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Alternator Not Charging Until 3k RPM

77 911S with external voltage regulator and Marschal alternator that was professionally rebuilt.

I've read through a few old posts and have some direction, however asking for any other input.

Situation is no charging when engine is first started and will not charge until engine reaches 3k rpm or higher. After this, charging output is 14-14.5 volts at the battery or cigarette lighter at all engine speeds including idle.

It sounds like the alternator field is not getting excited. Would one of the first thing to check be the connections and wiring (blue wire) at the charging system dummy light in the gauge? Any other suggestions appreciated. THX.
Old 06-17-2019, 01:02 PM
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do you have the ground connected to the engine case?

Ivan
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Old 06-17-2019, 01:05 PM
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Never say never and I will check that. The problem started with no warning. I use the car regularly and it was fine, and then not the next time I started it the same day.
Old 06-17-2019, 01:34 PM
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does that charge light illuminate with key on before starting?
Old 06-17-2019, 01:39 PM
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My suspicion also was the excitation circuit/warning bulb, as it has seemed to me that eventually things start working even if there isn't enough excitation current.

I think the external VR is not solid state - it uses a coil or coils. Which means points to wear. The 914 external VR is potted solid state - I use one on my race motor. Not expensive, and the same 3 slot plug will plug into it. Might be easy to try even if it might not be the exact match for the alternator.
Old 06-17-2019, 03:30 PM
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Thanks. Ground strap OK and yes, the light does go on when the key is on before cranking.

When I had the alternator rebuilt by a local electrical shop 3k miles and 5 years ago. I installed a new solid state Echlin regulator from NAPA. I still have the old Marschal regulator which is good and swapped it out with no improvement. Put the Echlin back on.
Old 06-17-2019, 03:54 PM
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There are three grounding wires that must be clean. One is the wire from alternator to engine, the next from transmission to body (passenger side of car) and the last is the ground at battery. They must all be spotless and never trust a ground wire by anything but date - they can look fine and be full of powdery white corrosion.

Beyond this I cannot help - this was my issue after putting my rebuilt alternator in. The light would not die, but over a certain RPM (resistance?) everything began to work. Replacing those ground wires on advice was the solution. When I cut the alternator to engine wire (which looked good) it was hard metal and white powder.

Good luck!
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Old 06-17-2019, 06:29 PM
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Thank you Jdub. Good advice.
Old 06-17-2019, 07:18 PM
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there is one more ground if you have checked the other.
Behind the fuel filter on left side..
You are sure the ground behind the alternator is attached to the case?

Ivan

i would get an original regulator it is not expensive....
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Old 06-18-2019, 12:37 AM
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Did you change the pulley size?

I ask as I did on another car and it did exactly what you are talking about...
Old 06-18-2019, 10:47 AM
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Thanks,

I will have time to check the ground on the alternator tomorrow. I can't remember if it is visible or I have to do a partial alternator removal to see. The pulley was not changed, however, the belt is tight but does slip a little when the the pulley is turned by hand. I thought about changing the belt to rule it out but doesn't seem like it would make a difference. Also, when the alternator does begin charging its seems too consistent to be a slipping belt which I would think would be less consistent, i.e. temperature, moisture, etc.
Old 06-18-2019, 04:20 PM
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you have to tilt the alternator to see the ground.It should go to the 2nd or 3rd nut on the case like here ...this one is melted never mind that condition...

Ivan
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Old 06-19-2019, 12:46 AM
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There is an easy way to check the ground straps if the VR plug is available. The D- wire to the VR is connected to the alternator case. With the engine running, check the voltage from the D- wire at the VR plug with respect to the chassis. Not the engine but the chassis. If this voltage is near zero (0.1Vdc is ok), then the ground straps are ok.

The 914 charging system is very similar to the 911 w/external VR. Notice that D- goes straight to the alternator case.

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Old 06-19-2019, 09:36 AM
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I appreciate the suggestions and it appears grounds are some or all of the problem. I started with the easiest one, the negative battery cable. The chassis side connection of the cable had some corrosion and the cable itself is one that I have been meaning to replace because it is red in color not black. After replacing the cable and cleaning connections, the situation is improved. After starting, the charging light does not glow as bright and a quick rev of the engine to only about 2k or less rpm turns the light off and the alternator charges. Tomorrow, I will check the other grounds.

One mention though. Earlier when asked, I said the red charging light comes on when the key is on but engine off. It is but not all the time. Sometimes when I turn the key to the on position engine off, the red charging light is on and sometimes not.
Old 06-19-2019, 10:59 PM
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