[jsdsm]

New to site. Original owner of 1989 Mustang GT. Recently observed temperature gauge quickly goes to hot when lights and/or other electric accessories turned on. When turn them off temperature gauge drops to normal. Any ideas out there of what is going on?

 

[ALMOST STK]

I also have a 89 and I still have the stock radiator and water pump ans I see a very slight increase in the temp when I turn on the A/C but nothing I'd worry about.
Have you flushed your cooling system lately?

 

[David.kinter]

Are we talking quickly as in a few seconds like there is something wrong with the gauge or over the course of several minutes as if the extra accessories are making the engine run at a higher temp?

 

[curtisboyte]

IVR is bad if the temps jumps immediately. I had a 83 mustang to do it and i changed it out and now my 93 is doing it now.

 

[jsdsm]

Thanks for quick feedback gents.
In my situation the temp gauge jumps quickly (within 5 seconds) to high and likewise drops just as fast when I turn off any accessories.
@ curtisboyte - pardon my ignorance...what is IVR?

 

[jsdsm]

IVR = Instrument Voltage Regulator. Ha...gotta love the internet. Now...how do I get part and where is it located?

 

[5.0Mustang1989]

Sounds like you have a bad ground. The headlights and instrument cluster are grounded through the same wire, and when that ground starts dropping volage the gauge will go up. Does your tach move too when you turn the lights on at idle? or turn on the wipers.

My ground got corroded, and when I turned the lights on the wipers didn't work, the tach didn't work, and the temp gauge went all the way up... pulled the ground off and cleaned it and all is good now.