electrical failures, current vs voltage

[Huw Powell <human@nh.ultranet.com>]

There have been a few threads recently dealing with miscellaneous
electrical failures where the contacts yield 12v to a meter but won't
run the device.

I'm out in the 90+ heat today zipping around a few errands, when I
notice the coolant gauge reads a little higher than normal.  The oil
temp was ok so I cranked up the heater (ouch!) which brought the coolant
down to normal to get me where I was going.  A little testing... 12v at
the rad. fan connector - must be a bad fan motor, right?  I drive home
carefully using the heater to keep the engine temp ok and start
fiddling.

I try running two potential replacement fans off the connector - no
dice!  Are they both bad?  No, the measured 12v turns to nothing when
any load is connected!

Go to the fuse... (where the heck is the rad fan relay, anyway?  I can
hear it clicking!) I wiggle the fuse with a needle nose to remove it,
and sparks! and the fan comes on.  So I clean the contacts and put in a
new fuse.  All better now.

As has been pointed out before, never trust a meter only reading of 12v
to diagnose a failed part.  Make sure there is some current behind that
voltage!

-- 
Huw Powell

http://www.thebook.com/human-speakers/audi-main.htm