.boB

I returned from a road trip last night. https://www.dieseltruckresource.com/...d.php?t=241235

During that trip, the truck performed almost flawlessly, and returned 18-19mpg. I was quite impressed, and really enjoyed driving the truck - especially through the mountains.

But, the one thing I noticed was that the trans temp never got very high. I never saw it over 100*, and it was usually 75-80*. But coolant temp was a consistant 190-195* in all conditions. 2 weeks ago I noticed the same thing, even towing a light trailer with the winter cover on, blocking the trans oil cooler. This concerned me a bit for a variety of reasons.

This morning I read through the service manual to get an idea of how it works. The fluid moves from the pump, through the valve body, and out the cooler lines. The fluid first enters the water/oil cooler on the side of the block, then to an in-line thermostat, and then to the air/oil cooler behind the grill. If the oil is not up to temp, the thermostat routes it directly back to the trans. The manual does not say how much of the oil might be directed back, some or all. In any case, during light duty operation you would expect the oil to be fairly close to coolant temp.

BTW - The 6.7 does not have an oil/water cooler (heat exchanger).

The test procedure requires the engine coolant temp to be <85*F. Start the engine, and idle in Drive. Oil temp should rise at least 15* in 10 minutes.

My oil and water temp started at 48*F. Within 1 minute, oil temp rose to 95*. Over the next few minutes it dropped down to 52*. When water temp was 85*, oil temp stayed at 52*. They should be within 9* of each other. As water temp rose >100*F, oil temp stayed at 52*F.

That leads me to believe the temp sensor is faulty.

The temp sensor is inside the case, immersed in fluid all the time. It's part of the governor assembly. I have also recently noticed the 2-3 shift shuttle people have been talking about.

In the next couple of weeks, I'll take it back to the dealer and have them replace the sensor and the governor.

Ace

If that is the problem, at least it appears to fail open. But how would you exlpain the fact that it seems to have been closed and opened once at 95*? I'll bet it's an all-or-none cyclic device intended only for cold warmup. i.e., resets each time the truck is shut off and re-opens once after the oil reaches temp. That's probably all it is supposed to do.

Just a guess.

.boB

The service manual gives a chart showing resistance at various temperatures. That would lead me to believe that it's a variable resistor like a gauge sensor, and not an on-off switch like an idiot light.

That would also lead me to believe that it can be simpley tested with a pan of hot water, a thermometer, and an ohm meter. But it has to be removed from inside the pan to accomplish that. Since it's under warrenty, I'm not going to bother doing that.

If the proper resistance is not met at the proper temperature, "Replace the Governor Pressure/Transmission Temperature Sensor per the Service Information. "