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Transmission overheat

Tuesday, December 2, 2014 - 09:00
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In a previous newsletter called “A Whale of a Problem”, the subject of fluid blowing out of the vent or filler tube was discussed. One of the causes can be due to an over heated transmission. With several possibilities being the cause to transmission fluid overheating, the subject was reserved for a newsletter all its own.

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One cause can be attributed to the torque converter which is a general way of saying in one broad stroke, a malfunctioning torque converter, an issue with the operation of the converter clutch, or a problem with the hydraulic circuit of the converter or the converter clutch. These possibilities would have to take into account the converter itself as well as converter related bushings and sealing rings. This would also include valves such as a TCC regulator valve, switch valve and checks valves. The filter and main line pressure control can also be contributing factors. If main line pressure remains very high, this will restrict or shut off converter flow generating high temperatures rapidly.

Cooler bypass valves are another possibility. These can be located inside the transmission (figures 1 and 2), on the transmission (figure 3), somewhere in the cooling 

line between the transmission and radiator (figure 4), or they can be in the radiator itself (figure 5). When these malfunction, they prevent the transmission fluid from being cooled by the radiator or heat exchanger.

Radiators (figure 5) and heat exchangers (figure 6) can be another cause to transmission fluid overheating. If the cooler flow itself was restricted where it could not pass through the radiator or heat exchanger to be cooled, for some configurations, this would negatively affect lube causing catastrophic failure to the gear train.

Another issue that comes up with the radiator or heat exchanger is cooler flow is perfect; nothing wrong there. The problem is the coolant passages are restricted preventing the coolant from extracting the heat from the transmission fluid.

It’s quite a laundry list of possibilities and a general one at that as many transmissions have their own unique designs preventing the list from getting into the finer points. One example would be how Mitsubishi had issues with their Montero vehicle overheating when under heavy load conditions or when trailer towing. They provided a new expanded capacity AT oil pan kit to resolve this issue. It included an expanded pan, a larger oil filter and a lengthened dipstick. Other manufacturers had similar issues where the sump would run low which would interrupt cooler flow. It is always a good idea to check for any factory bulletins regarding over heat problems which will focus on their unique system. And although this is a general list, it still is a comprehensive one and should aid in diagnosing an overheat problem should one roll into your shop.

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