Transmission issue, help!

[Navy dude]

I searched the forum, got some good reading in but have a strange symptom with the trans and truck is disabled. I have the old school 3 speed 3L80.I will do some troubleshooting but maybe someone can narrow down my search. I was driving the truck and it started slowing while the rpm's increased. Pretty soon it acted like it was in neutral and would not move forward or reverse. I checked the fluid, it's red, full and no burnt smell. I checked the the transfer case by shifting into different ranges, no change. The drive lever linkage is connected and the rod underneath is being actuated normally with the lever. The symptom that someone might make sense of is that if I shut down the truck and restart (either hot or cold) it shifts normally and will move normally (in either forward or reverse) a short time... but will immediately not work if I shift from drive to reverse or the opposite. I can then shut it off, start it again right away, it goes into gear normally then the trouble will start again. once the issue has started it doesnt matter how much I move the lever it doesnt catch the slightest in any position. Shutdown and restart motor, acts normal again for short time! I know little about trannys but could the fact that it works on start but quickly fades be some symptom that enough fluid is pooled somewhere on start to function but not replenished somehow by the system? Thank you so much for any inputs!

 

[98G]

Which transmission?

 

[Navy dude]

So sorry, I have the old school 3 speed. TH400 I believe.... I'll edit above.

 

[Wire Fox]

Run your engine, shift though all the gears, then place it in park while on a level surface. With the engine still running, tell us what the fluid level is like. This really sounds like once you get running and the fluid passages fill up, your transmission is starved for fluid and loses engagement. This would make for a delightfully easy fix if it's the case.

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[Navy dude]

Run your engine, shift though all the gears, then place it in park while on a level surface. With the engine still running, tell us what the fluid level is like. This really sounds like once you get running and the fluid passages fill up, your transmission is starved for fluid and loses engagement. This would make for a delightfully easy fix if it's the case.

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It's full when running, if anything slightly overfull. Nice pinkish red color.... My brother in law the mechanic says he will test the pressures says it looks like some type of "apply" issue. He has never worked on a HMMWV but says the th400 is simple and easy. My next plan is to download the TM for troubleshooting so that anything specific to a HMMWV could be helpful to him. I think I will first drop the pan and service the filter to start, even I can do that. He said that he traced the trans lines to the cooler and opened the purge valve (my interpretation) and no fluid sprayed out as he expected it would (once again my interpretation over the phone... I limped the truck into my father in laws driveway and left it, and my brother in law stopped by quickly to take a look). He also just reported that even when it first starts it barely goes into gear.

 

[papakb]

Unless it's been added there is no PARK in a HMMWV TH400 transmission. Here's a link to a You Tube video on pressure testing a TH400 trans: https://www.youtube.com/watch?v=SADOTGVJjtM It could be something as simple as the pressure relief spring on the pump breaking but I don't think it's going to get fixed without dropping the trans.

 

[nonnieselman]

I've seen a partially clogged filter have these symptoms.

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[tobyS]

I'll put my wager on clutch pack piston seal giving up. The more you try, especially if you give it RPM, the more damage it will do to the clutches and particularly the aluminum piston. Check for grit in the pan oil from the decaying frictions and (hope not) aluminum balls from the melting aluminum piston. This would seem to have the smell of burnt oil, so if tranny oil is not carmel, it may not be the problem. Pressure testing will tell if the seal(s) is leaking or worse.

 

[Navy dude]

Thanks so much, guys. I have a filter on order will see what’s in the pan once it gets here. I’ll follow up with results....

 

[snowtrac nome]

to me it sounds like a bad pump or torque converter but the lack of burned oil leads me to think pump also if the pump goes bat there will me no circulation through the cooler

 

[papakb]

If the pressure relief spring in the pump is bad it'll do this by bypassing all the pump output back into the case.

 

[Navy dude]

Just wanted to thank all who answered. I finally got the time to drop the pan, replaced filter and o-ring. The tranny pan fluid was glistening with metal sparkles, a little smattering of tiny metal fragments it the pan too. I added 7 quarts DEX3 and fired it up..... worked good as new again! I guess the filter was clogged. I will flush the system again now that I know the problem and hopefully it will live a bit longer now. I did shear a bolt taking it off, so that will be something else to address but at least the pan isn't leaking with the single bolt missing! Thanks again, guys.

 

[tobyS]

Just wanted to thank all who answered. I finally got the time to drop the pan, replaced filter and o-ring. The tranny pan fluid was glistening with metal sparkles, a little smattering of tiny metal fragments it the pan too. I added 7 quarts DEX3 and fired it up..... worked good as new again! I guess the filter was clogged. I will flush the system again now that I know the problem and hopefully it will live a bit longer now. I did shear a bolt taking it off, so that will be something else to address but at least the pan isn't leaking with the single bolt missing! Thanks again, guys.

That "glistening with metal particles" is the clutch friction material. If you keep going with slipping clutch, eventually it will burn up the aluminum piston. Does the "smattering of tiny metal fragments" appear to be aluminum? Stop now if it's aluminum. New fluid does not solve the problem of piston seals not holding. Plan for a rebuild soon.

 

[Navy dude]

I was just happy to get the thing running again since I am just finishing up a resto on my nicer one shown here. I plan on a 480 trans swap and frame off resto on the one with tranny issues but I need to be able to move it around so hence I am super happy with the quick fix.

 

[tobyS]

Very nice HMMWV. Sounds like you have a plan. If it's the fourth and reverse clutch, try not to use reverse any more than you have to if you plan to re-use it.

 

[Navy dude]

Very nice HMMWV. Sounds like you have a plan. If it's the fourth and reverse clutch, try not to use reverse any more than you have to if you plan to re-use it.

Yeah, just wanted to mention it doesn't seem to be slipping at all now in any gear. That issue hit immediately, not over time. I was out for a weekly short drive to exercise it and all of a sudden it started slipping. Luckily I made it one block to my father in law's house. It went from feeling like it was in neutral in any gear to completely back to normal after I changed the filter and o-ring. I still plan on flushing the system out completely this week since it doesn't take any effort. I didn't do it in my father in law's driveway because I just wanted to drive it 3 miles to get it home. worked like a champ thankfully! Yes, I am pretty pleased with how the green hmmwv is coming out, have a mk64 cradle to slap on, and waiting on a shield to arrive.... have a few interior insulation pieces to install and it looks brand new down to the data plates and bolts! The one with the trans issues is a National Guard 2 man tan 998 that went through the depot in 2005 and then sat. Maybe they didn't check the trans well during the service but it was 14 years ago. Anyhow, thanks for the replies, really like this hobby.