I am building a 914-6 and I have an issue where the battery does not charge. Basically everything is new on the car (alternator, voltage regulator, battery & cables). The car runs fine but just will not charge the battery. I have checked the grounds on the body and at the transmission. The belt seems fine and is new. I pulled the battery and the regulator and brought it to a shop where they said it was putting out 14.2V using my regulator.
My Alternator wires are as follows;
Red from B+ to Starter
Black from DF to DF on the Regulator
Brown from D- to D- on Regulator
Blue from D+ to D+ on Regulator and then to the Dash Battery Light. (which is on all of the time)
Ground from D- to engine case.
When I put the system back together, I checked the continuity of each cable to assure myself it wasn't just a pulled connector or broken wire. When the car is running and reved to 2,000 RPM I still only get battery voltage.

Please help with any thoughts on what I should check next. Thanks.

Try a different voltage regulator. It should be providing about 13.4 V IIRC.

but was that a load test ? should have been, if so that max output voltage is ok,I had a Hella VR and it didn't play well with my Bosch (rebuilt) alternator ,swapped out to NOS Bosch VR and all was good.
I have an AGM battery they tend to act a but funny but start really well even after sitting for a long time.
If you are upping the RPM to that level and only getting standing battery voltage I would suggest you have an issue with the alternator not getting excitation via your connections,its the finer blue wire in the engine charging part of the loom .
Is the ground strap from alternator to engine case in good condition ? ,mine was crappy and I replaced it for example.
Have you done a load test on the system ,maybe that might wake it up ?
Things to try good luck,cheers.

Thanks for the input. One other thought. I read on another forum that if the dash light is burned out it won’t excite the alternator. My light works but it’s led so it likely lights with very low voltage. Could this be an issue? I think I’ll switch the bulb and check.

The alternator light in the instrument CANNOT be an LED; filament bulb as original only. LED will not allow charging system to work!

@Schmaz356

With the engine running, measure the voltages on the VR to chassis ground, not the engine case.

Measure D+ to chassis: should be 14V
Measure DF to chassis: should be 5-10V
Measure D- to chassis: should be 0V
Measure Battery to chassis: Should be 14V

Here's a basic charging system used on the 914. The 911 system is very similar.

This is how it works:

There is a circuit in the alternator to control the charging system warning lamp that is on the dash. Part of that circuit is a set of diodes mounted inside the alternator called the diode trio. The diode trio takes current coming from the three stator windings and passes a small amount through three diodes so that only the positive voltage comes through. After the diodes, the wires are joined into one wire and sent out of the alternator over the blue wire. It then goes to one side of the dash warning lamp that is used to tell you when there is a problem with the charging system. The other side of the lamp is connected to the run side of the ignition switch. If both sides of the warning lamp have equal positive voltage, the lamp will not light. Remove voltage from one side and the lamp comes on to let you know there is a problem.


On a 914, the bulb must be an original 12 volt, 4watt incandescent bulb. An LED won't work due to the way the circuit is built.

Put an old light bulb back in the lamp and I will bet your alternator starts working again.

do you have a replacement alternator with a built in voltage regulator or the original bosch unit?

@ClayPerrine

I recall some threads on Pelican where there was a resistor added in parallel with the bulb because the bulb current wasn't enough for the alternator to bootstrap itself and turn on. I don't think it was all years 911's.

I remember that too. But I don't remember what year 911 it was.

Thanks to everyone. The issue was the fact I used an LED on for the alternator light. Now the battery checks out to be 14.2 volts which is what the shop that checked it got. I do have one more issue however and is when I turn the key off, the car keeps running as its getting power from the alternator. I have found that the correction for this is to add a diode so the current can only run in one direction. Once this is done I should be good.

You have to put a one way resistor into circuit
We did also in our conversion

I’ll try to pull my diagram today for the info

Here is our racecar wiring

Not pointing this out to shame you, just the facts.

There is no such thing as a one way resistor. All resistors are are bi-directional.

Are you referring to a diode? Small black, round cylindrical package with a band on one end. Diodes only flow current in one direction. Could easily be mistaken for a resistor to anyone not intimately familiar with electronics.

I don’t have schematic in front of me but i do know there is a diode used in my 73’ behind the instrumentation.
Click to view attachment

Correct. Diode

Old man’s disease
Plus I really don’t love wiring

My dad wired the racecar and also hand drew the schematic
Looks like he drew a diode even

When I saw the symbol I knew it was a diode. Thanks again

Brant;
One more question.
I installed the diode like you have shown in your drawing. The band is toward the regulator. The engine now shuts off but the generator light does not light at all. Am I missing something?

I don't know...
thought that the light would excite... but not keep the motor running
sorry not certain
brant

Brant;
One more question.
I installed the diode like you have shown in your drawing. The band is toward the regulator. The engine now shuts off but the generator light does not light at all. Am I missing something?

Okay, Thanks.