Rebuilt TH400 will not downshift and will not rev out when WOT - Diesel Place : Chevrolet and GMC Diesel Truck Forums
 
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Old 07-12-2019, 06:01 PM   #1 (permalink)
87scottsdale
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Rebuilt TH400 will not downshift and will not rev out when WOT

Ok, Here is my dilemma. I've been trying to research this for quite some time. First of all it's a 1987 Chevy Scottsdale 3/4 ton 4x4 6.2 NA "J" code diesel with an almost new TH400 transmission behind it (which I know is the USHH) lol.

Anyways, My issue is that when I mash the pedal no matter what speed or when. It will NOT downshift and if I mash on it from a dead stop it will NOT rev out like it used to years ago. I am aware of the modulator at the trans and aware of the VRV as well as the vacuum pump.

I have checked vacuum at the pump and its right around 21-22 in.HG. I just hooked up a mityvac to the output to the transmission and it reads 15 in.HG at idle and will drop to zero if WOT. I am really stumped here as everything is working as it should as far as normal shifts go.

I have been messing with the plastic adjuster at the top of the VRV and it DOES change softness and hardness of shifts as well as when it shifts. I have read there is some kickdown switch of some sort at the pedal but cannot see one what so ever. I can post pics if need be. Its just driving me INSANE because it used to downshift no problem and now it doesn't even do it on a hill when mashed.

Thank you for any help you guys provide and hopefully I can solve this soon!

__________________
1987 V20 Chevy Scottsdale
n/a 6.2 VIN-J nothing done to it "yet"
TH400
4" lift runnin 33's

1960 Willy's Wagon 6" lift on 33's with a 350ci and TH350 trans original tcase.

1987 Toyota 4runner SR5 3" body on 33's
lightly buillt 22RE maybe a 4 cyl diesel swap in the future?

1984 Ford F350 XLT bone stock with a 460ci - SOLD

Last edited by 87scottsdale; 07-12-2019 at 06:02 PM.
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Old 07-12-2019, 07:32 PM   #2 (permalink)
legendman
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From what I remember the downshift is electrically operated....


The orange wire on the LH side of the TH400 (single wire) is a 12V power wire that activates a little solenoid on the valvebody to cause the 3-2 forced downshift.

This orange wire connects to a little switch mounted above the throttle pedal.

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Quote:
Originally Posted by Diaric View Post
What your saying happened, should never be able to happen.
Quote:
Originally Posted by smackzed View Post
I put a chevrolet tailgate on my last truck and it started acting like a girl...love me, need me, want me pay attention to me. Took it off and put the GMC back on (fixed the latch) and it went back to being the same old dirty bastard it used to be
~ Bob
1987 K20 5.7 TBI 700r4 np208
1990 C1500 5.7 700r4 Short stepside
1999 Chevrolet K2500 5.7L Auto
2006 Jetta TDI 5sp

Do yourself a favor and read the 6.2 Diesel Student Training Manual ... lots of good info...it's in PDF format located here >> PDF Manuals
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Old 07-12-2019, 08:35 PM   #3 (permalink)
87scottsdale
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Hmmm..... From what I've read the 6.2 diesel is purely vacuum actuated. Would it be a wire on the LH (drivers side)? Because I do have a plug there with 1 wire if i'm not mistaken I will have to go out and take another look. But I was just under the dash all over where the throttle pedal is and there is no switch of any sort there.

I did have someone go inside the cab and put the pedal to the floor to double check that the IP is opening fully, and it is. Weird thing is I just went and ran an errand and got it to down shift but only at like 20-25 mph. I am just totally lost here.

Before the tranny blew it worked fine it seemed..... Go to get on the freeway and mash on it then it would down shift and get up to 55 no problem. Hard for these diesels to do when in 3rd gear from 30-35 mph lol.
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Old 07-13-2019, 01:46 AM   #4 (permalink)
KenDDiesel
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All turbo 400's use an electric kickdown. There a single spade connector on the drivers side of the case.
And you should have a kickdown switch attached to a bracket up above the gas pedal a bit.
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1991 k30 cclb 6.2l 4l80E
1986 k30 cclb 6.2 n/a th400 dually
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Old 07-13-2019, 09:03 AM   #5 (permalink)
legendman
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Quote:
Originally Posted by 87scottsdale View Post
with an almost new TH400 transmission behind it (which I know is the USHH) lol.

How much do you know about the rebuild?
I am wondering if by chance it has a gas motor governor and modulator.


The vacuum side of things regulates shift points and line pressure.
__________________
Quote:
Originally Posted by Diaric View Post
What your saying happened, should never be able to happen.
Quote:
Originally Posted by smackzed View Post
I put a chevrolet tailgate on my last truck and it started acting like a girl...love me, need me, want me pay attention to me. Took it off and put the GMC back on (fixed the latch) and it went back to being the same old dirty bastard it used to be
~ Bob
1987 K20 5.7 TBI 700r4 np208
1990 C1500 5.7 700r4 Short stepside
1999 Chevrolet K2500 5.7L Auto
2006 Jetta TDI 5sp

Do yourself a favor and read the 6.2 Diesel Student Training Manual ... lots of good info...it's in PDF format located here >> PDF Manuals
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Old 07-13-2019, 05:55 PM   #6 (permalink)
87scottsdale
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It was rebuilt years ago by a transmission shop here in my city, But only has about 3000 miles on the rebuild. I guess I forgot to add the truck HAS been sitting for about a year.

(Update!!!!) I have been driving it daily for a few days now. I just took a trip to the store and it downshifted at about 30mph and will only downshift around there every time (so far) I am about to take another trip and will see if it continues to do so. Is this normal for these diesels since the RPM band is totally different? I just find it weird that it wont rev out as well when you mash on the pedal or is this normal also? Because I swore it did before.

It does indeed have an electronic sensor on the drivers side of the transmission with 1 wire coming out of it. Only thing weird is there is not a switch at the throttle pedal anywhere. Ill grab some pictures and throw them up so you can see what im talking about.

Appreciate all the input guys! Getting closer to knocking this one out (hopefully) lol
__________________
1987 V20 Chevy Scottsdale
n/a 6.2 VIN-J nothing done to it "yet"
TH400
4" lift runnin 33's

1960 Willy's Wagon 6" lift on 33's with a 350ci and TH350 trans original tcase.

1987 Toyota 4runner SR5 3" body on 33's
lightly buillt 22RE maybe a 4 cyl diesel swap in the future?

1984 Ford F350 XLT bone stock with a 460ci - SOLD

Last edited by OkDually; 07-13-2019 at 06:09 PM. Reason: sig
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Old 07-13-2019, 06:50 PM   #7 (permalink)
87scottsdale
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Ok, Just went for another drive like I had said. Still down shifts right at or barely under 25mph which is probably nearly 30mph with the increased tire size. It still will not down shift anywhere after that. Thanks again guys! grabbing pictures is next step. Unless someone has any other helpful input.
__________________
1987 V20 Chevy Scottsdale
n/a 6.2 VIN-J nothing done to it "yet"
TH400
4" lift runnin 33's

1960 Willy's Wagon 6" lift on 33's with a 350ci and TH350 trans original tcase.

1987 Toyota 4runner SR5 3" body on 33's
lightly buillt 22RE maybe a 4 cyl diesel swap in the future?

1984 Ford F350 XLT bone stock with a 460ci - SOLD

Last edited by OkDually; 07-13-2019 at 07:20 PM. Reason: sig
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Old 07-14-2019, 09:41 AM   #8 (permalink)
legendman
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Quote:
Originally Posted by 87scottsdale View Post
Only thing weird is there is not a switch at the throttle pedal anywhere. Ill grab some pictures and throw them up so you can see what im talking about.

Look around the accelerator pedal bracket.

https://www.gmsquarebody.com/attachm...dal2-jpg.2476/

__________________
Quote:
Originally Posted by Diaric View Post
What your saying happened, should never be able to happen.
Quote:
Originally Posted by smackzed View Post
I put a chevrolet tailgate on my last truck and it started acting like a girl...love me, need me, want me pay attention to me. Took it off and put the GMC back on (fixed the latch) and it went back to being the same old dirty bastard it used to be
~ Bob
1987 K20 5.7 TBI 700r4 np208
1990 C1500 5.7 700r4 Short stepside
1999 Chevrolet K2500 5.7L Auto
2006 Jetta TDI 5sp

Do yourself a favor and read the 6.2 Diesel Student Training Manual ... lots of good info...it's in PDF format located here >> PDF Manuals

Last edited by legendman; 07-14-2019 at 09:43 AM.
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Old 07-14-2019, 12:57 PM   #9 (permalink)
87scottsdale
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I don't have a kickdown switch at the pedal. I am going to try these pictures I just took, Hopefully they work!






I thought the VRV is what controls the kickdown on a 6.2 diesel with manual IP? I am beginning to wonder if my VRV is starting to go out maybe? I know all vacuum lines are good and solid.

Last edited by 87scottsdale; 07-14-2019 at 12:59 PM.
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Old 07-14-2019, 02:16 PM   #10 (permalink)
87scottsdale
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BTW the clear module you see there is not a switch. Pretty sure it was part of the alarm system that was in it at one point in time. Not to confuse with any sort of kickdown switch.

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