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1940 Transmission issue
#1376504 Wed Sep 09 2020 04:25 PM
Joined: Aug 2004
Posts: 56
Gelee Offline OP
Wrench Fetcher
1940 GMC 3/4 ton transmission keeps locking up. I have to remove the shifter unit and somehow get gears back in proper position. Anyone experience this. 4 Speed.

Re: 1940 Transmission issue
Gelee #1376515 Wed Sep 09 2020 05:24 PM
Joined: Jul 2018
Posts: 48
Wrench Fetcher
I am not familiar with your particular transmission but is it a "top loader" or does it have "levers" on the side. The gm 3-speed transmission can experience the same type of problem. Sometimes this can be from your linkage not being adjusted properly thus allowing you to put one in gear while the other gear is not fully out of gear. I know its a little confusing but def check your linkage adjustments.

1948 5 Window Chevy 3100 an absolute rescue that is now back on the road
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1956 3200 Big Window

Re: 1940 Transmission issue
Gelee #1376516 Wed Sep 09 2020 05:30 PM
Joined: Feb 2004
Posts: 21,062
If you have the "crash box" 4 speed with the reverse lockout lever on the shifter, those transmissions are pretty flimsy, and are subject to wear. Chances are the shift rails in the top cover are the problem. I'd suggest removing the cover and disassembling the shift rods, looking for wear and/or broken parts such as springs and detent balls. It's also possible for the bearings and shift collars in the main gear set to get worn and hang up while they are being moved in and out of mesh. That box has no synchronizers to give trouble, which can be a good thing!

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Re: 1940 Transmission issue
Gelee #1376536 Wed Sep 09 2020 08:19 PM
Joined: Apr 2007
Posts: 791
Shop Shark
In addition to problems with shift rails mentioned by Jerry, the shifter guide plate riveted to the shift tower may be worn. The guide plate is made of mild steel and they seem to wear a great deal. To change the guide plate you grind off the rivets holding it to the tower, however, I have never seen a new guide plate for sale, either new or new old stock.


1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix
Re: 1940 Transmission issue
Gelee #1376541 Wed Sep 09 2020 08:35 PM
Joined: Feb 2001
Posts: 1,136
Shop Shark
I have taken the guide plate off , welded the corner shape and filed it square again , works real good now . I've done 80k miles since and no sign of trouble .

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