPM. I have no reference specs for D+, but it seems like D+ is a bit high too. Do these readings suggest that the voltage regulator has failed and is driving excessive output by feeding too much current to the alternator through DF?

Any thoughts would be welcome.

[Early_S_Man]

John T,

You are very fortunate to have a Fluke 88 ... a model they shouldn't have discontinued!

Your system seems to be suffering a bit from old age, not any failures, per se ...

The blue wire does seem to me to have too much resistance in it, which is causing the dimly lit warning lamp above 2000 rpms. Try cleaning all of the connections from alternator brush holder to warning lamp bulb holder with Q-tips dipped in vinegar, followed by a rinse with distilled water, blow dry, and final rinse/brush with fine, soft toothbrush [particularly at the two 14-pin connectors, which have silver-plated connector terminals -- clean female receptacle with pipe cleaner, male terminal with toothbrush] and 91% isopropyl alcohol. After the circuit is performing normally again, all connections should have a coating of Dow Corning 4 or 111 applied to protect from moisture and oxygen ingress for the next 20 years!

After cleaning, all of the female Faston connectors should be re-tensioned on both sides with needle-nose pliers -- just slightly, not enough to close the gap to zero! With clean, tight connections along the blue wire, I don't think there should be more than 0.4 Ohms in the D+/61 lead end-to-end from brush holder to bulb holder, ald the Voltage drop should be significantly less than 1.0 Volt, say 0.3 - 0.4 Volts, maximum.

Your Voltage regulator is not failing, just a bit out of adjustment due to wear on the 'points' [contacts] ... and being a mechanical relay-style regulator means it can be adjusted, after the 'points' are cleaned and burnished. At 4000 rpm it should not put out more than 14.5 Volts at the battery terminals.

One final comment ... alternator measurements and adjustments should be made with engine fully warmed-up, as Voltage output tends to drop measureably when engine is hot at running temp.

[Christien]

I've been following this thread and re-reading it several times as I'm thinking of finally tackling this problem. I've been driving with the dim light (bright at idle) for a couple months now with no ill affects at all. But now something new has cropped up. For days at a time, the light will be completely out, regardless of idle. It's not a loose connection, because it still comes on when the key is in the last position before starting the car (CD box and fuel pump running) but then goes out when I start the car and while I'm driving. Then a couple days later, it'll do the bright at idle/dim above 2000 rpm.

What gives? Does this sound more like a bad ground or electrical connection somewhere? Maybe just the wires at the bulb itself need cleaning, as someone suggested above? I haven't had a chance to try anything out, aside from cleaning grounds around the coil, CD and voltage regulator when I changed out the coil and CD.

[Christien]

bump - anyone?