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HT-741 Thermodynamics - Printable Version

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HT-741 Thermodynamics - 77newell - 05-08-2018

I've learned a lot since I've wrestled with two instances, on pretty much level ground no less, of the trans temperature gauge indication very slowly but persistently rising until I got uncomfortable. I had the transmission checked by 2 Allison service techs who found the transmission to be in excellent condition and operating just fine. So I changed out the trans temperature gauge and sensor. I've subsequently driven two 230+ mile legs (length similar to when this problem arose) with nothing but sensible readings. Along the way with lots of research I've learned a lot about how all this stuff works together for managing the transmission operating temperature. What follows is for posterity.

Transmission heat is exchanged between a flow of transmission fluid flowing from the tranny through a coil in the bottom (the coolest part) of the radiator. Since these transmissions don't work as well in really cold weather (in fact they may refuse to shift into gear at all) using warm coolant from the engine allows faster warming of the transmission to an acceptable temperature. Then as the transmission heats up due to driving it is then cooled by the engine coolant. The amount of trans fluid flowing through this circuit increases a lot when the converter or retarder is active since these services generate a lot of heat.

The temperature reading on our dash is reporting on the fluid coming out of the transmission after that fluid has passed through the converter, whether active or not, and is on its way to the radiator coil for heating or cooling. When that fluid returns to the transmission its pressure is regulated and then used for lubrication after which it drops into the oil pan for recycling by the transmission oil pump. The external oil filter operates on an entirely separate branch of flow off the oil pump. There is a sensor in the oil pan that is used by the ECU to trigger the check transmission light and prevent shifts when it senses above 270F.

On a 70 degree day on flat conditions at 62mph the temperature gauge varied between 135-150G with the Silverleaf reporting an engine temp of 178-182 and a manual IR gun check of 116F at the bottom of the radiator (obviously is was not while on the highway, I can't run that fast especially while reaching under the back).

Thermodynamically, the engine and transmission are a complex network of heat loads and rejection. In highway driving conditions, heat is added to the engine coolant, in order of occurrence, by the transmission coil, the engine oil cooler, and the engine water jackets. Depending on the coolant temperature the thermostats then bypass some of the coolant around the radiator in an attempt to maintain a temperature of 180F at the inlets to the thermostats. There is no thermal control of the transmission fluid except through the relative temperatures of the of the fluid and the engine coolant at the coil in the bottom of the radiator. Ordinarily, the heat generated in the transmission is quite low compared to the engine so the transmission has little overall effect on coolant temperatures, except when the converter is active and is so for an extended time such as climbing a mountain. In those circumstances the engine is also generating a maximum of waste heat so the combination can exceed the radiator's capability. In those circumstances we need to slow to the point where the transmission will work in a converter lockup mode to minimize the heat creation.

And this brings me to the latest thing I've learned. On my dash is an indicator telling my the gear selected on the gear selector panel and the gear the transmission is currently in (labeled "attained" on the indicator). Between those two indications is a symbol I never noticed before that tells if the converter lockup is on or off. This is really helpful in knowing the full shift status, especially in long uphill pulls as noted above.

I've also learned there are three shift patterns ( I thought there were only 2) for the HT-741 for on-highway use:

1C-2C-2L-3L-4L  Used primarily on buses and RVs

1C-2C-2L-3C-3L-4C-4L  Used primarily on trucks

1C-1L-2C-2L-3C-3L-4C-4L Used primarily on high gross trucks

I've experienced the top two and personally prefer the second one, the shifts are less harsh and the engine RPMs are kept higher and there is less a sense of lugging. The second one will shift quickly to lockup mode with light throttle so not a lot of fuel mileage is lost with a light foot on the throttle if desired.

Now I know more about this than I ever knew I wanted to know. If my explanation is somehow not clear, no big surprise here, please feel free to ask.

RE: HT-741 Thermodynamics - 77newell - 05-08-2018

I should also note that the mechanical versions of this transmission, the HT-740 and HT-750, have heat flows identical to what I've noted above with the only difference being that the mechanical transmissions don't have the "check transmission" light trigger (as far as I know, it's been too long since I owned the 77 coach).

Finally, these are elegantly designed mechanical devices with a ton, almost literally, of moving parts with fluid flying all over the place at various pressures. Really amazing stuff.

RE: HT-741 Thermodynamics - Fulltiming - 05-08-2018

Jon, thanks for the information.

The HT-740's in Newells typically only show the gear selected, not the gear obtained. Since they are mechanical, no information shows on the VMSpc related to the transmission either.