Make sure you have no corrosion on the terminals on the wires. My 86' 302EFi would do crazy things to the temp gauge but I knew the engine couldn't get that hot or cold that quickly. I changed the temp sending unit above the T'stat and its connector.


But it wasn't until I changed the corroded coil terminals that the temp gauge began to read normal. That was a few years back, it's been fine since.






Make sure you don't have corrosion on the connectors.

 

Finally got back to this after the holidays. I switched the oil and temp gauges. When I did this the oil guage(now temp gauge) was reading normal. The needle would point to the ‘a’ in ‘normal. When I switched the temp sender back to the temp gauge, the needle would, again, point to high. Sometimes it’s even higher than in the pic. How can I switch out or fix the gauge?


thanks

 

You only tested it half way. You know the temp wire and motor temp are good using the oil gauge as a temp gauge.

When you put the oil wire on the temp sender did the oil gauge read right or high like when the temp wire is on it?
If it reads high and you have not changed out the temp sending unit I would and see what happens.

If you have changed the temp sending unit and both gauges reads high when on the temp sender then I would think the new sender is bad.
Buy a different brand from a different brand store and see what happens.
Dave ----

 

I did hook the oil pressure sender up to the temp gauge. I thought the results were inconclusive. This is because when I would start the truck cold the temp gauge(which now shows oil pressure) would go very high, but my understanding is that this is normal on a cold engine. As the truck warms up, the oil pressure would go down to normal range. I felt this test inconclusive because the temp gauge might be showing higher oil pressure than there actually is, while still being in the normal range. Or perhaps the gauge is just inaccurate in lower ohm readings.

So, while I can’t say anything for certain about the oil pressure being hooked up to the temp gauge, what I can say with some certainty about the engine temp is that the temp gauge shows the engine overheating when it’s not and the oil gauge reads the temperature accurately.

I’ve attached a pic of when the wires were switched.

Im not trying to argue anything. I’m a rookie at this and am open to suggestions, so I can move on to the 500 other issues to work on.

 

I didn't see where you were totally against aftermarket gauges. Some people are, some people are not.

 

im not, but I’d prefer to fix the existing gauge if possible.

 

It's all good just trying to under stand what it going on is all.
Is the picture above with motor cold and with the
temp gauge + oil sender = OK
Oil gauge + temp sender = A Little high
or something different?

Here is what you can do, everything hooked up normal and get the motor up to temp
If I under stand it right the temp gauge would read that it is over heating like in your first post, and the oil gauge reading normal.
Shut the motor off and swap the oil & temp wires / senders so the oil gauge is on the temp sender and the temp gauge on the oil sender.
What do the gauges read? Is one reading higher than the other?

If the oil gauge, now hooked to the temp sender, is reading really high then the new sender is bad.
I know you changed it out once did you also change that for a 2nd TU25 from the same store? If so hit a different brand store for a different brand sender and see what happens.

If they both are reading normal then swap the wires / senders back and tell us what happens.
If the temp gauge, now back on the temp sender, reads high again I am at a lost
Dose it happen as soon as you turn the key on or after a bit like normal warm up?
Check the temp wire for any broken insulation that could be shorting to ground on the back of the motor as it makes its way to the temp sender.

Again I am just trying to under stand what happens using the different senders for testing and when.
Dave ----

ps the picture above looks normal for the oil and temp. Well the oil maybe a little high but normal for a cold motor.

 

Hmm, my brains hurts trying to unwrap all this. I've had to delete some pleasant childhood memories to free up some cerebral computing ability.

One thing I did notice which may skew the results: The normal change in oil pressure when cold versus warmed up. You've introduced a variable into the test method. Ideally, we want things as equal as possible between the two systems. I would suggest repeating the switcheroo test, but this time with the engine already warmed up. The signal from the oil pressure sender (a reasonably trustworthy value) should now be the same, whether driving the temperature or oil pressure gauge.

Additional bonus thought (no extra charge): Do not change the switch position for the headlights or heater fan during the test. We've had several instances of a marginal/missing ground for the cab causing the temperature gauge to read high when other electrical loads are high. I'd make sure the ground between the cab and engine is present and properly connected. It runs between the firewall (near the throttle cable passthrough) and the rear of the engine. Pour yourself a strong coffee or other drink of choice and peruse this LarryThread™ for more details. The fix was eventually documented in post #59:


https://www.ford-trucks.com/forums/1...l#post21073752