Dennis Haynes' post hit upon the likely cause of the higher temperature
reading when the headlights are on.

It's called voltage drop, in that the headlights take enough power to run
that the resistance of the wiring between the battery and the instrument
panel causes the panel's voltage to be less than when the lights are off.

This in turn causes the guage to read differently when the lights are on.
Two ways to test this theory and find out exactly what is going on are to
measure the temperature of the engine with lights on/off, with no other
conditions changing, and compare the voltage at the guage, as measured with
a high-quality meter, with lights on/off.
Also, if the temp guage changes reading instantly when lights are switched
on, this semi-proves the voltage theory, since the water temp can't change
that quickly.

Ron Antaki
ex-electronics tech
now wrecking computers for fun and prophet...

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM]On Behalf
Of John Baker
Sent: Friday, September 07, 2001 9:00 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Is there any way...

Howdy All,
This sounds stupid even as I type it, but Is there any way that driving my
83 water cooled Vanagon with the headlights ON, would affect the water temp
gauge reading?? Stupid, right?  But I swear i have noticed that when I drive
with headlights on, even if the outside temp is a little cooler, my temp
gauge reads higher!  I have noticed this at night and in heavy fog when i
have had lights on.  Can outside humidity also affect this reading.  Is it
possible the head light wiring is arcing with the temp sender wire or
something, Or is it JUST A COINCIDENCE!?!?!  PS, It's OK if you laugh at me
for asking this question ;-)

Peace, JB
'66 Split Window Camper
'83 H20 Vanagon

"You cannot simultaneously work for peace and prepare for war..."