I mistakenly posted a question at on the '80 - '86 forum for this problem and the guys there helped me out. Short story is that for my '88 F-250 4.9/M5OD, the Ford dealer's gauge temp sensor, #F1SZ10884A, and NAPA's and Advance Auto's sensors don't work; the resistance is too high. With those sensors the gauge needle won't ever touch the "N" in NORMAL, even in 90F+ ambient temps. I did find a sensor that works better, a Borg Warner (BWD at O'Reilly's/Checker) #WT386. That sensor reads at "O" when the 195F thermostat opens.

The longer story is that I pulled the temperature/voltmeter cluster and hooked it up to a 12.5vdc power supply and a pot. This is what I got (N, R, and L are the letters in the middle of the gauge: NORMAL.)

C: 32 ohms
N: 20 ohms
R: 18 ohms
L: 14 ohms
H: 8 ohms

I realize that my gauge might be the cause of the problem but these readings are pretty linear so I'm not replacing the gauge. The Ford sensor and the other sensors read > 300 ohms at room temp. Too high. There is another Borg Warner sensor that would probably work even better, #WT324, but the stud size is 10-24 instead of 8-32 and I don't want to have to adapt it; the sensor is too tough to get at to be messing around with it. I found this info at a Ford diesel site F250 4BT Buildup I might play around with putting some resistance in parallel with the sensor lead behind the gauge to get the 195F thermostat opening sitting right in the middle at "R".

I don't want to get an aftermarket gauge because lots of different people are going to be driving the truck, people who don't know anything about cars, and I'll be 10 hours away while they're driving it. I want the truck to look bone stock. Two temp gauges would be a distraction for them.

 

I noticed my cluster also is at operating temp before the N. So I swapped clusters first, didn't change anything. Oil pressure gauge now reads higher than it used to, and with a new oil sending unit I'm guessing my old cluster was worn to some extent.

I have yet to swap the coolant temp sending unit, but I did swap the ECT sensor. That of course doesn't factor in with the gauge reading but when it failed an ohm test I replaced it. I don't think I'm gonna bother with that coolant temp gauge. I also have a 180 stat in my truck so until I switch it back it'll be off anyways.

You know what I'd do? Tell them don't even worry about the temp gauge, it's broke, just drive haha.