Hard Shifting into 3rd 4th & reverse

[royalglen]

I am having trouble with 1990 chev 2500 Muncie 4 speed (m20??) not easily shifting into 3rd & 4th and reverse. It seems you have to wiggle the shifter into the shift gate and wiggle it into reverse. The problem happened all of a sudden so its not regular ware. It is a snow plow so we need reverse quite a bit. With the shift stick out you can site down and see the shift gates and they look lined up OK. Need help. Any ideas????

[DmaxMaverick]

It may be more simple than it appears. When it gets cold and the shifts get hard, it's time for a lube service. Replace with a GOOD synthetic lube and it should improve. If not, it's time for overhaul. Synchro's are gone. Continuing will only make it worse, and the overhaul will get more costly.

[royalglen]

Not what I wanted to hear but you are probably correct---- Syncro ring may be broken on 3rd. Strange that it effects it getting into reverse ?? Any ideas .

[DmaxMaverick]

Are you sure it's an M20? I thought they quit those in the 70's. The M20 (or 21) seem awful light for a heavy truck.

If it's an M2x, reverse isn't synchronized, or none were that I recall. The boxes are very tight, in that there isn't much room inside between parts. If something is actually broken, like a synchro, it may be interfering. They usually just wear beyond a point of engagement. Or, the reverse gear is worn flat, making engagement more difficult ("can't find 'em, grind 'em"). Frequent reverse use, such as plowing, would exaggerate that. I haven't messed with Mxx trannies in years, but did a bunch when I did (mostly rock crushers).

Switching to synthetic lube after running dino lube for years will usually cause the synchro's to quit. Worn too smooth, with lube too slippery. Also, lubes, even GL-5, labeled "no friction modifiers necessary" will also cause issues with synchro's (the friction modifier is for L/S rear ends, but works against you in a synchro'd tranny).

[royalglen]

Thanks for your advice ---I will drive it a little and take it out and repair what it needs----I always like to know where I am headed before I strip it. It just seems hard to find the shift gate for 3&4&reverse but maybe a broken syncro ring would not allow proper alignment of collars ----When I find out I will post results ( maybe ask for a loan $$$$$ )

[DmaxMaverick]

Now I'm curious......

Unless you installed an M2x tranny in the truck, it doesn't have one. If you did, it was a very poor choice, in my opinion, considering the capacity of the tranny, and the odd fitment, especially in a 3/4 ton 4x4. I don't even know what or how any transfer case would match, unless it were divorced.

OK. Assuming you do have an M2x tranny, there are a couple things that will cause what you are seeing, not related to internal problems. Late model GM HD trannies are top loaders. Meaning, the shifter is part of the tranny, and the shift lever actually enters the tranny at the top. The Muncie M2x trannies are not top loaders, and have a stand-alone shifter, that operates the 3 input levers on the side of the tranny. Having trouble finding the "gate" of a gear is almost always a shim in the shifter, and the only current replacement I know of is from Hurst, but only for complete compatible shifters, or parts for currently offered models. Parts are not compatible with older models (1960-70's). Many shifters from Saginaw trannies of similar capacity will fit the Muncie trannies. The Saginaw tranny cases of similar capacity were cast iron, vs. the aluminum Muncie case. Another possibility would be the linkage input pins and/or the input levers. The tranny would be near/over 40 years old, so the pins and/or levers would be worn very badly (read: very sloppy).

The M20's and M21's were actually weak transmissions, compared to the HD light truck trannies that were used. The M22 was much stronger (nicknamed the "Rock Crusher"), but still not "heavy duty" by truck standards.