Adding Park to a 3L80 trans UPDATED

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Adding Park to a 3L80 trans UPDATED

Post by Kurt Lesser » Wed Sep 09, 2015 7:08 am

PARK YOUR HMMWV (Original article 2001)


The High Mobility Multi-purpose Wheeled Vehicle (HMMWV) really is a spectacular vehicle and was an impressive addition to the military assortment of tactical vehicles. While it’s opponents talk about the HMMWV being too wide and not being able to go where the jeeps went, the truth is that the HMMWV can carry 4 men and their equipment in a single vehicle without a trailer into most areas the jeeps could go. It is wide, 7 feet. But is has power the jeeps never had and flexibility that allows it to be configured for multiple mission specific purposes. That said, one of its serious shortcomings in my opinion has always been the lack of a “park” position in the transmission. For whatever reason Tacom decided not to add this to their specification for the vehicle, for street use it needs to be there. The newest A2 variants have restored “park” to the transmission and so I decided to retrofit mine for safety reasons.

I spoke to dozens of HMMWV owners and drivers about the necessity and the requirements to add a park position and the answers I received were as diverse as the HMMWV’s missions. I was told that they needed a park position. I agreed. I was told that the transmission needed to come out for this to happen. I was told that the job had to be done by a transmission shop and it was beyond the skill level of the collector. I started to investigate and what I determined was that the average mechanic/collector shouldn’t have any problems installing the components provided they had a guide to work from, and that the job could be done without removing the transmission from the HMMWV. Based on this, I went looking for a guide on how to do the installation. Not finding one, I decided to write one.

I bought the Haynes shop manual on the General Motors THM400 transmission. If I were rebuilding turbo hydramatic transmissions it would have been useful. In the area of the adding the park components it was sorely lacking. Interestingly enough, the 1996 -24P Parts TM shows the components required in figure 96, but when you get into the transmission you’ll find they aren’t actually there. I scouted the local transmission shops and found one that sold me the internal parts from a salvage yard transmission for $5.00. I think this is on the low side but wouldn’t expect to pay more than $25 for them. The other parts required for the installation are the shifter assembly, a new filter, a pan gasket, and a six quarts of Dextron III fluid. If you’re going into the transmission this far, new fluid and filter are logical steps to avoid trouble in the future. Keep in mind that the breakdown you avoid may be your own. The parts you need to get from the transmission shop are shown in figure 1 and include the detent selector, park actuator linkage, retainer plate, park pawl, pawl spring, pawl pin, hitch pin clip, pan gasket, and two 5/16 – 18 x ¾” hex head bolts.

The shifter assemblies are the hardest piece of the upgrade to find. There are also 2 models out there that you should be aware of when you go searching. The early civilian Hummer had the same transmission that the majority of the early year HMMWV’s had, the THM400HD. The later Hummers had the 4L80E transmission that is an electronically controlled 4-speed overdrive transmission. This article focuses on the THM400HD. The early civilian shifter has the 1,2,D, N, R pattern that you are familiar with but it also has the “P” position. The post 1994 shifters are 1,2,D,OD,N,R,P. This shifter can be used with the THM400HD but won’t put the transmission into gear in the “D” position. The transmission will be in Drive when the shifter is in the “OD” shifter position. Everything else works normally. The 4L80E electronic transmission requires an additional computer and sensors to control it and that change out is much more complicated. Both the transmission and the shifter assembly have detents in them that help in setting the linkage when you get that far along.


THE TRANSMISSION JOB ITSELF:

A) Remove the linkage connecting the shifter assembly to the transmission by removing the cotter pins that hold it in place. This is a round “J” shaped linkage about 8” long.
B) Drain the transmission fluid into a clean bucket.
C) Support the transfer case and remove the transmission support frame rail. Remove the transmission mount from the transmission. Remove the transmission pan. This may not be necessary if there is sufficient clearance to remove the pan.
D) There’s one 13mm (1/2”) bolt that holds the filter in place. Remove it being careful not to lose the spacer on the top side of the filter. Remove the filter by pulling it downwards.
E) Remove the plastic pickup tube and o-rings from the right front corner of the transmission.
F) Loosen the bolt that holds the detent spring and roller in position and swing it out of the way. It doesn’t need to be removed.
G) Remove the retaining pin (item 4) on the inside of the case that keeps the selector linkage shaft from slipping outwards. Slide the shaft far enough out to remove the detent selector nut.
H) Replace the detent selector (item 5) with the replacement from the transmission shop. The pin on the detent faces inwards and rides in a groove on the selector valve.
I) Replace the retaining nut, slip the shaft back into position, and replace the “nail” with a 1/16” cotter pin.
J) Slip the end of the park actuator linkage (item 2) into the hole on the upper section of the detent selector. The spring-loaded cone will now be hanging downward. When pivoted upwards the linkage should fit inside the right side of the transmission housing without binding.
K) Disconnect the pink wire that goes from the downshift solenoid to the feedthru on the casing side at the feedthru. This is a slip on connector.
L) On the rear side of the transmission casing there is a small depression that has a 3/8” plug in it (item 12). This is where the park pawl pin (item 11) will be installed. Clean the area well and using a small pin punch, gently tap the plug into the transmission and catch it when it comes through. It will be reinstalled after the pawl pin in installed.
M) Hold the park pawl (item 13) in position and slip the pin into the housing to hold it in place. The “tooth” on the pawl faces upwards toward the output shaft of the transmission. Install the pin with the groove for the hitch pin clip (item 10) facing the front of the vehicle. When it is fully installed put the hitch pin clip in place with a pair of needle nose pliers. After the pin is installed gently tap the plug back into its hole flush with the case surface.
N) Attach the spring (item 9) to the pawl with the square end on the pawl and loop the curved end around the post on the transmission housing casting. Hold the park pawl up into the transmission.
O) Swing the park actuator linkage upwards making sure it fits into the right side of the transmission and doesn’t bind anywhere along its length. Reattach the pink wire to the feedthru.
P) Install the retaining plate (item 14) with the two 5/16 – 18 x ¾” bolts (item 1). This plate holds all of the components in place and provides the ramp that moves the pawl into the output shaft that “parks” the transmission. Mount this with the "finger" pointing forward toward the downshift solenoid.
Q) Remove any traces of the old pan gasket from the housing and the pan.

You might want to stop at this point until the shifter is installed so that you can watch the mechanism during shifter adjustment.

R) Re-install the plastic pickup tube and the two o-rings. The tube is labeled pump (upward) and filter (downward).
S) Re-install the filter by slipping it onto the tube, hold the spacer on the top of the filter, and attach it with the long 13mm bolt.
T) Re-install the transmission pan.
U) Refill the transmission with a 6 quarts of fresh Dextron III. Check the oil level with the engine running after it reaches operating temperature. The -10 manual shows the capacity as 6 quarts so top off as necessary.

SHIFTER REPLACEMENT:

A) Disconnect the linkage to the transfer case by removing the cotter pin retainer.
B) Disconnect the wiring for the lamp, neutral starter switch, and back up light switch if fitted.
C) Remove the four bolts and nuts that hold the shifter assembly into the transmission tunnel.
D) I installed three ¼-20 riv-nuts in the floor panel to make it easier to bolt the new shifter into position. Use three ¼-20 x 1/2” allen head cap screws to install the shifter assembly.
E) Re-connect the wiring. . It is important to re-connect the neutral starter switch so that the HMMWV only starts in park and neutral. These wires are numbered 14 and 14A. The new shifter will have two wires for the back up lamps (#67). If you don’t have them then leave them disconnected. The lamp power wire is #40.
F) Moving the lever on the side of the transmission by hand put the transmission into neutral. With the top of the linkage arm fully forward (toward the front of the vehicle) it will be in the 1 (or low) position. Rotate the lever towards the rear through the 2, and D positions into neutral. The third detent felt will be the neutral position
G) Put the shifter in the neutral position.
H) Adjust the connectors on the end of the round “J” shaped linkage by turning them around the linkage shaft so that they easily line up with the holes in the shifter and transmission levers. I had to run a 3/8-16 die down over the existing threads on the longer side of the linkage to add another inch of threaded rod to give me enough adjustment range. Re-install the cotter pins to hold the linkage in place.

The legend lighting in the HMMWV shifter is a 24 volt Light Emitting Diode (LED). The legend lighting in the Hummer shifter is a 12 volt incandescent lamp. The power comes to the shifter from the Bright/Dim position on the lighting control switch on wire #40. I tee’d into the power from the switch to power my back up lights reasoning that they’re needed when the switch is in the service brake position and the dash lights were set to bright . You will have to change the legend lamps to 24 volt bulbs (Chicago Miniature #2314) or add a resistor (220 0hm 5 Watt) to drop the voltage to 12 volts for the original lamps. Your other option is to change the lamps to LEDs with the appropriate dropping resistor.

The entire change can be done in one day if you have everything on hand when you start. I replaced the emergency brake handle with the newer latching model at the same time. It’s the opportune time to do it since the shifter has to come out to gain access to the brake handle mounting bolts. I believe this is one of the best safety modifications you can make to the HMMWV.



Kurt Lesser
President, Military Vehicle Collectors of California (MVCC)
3/18/01

Figure 1
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Re: NEW Adding Park to a 3L80 trans

Post by Action » Tue Sep 22, 2015 9:07 pm

The link works.

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Re: NEW Adding Park to a 3L80 trans

Post by Clarke » Wed Oct 14, 2015 3:44 pm

Many people would say that the drive-shaft e-brake on the older trucks is inadequate - certainly I've found it so offroad. The A2 e-brakes in the calipers are infamous for sticking and dragging the rotors. Park gives you an option for when you aren't on an incline to not use the e-brake if you don't want it.

As far as why not just upgrade to the 4L80E - because that's a $3000 upgrade in parts alone. $800-900 for the A2 body harness (assuming you already have a 12/24V alternator and don't need to buy one of those too), $1200-1400 for a reman trans and a couple hundred for the TCM, and 500-800 for the shifter assembly.

$850 (shifter and internal parts) or $2200?

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Re: NEW Adding Park to a 3L80 trans

Post by claptonisgod » Wed Feb 17, 2016 5:28 pm

How readily available are the internals for the trans and 24v/LED bulbs for the shifter? I see the tutorial was written some years ago, and quick Googling didn't turn any of the parts up.
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Re: NEW Adding Park to a 3L80 trans

Post by Kurt Lesser » Thu Feb 18, 2016 8:21 am

To answer your question Fenris, the E brake does not work fine other than on flat, level surfaces. That's why the military chocks their wheels. Even on my slightly sloping driveway the E brake would not hold the truck in place unless I tightened it so much it would overheat and warp the rotor. Don't ask me how I found this out. They left the Park position in the same trans in the CUCVs, so why they removed it from the HMMWV is a question some no longer employed designer might be able to answer.

Admittedly I live in San Jose and there are plenty of shops that rebuild transmissions and was able to buy all of the parts necessary to do the upgrade for around $15 and have never gone out and asked what all new parts would cost but if you look at the parts you need to collect they're all small stuff. There are no hydraulic valve bodies or solenoids, just a pin, a pawl, a shaft, and a selector detent. Even if you went to Chevrolet and bought them I wouldn't think you'd be into them for over $100. As with most transmission jobs the money is in the labor rather than the parts. If you have a "Pick-n-Pull" type wrecking yard around you might find a core transmission that you could strip the parts out of. If you need some help just shoot me a PM with what you need.

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Re: NEW Adding Park to a 3L80 trans

Post by GPW43 » Fri Jun 03, 2016 7:59 am

Gents
Attached is the picture of the failure mode Kurt was presenting. I experienced it first hand and didn't even noticed it. I parked the HMMWV after a 20 min drive. Came to move afterward and it locked up. I thought it was the X-fer case at first, I spent few minutes troubleshooting it and couple of back and forth later ... BANG, the issue went away. Then I would hear this rattling noise and I thought I let go something in the rear Diff. My good friend Cam jumped on the back seat and we drove for 1/2 mile; got out and looked underneath. We got the picture, literally and physically.

For those who I am sure will comment - The brake was neither ON nor I noticed any loss of power but there was enough friction left on those pads to cause the disk brake to melt and warp to the point it locked up the shaft after I parked the car. This is from the tendency of increasing the tension on the E-brake to get it to hold the truck on an incline. In San Francisco area that is a NECESSARY undertaking.
Z
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Brake Failure Mode
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Re: Adding Park to a 3L80 trans

Post by progun3400 » Mon Dec 17, 2018 7:04 am

Hello Kurt.
We’ve just completed (this past weekend, trans still in vehicle) the park components upgrade on an early M998 equipped with the TH400/3L80 transmission. I have at least one more to upgrade soon. The plan (if time allows) is to shoot some video of the steps involved, hopefully upload the video to YouTube? I’m very familiar with the internal workings of the TH400/3L80 transmissions, however your excellent article/instructions were definitely helpful.
Thank you for sharing, very much appreciated. Paul

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Re: Adding Park to a 3L80 trans

Post by Kurt Lesser » Mon Dec 17, 2018 8:33 am

Some videos would be a great addition to the original post. I did this job in my driveway and didn't have much room to get a camera into play.

I'm glad this worked out for you, it isn't the complex task a lot of people think it would be and to be honest this was my first adventure into an automatic transmission.

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Re:2020 Update to adding PARK

Post by Kurt Lesser » Sat Mar 28, 2020 10:52 am

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. Check the -24P manual, it shows the proper installation. 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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Re: Adding Park to a 3L80 trans UPDATED

Post by rhodesniper » Tue Mar 31, 2020 7:47 pm

I am assuming this is what you are referring to that is needed to add to your turbo400. Found it in TM 9-2320-387-24 Volume 3 Pg 286.
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Re: Adding Park to a 3L80 trans UPDATED

Post by rhodesniper » Tue Mar 31, 2020 8:10 pm

I am not certain about the shifter replacement. Are you saying to use the early hummer H1 shifter? Could not find one on the internet. Sounds like the Newer 4 speed HMMWV shifter would need a lot of modification to work.

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Re: Adding Park to a 3L80 trans UPDATED

Post by Kurt Lesser » Wed Apr 01, 2020 7:21 am

The newer 4 speed shifter doesn't need anything to work, just drop it in and do the linkage adjustment. The issue with using the 4 speed shifter with the 3 speed transmission is that the gear indications will be off for the forward gears. The displayed OD position will equate to Drive. In the displayed Drive position the trans will be in 2nd gear and when the shifter shows 2nd gear the trans will be in 1st (Low). The 1st gear position on the shifter can't be reached because the transmission will already be in it's lowest gear so it's a dead spot. Park, Reverse, and Neutral all indicate normally.

The 4 speed shifters are available in lots of places, finding an early H1 3 speed shifter with a Park position is going to be difficult since they were only used for a few years. I originally thought you could use the 4 speed gear indicator and just relabel it to show the correct gear by shortening it but the display is a fiber optic unit and to try to shift the legend plate would mean cutting it and removing the OD position but that would destroy the fiber optic parts of it. Now if the gear indicator could somehow be pealed away from the fiber optic section it could be done but I'm not ready to tear up a shifter to find that out.
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Re: Adding Park to a 3L80 trans UPDATED

Post by progun3400 » Sat Apr 25, 2020 9:54 am

rhodesniper wrote:
Tue Mar 31, 2020 7:47 pm
I am assuming this is what you are referring to that is needed to add to your turbo400. Found it in TM 9-2320-387-24 Volume 3 Pg 286.
I’ve converted/upgraded one M998 (two door) TH400/3L80 transmission by adding the internal park linkage. The two door M998 that I installed internal the park linkage, has an aftermarket Hurst shifter installed. I have another M998 (four door) waiting for this same trans linkage upgrade but that will get the later style 4 speed 4L80E shifter installed as well. I read Kurt’s instructions a long time ago and they were extremely useful. Being very familiar with the TH400 transmission, his instructions were easy for me to follow.
I seem to recall Kurt suggesting - adding an inch or so of threads to the long end of J shaped linkage rod for greater range of adjustment, but I’ll let Kurt confirm that?
Rhodesniper attached the complete parts breakdown of the TH400/3L80 internal parking linkage. The specific parts needed for the upgrade are numbered as follows:
1) 5/16-18 x 5/8” hex bolt (hardware store items, might even be metric?)
2) Parking lock actuator
5) Inside detent lever (commonly referred to as rooster cone)
9) Parking pawl return spring
10) Parking pawl shaft retainer
11) Parking pawl shaft
13) Parking pawl
14) Parking lock bracket
Note:
12) 3/8” cup plug is already installed in the case. It will have to be removed. I used a pick to clean/scrape/remove debris from the outside of the bore. After driving the cup plug out (I drove mine into the case with magnet waiting to retrieve it) I followed up with a 9mm bore brush to clean debris from parking pawl shaft bore. You can reuse this cup plug, but you will be installing it from the outside of the case, inward towards the parking pawl shaft once it’s installed.

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Re: Adding Park to a 3L80 trans UPDATED

Post by progun3400 » Sat May 13, 2023 10:32 pm

Correct parking linkage rod bracket orientation. As Kurt pointed out, this bracket can be installed upside down if you’re not careful
Image
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