TH400 shift problem and questions

[texasag91]

Well I need a little help. I am not at all a transmission guy so I need to be educated on the TH400 transmission. First I have a 1990 454 with a TH400 transmission. Second I want to learn a little more about the specs on how this transmission shifts. Third help with diagnosing a gear shifting problem.

What is the proper speed does this transmission shift through each gear (stock setup)?

Does the shifting have much to do with rpm?

Need some specs on this tranny?

My problem: Transmission shifts through 1st and 2nd fine at about the proper speeds but when reaching 3rd I have to be running about 70 mph to get it to shift to 3rd. Now this is not punching it but the gradually going up in speed on the highway. The engine runs about 3200 to 3500 rpm on the highway but the last few days to get it to shift the rpms are going to about 4500. Then once shifted goes back down to 32-3500 rpm. Also slowing down below 70 mph shifts back to 2nd. What's my problem you think? Vacuum Modulator? Vacuum problem?

Now I have only had this truck about a month so I am still getting the feel of it. According to the guy I purchased it from he did not mention anything about putting a shift kit or anything in the transmission. The engine is upgraded with performance intake and headers.

Sorry so long but I am just a dummy when it comes to transmissions but I am learning.

[Greenlee]

The shift points are contolled by vacuum on the TH400. Sounds like a vacuum modulator problem. That is the cheapest and easiest thing to check. Look for vacuum leaks or replace the modulator if necessary.

I have a TH400 in my truck. It shifts to 3rd at about 45 mph. Sometimes I wish it would hold a little longer. You can buy modulators that are adjustable. They have a screw inside that you can turn for adjustment. I think from the factory they are not adjustable, but there were something like 3 different versions with different shift points. Seems like I read that they are different colors to distinguish the different versions, but I can't remember the details on that.

There is a relay on the firewall that controls your kickdown via computer. The older th400's had a switch behind the gas pedal to control kickdown. However, I don't think this has anything to do with your problem. It should shift fine without the kickdown even hooked up.

[texasag91]

Thanks I will check that out.

[GRADYS Performance]

Shifts are also controlled by the Governor behind the 4 bolt cover near the rear yoke. You may need to inspect and clean it.

[texasag91]

Could the problem I having be the kick down switch? Read this on another forum.

Where is the kick down switch on the 1990 TH400 454?

Again the problem is when shifting from 2 to 3. Sometimes it will rev to high rpms then slowly go into 3rd. Then wants to kick back and forth between 2 and 3.

Could I just disconnect the kick down switch?

[Just call me Sean]

1990 is fuel injected, the computer controls kickdown. You said performance intake, so is it still fuel injected? If not, the computer probably isn't controlling that anymore, nothing is. I believe the switch is on the gas pedal, but mine doesn't have one there because it's fuel injected.

[texasag91]

Mine is a TBI. Only thing performance is the Edelbrock intake. Think the TBI throttle body is stoc. Checked the gas pedal no switch that I could find.

Well I think I found the kick down switch wire on drivers side of trans. I disconnected it but ran worse disconnected. Didn't shift well at all. Sounded like it would barely shift into each gear. Got home plugged back in the wire back in. Went for another drive and 1 to 2 shifted pretty good but still revs to high rpms to get 3rd. Now this was just driving normally and not accelerating fast. Did another test drive later. This time wide open throttle from dead stop. This time it shifted quickly through the gears and ran good. More like it should. Held at 75 going down highway everything seemed fine. Down shift fine coming into town to 30 mph.

Trying again tomorrow. Still confused with what is happening. Other steps would be to inpect governor as others said. I did check vacuum line but looks ok.

[Greenlee]

On square bodies, the relay for the kickdown is on the firewall next to the fuel pump relay. Not sure about the newer body style. The computer sends a signal to the relay. Non computer controlled vehicles had the switch on the gas pedal. You could unplug this and it should still shift normally, you just won't have kickdown.

I don't think this has anything to do with your problem. TH400 shift points are controlled by the vacuum modulator.

[Chevy3+3]

Just put a new vacuum modulator on it first and see what happens. Like others have said. That controls your shift points on a TH400. I just replaced mine and it cost around $12. I bought the adjustable one and it had a red stripe on it

[texasag91]

That was my next step. Will try to get one tomorrow. Also going to pull out my governor to and inspect it.

[cadillac_al]

It's hard to diagnose a trans problem when you don't know what the fluid looks like or when it was last changed. If that info is unknown then it could be a 20 year old filter getting plugged up.

[texasag91]

Well I swapped out the vacuum modulator and added some Lucas transmission treatment today. Drove up and down the road starting and stopping about 8-10 times cycling through the gears. Runs pretty smooth right now. Only revved up on the rpms one short time. Guess not warmed up yet. I really think it was a combination between both modulator and additive. Tomorrow head 20 miles to work. Let you guys know how it does. Ya'll were a wealth of knowledge on the subject. Thanks

[texasag91]

Oh yeah. Some time down the road I need to do a filter change.