BW M12 Shudder

Had an issue with my BW M12 (possibly M11B) shuddering under hard acceleration, i.e. at the drag strip. The transmission fluid appears to be full of friction material. Looks like I will be pulling it our for another rebuild, any thoughts on the cause and possible remedy. The transmission hasn't done a lot of work since the previous rebuild.

I've only worked on the smaller cousin to that but if the clutches are slipping it kind of indicates the pressure is low. Do they also have rings that contain the oil from bleeding off? Maybe one got broken during assembly. Leaking servo? Crud in the valve body? The TSM probably asks for a pressure check and I would do that before taking it out of the car just to see if it is low and again when re-installed.

IIRC the TSM does have some pressure checks... look in the troubleshooting section, but also through the "service in car" section.

How much power is in front of it? It should handle a bit more power than stock, but if you think you have over 450-500 hp at the crank that is about the limit. That and running hard combined will kill it quick. Are you running a cooler other than the stock radiator tank cooler? If not you need too! Heat kills an auto trans quicker than anything else, and drag racing will produce a lot of heat. The more it slips (regardless of reason) the more heat and the shorter the life span.

Could have been old stock clutches not stored properly, old stock rubber servo seals that hardened quicker than new production would, bands not adjusted properly, or springs in the pressure regulating part have lost tension over time.

If you're going to race the thing a lot you really need to invest in a later model trans, either a built 904/998 (preferred for drag racing but can be costly), a 727, or a Jeep TH-400 converted to 2WD. Rear drive AMC trannys are getting hard to find, but the full size Jeep Wagoneer/Grand Wagoneer was made through 91. The 4x4 trans (TH-400 through 70-79, 727 80-91) can be converted with standard GM or Chrysler parts -- the trannys are the same except for the case and bell -- but has to be torn all the way down to change the output shaft. You probably knew that, just adding for other readers.

There's an old Hot Rod magazine (or one of the rodding rags) article about it, but it's really an "infomercial" article for B&M. Goes on about a B&M spring kit and converter. You can shim the main pressure spring for more pressure, or replace it with something a bit stiffer. Someone had the number of a hardware store spring that helped a few years ago, but no telling what the current spring is made of. Might not hold up long term, might not even be any stiffer now.. if the same company is even still in business. I didn't keep the number for that reason.

The stuff vinny has been posting on rebuilding his M-35 will work on the M-11 also. They are basically the same trans -- at least the rotating assembly. The M-11 is heavier and stronger, and therefore a little larger in bulk, but made the same way. There are some differences in the VB, but not much. Main difference is the cast iron case vs. aluminum. One of the servos pushes against the case. Too much pressure and an aluminum case will crack in that area. I had one with a defective VB do just that. I'm sure there are other areas the case needs to be stronger to handle more power as well.


Edited by farna - Nov/24/2014 at 4:37pm

Dexron is what I ran in my M-35 for 14 years, putting 6-7K miles a year on it.

What you really need is a trans book published by one of the trade publishers. Something like a Mitchell's Professional manual. Would have to be an old one, of course. I think you could go back as little as 82. AMC stopped using BW autos in 72, but a lot would be on the road and needing maintenance at least 5 years later, most likely as much as 10 years after (1982).  I don't have one, but I have seen one in a trans shop. Hmmm... actually, I think it was in the auto hobby shop at Mtn. Home AFB, ID. They had a Mitchell's Professional set for us GIs to use.