Turbo Hydra-Matic 400 Problem! Help! Calling Rick, Greasy, Willys and the Usual Suspects!

Turbo Hydra-Matic 400, installed in a '73 Jeep, AMC 304 (might be 360, but same-same).

History: Linkage ALWAYS been sloppy. Park was often hard to find, so care was taken where I stopped. Vehicle is only driven off-road for short distances/duration, maybe 10 miles or an hour at a time. Rough terraine. Rear axle was locked permanently recently. Been driving this POS for near 20 years. Recently, the PARK detent stopped, well, detenting. Then, a leak, leading to a near pour. Leak is now intermitant, which make NO sense! Keep readin’…

Symptoms: Only works intermittentaly. Weirdist thing: When topped off with fluid, will run for a mile or so, then lose drive. Turn off the engine, wait a minute, and fire it up and it will go in whatever gear you have selected (no safety switch). Leaks sometimes, but doesn’t act like its out of fluid, until it stops driving. Shut it off, wait a minute and off it goes! It will go for maybe a minute (or a half-mile or so) and then slowly lose all drive. However, if you move the shifter from Drive to N, stop and try to engage R, nothing! Sometimes it will go from R to Drive (again, fluid level seems to be okay).

I admit, I know little about Auto Trans. Seems like VooDoo or Black Arts to me. Lots of little tubes and valves. I prefer Flay Rods.

Is it the Torque Converter (which I know nothing about)? And how does that work? Why would it quit drive, then after stopping for a minute move again? Fluid leaking down in the converter?

When it leaked, it seamed to come from the upper side of the passenger side. Can’t seem to duplicate it now. Had the Jeep up on blocks, running in Drive with me lying underneath it (suicide note, this is!).


But I know a couple of Dopers who can steer me in the right direction.

Thanks in advance!

P.S. The e30 is singing like a songbird!

I am not very familiar with this transmission. A few questions if I may?

  1. The fluid is full, right?
  2. When you were under there did you check the lines going from the transmission to the radiator for kinks, leaks, +/or crushed portions? If not please look for these. I do not know where these lines are coming out of the TH400, but maybe the leak is at one of these fittings.
  3. Did you change the filter yet? It is about $15.00 plus six quarts of ATF. This is the first thing most mechanics do when troubleshooting a transmission. When you do, pull the pan and filter, then let it drain overnight. In the morning, look at the transmission with the cover and filter off. Use safty glasses or goggles. You do NOT want ATF in your eyes! Do you see any obvious damage? When you put this back together, do not over-tighten the pan bolts!

Now for some observations that may, or may not, apply to your rig.

  1. Automatic transmissions worst enemies are Heat and Dirt. This applies to ALL Automatic transmissions.
  2. Sloppy linkage can cause all kinds of symptoms. Slop can be in the shift linkage +/or the kick-down linkage, check and repair both. The slop is usually caused by worn or missing bushings (plastic or brass). Look close at this.
  3. My 727 Dodge automatic transmission acted a lot like your description after it overheated due to crushed cooling lines.
  4. Running an automatic transmission low on ATF will cause it to overheat! Try not to do this!

I am concerned about the lack of a working PARK position. I have heard of the parking pawl breaking out a section of the transmission case when it was engaged before the rig was at a complete stop. Very bad! Do not use PARK to stop the Jeep.

Now that I have thought this through, do these in order:

  1. look at the transmission case for cracks or holes! There is no sense in changing the filter if the case is broken and needs replaced!
  2. look at the cooling lines. Look closely where they go between the frame and anything. You may have to use the Brail method and feel with your fingers. Repair as needed.
  3. Repair the linkage.
  4. Change the filter.
  5. Get back to us!

I’d be somewhat concerned that the shifter is only putting the transmission part of the way into drive. When you shift all the way through, does it feel like the notches match up with the markings on the shifter? Another thing to check is when you first start out, try moving between P and D verrrry slowly and see if the truck starts moving before the shifter actually clunks down into D. Also, when you’re dead in the water, does dropping it into 1 or 2 or L or whatever make a difference? Alternatively, does going your mile or so down the road in a lower gear seem to make a difference in how far you get?

Otherwise I’d agree with dropping the pan and changing the filter if that hasn’t been done lately. I actually had an old Dodge truck (also with the 727, although I can’t imagine one huge Big 3 slushbox is all that different from another) that did pretty much the same thing-- it’d work fine until you got a few miles down the road and then slowly quit going until you gave it a rest. Changing the filter fixed it up good as new. As far as I could tell, it was the original filter on the 30 year old truck. Amazing what kind of abuse those old transmissions would take!

Well leaks may let dirt in, or if the leak is due a gasket, the gasket failure may letting out the pressure from the control conduits.
The friction material is what changes gear… The bands are like a brake pad on a gear… They are driven by fluid pressure… so perhaps the gasket failure is causing the problem.

Ooops, I may have just revealed how infrequently I drive automatics these days. Anyways, just slowly shift to see if the gear changes are actually happening between the detents on the shifter, so P to R, R to N, N to D. (I’m prettty sure that’s the order they go in :))

If it has no drive (forward, reverse, or both; sometimes or always) when the fluid is full, it has significant internal wear and is on its last legs. This is not at all surprising given its age. The cure is a transmission overhaul or replacement.

Apparently it also has a faulty neutral safety switch and problems in the shift linkage. Fixing these is additional to a basic overhaul, but of course should be dealt with as well.

I seriously doubt the torque converter is causing any problems, but it should be replaced along with the transmission repair. It’s almost certainly contaminated with debris that could jeopardize a new/rebuilt transmission if it’s not replaced.

A leak that stops leaking at times is not terribly common, but is not unknown either. With a tranny that has internal problems, there’s really no point in pursuing it separately. It will be fixed with the overhaul.

From the various symptoms described, I think it would be unwise to spend time and money trying to repair individual issues. It would be like trying to put little patches on a roof that’s on the verge of collapsing.

I guess phase one will be pressure-washing the 600lbs of mud and 30 year build-up of oil and various fluids from the underside of this heap. Not to mention the sticks and sagebrush. I guess I’ll drop that pan and take a looky, swap the filter and toss on a new gasket. Much more and it might not be worth the effort or expense.

I had to break it to the dogs that the Jeep was out of commission for a while. They love going for rides and were pretty broken up by the news.

Thanks for the info so far, I’ll keep ya all posted.

Can’t see the line (yet) from the mud caked up there, but no leaking coming from “upstream”. Trans had custom mounts/supports made not long ago (when the rear was locked up and re-installed) and everything is still sound, so I don’t think its been wigglin’ around too much. Will go for a filter change first thing.

Markings? :smiley: Long gone! Very little detent in any of the positions. The lever just kind of flops around. Doesn’t matter what gear you start in, it goes for a while then slowly loses drive. Shut it down for a minute and off it goes again!

If it helps at all with the diagnosis, it didn’t do this gradually. Friday it worked, Saturday it didn’t.

Having internet issues. Will be back…

Mid-repair thoughts:

My pressure washer ain’t “all that”

What kind of Special Asshole at GM does not provide for a drain plug on a pan holding 6 qts of nasty red fluid?

The thirteen bolts holding the pan broke free very nicely. It* looked* like the pan would drop without disconnecting the exhaust, but NO! I’m sure the 4 adobe-looking exhaust bolt won’t be so easy. Picking up the new filter kit tomorrow, will tackle Saturday.


On the positive side, I was wondering around the garage and think I stumbled on a good TH400 that I pulled out of a Oldsmobile years ago. So, if the Jeep tranny is Tits Up, I might have a go-to.

You’ve just described my life. Sorry for not putting you in the thread title. I can’t remember all you guys!

Although GM made the TH-400 unit for applications across the GM divisions, and for other car companies (AMC Jeeps, Jaguar, Rolls Royce) Your AMC has a different tranny bolt pattern than the Olds.

Unless you have an adaptor plate…or switch the internals from the olds to the AMC trans.

The same kind who designed all the tranny pans for Ford and Chrysler products. Apparently a breed unique to America, as Asian and European tranny pans have drain plugs. But, hey, I’m sure it saved them a dime per car, and that adds up. :frowning:

If the leak is on the passenger side it is likely the vacuum modulator a screw in part on that side of the trans maybe 1/2 way back.
Also if the bell housing is different on the two trans you can probably switch them. The entire front half the trans slides off forward and has a couple of very large O-rings for sealing. So don’t sweat the adapter plate just yet.

Yes, what Rick said about the Jeep to Oldsmobile conversion is correct. I do understand where you are financially. Been there, done that.

Keep us informed, Thanks, 48.

Picked up the filter. Doing the job today. Will update this afternoon.

Slight chance you may have a bad oring where the dip stick tube goes into the trans. Not too uncommon where tranny’s have been changed and the stick didn’t line up just right.

update coming…

Well? Inquiring minds want to know.


Once the pan was off, I was able to check out the linkage and adjust the shaft from the lever properly. Tightened up a nut and it works like a charm! Park works again, and able to shift cleanly thru all gears.

Doesn’t seem to leak anymore, either.

So what was it? Was that filter all plugged up and not letting fluid flow, maybe backing up and leaking out?

I have to go out and re-install the exhaust on the passenger side. Didn’t seal up tight and very noisy.

But I’m pretty happy overall, so far.

Happy ending, good job!!