1972 Chevy Truck THM 400 Transmission Problems

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  #1  
Old 09-17-08, 07:31 AM
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1972 Chevy Truck THM 400 Transmission Problems

1972 3/4 ton, 2 wheel drive with a 402 big block engine and a 400 trans.

My truck sat for 15 years straight. I recently revived her after a long rest, but the transmission will not respond. I changed the fluids and put a new filter, but still nothing. The linkage is still in tact, no smells or sounds... just nothing.

What do you suppose is the problem?

The oil that I removed for her was a dark red, no smells, but heck, she sat for 15 years. Maybed it shot?

Ron in Washington State
 

Last edited by 72CHEVYTHM400; 09-17-08 at 08:21 AM. Reason: More info about truck
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  #2  
Old 09-17-08, 08:25 AM
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Ron
2 things come to mind. First, the lines could be air bound preventing fluid from flowing (unlikely though). Second, pump behind the TQ converter could be bad from sitting. I would pull the lines going into to the radiator out and try to crank the truck to see if fluid comes out. If it does, move onto something else. If it does not, time to pull tranny and replace pump. If fluid came out, I would repull the pan and yank out the valve body and give it a good inspection. May have gum or varnish buildup in the passages prohibiting the fluid from being correctly routed.
Good Luck

Ben
 
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Old 09-17-08, 08:52 AM
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Ben

I will give it a try tonight and let you know. To change the pump, are any special tools required as I am only a backyard mechanic. Thanks for your help.
 
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Old 09-17-08, 09:04 AM
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Hi ron
actually, I am a backyard mech myself too. I personally never took one apart except for installing a transgo shift kit in the valve body and replacing a few servos. I think, although I am not 100% sure, that you pull off the TQ conert, and the pump is right there..say 6 or 8 bolts and its out. To rebuild them, yes i think you need special micrometers to measure clearances ect. But I believe that they are core exchange items. Another thing to check too, is the band adjustment. I am not sure if the TH400 has such adjustment but any Haynes manual will tell for sure OR you can google it and see what you come up with. I was thinking that the clutches could be suspect but if the truck was running fine before it was parked and it was not slipping or doing those 'runaway upshift/downshifts' I wouldnt focus too much on the clutch assmy. Give the pump a good test and go from there!
 
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Old 09-17-08, 10:55 AM
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I would start off by letting it run at least half hour.
Feel the trans cooling lines once in a while and see it there is a difference in temperature after it has been cooled by the rad. (This would indicate flow)
During this time slowly shift thru all gears (R.N,3,2,1) to get flow going. Just once or twice should be good.
After the half hour is up see if you have anything out of reverse gear. Rev it just some but not for long incase it is just starting to hook up
Get back to us
 

Last edited by frankiee; 09-17-08 at 12:56 PM.
  #6  
Old 09-17-08, 03:02 PM
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Frainkiee,

Will try tonight and let you know in the morning. Thanks for the advise.

Ron
 
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Old 09-17-08, 09:01 PM
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the pump is direct drive off of the torque converter if it is bad you will usually know right off.ie fluid everywhere, most likely you have a stuck pressure regulating valve the bad news is it is in the pump body and you would have to pull the trans, i would try the running and shifting with the rear axle up on jack stands first. did you run it for a while and then check the fluid to make sure it was not low on fluid? more than likely you have multiple problems after sitting that long, ie stuck valves, stuck governor, but most might work out as long as nothing internal is rusted or corroded.

are you going to cowboy up or just lay there and bleed?
 
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Old 09-18-08, 07:29 AM
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All,

The Trans pump is pumping. With the engine running, I took off the top oil line leading into the radiator and trans fluid stated to spew. The pump is good.

I then allowed the truck to run for a while, at least a half hour. I then checked the fluid and it was around a pint over. Unsure if this could cause the problems I am having. Anyway, I then did as recomended by Frankee above, and slowly shifted into each of the gears a few times. Let her rest and did it again. No luck. Alls I got was a slight, very slight, shifting sound, but absoultely no movement. None of the gears work.

What's next?
 
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Old 09-18-08, 09:09 AM
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The Trans pump is pumping. With the engine running, I took off the top oil line leading into the radiator and trans fluid stated to spew. The pump is good.

I then allowed the truck to run for a while, at least a half hour. I then checked the fluid and it was around a pint over. Unsure if this could cause the problems I am having. Anyway, I then did as recomended by Frankee above, and slowly shifted into each of the gears a few times. Let her rest and did it again. No luck. Alls I got was a slight, very slight, shifting sound, but absoultely no movement. None of the gears work.

What's next?
 
  #10  
Old 09-18-08, 09:14 AM
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Ok, I will lift the rear wheels tonight and try it all again. I will let you know in the morning.
 
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Old 09-18-08, 03:12 PM
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if you still can't get it to move then you will at the least have to r & r the valve body to make shure that all of the valves are free in their bores, but you more than likely at this point just need all soft parts,(ie piston seals and gaskets ) probably the clutches are not engaging with sufficent pressure to work it. I think that if you can you should pull the trans and either go through it your self or find someone to do it for you.

Murphy was an optimist
 
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Old 09-18-08, 03:41 PM
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Seals could be distorted/cracked/not there anymore. They dry out after sitting. Also as noted above they could be stuck.
At this point I would try some mechanic in a can such as "Transmedic" or such. I idea is that it will soften up the seals and slightly swell the seals and that would help make them seal again. After I put it in I would do the same manouver as I noted earlier and then leave it overnite. Give it a chance to work into all the seals. Them try agian it the morning.
Keep in mind that swelling the seals could wreck them and make a bad situation worse. This is just what I would do.
I would not tell you that I have had luck over a half a dozen times by putting brake fluid in the trans and leave it over night after running it thru the gears. Remember I would not tell you that. Thats just what I have done in the past. I have in the past started off with 250ml then went to 500ml of brake fluid.
I kept in mind that brakefluid eats seals for a midnite snack.
Thats only what I have done in the past and would not recommend it to anybody unless they have completely given up on the transmission and it was considered junk.
 
  #13  
Old 09-18-08, 07:04 PM
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truck mech

ron . if your truck sat that long . I would say you have stuck valves and check balls in the valve body . if you try the brake fluid witch i dont recomend you can run trans conditioner and drain out a quart of fluid before you put a quart in and also put in friction modifier for clutches . and do all testing on jackstands.and warm up tranny if this dont work and you have no tranny experince. and you want to pull valve boby GET the book and dont try with out it . you can drop pan and move some valves by hand or a dull screwdriver never nick a valve . but if you do it your self you have to know wkere to put back check balls. so take your time and move slow wehn you pull it and try and keep it straight and level . yhe check balls are in tranny above spacer plate and gasket. but ypu need book to go back on with it. if you need to buy a new tranny GM has the best warrenty.
 
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Old 09-24-08, 08:08 AM
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Continuing saga

Hello helpers.

I have been computerless for the past week due to a virus. Anyway, now fixed.

I tried Mech in a Can.... I managed to get a slight movement on the rear wheels, but nothing more than a slight move.

Anything else I should try before I hire a Mechanic to rebuild?

Ron
 
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Old 09-25-08, 04:10 PM
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Red face shadetreeall

if you dont have movement than i recomend a new tranny. check with dealer befor you do any thing, they have some good deals and the best warrenty ive found. remember you get what you pay for,
 
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Old 09-26-08, 05:58 AM
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If you find yourself scraping the old transmission and going for another one, finish off the old one by dumping 500 ml of brake fluid into it and running it thru the gears and then letting it sit over night.
It you plan to fix the old one, dont do this
 
  #17  
Old 09-26-08, 07:49 PM
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what the heck for?

Murphy was an optimist
 
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Old 10-02-08, 07:01 PM
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After 15 years of sitting this transmission will surely have dried and cracked seals. Not shaft seals, but the internal lip seals on the clutch pistons. When these go bad, you aint goin nowhere. The 400 is reasonable and easy to overhaul. You probably have other problems too, after so many years of sitting.
 
  #19  
Old 10-02-08, 07:06 PM
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You hit the nail on the head - Overhaul is due
 
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