Hopefully this is helpful; I hope you are monitoring your retarder temperatures with a retarder temperature gauge. The transmission gauge never shows what the retarder gauge shows. It is much faster to rise up into IMO higher than acceptable temperatures. I doubt it will hurt the retarder, but the oil can be damaged operating at hi temperatures. I was truly surprised to realize the difference between dash indicated and actual temperatures once I got a actual retarder operating temperature indication. Yes the retarder is a wonderful tool. Hi rpm is your friend.
Scott
I'll second Scott's advice. Before I had a VMSpc and could see the retarder temp I used the trans temp for the information. After going down the same grade I had in the past and using the same gears and retarder as I always had the retarder temp was at 260 deg. and climbing before I shifted down 2 gears and started using the brakes periodically. I had no Idea how hot the fluid had been getting so I changed it out with fresh Transynd before this trip even though I had changed to Transynd fluid when I bought the coach.
This begs the question: With Transynd transmission fluid, what should be my personal caution and red line temperatures for the retarder temperatures?
All I could find was opinions, some seemed accurate and some were crazy low. it also depends on how long the temperature stays at the high temp.
What is max Temperature for Transynd in Allison 1000 - iRV2 Forums (https://www.irv2.com/forums/f125/what-is-max-temperature-for-transynd-in-allison-1000-a-160596.html#:~:text=I%20developed%20TranSynd%20for%20Allison,temperature%20is%20330F%20)(intermittent).
Since temperature-caused degradation of transmission fluid is cumulative, not sure there IS a finite answer.
I can tell you that Allison considers any of their transmissions with a retarder as SEVERE DUTY, and fluid recommendations (Transynd or perhaps the new fluid they just released) and filter change intervals are quite a lot more frequent than for the same transmission without the retarder.
As already stated, use high engine RPM when using the retarder for lower fluid temperatures.
What I have read about Transynd is that occasional high temps (275°-300°) will not hurt the fluid. 250° is OK. High fluid temps for extended periods will shorten fluid life and very high temps can start to damage the transmission.
Retarder temps tend to run about 10° above transmission temps until you start using the retrder then they go up very fast. I set the yellow warning light on retarder temps on VMSpc to 235° and the red warning light at 250°. 235° is conservative and just lets you know that some change in operation is indicated.
Using the brakes with the retarder on will spike the retarder temp. Using a notch or two on the joy stick and a lower gear will mange speed on long downhill grades with very little service brake use and keep retarder temps around 235°. Going to a lower gear increases engine and transmission cooling and helps manage speed in conjunction with the retarder.
http://ddwt.us/Allisson_4000MH_Transmission_Owners_Manual.pdf
My personal high temp cut out is 250 deg.
Here is a chart to go by for conventional fluids.
Pierce
Split side discussion of fluid temperature out from air leak topic. Thank you to the member who reported it - Michelle
I've read this here, I think. And have verified that when the retarder temp as reported by VMSpc gets to 254º or a bit above an internal valve will open and flush the hot coolant through the transmission. I've seen it a couple times and the temp will drop down to 220º or so. Not positive of the temps but it seems to be a design to protect the coolant and tranny.
I agree with the above and is how I have found to best manage retarder temp, transmission temp, gear selection, and speed going down hills. 2 clicks on retarder, lower gear and sharp/quick brake pressure if needed but if gears and retarder are correct you usually don't need the brake.