Calling all electrical gurus........., my alternator saga continues Options

[East coaster]

I had a strange electrical problem with my 914 and I thought I had it resolved, but now I'm wondering.

A while back, I was driving to work and the car died. I found that the battery was dead. I removed the battery, charged it, and made it to work/home. I assumed my alternator was bad, so I ordered a new alt (alt/reg) and replaced it. I'm running 911 gauges and don't have a volt meter/amp gauge and the idiot light was not working, so I really had no indication while driving other than my stereo would crap out due to low voltage a few minutes before the engine would be affected. This gave a little warning that something was awry.

Once the alt was replaced, things seemed good for about a month or so and then it happened again. I inspected every ground and found nothing. I restricted my driving to my local hood until I figured this out and I finally thought I had.

I fired it up for a drive a few weeks back and it cranked over rapdily and started right up (as it usually does). I proceeded to back out of my driveway and started heading down the street and within 3 blocks (<1/16 mile) it began to die. I reasoned that no electrical problem can take a fully charged battery down that quickly without frying something? I could arc weld with a battery and not kill it that quick! So, I thought the battery itself is the only thing that could cause this symptom.

So, I got the battery replaced under warrantee as it checked "bad" when I returned it and it's been working OK since.

Here's my dilema (after the long winded above description!), I replaced the bulb in the idiot light and now I have a new concern. When I start the car the light is off and remains off until the rpms hit 3800 or above and then it comes on and stays on until I shut the engine off and restart. once restarted, it stays off until I hit 3800 revs. With the light on I metered the voltage and it looks good (14.2-14.4), so it seems the alt is working fine........so what's up with the damn Alt light??????

[Spoke]

When you start the car and the gen light is off, what is the voltage at the battery? Is the system charging at this point?

After first starting up and gen light is off, rev to 3800 while measuring the voltage at the battery. Does the battery voltage change when the gen light comes on?

If the Gen light comes on during operation, this tells you that the alternator circuit for the gen light and voltage regulator is not producing voltage. The gen light and VR have their own set of diodes (ie, positive voltage) from the ALT. Another set of diodes charges the battery.

No voltage for the gen light/VR could be the wiring or the diodes in the ALT. You replaced the alt so diodes should be ok.

Check all wiring to be sure they are securely connected.

One test you could do is check ALT by itself by removing the VR and jumpering lugs D+ and DF together. These are on the relay board where VR connects. This simulates the VR commanding the alternator for full output. You should see the battery voltage go up over 15V. This is not a test you should run for a long time (like hours), maybe only for minutes. This test will take the VR out of the equation. Rev engine to 3800 and see if the gen light comes on. Measure battery voltage before revving and after.

The D+ and DF lugs on the relay board are the 2 side-by-side lugs on the side of the board towards the front of the car. DF is the left one (driver side) and D+ is the one on the right (pass side). D+ is the one that connects to the gen light and the ALT output.

The other lug is D- which should connect to ground. Test this connection by putting your multimeter on OHMs and measuring from D- to the chassis. You should read 0 ohms or close to it. For safety of your meter, you may want to disconnect battery NEG before checking resistance.

BTW, the gen light plays an important role in the starting up of the alternator. With the gen light burned out, it is possible that the ALT will not produce voltage.

The VR gets power from either the ALT (normal operating condition) or the gen light (starting).

The gen light provides initial power to the VR when starting. The VR in turn powers the ALT armature (spinning part). Armature magnitizes with power from VR and thus the ALT generates voltage when the armature is turning. The ALT may work even without the gen light as the armature possesses a small amount of residual magnetism even w/o power from the VR.

The characteristic of gen light staying on after first startup before revving is because the gen light is like a resistor so the VR doesn't get full 12V and thus only magnetizes the armature a little bit when the engine is started w/o revving therefore the ALT produces little voltage....Until the engine is revved, the ALT spins faster, makes more voltage which powers the VR/armature and bingo: ALT starts charging and powers the VR full voltage and gen light goes out.