when driving at high speed over 2000 RPM like 2300 RPM the temp gauge goes to the high end of gauge range; if driving at 2000 RPM it stays in center of gauge. even with no load on truck at 60 MPH on blacktop the temp gauge goes to high end that is at 2300 RPM; if at 55 MPH on blacktop at 2000 RPM it goes to center of gauge or little bit above center of gauge. If with a load on and traveling on gravel road slow speed 10 to 15 MPH and going over 2000 RPM up to 2300 RPM the temp gauge goes high end of range, but if I keep the RPM under 2000 the temp gauge stays in the center. Any ideas what is happening did not do this until recently. engine is Ford 7.8 litter diesel 225 HP at 2300 RPM. Truck Ford F900 year 1982. miles 260,481. More then likely has original engine temp gauge in coolant system.
I think you need a new temp sensor in the engine for your dash gauge. First check the wire going to the sensor, make sure its clean no corrosion It should be a single wire coolant temp sensor, they are like $18? If you are really interested in diagnosis before throwing parts at it there should be a ohm value for that sensor and a resistance test: http://jimsprojectgarage.weebly.com/ford-coolant-temperature-sensor.html
My work f250 will jump the gauges all around if you pound on the top of the dash.... don't ask why am I pounding on the dash so much
X2 on the percussive repairs... On something as old as this, ball up fist, pound downward on dash, when the gauge is actin up, just hard enough to not break it. If you break it, well, needed fixin, anyway...
I have a '91 F800 and had an '84 F600, the oil and temp gauges are a joke, slow reactions and ambiguous indications. Do yourself a favor and put in a set of quality mechanical gauges.
How about VOLTAGE? what does the gauge do if you floor it sitting still and let it rev 2,400 RPM? I'd either check the voltage of the engine to ground, the alternator + to chassis, the alternator + to battery +, at idle and at high RPM. OR add a new clean ground wire from the engine to frame to battery at least 10 guage, and a new alternator to battery wire, also 10 gauge. Then see what it does. The other check is to take the wire loose at the temp sender/sensor, see if there's any change in the gauge at different RPM, then take that loose wire and clip it to ground, and again see if there's any change at different RPM.
This action also works for the radio in my work truck sadly it is not a permanent fix... so usually in the morning when it's still cold and I'm listening for a traffic report the friggin radio goes into stealth mode that's when I put some serious effort into persuading it to sounding off again
X2 on a set of mechanical gauges. Also, I would check your coolant level and water pump belts.
Yeah, the water pump belt runs the air compressor too if you have air brakes, I have to tighten mine frequently.
Yup, good reason to carry a spare belt.
The air compressor belt is a constant hassle, good advice above, carry a spare and wrench.
Oh I do, wrenches aren't a problem... its a service truck. It does suck when you realize you are running low on air and high on temp in city traffic.
My 2007 dodge with 6.7 does the same thing. I installed a manual temp gauge, it agrees. I thought thermostat, it’s been changed 2 times in less then 2 years, radiator is newer, did elect fan, changed water pump... same issue. No difference on behavior if pulling a trailer or no load.
Well I blew out and washed the radator, changed the thermostats and gave it a test drive on the highway. The temp gauge stayed just above the center of the gauge. Thank you all who shared your comments. This is a great site on the web.