[Johnc]

Normal Allison HD transmission operating temperature range is 160F to 200F. Anything over a sump temp of 250(f) is considered excessive. On newer transmissions the ECU will inhibit operation in higher ranges if sump temp reaches 262 F

[folivier]

I did clean the radiator with simple green before we flew back home so hopefully I'll see lower temperatures.
Is there anyway to tell whether I have Transynd or not?

[Johnc]

That's a very good question. You probably can't use color because of the mixing of the new with the old. Transynd is noted for its extreme heat tolerance (330F intermittent), so perhaps a frying pan heat test? Take a sample from your transmission and compare it to a standard oil sample as you heat the pan? I have seen heat tests with synthetic versus mineral engine oils, and the time difference in the oil starting to darken is fairly significant. Be careful whose frying pan you use

[LynnJeff]

Forest

We cored the radiator on our bus after you were here and took a 1000 mile "shakedown" trip. Tranny (740 4 speed ) had been running under 200 but on this trip ran up to 245. I talked to Allison and they told me anything over 220 and the fluid "cooks" and looses its viscosity. I thought maybe when we put the radiator back in that they had blocked a line (transmission coolant runs through a separate section in the main radiator) Long story short, we dumped the tranny fluid and also took the pan off and cleaned the filter in the bottom of the pan (which a 740 4 speed has0 that had some gunk on it. Took it out for a hard test run and firewalled it for 100 miles in 93 degree weather and never got above 190 with new Transynd and cleaning of the filter. Not sure if your 6 speed has a filter in the bottom of the pan (Allison could tell you), but if so, it may be partially clogged as mine was which would be an easy fix versus a new transmission.