What is causing this to happen...?
 

I never drive at night before until yesterday, so I have never experienced this problem:

When I turned on the headlight, the voltmeter automatically went from 12 v to 10 v. Also, the car idle suddenly drop, the air/fuel meter drop to lean, and the car drive a bit poorly. I do have the Crane XR3000 and PS91 installed (if this help). When the light is turned off, everything is back to normal...reading 12v, air/fuel ok and car drive good.

Any suggestion?

I would have the battery load tested...could be a alternator...You may also have a short in the headlamp curcuit...I would start witht eh battery.

Due to the location of the volt meter connection you will get a lower reading with the lights on. Our stock 1974 reads about 10.5 to 11 volts. This is with a new (not rebuilt) alt and regulator and new batt cables for both + and - connections. I agree with getting things checked out since the heat in the engine compartment can make things deteriorate faster than we like to admit. Good luck.

Installing new fuses (esp. #8 or #9, I forget which one) and cleaning up the fuse holders with contact cleaner and possibly emery cloth can also help a bit. They may help more with the running than with the voltmeter reading, though.

If you want to know the actual state of your charging system's health, you have to check across the battery terminals themselves. The stock voltmeter isn't that useful until you know how it "usually" behaves.

Brad Anders had a similar problem with his lights. He adjusted the idle mixture (I forget which way, richer or leaner) and the problem cleared up.

--DD

When I purchased my first 914 and decided to drive it home from Oakland to LA - I had simular problem - However my problems got me stuck!
It had a bad battery and voltage regulator (that was the real culprit). The other problem which is non related to what you have was the throttle switch. The car would start to sputter at a constant speed. To remedy my problems I replaced the ground cables, volt reg, battery, and throttle switch. Not that expensive of a fix in the grand scheme of things.
On my 75 2.0 which I currently own, I had some electrical problems due to the fact that the wiring harness that attaches to the relay panel in the eng comp. was loose. It plugs in underneath - I suggest checking that. When I figured that out - eureka!! I had dash lights again. Volt regs are cheap probally wont hurt to put a new 1 in anyway.

Good Luck

Check the ground cable on top of the transmission.

so you put a continous new load on the bat.is there no change in charging rate as rpm increase?start at the beginning,good grounds, the trans and where the bat connects to the chassis,good clean terminals and clamps on the bat.with the engine off reach in the cooling fan and check the alt belt tension. these four will take all of 20 minutes and you will have a good starting point.