Calling all HMMWV gurus! I need help!
So, my 1990 with a 6.2L has run with no problems for the 2 years that I have owned it. Out of nowhere, the truck stopped cooling properly and will eventually climb to 240 on the gauge if I do not shut her down or pull the TDM wire. It takes 10-15 minutes of hard driving before the truck spikes over 220. Once I disconnect the wire to the Time Delay Module, the truck starts cooling down immediately and will drop below 200 degrees with the fan continuously operating. So the long and short of it is that the fan is not kicking once the engine exceeds ~215 degrees. I have confirm the temp gauges accuracy by surveying various parts of the cooling system with a laser thermometer. I have researched this issue as best as I can and employed all the great advice on this message board to no avail.
I have performed the following maintenance (in order):
1. Replaced the coolant expansion tank cap. Result: no change
2. Replaced the thermostat and topped off the coolant (coolant looks okay and was serviced by the Military in Oct 2015). Result: no change
3. Swapped in 2 different TDMs from two of my other Humvees that operate as they should. Result: no change
4. Swapped in a NOS cadillac valve (then tried all three TDMs). Result: no change.
So I am at a loss. I do not believe it is a water pump/liquid cooling system issue as the truck runs plenty cool with the fan constantly on. What else could be responsible for the fan not kicking on? (I have confirmed that the fan extends and engages when I disconnect the TDM). The only item I have not tested is the temp sensor on the water crossover.
Thoughts?