I just had my engine rebuilt.

Drove the car once for 40 miles, and it was fine. The next drive was at night with all my lights working.

The following day I try to start the car, and the battery was dead. I noticed a faint light from the generator's idiot light bulb. At first, I thought it might be a sun reflection.

I removed the cables and charged the battery. I started the car, idled nicely, cleared the ground cable, and it died immediately.

I have a new voltage regulator installed as part of the rebuild.

I do not know my next step.
How do I diagnose what is back-feeding the idiot light?
I need a step-by-step guide for idiots.

p.s. I checked the bulb to ensure it was a 12V, not an LED installed by the PO.Click to view attachment

My guess is a bad diode in the alternator.

Un plug the 3 wire alternator connector. If the light goes out . It would point to the alternator or the alternator wiring.
Im thinking this would say the regulator and relay board is ok, or not keeping the light on

Like Dave said...Bad Alt/diode

Speaking of bad alternator diodes, it used to be common practice to test whether an alternator was working by simply disconnecting the battery while the car is running. Unfortunately this isn't a good idea with 914s, had a shop do that and it burned out the diodes in my alternator, which led to a chain reaction of other issues that eventually led to me putting her in storage back in 1986. Don't be me.

Oops. That is the first thing I did.

Remember, the GEN light is just a voltage difference gauge between the alternator and the car's main buss. If there's a voltage difference (about 6-7V), plus or minus, the light comes on.

Grab a VOM and find out where that key-off voltage is coming from.*

*Prob a bad alternator diode. Mine would glow MORE with more RPM but battery voltage was still ~13.5V at full tilt. Replacing the alternator fixed it.

@GregAmy . When I start the car the light goes away. I drove the car yesterday with a voltmeter attached to the battery. The voltage climbed slightly at idol. Then increased on my short drive. I don't remember the exact numbers but it got close to 14V

I don't know what to test. What do I have to remove around the alternator to test a diode? It's in a tight spot for sure. I think I am over my head.

agree replace the alternator. We have plenty of rebuilt BOSCH units on the shelf, becoming a more common problem these days as all of the 914s are or approaching 50

Here's a decent article on how to troubleshoot the alternator:

https://web.archive.org/web/20190510232710/...roubleshoot.htm

I had to send you to Internet Archive because their current web page is all messed up.


That means that there's voltage difference when the key is off, and none with the engine running. That's how the light works.

One side of the GEN lap is connected to the D+ side of the alternator and the other side is connected to Fuse 9. What's probably happening is that a diode on the D+ side of the alternator is open, so when you shut off the key the battery is providing power to that side of the lamp. That voltage can always find a ground somewhere (through the fuel sendor or gauge, other lights, anything on that same Fuse 9 circuit), illuminating the lamp. And that would slowly drain your battery.

You can test this hypothesis by removing Fuse 9; if the light stays on then it's leakage from the alternator and/or voltage regulator. Pull the harness off the VR; if it stays illuminated then it's an internal alternator issue, likely a diode that's supposed to block that voltage leakage that direction (which would be my wager). A rebuild is in order.

If you have to pull the alternator, just replace it or have it rebuilt (I know a guy in NH rebuilt mine for $125). It's such a finiggly PITA to get in and out you don't want to do it again.

Greg

I have a half dozen good stock (T4) 45A alternators here. Make me an offer & I'll get one in the mail