Alternator problem

[Zoglmann]

Dear all,

I wonder if anyone out there could help me with the following. Since two month I own a 911 T (US-type) from 1973. The first one and a half month I drove the Porsche several times without any problems. Took the car for a long drive south for a two weeks holiday in France. After about 400 miles the car died and the battery was almost empty. Only then I checked the alternator warning light and saw it glowing very faintly. The car went in repair at a classic car garage near Lyon (France) where the alternator brushes where renewed and this should solve the problem, according to the mechanic. Driving home the car nearly died again after 400 miles (luckily I could reach home). Problems started with the green direction lights in the dashboard blinking both when giving direction. On my doorstep, the car still running I tried to switch on as many 'users' as possible and nothing seemed wrong (motor running at idle) and everything working properly without any fading of lights when pushing the horn or switching the rear window defroster on/off. Nevertheless the battery was empty.

A few things: Stupid enough I only found out recently (because of these charging problems) that I only have one 36 Ah battery and not two. Battery shows 12,5 Volt with ignition off and starts with 13,6 Volt running at idle, but drops a little later to 12,6 Volt (still running at idle). With engine running and lights and ventilator motor on, the Voltage drops to 11,5 Volt. All the time the alternator warning light was not glowing (was out). The motor running at 3000 rpm doesn't seem to raise Voltage very much (approximately 13 Volt). Charging the battery with a 4 Ah battery charger for nearly eight hours did not lower the charging of the battery charger (normally when the battery is getting towards full charge the charging gauge reads 2 or 1 Ah charging capacity). What could be the problem? It appears that the car charges all right, but only at idle speed???

Open to all suggestions

Peter

[jclc]

Peter,
I had a similar problem on a 74 911 with a Paris-Rhone alternator. There was a bad solder joint on a connection between a wire and a diode inside the alternator. It looked ok visually but when you checked by grabbing the wire with a needle nose pliers, it had a tiny bit of movement.


Soldering the wire fixed the problem. I wouldnt expect this of a Bosch alternator, though, they seem pretty well put together.

Jeff Craddock
Orange, CA
74 911 Coupe

[Early_S_Man]

Peter,

Could it be that the shop that did the brushes repair ... put in the wrong brushes, or maybe defective ones?

I have seen multiple cases where alternator output was normal at idle, but at higher rpms the brushes would stick in the holder, and the field circuit would be interrupted, causing excitation surrent to go to zero, and alternator output likewise went to zero without any lamp indication ... because the warning lamp circuit depends on a complete circuit through the field winding (and brushes) to ground!

You may want to remove the alternator, and have the brushes inspected ... and the alternator tested.

[MDrive]

Seems kinda crazy that you would get 11.5 volts at idle. I was having starting problems (well, I am not entirely sure I am out of the wood just yet, ...I digress) and drove around with a voltage meter attached. Started at 12.7 volts when I first started the car up, but quickly increased to 13.8 and finally 14.09 and held there. Granted I only drove around for 10 minutes but it did not fluccuate much. Warren, does the alternator eventually back off on charging the battery after a while?

[jk911]

I have a 1975 911 and i have the exact same problem. The auto electrician reconditioned it ie replaced the diodes, brushes were very good. But it stil doesn't chrge unless i rev it over 3000 rpm's, at idle it charges at 12.5 and if the light are on at 11.8 or below.

The electrician said I can't do anything about it. its just an older design ??

Its a motolora type.

[Jorgeman]

Zog--I believe the battery is suspect. As you pointed out, the external charger was not able to effectively charge the battery. It was acting like a bad cell--which might also be indicated by the "in car" measurement of 11.5 volts while running.

[Zoglmann]

Dear all,

Thanks for the quick replies. It's amazing how good this forum works.

To get a better reading and a testing I went to a Bosch Car-electronics shop yesterday. I am puzzled, but they came up with something different. The Voltmeter showed a nice inclining line to approximately 14 Volt, which you would suspect to indicate good charging. However testing the charging by looking at the amperage showed nil (0) amperes. Indicator light for the alternator still not lighting with engine running (it does work when contact is on and engine not running). The car shop suggested that I get the alternator out of the car so that they can test it. Does this make sense and should it not be tested together with the regulator??

Again I would appreciate any help

Kind regards,


Peter Zoglmann

[KNIGHTRACE]

Peter,
One thing you need to check as crazy as it sounds is the ground from the eng to the alt and body to trans groung. 11.5 v usually wont be a bad cell. 6 cells times 2v equals 12v. bad cells will usually be in the low 10v area. Hope this helps, William