Transmission Oil Cooler Failure

The video below shows the condition of the engine coolant after the failure. The flushing of the cooling system takes a lot of time with at least 3 to 4 flushes which involves running the engine to a reasonably hot temperature using water and dish washer soap, thermostats removed….rinse and repeat.

The transmission oil on automatic transmissions need to be cooled and the best way is to run through the lower rad tank which is a separate core that allows the coolant to remove heat from the trans oil since engine coolant temperatures run lower than transmission temperatures depending on the conditions.

Automatic Transmissions running over 200 degrees is not uncommon and they can go much higher than that without causing damage. The cooler keeps the temperature down to a reasonable operating temperature which the factory engineers have calculated by the size of the cooler core and the estimated load on the transmission.

How The Transmission Failure Transpired

The driver of one of our 2008 Thomas C2 Saf-T-Liner school buses came in originally for an engine code that turned out to be a low coolant level and high coolant temperature. The sensors were not reading properly because of the contamination. That made perfect sense after observing the brown sludge that came from the coolant fill tank.

The surge tank was over full with additional transmission oil. The dipstick on the transmission side was not as bad as the coolant but the thought of what the transmission condition was in definitely crossed my mind.

The remanufactured price for an Allison 3000 series unit runs about $5600.00 so we were determined to try and save it at all costs even if it meant flushing it twice with synthetic ATF. We had this same incident happen before on another bus the exact year and specifications. There is a third bus in our fleet that was purchased along with these 2 units so a new radiator is coming to be replaced on the third bus asap. That’s why we call it preventive maintenance 🙂

The transmission on the first bus is still operating perfectly and I’m not saying that it will last as long as it would without this kind of failure but if it’s still giving us trouble free service. I’m all for giving it a college try and spending a few hundred bucks on ATF and filters.

Synthetic Transmission Oil Well Worth It!

Synthetic ATF is well worth the extra cost. We perform less services (every 3 years) and there have been no transmission faults until the radiator transmission coolers failed. An indirect failure and even after all the abuse the transmission received from coolant in the oil it’s still going strong. If you aren’t using synthetic oil look into getting it into your maintenance program.

Thanks for reading this post and I hope it has given you enough cause to check out your C2 conventional bus radiators and cooling systems. The failure usually happens suddenly without warning of sludge starting to form in the radiator surge tank.

These buses were manufactured in 2008 so I can’t be sure if the year of the bus is a factor. If you do see sludge building in the surge tank and the transmission oil level dropping you will be needing a replacement radiator.