Alternator charging

[jess p]

Hi Gang

I’m having a charging issue on my 70 914. The electrolyte has been boiled off, bone dry. I refilled my battery with battery acid and have charged it. After charging it’s a healthy 13.2 volts although I just might buy a new battery. I put the battery back in
the car and at idle my volt-ohm meter registers 15 volts at idle. As I rev the throttle
my volt-ohm meter reads 17+ volts.
This indicates a faulty voltage regulator but additionally my charging warning light bulb
burned out but have replaced it. So here are my questions.

1) I have read somewhere that a burned out bulb charging light prevents the alternator from excitation.
2) Are my findings indicative of a faulty voltage regulator only. Would there be any reason to suspect a faulty alternator given it is charging but needs regulation.

Thanks in advance.

[Dave at Pelican Parts]

Sometimes a failing VR can cause an alternator to fail. Or vice-versa.

If it were me, and if I could afford to have the car parked for a little while, I'd replace the VR and then test. If the voltage was fine at low and high RPM, engine cold or hot, I'd take the W and carry on. If the voltage was out of spec at any point, I'd replace or have the alternator rebuilt.

--DD

[jess p]

Ok Thanks Dave, I have a new battery and a good spare Alternator. I’ve ordered a new VR and it’s on the way. With regards to the warning light on the dash. If the bulb is burned out does that affect
function of the alternator.

[Dave at Pelican Parts]

If the charging system warning light bulb is burned out, in theory it would prevent the alternator from charging the battery at all.

--DD

[930cabman]

Sounds like a hell hole in the short future

Voltage is waay to high and with the acid disappearing, not a good situation

[djpateman]

The alternator requires the current passing through the bulb to create the magnetic field required for the alternator to function. An LED in place of the incandescent bulb will not work. I have seen a lot of alternator harnesses that are fried; at least 15 out of 20, and that is a very high percentage which does not bode well for yours. Good grounds are a must; the alternator grounds through its case to the fan shroud, to the engine case, to the transmission case, through the trans ground strap, the body of the car, the battery ground strap and the battery post. If the fan shroud or engine case has been painted, then that can cause problems. I have also seen the body studs for the ground straps not good enough. On one of my cars I put a new ground from the battery to the fan shroud (a very convenient hole in the fan shroud made an ideal location) and that solved a huge problem.
Boiling a battery dry is a very nasty problem and usually results in a battery of questionable reliability.
The voltage regulator needs a good battery connection (from a good battery) to function properly. It needs to see a proper voltage from the battery to regulate the alternator voltage to the proper level required to charge the battery correctly. The regulator is not very smart, and cannot tell if the battery is defective, nor if the current path is defective.

[jess p]

Thank you for all the great advice 930cabman, Dave and djpateman.

I restored this 914 from the ground up about 6 tears ago. New alternator, vr etc. This is my 5th
914 but this issue had me a little dumbfounded. I have a new alternator, VR and battery I’m going to
install today. At the time of restoration I also installed 914rubber’s new alternator harness.

Looking at the wiring diagrams I see that the alternator warning light has a wire going to D+ on the relay board in the engine compartment. As well as a power wire from fuse #8 from passenger compartment. I think those are the only relevant wires to that bulb. BTW I’m replacing that bulb with a new one from our host.

70 914
67 912
76 911S
82 911sc
C8 vett
C5 vett

[Rodsrsr]

I recently experienced an overcharging issue on my 914 which also has a new alternator, VR and harness. It turns out that the plug on the new harness was somewhat loose where it connects to the back of the alternator preventing a solid ground. I thought it was a one-off but maybe not. The connector never really seemed too secure but I never thought it would come loose. I ended up completely cutting it off and running new individual connectors. problem solved. Steady 13.8 volts at idle now.

You can trace this out with a meter before pulling the alternator. Just unplug the alt connector plug at the relay board and test the brown wire for a solid ground.