TH400 issues

chevy86 · 12-31-2007, 02:20 PM

I own an `86 C20 Scottsdale pickup 2wd 3/4 ton. The TH400 automatic transmission shifts up through the gears until the truck warms up to normal operating temp. From there it will not shift into 3rd gear but stays in 2nd gear, unless I let off the gas and then you can feel it shift into 3rd. If I accelerate to gain momentum again, it goes right back into 2nd gear. I was told it was gummy/dirty inside the trans the last time it was serviced (about 2 months ago) another said an inside valving system that causes it to go to 3rd gear is somehow stuck or a valve-seal is torn, another said it's the governor. The modulator is good and has adequate vacuum. I also put about a quart and a half of "Trans-X" into it which I was told to use because Trans-X is very thin. Being a thin type of trans-additive, it was supposed to help clean things up in there. So far with no results though.

Ultimately I know I have 3rd gear just not after it's warmed up.

Any thoughts before I go through the expense of a rebuild?
thanks
Chevy86

Case 7230 · 01-02-2008, 11:29 AM

My guess is that the guy who mentioned the seals is right. I would think that the servo seals are probably worn. Before I would go for a rebuild I would drop the oil pan, clean or replace the filter, refit oil pan and fill with new fluid along with a bottle of Lucas Auto Trans Conditioner. I use Lucas every time I do a trans fluid change. It may just get you another year or two.

chevy86 · 01-03-2008, 06:25 AM

I will look into the Lucas product. Obviously something thin in the way of a "cleaner oil" didn't or hasn't yet done the trick. Concerning the servo seals, are they something within reach once the pan is dropped???

Case 7230 · 01-03-2008, 11:20 AM

Dropping the oil pan only lets you clean the pan and filter, you will not be able to access the clutch servo seals. However the Lucas product may soften and expand them. It is not that difficult to do and will only cost you the new fluid and and the Lucas, so worth a try. Should take less than an hour. If you intend to keep the truck for a lot more years or will be doing a lot of towing maybe you should consider a trans rebuild. It`s your call but like I said, if it was me, I would try the Lucas.

phattysGMC · 01-03-2008, 06:03 PM

the same thing happened to my 86 when my son drove it on the highway it had 100,000 plus miles on it he didnt realize it didnt shift and BOOM the cam snapped so i put a new motor in and built the trans. i put it up on stands to make sure it shifted and it only had 2 gears checked pressures and were ok dropped the pan a million times couldnt figure it out was ready to pull it out and go thru it again had a friend w a tranny on the floor and i stole the valve body and swapped it and it was fine found one of the valves was stuck hope this helps

1979_4x4_Plow · 01-03-2008, 08:40 PM

reminds me....i need to do a tune up and a trans filter kit on the truck. if anything i know it needs the tuneup. i am planning on doing a trans filter kit to see if it will help the trans since it quit on me last winter but i went to move it this year and it acted fine so far.

mrjohnwayne · 01-08-2008, 07:57 PM

I'll have to agree. either a stuck valve in the valve body or a torn/wore out seal for the 3rd gear servo

12-31-2007, 02:20 PM	#1
chevy86 Junior Member Join Date: Jun 2007 Posts: 5	TH400 issues I own an `86 C20 Scottsdale pickup 2wd 3/4 ton. The TH400 automatic transmission shifts up through the gears until the truck warms up to normal operating temp. From there it will not shift into 3rd gear but stays in 2nd gear, unless I let off the gas and then you can feel it shift into 3rd. If I accelerate to gain momentum again, it goes right back into 2nd gear. I was told it was gummy/dirty inside the trans the last time it was serviced (about 2 months ago) another said an inside valving system that causes it to go to 3rd gear is somehow stuck or a valve-seal is torn, another said it's the governor. The modulator is good and has adequate vacuum. I also put about a quart and a half of "Trans-X" into it which I was told to use because Trans-X is very thin. Being a thin type of trans-additive, it was supposed to help clean things up in there. So far with no results though. Ultimately I know I have 3rd gear just not after it's warmed up. Any thoughts before I go through the expense of a rebuild? thanks Chevy86

01-08-2008, 07:57 PM	#7
mrjohnwayne Moderator Join Date: Apr 2005 Location: Texas Posts: 3,104	I'll have to agree. either a stuck valve in the valve body or a torn/wore out seal for the 3rd gear servo __________________ John Wayne 2000 F-350 7.3L Powerstroke 4x4

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01-02-2008, 11:29 AM	#2
Case 7230 Senior Member Join Date: Dec 2007 Posts: 226	My guess is that the guy who mentioned the seals is right. I would think that the servo seals are probably worn. Before I would go for a rebuild I would drop the oil pan, clean or replace the filter, refit oil pan and fill with new fluid along with a bottle of Lucas Auto Trans Conditioner. I use Lucas every time I do a trans fluid change. It may just get you another year or two.

01-03-2008, 06:25 AM	#3
chevy86 Junior Member Join Date: Jun 2007 Posts: 5	I will look into the Lucas product. Obviously something thin in the way of a "cleaner oil" didn't or hasn't yet done the trick. Concerning the servo seals, are they something within reach once the pan is dropped???

01-03-2008, 11:20 AM	#4
Case 7230 Senior Member Join Date: Dec 2007 Posts: 226	Dropping the oil pan only lets you clean the pan and filter, you will not be able to access the clutch servo seals. However the Lucas product may soften and expand them. It is not that difficult to do and will only cost you the new fluid and and the Lucas, so worth a try. Should take less than an hour. If you intend to keep the truck for a lot more years or will be doing a lot of towing maybe you should consider a trans rebuild. It`s your call but like I said, if it was me, I would try the Lucas.

01-03-2008, 06:03 PM	#5
phattysGMC Junior Member Join Date: Jan 2008 Posts: 3	the same thing happened to my 86 when my son drove it on the highway it had 100,000 plus miles on it he didnt realize it didnt shift and BOOM the cam snapped so i put a new motor in and built the trans. i put it up on stands to make sure it shifted and it only had 2 gears checked pressures and were ok dropped the pan a million times couldnt figure it out was ready to pull it out and go thru it again had a friend w a tranny on the floor and i stole the valve body and swapped it and it was fine found one of the valves was stuck hope this helps

01-03-2008, 08:40 PM	#6
1979_4x4_Plow Senior Member Join Date: Dec 2007 Posts: 98	reminds me....i need to do a tune up and a trans filter kit on the truck. if anything i know it needs the tuneup. i am planning on doing a trans filter kit to see if it will help the trans since it quit on me last winter but i went to move it this year and it acted fine so far.