TH 400 reverse engagement problem

[Highpsi]

Guys, I need some advice on an issue I'm having with my '85 Burb. It has a Turbo 400 tranny & the shifting mechanism on the column is pretty sloppy for starters. When I bought it about a year ago the PO had a trusty bungee cord on it strategically placed so as to hold the lever with a bit of tension pulling forward so it wouldn't just flop around. (There's no forward spring tension left).

Needless to say this setup hasn't worked very well and I'd like to remedy that. However, now I think I might have more than just linkage and shifter issues. One day when I went to move the truck it sounded like it would go into reverse, but it just revved up and shuddered a bit. I shut it off and checked fluid level. Level is fine but the fluid definitely looks like it needs a change. I went to try it again and still no go. But after moving the shifter around and going back and forth through the gears it finally worked.

After that, I've not been driving it much as to avoid potential damage to the transmission if something really is wrong in there. The other day I had to use it and when I did I was driving along normally under a very slight uphill grade, and while in "D" the motor just suddenly revved up and it behaved like it was in neutral. I grabbed the shifter and went for "2" and no problem shifted into that gear – power again and when shift back to "D" its pulling fine again.

So last night I went to move it, and once again, sloppy shifting and no "R". Pulled it into garage for a look at the linkage at the tranny, and I was surprised to see the hokey connection at the transmission with that goofy 90 bend and no bushing?! Just slides back and forth in there loose? At any rate, I decided to unhook linkage and manually shift it through all gears. All feels well and stays nicely engaged at the various detents, so it seems like it is fine for holding in the gear it is supposed to be in when linkage unhooked. I cranked up the truck in "R" with wheels blocked and found that the reverse was pulling fine.

I've done quite a lot of searching online and found several other cases with people having similar TH400 issues. Most of the time people seem to report a loss of R & 3rd gear together because they use the same drum or something? The PO said that the tranny was rebuilt not too long before he sold to me, so now I'm worried something in there was done wrong.

I should also mention that I've only put about 500 miles on the truck since purchased, but it ran great with not even such a sign of any problem until just NO reverse suddnely that first time.

I guess it is worth sucking the fluid out and then dropping the pan to see if there's clutch material and debris in there. Probably should have done that prior to posting but I wanted to know if anybody has modified their shift setup so there's something over the transmission hump / custom shifter or any other ideas to get rid of the shift on the column garbage. I really don't want to have to pull a steering column and try to fix that part of it (assuming this is pretty complicated repair). I'd almost prefer to just have a simple ghetto floor shift setup for time being to isolate the behavior.

Any thoughts or opinions / experience appreciated…

 

[diesel4me]

Try going on google and type in "TH400 Diagnosis"....the first listing that comes up is from a Buick Riviera site link,that has a PDF file of the Motor's manual diagnosis,rebuilding and service procedures there you can download...(I tried too,but it gave my old computer a hemmorage!)..look at page 16 & 17 for the "no reverse/slips in reverse" and also the symptom "no engine braking" ,there is a list of all the possible things that can cause that--which includes linkage problems,vacuum modulator defects,servo stuck,etc...

 

[folkenheath]

The direct drum holds clutches that are applied in both reverse and 3rd gear. So if its really slipping then you may have lost your direct drum clutches. There is an inexpensive mod to prevent this for less than a dollar but it is too late for that if the clutches are shot. However, I would make sure you get the linkage fixed first as that could be your only problem, if the manual valve is not in the correct location all sorts of funny stuff can happen. Also, just because it spins the wheels with no load doesn't mean the clutches will hold a load when the truck is on the ground. So you really need to fix the linkage and then try driving it. Good luck with it.

Also, there should be a spring and bushing in the linkage keeping it from rattling around, I have been using the factory column shifter without any issues even at the dunes (I did have an issue when my motor mounts were shot, then the engine moved around enough to change gears on me). Anyway, with good motor and trans mounts it works fine. Also, the actual detents are in the trans, not the column.

 

[diesel4me]

Care to elaborate on the one dollar mod to keep the clutches intact?...--if it doesn't involve dropping the valve body I might be interested in that...

 

[Highpsi]

Ok here's the update. I went to auto parts store and got my filter change kit and fluid just in case I need to go that route. I went to pull the truck into the garage from out in the driveway and again, no reverse at all. Shifted to neutral to let it roll to bottom of driveway, then tried for "D". Nothing. No forward movement. Shifted all the way down into first and it pulled up hill into driveway. Trying to suck the fluid out of the trans dipstick I only was able to get about a quart but the stuff looks kinda nasty.

Looks like she's toast. Fabulous!

Now I have to decide what to do. I happen to have another TH400 sitting in my shed, but I don't have any clue if it is compatible or anything. I know it was from a K truck but I don't really trust it as it was all just part of a big buy I did with my axles and other 1 ton swap stuff. I wouldn't really trust going through all the work to swap it in and then have it just let me down, or worse yet not work at all.

The direct drum holds clutches that are applied in both reverse and 3rd gear. So if its really slipping then you may have lost your direct drum clutches. There is an inexpensive mod to prevent this for less than a dollar but it is too late for that if the clutches are shot. However, I would make sure you get the linkage fixed first as that could be your only problem, if the manual valve is not in the correct location all sorts of funny stuff can happen. Also, just because it spins the wheels with no load doesn't mean the clutches will hold a load when the truck is on the ground. So you really need to fix the linkage and then try driving it. Good luck with it.

Thanks Heath- that's right about what I'd read previously, and coincidentally even in one of your posts in the past.

Also, there should be a spring and bushing in the linkage keeping it from rattling around, I have been using the factory column shifter without any issues even at the dunes (I did have an issue when my motor mounts were shot, then the engine moved around enough to change gears on me). Anyway, with good motor and trans mounts it works fine. Also, the actual detents are in the trans, not the column.

Right on. There was a spring and bushing on the far left side, down by the transmission. And once I removed all linkage from trans, and shifted the shifter on the trans through all the gears, the detents felt OK. However the column shift is bricked somehow but I think I've eliminated any blame there at this point for the current problem. (Though that might be some of the root CAUSE).

It is very strange though, I was literally driving it last night just fine. Into reverse, down the road letting it shift and it ran great. Just now it sits overnight and POOF. Heck, I'd think if the clutches were shot it would not work EVER.

I think more reading is in order.. Maybe I'm just loosing fluid pressure or something. Trouble is I'm so nOOb when it comes to auto trans - I've never worked much with them ever.

Guess I'll drop the pan at least.

 

[folkenheath]

Care to elaborate on the one dollar mod to keep the clutches intact?...--if it doesn't involve dropping the valve body I might be interested in that...

Well, its a cheap mod, but unfortunately only easy if you have the trans apart. Involves leaving out one or two seals in the direct drum and/or center support and putting a very small freeze plug in the case under the valve body. So not only does the valve body have to come out, but the pump, forward clutch, and direct drum. But the freeze plug is less than $1!

If you look here, the last 3 pictures show the 2 seals to leave out (one or both will do it) and the final pic shows where the plug goes.

http://www.jakesperformance.com/TH400_Rebuild_Tech.html

Trying to suck the fluid out of the trans dipstick I only was able to get about a quart but the stuff looks kinda nasty.

...

It is very strange though, I was literally driving it last night just fine. Into reverse, down the road letting it shift and it ran great. Just now it sits overnight and POOF. Heck, I'd think if the clutches were shot it would not work EVER.

I think more reading is in order.. Maybe I'm just loosing fluid pressure or something. Trouble is I'm so nOOb when it comes to auto trans - I've never worked much with them ever.

Guess I'll drop the pan at least.

If the fluid is clean then yes, you are probably losing pressure, internal leak, etc. But if the fluid is toast with clutch material in it, then even if you fix the leak it won't fix the clutches.

 

[diesel4me]

I'd read up on that PDF file I mentioned--I dont see how it could "die" sitting overnight--something "simple" could be wrong with it,like the modulator,a valve body valve sticking,bad gasket between that and the case,or a servo stuck,etc..
I would at least try checking the line pressures before removing it --..dont condem it too hastily--just low fluid or a plugged filter can make a tranny seem "junk"..usually a tranny doesn't just lose gears without warning,or acting weird,making noises,etc..

I dont know that much about automatics either--anything more than changing filter & fluid or a modulator is pretty much above my experience...but I have brought a few "dead" ones back to life--I was ready to yank the TH350 out of my van,I bought it for 500 bucks with a "blown tranny"--it had only first gear and reverse...it would "almost" shift into second sometimes,then drop back into first...turned out all it was ,the plastic govenor gear had stripped its teeth (apple cored!)--a junkyard govenor I got free took about 10 minutes to pop in,and I drove that thing 49,000 miles since then with no trouble!..

 

[diesel4me]

Well, its a cheap mod, but unfortunately only easy if you have the trans apart. Involves leaving out one or two seals in the direct drum and/or center support and putting a very small freeze plug in the case under the valve body. So not only does the valve body have to come out, but the pump, forward clutch, and direct drum. But the freeze plug is less than $1!

If you look here, the last 3 pictures show the 2 seals to leave out (one or both will do it) and the final pic shows where the plug goes.

http://www.jakesperformance.com/TH400_Rebuild_Tech.html



If the fluid is clean then yes, you are probably losing pressure, internal leak, etc. But if the fluid is toast with clutch material in it, then even if you fix the leak it won't fix the clutches.

That's an interesting read!..thanks...

I've had a TH400 in my '69 GTO that saw daily smoke shows and long distance commutes ,and one day I got it stuck in my sister's yard after a deep snow fell,I had to rock it from reverse to drive for a good half hour,before we finally got it out of there!..the tranny started whining like a siren or P/S pump low on fluid,and I noticed the next morning when I started it,it hesitated going into reverse and drive--kind of "slid" in,instead of the usual firm "thump" it always had...and the siren noise was louder until it warmed up a bit,but it never went away either--thought sure I fried it!..

That night I went to a guy who ran a small garage,he used to hang around after dark on hot days and work at night--he was not busy,and he let me use his lift,to drop the pan and put in a tranny filter--the pan was full of what looked like burnt coffee grounds,with brass sparkles in it...he said it was clutch material....not having any cash to buy another tranny,I just put the filter in it,the pan back on,and refilled it...the filter was pretty black too..

When I started it up,the whining noise soon went away,and it shifted into reverse firmly,just as it had before...after a few days of babying it,afraid it would blow,I started driving it like the usual leadfoot I was in it,and it never gave any trouble as long as I had the car,about 3 years...
So I wouldn't freak out if you see some clutch material in the pan--"some" is normal...what I saw in mine looked more like a pound!....

 

[Highpsi]

I think I have some good news!

I went ahead with a fluid and filter change. Ended up finally getting about a gallon from the dipstick tube prior to dropping the pan and it looked pretty nasty. The pan was very dirty in the bottom but no debris.

Topped it off and ran it and all is working fine. The real test will be tomorrow morning when it cools down and sits. I think if I have reverse then it was just a dirty filter and possibly low fluid level. (Though I checked it a few times but it was never exactly HOT and on a perfect flat spot).

This was a great experience - I love learning stuff like this about things that have always seemed so mysterious to me before like the dreaded automatic transmission. Once again the lesson learned - take it slow and don't over complicate something simple!

 

[folkenheath]

That's an interesting read!..thanks...

I've had a TH400 in my '69 GTO that saw daily smoke shows and long distance commutes ,and one day I got it stuck in my sister's yard after a deep snow fell,I had to rock it from reverse to drive for a good half hour,before we finally got it out of there!..the tranny started whining like a siren or P/S pump low on fluid,and I noticed the next morning when I started it,it hesitated going into reverse and drive--kind of "slid" in,instead of the usual firm "thump" it always had...and the siren noise was louder until it warmed up a bit,but it never went away either--thought sure I fried it!..

That night I went to a guy who ran a small garage,he used to hang around after dark on hot days and work at night--he was not busy,and he let me use his lift,to drop the pan and put in a tranny filter--the pan was full of what looked like burnt coffee grounds,with brass sparkles in it...he said it was clutch material....not having any cash to buy another tranny,I just put the filter in it,the pan back on,and refilled it...the filter was pretty black too..

When I started it up,the whining noise soon went away,and it shifted into reverse firmly,just as it had before...after a few days of babying it,afraid it would blow,I started driving it like the usual leadfoot I was in it,and it never gave any trouble as long as I had the car,about 3 years...
So I wouldn't freak out if you see some clutch material in the pan--"some" is normal...what I saw in mine looked more like a pound!....

TH400s are tough! There is a reason they put them in so many cars. Many people don't know it, but in addition to Chevys, GM, Buick, Pontiac, Olds, Cadillac, Jeep trucks, etc. They also put them in Rolls Royce, Jaguars, and even Ferrari's!

I think I have some good news!

I went ahead with a fluid and filter change. Ended up finally getting about a gallon from the dipstick tube prior to dropping the pan and it looked pretty nasty. The pan was very dirty in the bottom but no debris.

Topped it off and ran it and all is working fine. The real test will be tomorrow morning when it cools down and sits. I think if I have reverse then it was just a dirty filter and possibly low fluid level. (Though I checked it a few times but it was never exactly HOT and on a perfect flat spot).

This was a great experience - I love learning stuff like this about things that have always seemed so mysterious to me before like the dreaded automatic transmission. Once again the lesson learned - take it slow and don't over complicate something simple!

Awesome, good to hear you got it working, dropping the pan is the best thing you can do in that situation, then you know what you have to work with.