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Water Temp Gauge reads incorrect when Headlights on 700 1985

posted by  sdodd on  Wed Jun 22 05:50 CST 2005
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last visit: Sat Jan 23 00:38 CST 2010
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All,

I have a 1985 745td vehicle that I put a V8 into. I *believe* I have an instrument cluster problem. Basically, my water temp gauge reads OK when the headlights are off, but reads high (pegged) when the headlights are on.

So far I have done the following things:
1) verified the engine water temperature is OK, using a VDO electrical gauge and a separate sending unit. The car generally runs between 170 and 180F.
2) replaced the temperature gauge unit with another used gauge. No change.
3) replaced the rheostat (behind the fuel gauge) with another used unit. This rheostat controls the IC dimmer. No change.
4) removed the entire flexible circuit from the back of the IC. Reflowed every solder joint. Changed the intermittent nature of the problem to a very definate HL on, Bad; HL off, OK.
5) Moved the sending unit to a different water jacket boss. No change.

This problem SOUNDS very much like a busted voltage regulator in the 240 series. Is there such a thing on the 700 series?

Anybody have any thoughts?

thanks,
simon

simon dot dodd at alum dot mit dot edu






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