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??????

[East coaster]

Idiot light was not installed when I first put the car on the road (guess I'm the idiot!). I never knew the light was a critical part of the circuit. I still have trouble believing that a stupid a$$ bulb which could burn out at any time would be a consciously critical component of a charging system!!! Why wouldn't they use a shunt res or something??

When I wired the car I didn't have the correct bulb/housing for the alt light because I'm using 911 gauges now and the 914 light was totally different. I figured I would get the correct bulb/housing someday and when/if I had to pull the gauge panel out I would addres it then. Having learned that the bulb may be a critical part of the system, I recently pulled the panel and put the bulb in.

When I start the car, the voltage is about 12.8 and the light is off. When I rev the motor the voltage climbs to 14.2 -14.4v and the light kicks on at 3800 rpm. When the light is on I'm indicating 14+ volts and when the rpms drop the light stays on and voltage is still looking good.

[Spoke]

Why wouldn't they use a shunt res or something??

The gen light is simple and provides instant notification of ALT failure. It's a VW...think KISS.

When I wired the car I didn't have the correct bulb/housing for the alt light because I'm using 911 gauges now and the 914 light was totally different. I figured I would get the correct bulb/housing someday and when/if I had to pull the gauge panel out I would addres it then. Having learned that the bulb may be a critical part of the system, I recently pulled the panel and put the bulb in.

Has the car worked correctly since the 911 gauges were put in?

When I start the car, the voltage is about 12.8 and the light is off. When I rev the motor the voltage climbs to 14.2 -14.4v and the light kicks on at 3800 rpm. When the light is on I'm indicating 14+ volts and when the rpms drop the light stays on and voltage is still looking good.

When the gen light comes on, this indicates that something is not right with the ALT +VR. With the light on, the VR is not powered much at all and one could assume the ALT is also not working. When the light comes on at 3800, does the battery voltage drop even a little bit? This would indicate ALT not charging and the longer you run the car on battery power, the lower the battery voltage will get.

Before the gen light comes on, does battery voltage vary with engine speed? You know, like 14.2 at idle, 14.4 at 2000? If so, once the gen light comes on, does the battery voltage also show the same voltage vs RPM characteristic? Also try this experiment with the lights on. This will give heavy electric load and will hasten the dropping of battery voltage if the ALT is not functioning properly.

If you have a jumper wire, try the test with VR removed and D+ and DF shorted. Rev engine to 3800 and see if the gen light comes on. If you don't have a convenient jumper wire, you can make one with a short piece of wire and 2 crimp-on spade connectors.