Gearbox overheating
Q I have heard stories about potential failure of the oil cooler within the coolant radiator on my 1998 Astra 1.8 auto estate. To avoid any future problems, I have mounted an oil cooler in front of the existing radiators (coolant and aircon) along with a fan for the cooler. I have fitted temperature switches for the fan incorporated in the pipework. The thermostatic switches come on at 105°C – I chose this temperature as I thought that if the existing system uses the coolant to cool the oil it shouldn’t rise above this. Monitoring the oil temperature, it hasn’t gone above about 84°C, but I would expect this to be higher in warmer weather. I guess that, while on the move and with the torque converter ‘locked up’, the temperature would be fairly constant, rising with acceleration and in slow-moving traffic.
Is there a good working temperature for autobox oil? Alan Worland
A The normal running temperature of the automatic fluid should be around the 80°C level. This is the optimum operating temperature of the transmission fluid to ensure its longevity.
The temperature of the fluid can easily rise during normal driving conditions due to the friction experienced within the torque convertor and clutch packs. Once the temperature of the fluid begins to rise, the transmission fluid will oxidise, giving it that brownish tinge and a slight smell of burning. When this happens, it will begin to deposit a varnish-like substance on the internal components of the gearbox.
Fluid which continually runs at 95°C will last around half the time of fluid kept to around 80°C. This will be further reduced if the fluid is allowed to reach 110°C. If the temperature reaches 125°C the internal seals will begin to fail. As it is not always possible to maintain the optimum temperature, transmission fluid should be changed at regular intervals to maintain its integrity.
The existing cooler is placed in the base of the vehicle’s radiator, which should be the coolest section of the radiator, at the point just before the coolant is reintroduced into the engine. With an engine running at an average of around 90°C, the coolant at the lower section of the radiator should average around 8°C lower than the engine temperature. If you can maintain the 84°C you have at the moment, this would help protect the oil and the gearbox from undue stress.