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Alternator problems

Any experts on alternators. My red light came on showing no charge on my 71T. I removed the alternator and separate voltage regulator and took to the rebuilder.
I was told the rectifier was bad and there were none available for these old Bosch units.
I was given a used Seva unit but I don't know if this is a direct plug in and go or do I require a different voltage regulator. I took a picture of the used one they suggested to try but I don't want to cause new troubles.
Anyone familiar with this?
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Old 08-31-2018, 08:43 PM
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That Marchal / Motorola alternator was common in the old 911s. Yes, it is my understanding that you will need a different regulator than what the Bosch used.

I would recommend one of the new solid state units that our host sells.
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Last edited by Trackrash; 08-31-2018 at 09:57 PM..
Old 08-31-2018, 09:54 PM
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Thanks Gordan, that's what I read somewhere but wanted to confirm. They a supposed to be fixing my Bosch unit so I will see what happens before switching out.
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Old 09-01-2018, 09:08 AM
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Quote:
Originally Posted by Trackrash View Post
That Marchal / Motorola alternator was common in the old 911s. Yes, it is my understanding that you will need a different regulator than what the Bosch used.

I would recommend one of the new solid state units that our host sells.
Actually, any voltage regulator will be O.K. to use as long as it has the proper connections; D-, D+, DF, B+.
Solid state regulators are a bad choice for the older SEV/Bosch alternators, e.g. 35/55/70, as they can't be adjusted
to prevent excessive battery charging currents exceeding the alternator's rating.
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Old 09-01-2018, 09:17 AM
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The rectifier, aka diode pack, is not the voltage regulator. The red light you are seeing indicates a short in one of the diodes (there are three). All the parts are available on line for you to rebuild your own alternator. You can replace the diodes yourself or you can go to a shop that does these rebuilds. There is a nice tutorial on this forum detailing the steps in the alternator rebuild. Otherwise, you can do what I did which was to purchase the Valeo replacement from Pelican.
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Old 09-01-2018, 01:03 PM
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Quote:
Originally Posted by Targalid View Post
The rectifier, aka diode pack, is not the voltage regulator. The red light you are seeing indicates a short in one of the diodes (there are three).
Remember, the red plate on the alternator has two diodes, either of which if shorted can cause the alt light
to glow with the key off.
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Old 09-01-2018, 01:38 PM
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Quote:
Originally Posted by mysocal911 View Post
Actually, any voltage regulator will be O.K. to use as long as it has the proper connections; D-, D+, DF, B+.
Solid state regulators are a bad choice for the older SEV/Bosch alternators, e.g. 35/55/70, as they can't be adjusted
to prevent excessive battery charging currents exceeding the alternator's rating.
FWIW. Bosch alternators on the older cars are wired internally differently than the others and use a different regulator.

See here. https://www.pelicanparts.com/catalog/SuperCat/911E/POR_911E_ELCHRG_pg2.htm#item5

I and others have used a solid state regulator with Marchell / Motorola alternators with no issues.

I am not sure why you need to adjust a regulator on one of these cars.
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Old 09-01-2018, 01:53 PM
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Quote:
Originally Posted by Trackrash View Post
FWIW. Bosch alternators on the older cars are wired internally differently than the others and use a different regulator.
Not really. Example please.

Quote:
Originally Posted by Trackrash View Post
I and others have used a solid state regulator with Marchell / Motorola alternators with no issues.

I am not sure why you need to adjust a regulator on one of these cars.
Then you run the risk of damaging the alt with a strong battery and a SS regulator that's set for 14.5,
which results in the SEV 55 amp exceeding its rating.
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Old 09-01-2018, 04:13 PM
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Quote:
Originally Posted by mysocal911 View Post
Not really. Example please.



Then you run the risk of damaging the alt with a strong battery and a SS regulator that's set for 14.5,
which results in the SEV 55 amp exceeding its rating.
I guess Porsche, Pelican and I could be wrong.


A while back someone posted the internal wiring of Bosch and Motorola alternators. IIRC Bosch has an extra diode to prevent battery discharge.

Can you school me on how to adjust a regulator on a 911.

Please explain how a solid state regulator can cause the alternator exceed its rated output? I guess I don't fully understand charging circuits.
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Last edited by Trackrash; 09-01-2018 at 05:50 PM..
Old 09-01-2018, 05:03 PM
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Quote:
Originally Posted by Trackrash View Post
I guess Porsche, Pelican and I could be wrong.
That's not uncommon for a seller to package parts for a higher dollar sale amount.
You believe everything you read?

Quote:
Originally Posted by Trackrash View Post
IA while back someone posted the internal wiring of Bosch and Motorola alternators. IIRC Bosch has an extra diode to prevent battery discharge.
Please provide.

Quote:
Originally Posted by Trackrash View Post
Can you school me on how to adjust a regulator on a 911.
Please explain how a solid state regulator can cause the alternator exceed its rated output? I guess I don't fully understand charging circuits.
Bottom line: If possible use the original mechanical regulator if possible and avoid setting the charging voltage too high.

If an alt is only rated at 55 amps and;
1. The regulator is set for 14.5 volts and,
2. The battery requires greater than 55 amps to achieve at least 14.00 then,
3. The alt will exceed its rating.

Remember, when the SEV 55 Amp was designed for the 911, batteries in the early '70s
were not as current capable as today's batteries and as such they didn't require a greater charging
to achieve a desired voltage level.

A perfect example is a 911 3.2 with a 90 amp alt and with a monster battery (like many install).
Rev the engine to quickly charge a very low battery and alt will fail. The same goes for the 911SC
using the Valero (75 amp) internal regulator alt, which often fails because the reg is set for about 14.5 to 14.7 volts.

The worst case Porsche alt is the late '60s' Bosch 30-35 amp alt. That can easily fail with
the charging voltage set too high. The 356 6/12V generators are also prone to fail with some
SS regulators requiring too high of voltage when paired with a strong battery.

Bottom line: If possible use the original mechanical regulator and adjust its voltage to a conservative voltage, e.g. 13.5 -13.75V.
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Last edited by mysocal911; 09-01-2018 at 07:44 PM..
Old 09-01-2018, 07:38 PM
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Dave, good info. I still have a couple of questions.

So why are there two different regulators listed for the older cars? One for Bosch alternators, 901-603-206-02, and one for Motorola, 911-603-901-00?

I'm confused as how limiting the voltage output of the alternator can also limit the amperage output. Why wouldn't the alternator still exceed 55 amps at a 13.5 v limit?

Is there a way to limit the voltage to 13.5 v using the solid state unit? Say installing a resistor in the field circuit?

Thanks, BTW, I'm still searching for the internal schematics, like a needle in a haystack finding anything in an old thread here.
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Last edited by Trackrash; 09-02-2018 at 08:44 AM..
Old 09-02-2018, 08:40 AM
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Quote:
Originally Posted by Trackrash View Post
So why are there two different regulators listed for the older cars? One for Bosch alternators, 901-603-206-02, and one for Motorola, 911-603-901-00?
Have no idea what Peiican Parts marketing does.

Quote:
Originally Posted by Trackrash View Post
I'm confused as how limiting the voltage output of the alternator can also limit the amperage output. Why wouldn't the alternator still exceed 55 amps at a 13.5 v limit?
It's all related to how the battery functions, i.e. the higher the set charging voltage to a battery
the higher will be the charging current, e.g. set the reg voltage to 12V with a fully charged battery
and the charging current will be essentially zero. Some batteries while charging at 13.8V can draw
60 amps or more. It all depends on the battery.

Quote:
Originally Posted by Trackrash View Post
Is there a way to limit the voltage to 13.5 v using the solid state unit? Say installing a resistor in the field circuit?
There's no easy way. Adding a resistor would also reduce the alt's ability to produce its rated output.
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Old 09-02-2018, 08:59 AM
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Here you go.
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Old 09-02-2018, 01:09 PM
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Quote:
Originally Posted by Trackrash View Post
Here you go.
1. The diagram for the SEV is incorrect, i.e. the SEV has two 2 output diodes as I indicated and as shown by the OP in the posted image.
2. So what if the old SEV reg is different internally than the Bosch reg, i.e. both alts have the SAME pin functions for the reg (61/D+, F/Df, Grd/D-)

Take some time and look carefully at each diagram!
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Old 09-02-2018, 05:54 PM
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Quote:
Originally Posted by mysocal911 View Post
1. The diagram for the SEV is incorrect, i.e. the SEV has two 2 output diodes as I indicated and as shown by the OP in the posted image.
2. So what if the old SEV reg is different internally than the Bosch reg, i.e. both alts have the SAME pin functions for the reg (61/D+, F/Df, Grd/D-)

Take some time and look carefully at each diagram!
In this thread Bosch/Motorola voltage regulator compatibility you stated :

"Using regulators set for a regulation voltage of 15 volts will possibly result in damage,
i.e. failed alternator diodes, to the older SEV/Marchal alternators (55 amps) when
used with the later/higher capacity batteries which require high charging currents.
Remember, the SEV/Marchal 55 amp alternator must not only charge the battery but
also power the FP, the ignition system, and all of the accessories That's why it's better
to use the older adjustable regulator and set the charging voltage AT the battery to
about 13.5 to 13.9 volts at 2,000 RPMs. One also needs to test the alternator output
current to make sure that the alternator is not constantly exceeding it current rating.
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Dave"

Your earlier statement quoted above does make sense. Especially considering most of the batteries charging will be after one starts the car when the batteries charge is lowest. Your explanation in post 10 doesn't really make sense to me.

Yes, perhaps the Motorola schmatic is missing a diode but that does not change the fact as you stated in that earlier thread.

"In both mechanical/solid state regulators, the SEV voltage regulator needs to have the higher set point for the SEV 911 alternator vs the Bosch alternator due to the diode drop."

Has your opinion changed on that?

I guess my alternator is safe now since I only have one battery in my car and it originally had two?
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Last edited by Trackrash; 09-03-2018 at 08:30 AM..
Old 09-03-2018, 08:23 AM
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Quote:
Originally Posted by Trackrash View Post
"In both mechanical/solid state regulators, the SEV voltage regulator needs to have the higher set point for the SEV 911 alternator vs the Bosch alternator due to the diode drop."

Has your opinion changed on that?
No. But with the typical 14.5V SS reg set point, the SEV will be at higher risk of failure versus an adjustable reg.
Again, it's depends on the battery and its required charging current. One should always check the charging
current at the battery using a clamp-on ammeter.
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Last edited by mysocal911; 09-03-2018 at 02:41 PM..
Old 09-03-2018, 01:18 PM
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Not to highjack this thread but I have a related question...

I have been searching for a charging problem I've recently encountered. I have an original '70 914-6 that had a 911T 2.0l -6 and a Bosch voltage regulator. I have pulled that engine (for the time being) and replaced it with a rebuilt, ITB FI'ed, 2.7 from a '74 911S with a Motorola/Marchal SEV externally regulated alternator. I had thought that since the wiring was the same between the alternator for the original 2.0L and the 2.7L, I'd be okay to run the Bosch VR, particularly because of how it mounts on the stock -6 circuit board. However, after chasing this problem for a week or two and checking everything, I'm beginning to think it may be the Bosch VR with the Motorola/Marchal SEV alternator that's the problem.

Today, I looked on the Pelican site for the '74 911S, and they make a distinction between the two alternators when choosing a replacement VR for this model 911. The fact that the Bosch and the Moto/Marchal alternators could require different VRs has never been suggested to me but I've checked everything else (to the point of dropping the engine and having the new alternator checked that I just installed on the rebuilt engine). The Motorola/Marchal SEV VR is not packaged the same as the Bosch one that is stock on the -6 but the three connections (DF, D+, and D-) are the same, so I'll do the best I can when wiring it in to make it look stock.

Any comments or thoughts on this solution/line of reasoning?


911 VR for the Moto/Marchal SEV Alt.


914-6 Bosch VR

Last edited by db9146; 09-06-2018 at 07:44 PM..
Old 09-06-2018, 07:39 PM
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Ironically, I just got back from helping a friend of mine work on his 914-6. I noticed he had a Bosch regulator as well.

Let us know if that solves your problem.
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Old 09-06-2018, 08:26 PM
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VR for a SEV Marchal Alternator in a 914-6

Well, I think I figured it out.

All 914-6s were set up for the Bosch style VRs which work with the Bosch externally regulated alternators in the stock 2.0L engines. The Bosch VR's housing fits nicely onto the stock -6 relay board. However, if you switch engines to a larger -6 that has an externally regulated SEV Marchal alternator like to one in the first pic in this thread, you can't use a Bosch VR.....you have to wire in an SEV Marchal regulator. This was my solution to get my charging system to work after installing a '74 911S 2.7L in my 914-6.

I actually gutted an old Bosch VR and made up a small harness that could plug into the stock relay board connector under the VR but then it exits through the rectangular hole in the relay and wraps back up to make the connections on the new SEV Marchal VR. Doing it this way keeps any water out of the relay board connector under the Bosch VR.





Last edited by db9146; 09-11-2018 at 06:32 AM..
Old 09-10-2018, 08:33 PM
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Quote:
Originally Posted by db9146 View Post
However, if you switch engines to a larger -6 that has an externally regulated SEV Marchal alternator like to one in the first pic in this thread, you can't use a Bosch VR.....you have to wire in an SEV Marchal regulator.
Please explain why, i.e. using the diagrams in this thread. Thanks
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Old 09-10-2018, 08:43 PM
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