2005 sport trac
4.0 SOHC, Auto, 4WD, 220k miles
11-04 build date

Check engine light came on.
ECT sensor came up bad. Antifreeze is past due for a change anyhow.
Ordered Motocraft DY1269 Sensor (for computer, grey)
Motocraft SW6146 Sender (for gauge, brown)
Motocraft RT1167 Thermostat
Motocraft RG637 Thermostat O Ring
Standard stuff....drained, flushed, changed parts, no real surprises, with one exception.
The Thermostat Housing has only one sensor in it. The Grey one.
It has 2 sockets formed, the one on the LEFT (drivers side) is blocked off.
Apparently the part with one hole is RH258.
https://parts.ford.com/shop/en/us/co...onId:141077586
OK. No problem. The computer sensor is bad. The computer sensor is there. Replace it.
Filled with coolant, burped the system the best I could. Took well over 3 gallons, including the overflow bottle.
Initial test drive. Vehicle made tons of heat. Temp Gauge read about 1/3 into the normal.
Check Engine Light is gone. Neg Battery cable was disconnected, not a surprise.

Here is the problem.
A couple hours later, I go for a short trip and the temp gauge reads nothing. Truck makes heat.
I get home and leave it running. Hook up ELM327 and Forscan.
ECT reads 198 degrees. No relevant codes show up. (P1000)

I can find no other information on this particular setup.
There are 2 theories here.
1) The ECT sensor somehow also talks to the gauge. If this is true, then why does the ECT show 198 and the gauge show zero?
2) There is a dedicated gauge SENDER someplace buried under the intake or some other inaccessible location.

Anyone have any knowledge of this somewhat odd setup or maybe know if there is a sender somewhere i haven't found?
Many Thanks