re: basic (?) electricity

[William Elliott <William.Elliott@mail.mei.com>]

<<<What am I missing here?  The trunk light isn't working.  A DMM connected
across the empty socket terminals shows 12.4 V.   But when the bulb is
connected either in series or in parallel, the voltage drops to zip.  However,
when the ground is transferred from the neg terminal to a chassis ground, the
bulb goes on and the DMM shows 12.3 V. 

Can anybody explain what is happening, please?>>>

I've seen this sort of behavior a few times before. With my previous 
SAAB Sonett, it was a daily occurrence!  Test with a multimeter, get
12V.  Test with a test light, get nothing!  I've only seen it a few times
in other cars.

I can't describe the science of what is happening, but the reason
is a bad contact (in your case, it sounds like the ground.) For the
lack of a better explanation, it's like there's a "surface charge"
sufficient to register on the digital MM (which has nearly no 
resistance), but add the resistance
of even a small test light and the voltage drops to zero.  

On the SAAB, I would carefully take apart everything in the circuit
and clean it up. However, especially on the headlamp circuit, I
knew it would only be a matter of time before the same thing 
happened again. (Curiously, once the headlight was on, it would never
fail and turn the light off.  It was only on the initial application of power
which that it would fail to light.) I simply quit using the DMM since
it tended to make troubleshooting worse!

Any EE's care to explain?

Bill Elliott
Lake Mills, WI