[TGDianics]

In response to Chester's post:
(In a long downhill run, such as the 40 miles between Reno and Sacramento, the engine is basically idle but the transmission overheats due to the constant use of the Jake Brake, like 250 degrees. That is when it would be useful to have a switch to bypass the fan thermostat and cause the fan to cool the transmission.)

I am unable to find a "Jacobs Brake" connection to transmission or engine overheating. Are you using a retarder?

[Chester Stone]

I merely relate my experience and guess at what was taking place. With the fan shut down during the 40 mile decent, the Jake was operating on all cylinders and the transmission downshifted to 4th gear. I am not aware of the transmission having a retarder. Of course the engine temperature was below 190 and the transmission, with synthetic oil, gradually got up to 250 degrees. I can only assume that there was not enough air across the transmission section of the radiator to keep it cool. I suspect that the transmission would have been cooler had the fan been operating, which a switch on the dash would allow. On many other occasions while traveling in the Western mountains, the transmission heated up on long downgrades.

[77newell]

Chester: I am not disputing your observation, I accept you saw what you saw and am only curious to explain what brought that about. Two thoughts:

1) If the World Transmissions are plumbed the way my HT-741 is, the temperature on the dash gauge will be the temperature of the oil coming out of the converter and going to the cooler. The reason for this is that converter use is the most common source of heat generation in the transmission and the gauge gives us an indication of when we should back off the throttle since hard throttle is the usual trigger for the converter. I can only wonder if the downshift somehow managed to force the converter to unlock. I wouldn't think that would happen but maybe is does for reasons I can't conjure.

2) My HT-741 runs the transmission oil through the bottom tank of the engine radiator. If the newer coaches have completely separate radiators for the transmission oil then there are a variety of possibilities such as lack of airflow that excessive heat could overwhelm.

[TGDianics]

I merely relate my experience and guess at what was taking place.  With the fan shut down during the 40 mile decent, the Jake was operating on all cylinders and the transmission downshifted to 4th gear. I am not aware of the transmission having a retarder.  Of course the engine temperature was below 190 and the transmission, with synthetic oil, gradually got up to 250 degrees.  I can only assume that there was not enough air across the transmission section of the radiator to keep it cool.  I suspect that the transmission would have been cooler had the fan been operating, which a switch on the dash would allow.  On many other occasions while traveling in the Western mountains, the transmission heated up on long downgrades.

Makes sense. I like the switch idea.

I have been unable to determine if the torque converter plays a roll in trans overheating. Should it be locked or unlocked with JakeBrake use?

[77newell]

The function of the torque converter, aka converter, is to multiply engine torque delivered to the wheels when speed is modest for the current gear and a lot of throttle is input. When active, the torque converter is using pumping vanes to sling fluid through stator vanes to turbine that receive this high speed fluid and convert into torque at the entry to the transmission. All that shooting and sloshing of fluid heats it up. If, with the Jake active, the transmission has the converter active similar heat will be generated.

If it were me, I would try to ensure my converter was locked up when using the Jake. Honestly, I've never paid any attention to this yet, but I probably will from now on.

[Chester Stone]

I should mention that I discussed this heat situation with an Allison dealer and was told not to be concerned if the temperature goes to 250. Subsequent testing of the synthetic fluid reported that everything was normal.

[77newell]

What Chester said about heat limits is consistent with what I've learned from Allison sources. The temperature limit at which the transmission begins to limit operations is 270 degrees as measured in the sump. the temperature reported on our dash gauges is the temperature before the fluid goes to the cooler and is almost certainly the hottest point in the circulation, and is certainly hotter than the sump. I've heard that 350 or 375 is OK on our dash gauges, though frankly that would freak me out. It's nice to know the transmission itself has a temperature sensor linked to the ECU to warn us via the "check trans" light before things get expensive.