Alternator overcharging -- mismatched regulator?

logician · 12-29-2004, 06:59 PM

Is it possible to tell if the external regulator and alternator are matched in my car without pulling the alternator out? Can anyone tell from the attached pictures what regulator and alternator are in the car? If so, what regulator would I need to get to match it?

The regulator is marked Motorola, and does not match any of the pictures that I could find on the PP web site. The alternator shaft diameter is 16mm. I've been told that the solid shaft indicates a an alternator that requires an external regulator.

Here's the background: I'm trying to sort out a charging problem with a 911 that I just bought. The alternator is over charging (15.5-16.5V) and the alt light seems to not extiguish completely -- glows dimly. The car is a 1970 chassis, but has a rebuilt 2.7L engine in it. I don't know what alternator is in the engine, and I'm thinking the regulator and alternator might be mismatched. Would a mismatched regulator cause these kinds of problems?

Any help would be greatly appreciated!

-Juan

Wil Ferch · 06:22 PM

I'm waiting for others to come to the rescue...but the 16 mm x 1mm pitch shaft indicates an early ( pre-83?) alternator..the later used 17 mm x 1.5 pitch, I believe.

Also, I've come across posts tha the early units, with external voltage regulators mounted to the car chassis...indeed do need to be matched. Others more knowledgabe on the model year series should build on this....

Wil

CarreraS2 · 12-31-2004, 07:28 AM

Generally if you are overcharging it is the regulator.

At those numbers you are getting, you are definately overcharging!

Lorenfb · 12-31-2004, 08:08 AM

A regulator is a regulator. The key is the type of output; a pull to ground or a pull to
+V.

The regulator shown in the picture is the correct regulator for the early 55 amp alt.
This regulator can be adjusted easily to the correct voltage at the battery 13.5 - 13.7
and NO more or the alt. FAILS.

logician · 12-31-2004, 08:49 AM

Thanks guys for the replies. After a few inquiries, I'm pretty sure that the alternator was transferred over from the original 2.2L engine so it would match the regulator. I believe that would make that alternator one of the Motorola/Marchal (sp?) ones.

I've done a bit more investigation, and measured some voltages. I find that when the alternator is overcharging at 16.5V, the D+ voltage on the regulator is at 15.5V -- one volt lower than the B+. As I understand it, that really shouldn't happen, since the B+ is downstream of the D+, and after an isolation diode in the alternator. Thus the B+ voltage should be about a half volt lower than the D+, not the other way around.

Something is really fishy about the whole thing, and I'm suspecting a bad connection somewhere in the D+ wiring that is causing a voltage drop. I'm going to pull the alternator later today to have a look at the connections back there.

-Juan

Lorenfb · 12-31-2004, 10:33 AM

"I've done a bit more investigation, and measured some voltages. I find that when the alternator is overcharging at 16.5V, the D+ voltage on the regulator is at 15.5V -- one volt lower than the B+."

Not Possible! Unless the battery is charging itself. Re-check your voltages and the ground connections.

LakeCleElum · 12-31-2004, 11:16 AM

Juan - I've just finished swapping brands of Alt's in my '73.5 to track down a charging problem. My '73.5 has a Bosch Alt and VR. I pulled a Marschal (sp) from friends '72 E, put my Bosch in and it charged 15+ volts at idle. I then found my problem and put the Bosch in my '73.5 and it was 14.0 V at idle. So, the Alt and VR do have to be matched. Another thing I noted: There was a difference in pulley size and belt length between the '72 and '73. You may be spinning the mismatched Alt. faster than it should be turning....Good luck....

Lorenfb · 11:44 AM

"So, the Alt and VR do have to be matched. "

Nonsense! You didn't determine the condition of the switched alternators. Think!

A regulator senses the alternator OUTPUT (B+ which connects to the battery)
and switches the alt. on/off to maintain the voltage preset in the regulator.
The alt. regulator is neither a series-pass nor a shunt regulator as found in
electronics. It only acts as a switch.

"You may be spinning the mismatched Alt. faster than it should be turning.."

Some more nonsense!

logician · 01-01-2005, 02:11 AM

I pulled the alternator out today, and found that it was indeed a Marchal. I also discovered two problems. First the diodes going from the stator to the D+ post were blown and conducting in both directions. The diodes going to ground seem OK. Second, the ground strap was torn and disconnected from the engine case. It is possible that I pulled the strap off when I was pulling the alternator out.

Regarding the odd observation that B+ measured at a higher voltage than D+, I believe that is due to the rectification provided by the isolation diode between D+ and B+. I gather that the voltage at D+ had a lot of ripple in it due to the blown diodes. Thus the voltage measured at D+ was the average voltage, while the voltage at B+ was the peak voltage of that ripple.

In any event, it seems that I need to rebuild the alternator. Any suggestions for a good alternator rebuild shop in San Francisco?

-Juan

Lorenfb · 01-01-2005, 09:09 AM

"the diodes going from the stator to the D+ post were blown and conducting in both directions."

A common problem on this alternator when the regulator voltage is set too high,
i.e. > 13.8 at the battery.

logician · 05:14 PM

Loren,

Thanks for the reply. So you think my regulator is bad, and that is what fried the alt? OK, so I guess I should get a new regulator to replace the 34 year old one that's in there.

But, I can't figure out which regulator to buy from looking at the PP catalog, as none is listed directly for Marchal in the early 911 section. See attached picture of my alternator. Can you tell me which regulator to get for my alternator? Is the Motorola regulator that is in the car (in previous picture) the right one for this Marchal alternator? I'm starting to think that my Marchal alt is actually the same as the "Motorola 70A" unit that the PP catalog lists. Is that the case?

Confused...
-Juan

[Edited to include picture of my actual alternator.]

Lorenfb · 01-01-2005, 10:05 AM

Just clean the points on the old regulator and adjust its' voltage by varying the
spring tension. The one you have will work fine and is better since it's adjustable.

hkspwrsche · 01-01-2005, 10:20 AM

Hey Loren
If the pulley size is wrong, not adjusted, shim's missing etc... is it possible that it could affect the idle charging... That's what I was suspecting for my car as it seems to have an older style alternator and fan than a 3.0 SC should. I have a charging gauge and it will drop to 12v at idle sometimes...lights dim a bit and when I take off it brightens back up? normally idle is around 900 rpm.
thanks

Early_S_Man · 01-01-2005, 12:23 PM

Juan,

All 16 mm alternator shafts for 911 engines are Motorla/S.E.V. Marchal ... markings vary from year to year, but a Motorola regulator is correct for a Motorola or S.E.V. or 'Merchal' alternator ...

Lorenfb · 01-01-2005, 04:01 PM

"If the pulley size is wrong, not adjusted, shim's missing etc... is it possible that it could affect the idle charging."

Not likely. The older alternator's output current is fairly low until RPM>1200-1500.

logician · 01-01-2005, 05:11 PM

I have done a lot of searches about alternators, and learned a lot about them. One thing I've found is references to a "red plate" Marchal. However the one that I pulled out of my '70 911 appears to be a "blue plate" Marchal. See attached picture. Is there any difference between the blue and red plate Marchal alts?

Actually, turns out that I have a red plate Marchal sitting around from a previous car. Both red and blue have same part number. Is one a better candidate for rebuild than the other?

-Juan

Blue plate Marchal out of my car:

Old red plate Marchal that I happen to have sitting around:

Lorenfb · 01-01-2005, 06:08 PM

They're the same. Someone liked blue. See the overspray on the diode plates.

logician · 01-02-2005, 02:38 PM

Thanks for all the replies guys. So, I have read that the Marchal alternator was one of the worst ones. So, if I update to a different type, what should I get?

This is a race car application, so I don't think it will need a lot of capacity. Another racing issue is the alternator's survival when the ignition and battery are disconnected with the master kill switch while the engine is running.

I am also concerned about being able to fit an updated alternator into the existing Marchal compatible fan and housing.

Any advice?

-Juan

ASD · ASD's Garage

Quote:

Originally posted by Lorenfb
Just clean the points on the old regulator and adjust its' voltage by varying the
spring tension. The one you have will work fine and is better since it's adjustable.

Could you elaborate a little on this?
My 78 has been intermittently overcharging. New regulator did not help, although when you first start and drive it the system acts fine .
As it gets hotter the tach starts jumping and ther volttage spikes up to 17. If you restart the car when hot it immediately starts overcharging. Thought maybe that indicated belt issues...
The alt light glows off and on when this happens.
The belt is tight, shims ok, battery is good. Grounds have been checked. Nothing else seems to be not working or quirky.
I will be pulling the alt and still wonder about a bad alt overcharging versus undercharging..
Spring is coming I need to get this right soon. Thanks for the help .

vash · vash's Garage

juan, if you decide on getting the alt rebuilt, you have a great shop. RITE WAY, south of market. 6th and folsom. i just picked up mine yesterday. havent tried it, because i am waiting for my fan and housing to get back from the powdercoater. but this is the second one i had done, and no problems from the first one. one day turn around. they like to see your regulator too, so make sure to bring that in and they can test everything. my alt is so damn clean, i am gonna hate putting it back in.

cliff