Alt charging battery at 16.7V! 911SC 1981

[dazandkim911SC]

Hi all,

Have search forums for specific problem to no avail. Hope someone can help me out.

I have a 911 SC 1981 with external Marchal Voltage regulator.

The ALT light came on and then a short time later (2-3 weeks) the battery died completely.

The battery has been replaced but the ALT light is still on dim with engine running. Also the tacho fluctuates erractically at idle and in rev ranges up to 6000rpm.

The charging voltage with the engine running is 16.7 volt, which is obviously too high, thus cooking the last battery.

Can someone give me a clue as to whether this would be voltage regulator problem or alternator?

Or has anyone got a specific troubleshooting guide for the voltage regulator. I have the Bently book and 101 projects, which suggests it is the Voltage regulator.

I have had the voltage regulator apart, and checked the internal resistors to correct values and the continuity of the coil, also cleaned the contacts.

Thanks

[Porboynz]

First off, no more running the engine until this is fixed, bad things can happen to things like Tacho's and clocks and CDI units with excess volts, not to mention wrecking your new battery. I have never worked on a voltage regulator but theory suggests it will have a coil that operates some contacts that regulate the alternator output. The armature for the coil will be set to operate around 13.8V/14.2V, I presume there is a tension screw adjustment there somewhere. I would just swap the regulator for a good one. But then I have a spare regulator or 2 sitting on the shelf. Actually I have spares for most everything on the Porsche sitting on the shelf, part of the charm of owning an older car.

[JJ 911SC]

BTDT the V.R. is gone

[toddu]

If your alternator has ever been replaced, you may have the on board style VR instead. Happened to me when I changed the external one and it didn't solve my issue.

Just in case.

Todd

[neighbour]

Good points:

Also,boiling the batt could cause fumes to seep into the passanger compartment
and send the recipient off to a hospinal.Happened to a relative in an 88 911.

[wwest]

A shorted section, diode, within the alternator can cause this effect. The regulator wants to keep the alternator output at ~13.5 volts on the average. With one winding acting as a short the VR will pump up the other 2 windings in order to have an AVERAGE of 13.5 volts.

The dimly lit charging bulb is an indication that the VR is pumping a LOT of current into the rotor windings.

But whatever, do not rule out the VR.

[lupin..the..3rd]

VR is easy to change, I changed the one in my '87 Carrera when I had too-high voltage. Replacing the VR fixed the problem. It wasn't a difficult job at all.

[wwest]

But, as someone already said, be careful to ascertain that the external VR is still in use....

[acapella8]

Pelican Technical Article: 911 Alternator & Voltage Regulator Removal/Replacement