Allison Transmission Fluid Leak

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During cold weather, a small, red fluid leak may be noticed on some 2001-2010 Silverado and Sierra 2500HD-3500HD series models equipped with the Allison LCT1000 transmission (RPO M74, MW7). The fluid leak may be at the back of the transmission by the vent.


This condition may be caused by fluid being discharged out of the transmission vent during extreme cold weather (approximately -15°F or -25°C or colder), at which point DEXRON® VI and other conventional transmission fluids become extremely viscous.


When the viscous fluid is circulated into the vent area, it becomes trapped and is not able to drain back to the pan. As the transmission warms up, the expansion of the air inside the transmission can force the trapped fluid out of the vent, leading to small drops of fluid being discharged.


To correct this condition, remove the stock vent assembly and run an external tube (Fig. 2, A) and vent (remove the top of the vent assembly) (Fig. 1) up to the front of the transmission to a mounting bracket. (Fig. 2, B)

F01 vent assembly.jpg

F02 vent tube.jpg

The extra length of the tube will allow the expelled fluid to be trapped and later drain back into the transmission.


In addition, on 2001-2004 models, replace the transmission fluid indicator (dipstick) with a new vented indicator.


Refer to bulletin #10-07-30-001A for additional information and installation procedures.


- Thanks to Al Ferry

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This page contains a single entry by Blog Post published on March 1, 2010 11:00 PM.

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