So I bought this f150 just a while ago and it's always been showing pretty high temp. Gauge would start off in the middle when car was cold and move to near red when warmed up. I figured maybe i just need a thermostat, so I replaced it (it was bad, btw) while also doing a flush of the system. This got a lot of seriously dirty stuff out of the system. Well since then it gets to red even faster and goes pretty much all the way. It still starts in the middle when cold.

I thought maybe the temp sending unit is bad in the back of the block, but it measures resistance within limits. I then measured resistance on the wire that's on the unit to the ground and measured only 20ohms. There appears to be a short on the wire somewhere. I tracked it down to be past the firewall somewhere inside the cabin. So I went ahead and pulled the gauge cluster out and started measuring resistances on the needles. Each needle has 3 terminals...GND, IGN and SIG. Ground to ignition resistance on both coolant and oil gauges is ~58ohms. Ground to signal is ~31ohms (pretty much the thing I measured on the temp sending unit side) and ignition to signal is ~60ohm on coolant and ~48ohms on oil gauge.

I never paid much attention to the oil gauge behavior, but it does hover around the middle most of the time...not sure if that one is bad, too.

Last I measured the resistances on the fuel gauge and they're very different: Ground to ignition is 290ohms, ground to signal is 200ohms and ignition to signal is 100ohms.

These values make much more sense since the temperature sending unit spec is 74ohms (cold) to 9.7ohms (hot). When you have a 200ohm parallel line to ground it's not as bad as a 31ohm parallel line to ground. Then again, maybe it's all calibrated for it.

I also measured the resistance to ground on the wiring harness at the gauge cluster and it's open so it really does seem like an issue in the cluster.

Does anyone know what the correct resistances for this are supposed to be? Thanks!

 

Not sure what you are asking but the back of the cluster at the temp gauge is this.
SIG to GND 27.8 ohms
SIG to IGN 37.6 ohms
GND to IGN 47.4 ohms

I do not know how to take the gauge out to take a reading on it.

You should be able to ground the wire at the engine with the key on and the gauge should peg one way and when not grounded it should peg the other way.

I hope this helps.

 

Thanks, that's where I was measuring, too.

I did ground the wire at the engine bay and the gauge pegged one way (hot), but it didn't peg when it was not ground, just disconnected, it just sat in the middle.

 

I would say bad gauge then.

 

i have a problem with mine too... it wont get hot. i thought the sensor was bad so i changed it, still no needle movement. then one day in cold weather i discovered that the truck doesnt get hot enough for the heater to work very well. so i think im at the conclusion that someone removed the thermostat. i know they have been into it because it has permatex around the housing...

 

Do the check in post #2 above.

 

Ford gauges are crap, they're disguised idiot lights. Not only do they not tell you anything useful, but they break often as well. My oil pressure "gauge" broke, it just barely touches the "low" part of the normal band when the motor is running( it used to go right to the middle). The oil pressure is fine in reality. My temp gauge also rarely ever gets to the low mark of normal(it used to go just under the middle), if it's hot weather with a trailer it'll get a little past but that's it. Thermostat is good, and the little dial on the radiator cap shows perfectly normal operating temps.

I'd get a gauge out of a junkyard, or better yet, just install a real temp gauge. Capillary temp gauges are cheap ($20 or less), easy to install, and actually functional. Electronic ones are fine too, but then you have to deal with a bunch of wires and they cost 5x as much.

 

I read about somebody complaining of this on his Ford trucks' oil pressure gauge in a popular mechanics magazine many years ago. The magazine responded telling the reader that it wasn't so much the gauge as the sending unit. They said that the sending unit is basically on/off. They said that a sending unit from a certain earlier model would plug right in, and actually drive the gauge correctly.

Unfortunately, the magazine is long gone so I have no idea what sender one really needs.

I found the article, but it's not quite as helpful as I had hoped http://www.popularmechanics.com/how_...51.html?page=2