Temp gauge reads hot when lights switched on....?

Before I replaced the leaking water pump hose I had a new issue of the temperature gauge (VDO) reading 90C+ when it normally reads 83-85C at running temp, and thought the two were connected.

Having replaced the leaking hose, refilling and going for a drive today I noticed the temp gauge was fine, up to high 80s then down to ~83 once the stat had opened. I then switched the side lights on and the temp gauge went up to 90, lights off and back down to low 80s. Side lights and headlights produced a reading of ~100 but back to normal once off.

Got home and checked the engine bay, rad, hoses with an IR thermometer and nothing indicating an overhot engine. Top pipe hot to touch firm but squeezable, rad hot at top but cool at bottom and return hose was warm.

Any ideas? Earth issue, leaking current from lights? The gauge is powered from an ignition switched live with an earth to the instrument cluster ground as per the original LR one.
Not sure if this has any relevance but the same day it first happened the radio wouldn't switch off, couldn't change channels or anything, did nothing but stay on. Removed it and the power plug and saw it was damp, dried it and no further problems.

TIA

What year/model truck? Does sound like a wiring issue though. Chance there is a water leak into dash area?

tottot said:

What year/model truck? Does sound like a wiring issue though. Chance there is a water leak into dash area?

Click to expand...

It is a 1985 Ninety, originally a 2.5P/LT77 but now a 3.5V8 Efi/ZF4HP22.

I agree about a dash leak, a bulkhead grommet or two are missing or shot, probably how water got into the radio plug.

I think I need to check:
Sender wire might be dodgy, or the gauge earth (B) or switched pos (G).
As it is affected by the light switch it could be the feed from the light switch (R/W) or the gauge light earth?

Not been a problem until last week; had sat unused in the cold and rain for a couple of weeks.

OK dash out and multimeter when I CBA!

Haven't been able to check the earths or wiring yet.

Drove today and gauge was spot on at ~85C, saw it slowly rise to just below 90 then back down to 85 as the thermostat opened.
However with the side lights on it went up to 90, switched off and back to 85. Same with the heater blower, headlights, any extra load and the gauge started to rise. I know from the IR thermometer and touch that she is not overheating.

The association between the current draw with ancillaries on and behaviour of the gauge is telling me something but I don't really know where to look.

This is a new phenomenon and first occurred on the day the water pump to heater hose sprung a leak and I lost some coolant. Sat for a couple of weeks waiting for the hose, then fixed, refilled, air purged and running again with no leaks; just the erratic behaviour of the gauge.

It may be possible that the voltage regulator of the alternator is playing up. Did it get soaked? Should not matter but. Check the voltage output, should be no more than 14.2 ish volts when running.


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My 300 does the same ,gauge goes up when lights turned on and goes down when lights turned off pretty sure its a earth issue but not looked into it

Weird stuff with electrics is (nearly) always a bad earth.