papakb
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PARK YOUR HMMWV ADDENDUM 2020
Back in the olden days (2001) I wrote the article on adding park to the HMMWV TH400 transmission in the HMMWV and I’ve seen it republished several times since then. I was recently working with Glen on adding park to his M998 and realized that there were some errors and additions that needed to be made to that original article. So here goes:
After installing one of the scarce 3 speed park shifters out of the civilian ‘91-‘92 Hummers we realized that even though the transmission would go into 1st gear there wasn’t enough travel in the linkage to allow the shifter to reach and latch into the 1st gear position. The shifter would sit between the 2 and the 1 on the gear shift indicator even though the trans itself was in 1st. Glen had found an article on the internet about someone adding an inch to the shifter to resolve this issue and I had to stop and wonder, “Why would you modify the shifter?” There’s a much simpler solution for the problem. I removed and measured the lever arm on the side of the TH400 and found that by simply adding 0.75” to its length would accomplish the same thing and the shifter itself wouldn’t need to be modified. I made the new arm from a 0.1875” (3/16) x 0.75” steel bar stock 4.75” long. The round linkage hole is 0.422” and the rectangular hole where it attaches to the trans is 0.32” x0 .50”. I’ve done this now on 2 trucks and the mod is simple and takes about an hour to install and adjust. It does require you to cut off about 1.50” from the long end of the “J” linkage to keep it from hitting the bell housing.
Something else I found in my truck was that occasionally the trans didn’t always want to go all the way into park and would “ratchet” if the truck moved on an incline. This was caused by my having reversed the small plate that pushes the linkage arm upwards and forces the park pawl into the output gear. It’s a poor design that can be installed either way and it’s important that it gets installed correctly. The image below shows the correct orientation for this plate. In the original article I said to attach this plate with 2, 8mm x 13mm bolts. I stand corrected, use 2, 5/16-18 x ¾” bolts
I was also noting that the 4L80 4 speed shifter may work with the TH400 but the gear indications will be incorrect. Overdrive would become Drive, Drive would become 2, and 2 would become the 1st gear indicated. If the legend wasn’t fiber optic it would be easy to simple cut out the OD position and slide the D up into its place. The original 1st gear position would be unused. I haven’t done anything more than make measurements on this but mathematically it looks like it should work. If anyone has done this please drop me an email and confirm it. THANKS
Kurt Lesser (papakb@yahoo.com) 3/14/2020
Back in the olden days (2001) I wrote the article on adding park to the HMMWV TH400 transmission in the HMMWV and I’ve seen it republished several times since then. I was recently working with Glen on adding park to his M998 and realized that there were some errors and additions that needed to be made to that original article. So here goes:
After installing one of the scarce 3 speed park shifters out of the civilian ‘91-‘92 Hummers we realized that even though the transmission would go into 1st gear there wasn’t enough travel in the linkage to allow the shifter to reach and latch into the 1st gear position. The shifter would sit between the 2 and the 1 on the gear shift indicator even though the trans itself was in 1st. Glen had found an article on the internet about someone adding an inch to the shifter to resolve this issue and I had to stop and wonder, “Why would you modify the shifter?” There’s a much simpler solution for the problem. I removed and measured the lever arm on the side of the TH400 and found that by simply adding 0.75” to its length would accomplish the same thing and the shifter itself wouldn’t need to be modified. I made the new arm from a 0.1875” (3/16) x 0.75” steel bar stock 4.75” long. The round linkage hole is 0.422” and the rectangular hole where it attaches to the trans is 0.32” x0 .50”. I’ve done this now on 2 trucks and the mod is simple and takes about an hour to install and adjust. It does require you to cut off about 1.50” from the long end of the “J” linkage to keep it from hitting the bell housing.
Something else I found in my truck was that occasionally the trans didn’t always want to go all the way into park and would “ratchet” if the truck moved on an incline. This was caused by my having reversed the small plate that pushes the linkage arm upwards and forces the park pawl into the output gear. It’s a poor design that can be installed either way and it’s important that it gets installed correctly. The image below shows the correct orientation for this plate. In the original article I said to attach this plate with 2, 8mm x 13mm bolts. I stand corrected, use 2, 5/16-18 x ¾” bolts
I was also noting that the 4L80 4 speed shifter may work with the TH400 but the gear indications will be incorrect. Overdrive would become Drive, Drive would become 2, and 2 would become the 1st gear indicated. If the legend wasn’t fiber optic it would be easy to simple cut out the OD position and slide the D up into its place. The original 1st gear position would be unused. I haven’t done anything more than make measurements on this but mathematically it looks like it should work. If anyone has done this please drop me an email and confirm it. THANKS
Kurt Lesser (papakb@yahoo.com) 3/14/2020
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