'78 F150 4x4 351M-400/C6

The previous owner had an aftermarket temp gauge screwed in under the ashtray. I hate it and want to get my instrument panel gauge working.

Whoever put it in ran a metal braided sleeve w/wire to a screw in temp sensor at the top of the waterpump.

The Chilton wiring diagram shows the Red/White going from the instrument panel to the coolant temp switch. I assume it's snipped off somewhere outside the firewall, but haven't located it yet.

Motorcraft has this available:
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SW2328 Coolant Temperature Switch
TEMP SENDING UNIT - SINGLE CONTACT - GREEN INSULATOR 1.5" LONG - w/GAUGE.
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Assuming I can locate/get continuity/or splice a section of wire back to the instrument wire harness, would the above SW2328 W/Gauge be what I'm looking for to get the instrument panel temp gauge working again?

In other words, if the gauge stopped working and they replaced it w/aftermarket, wouldn't it seem logical I need to replace the gauge on the instrument panel?

 

Probably NOT. You need to know what the voltage to temp numbers are for the original gauge sending unit. ie: 1v=10degrees - or whatever that happened to be. I would go either to an auto graveyard and get one from alike model year vehicle or order one from Ford or other vendor. Yeah, I know...are they even available???? That is prob why the previous owner installed the aftermarket one. Sorry!

 

The factory gauges are very reliable and usually are not the problem.

I would find the wire. Once you find the wire, turn the key to run without starting the engine, and touch the wire to the engine block. The temp guage should peg full scale in one direction. Then take the wire off the engine block ground and let it hang in the air touching nothing. The gauge should swing full scale in the other direction. If it passes this test, the gauge and the wire are good, and all you need is the sensor. Make sure you do not use teflon tape on the threads, and very little sealant if any. The threads need a good ground to the engine block.

 

Thankyou, I'll try that. Now to find that wire...they're all a dirty black right now, so I've got some cleaning to do. Hopefully I'll just run into it.

 

351M/400 temp sensor is on the front of the block on the passenger side, near the top and
also near the water pump.

It's a PITA to get to, will probably require removing the smog pump & alternator.

On my 1981, the wire is contained in the same harness that contains the other sensor &
ignition wires... The temp wire is something like red/white stripe (your book is prolly correct)
and the oil pressure is the opposite - white/red stripe in this example. Problem is, it's
sometimes awfully difficult to discern which one is which, they both look very similar. If you
have one going down near the at the front of the engine (in the same harness with the dizzy
wires) that's it; the OP sender is on the top rear of the engine, behind the carb.

 

You can run the same "ground the wire" test on the oil pressure wire. So that may help you figure out if you have found the correct wire.