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Transmission Discussion of preventative/corrective maintenance and other technical issues regarding your coach's transmission.

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  #1  
Old 2 Weeks Ago
Mustang88's Avatar
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Default Allison MT-643 Leaking @ Pan Seal / Modulator

The transmission (MT-643) is leaking fluid transmission fluid - appears to be around the rim of the transmission oil pan and from the o-ring seal for the throttle modulator.

I replaced the internal filter and installed a new gasket in 2015. There was a leak before I replaced the filter/gasket and it remained afterwards but was not much. I also replace the o-ring on the modulator at that time. Leak is more noticeable now and needs to be fixed. Looking at the pan, the sealing surface does not look flat / straight. What is the best way to fix this... or should I try to find a new pan. There are no sighs of cracks in the transmission and fluid is red, after cleaning fluid is only dripping from aft edge of pan.

How often should the internal filter be changed - and can I re-use the gasket?

I do not have a secondary filter on the transmission - should I install one?

What is the correct size / type o-ring to use for the modulator?

Thanks,
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Old 2 Weeks Ago
markusfmeyer markusfmeyer is offline
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Every time I change the trans fluid I also do the filter and gasket, usually come as a kit. I change my external filter annually.
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  #3  
Old 2 Weeks Ago
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Take the pan off and use a hammer and a big block of wood and massage that lip.
Lots of times the lip gets bent around the bolt holes from guys that don't know how strong they are.
They dimple the pan around the holes and the gasket will not seal.
Looks good under there,and the pan has no rust,get a gasket and get to work!!
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Thanks Randy and Marcus. Much appreciated... got to be ready to travel when the travel restrictions get lifted here.
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  #5  
Old 2 Hours Ago
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Default MT 643 Governor Filter

Cleaned up my Transmission, change the internal filter, replaced the pan gasket after straightening the rear corners a little bit and torqued all 21 bolts to 12 inch-lbs - no leaks now! No pictures yet - still trying to get it painted again.

Found this link to a copy of the MT 600 Service Manual - can be downloaded (for a price, but could read it online (didn't find link to this document in Allison Transmission Documents sticky).

https://www.scribd.com/document/3260...-SM1317-199907

Service Manual recommends cleaning the Governor Filter - easy enough to do, but it is a small, somewhat fragile steel mesh filter so must be careful. Pictures included to help
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