Hi Wayne It's possible that your problem may be a combination of things. I've seen partially clogged radiators that don't let enough heat thru to the fan clutch. It's easy to check for a clogged radiator( no tools req'd ). With the engine at operating temp, hold it at about 3000 rpm for a minute or two then let it return to idle. Shut the engine off immediately and reach in between the fan and the radiator. If the center of the radiator is barely warm or even cool to the touch it's clogged. In that condition it won't let the viscous fan clutch heat up so it's like having no fan at all. I know it sounds scary but it works! As far as your auxiliary fan goes, both fans should work at the same time. It probably has a two-stage setup. The first stage( lower speed) comes on with high engine temp. The second stage( high speed) comes on with high A/C pressure. One other thing to keep in mind. Diesel models usually have insulated encapsulation panels beneath the engine and transmission. If you remove these in the summer it will probably lower the temp a bit. My guess is that you'll need a radiator and a fan motor. Let me know how it goes.