Transmission Retarder OverTemp

[davidbrady]

How hot is too hot? I pulled this from an allison HD4000MH manual:

These temps define transmission overheating:

Sump fluid 121°C (250°F)
Converter out fluid to cooler 149°C (300°F)
Retarder out fluid to cooler 165°C (330°F)

A transmission tech at Prevost once told me my 2003 chassis H345 VIP will throw a transmission temp code at 150 degrees C. That must be the converter out fluid to cooler temp of 149 deg C spec-ed in the Allison manual. I think on my bus the retarder out of 165 deg C (330 deg F) is considered a transmission overheat event. I try to keep my retarder below 230 deg F, but I have gone as high as 250 deg F. At around 230 deg F the S60 cooling system fan comes on full speed. The Prevost Tech's also told me if your transmission fluid hits 300 deg F you need to dump it. But, Tom Johnson, the Allison fluid engineer who wrote the TES-295 spec of which Transynd belongs, says Transynd can take 330 deg F intermittently:

http://www.irv2.com/forums/f125/what-is-...60596.html

[ERNIEE]

I have noticed my temps get around 230 when in full retard, then I back off it and it goes back to "normal"

[davidbrady]

Ernie,

I find it nearly impossible to keep it below 230 deg F on a 6 percenter of any length. The fan turns on at around 230 deg so there's lot of thermal mass and resistance to climbing at that temp. I find it usually settles in a little above that temp in fourth gear. But there are some grades where you need more. Descending into Death Valley from the west is an example - something like an 8000' drop in just over 5 miles or so. There I hit 250 deg F but it didn't throw a code as it shouldn't. Coming down Eisenhower Pass east bound on I70 I had to descend at 35 mph to keep it at or below 230. I know today that I was babying it too much and I should have been able to descend at 40 to 45 mph. The transmission can easily handle 250 deg F. That's my new self imposed limit before getting on the service brakes, or attempting to let it cool down.