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'91 F250, 7.3L idi. On my way to work, idiot light "Engine" came on. I glanced at it, then saw the Engine temp gauge BURIED to the right. Truck was running normal, but I panicked, pulled over and killed the engine. Popped the hood, first think I noticed was the engine didn't seem hot. I could hold the upper hose with my hand. No steam, so-- feeling brave, I loosened the radiator cap. Slight venting- no boil over-so I removed it to inspect the fluid. Radiator fluid good, good running temp, slight radiator pressure- nothing unusual. I pulled off the connector on the single wire sensor just forward of the exhaust manifold on the driver side, gauge went to a normal reading and the idiot light went out. I plugged it back in, and gauge went full hot, but flicked back a few times (erratic) and the idiot light went off and on. This confuses me because I thought there were separate sensors for the gauge and the idiot light. I thought the gauge sensor was a two-wire sensor on the front driver side of the engine, just to the left of the AC compressor?
Obviously I do not know how these interact. I DID (on the day before) spray oven cleaner on the single wire sensor while I was attempting to clean up the area around the power steering pump (slow leak somewhere), and then sprayed the area with a water hose.

Can anyone tell me how the temperature gauge and engine idiot light work off these sensors?

Thanks all-
 

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Update- trip home from work- the gauge came off full hot, enough so that the idiot light went out, then it dropped more, now back into normal range.. what the heck?? LOL - no more oven cleaner on the sensor! .. course- I DID wash it all down hard with a garden hose.. coulda done it I suppose- washed some junk over the sensor top and lowered the resistance?? Now it's drying out?? ha - quite a scare tho-
 

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Temp. unit grounded, the gauge goes hot. I don't remember where the idiot light ties in, but it is separate. Grounded, it comes on. Is it possible that they both grounded at the same time? Oven cleaner probably isn't the best thing to use as a degreaser, especially around sending units, relays, and the like.
 

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Update

Thanks Oldrebuilt- I am now a believer- LOL. Well, turns out this issue was a faulty Engine Overtemp sensor that I sprayed with oven cleaner and then a garden hose (stream). I likely damaged it. It worked off and on after drying out, producing the symptoms mentioned.

Still- because the gauge was going crazy, I replaced the actual Water Temp Gauge sensor (single wire on this truck) located to the left of the AC compressor in a water jacket. This is the smaller of the two sensors. Made no change. I then replaced the larger overtemp switch above and to the front of the driver side exhaust manifold (the one I sprayed). Bingo-perfection. This is a large sensor, single wire, requires a 15/16 to remove. I still don't get how it interacts with the temp gauge sensor, but it definitely does. Hope this helps someone with temp gauge issues.
 

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I could be completely wrong, so don't hold me to this, but I believe that the smaller temp. sensor is the one that controls the fast idle/timing advance for when the engine is cold. The only one I ever messed with was on the 89 that I had. It was located driver side rear in the head. there was another one driver side front as well, and then the one for fast idle. The 89 didn't have any timing advance. I really don't remember the locations, (it's been abut 15 years, and I've slept since then) but I believe what I told you is right.
 
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