Th400 issue

[79K20M]

Just bought my first square body. 1979 k20. The guy took out the manual trans and put a TH400 in. The issue I'm having is when you give it gas the truck crawls forward give it more gas and it still crawls as of it was just ideling till it shifts. When it shifts to second it will pick up speed like normal. I replaced the fluid, filter, vac modual, checked the gov. The trucks got a base 350 gm crate, lifted 6" on 35x12.5r15s, np208. The trans tag says FWA 90-FWA. I'm thinking either the trans is fried or the TC is for a big block.

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[chengny]

First, be sure your shifter linkage is correctly adjusted so that the shift lever (item A) hits exactly on the various gear detents. Only the neutral detent is set - if that is correct, the other detents should fall into place as well.

Adjust

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• Apply the parking brake.

1. Loosen the screw (242) or the nut (236), as used
2. Put the column selector lever in the "N" (Neutral) position.

• Put the lever into the neutral gate. Do not use the indicator to find the neutral position.

3. Put the transmission in neutral (this means down at the transmission using the shift lever "A" - see text below the diagram).

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• Move the shift lever (A) to the forward position. Then back to the second detent.

4. Hold the rod (240) tightly In the swivel (232).


Tighten

• Nut (236) or the screw (242) to 23 N'm (17 ft. lb.).

5. Put the column selector lever In the "P" (Park) position.

6.Check the adjustment

• The column selector lever must go Into all positions.
• The engine must start in the "P" (Park) or "N" (Neutral) positions only. Adjust If needed.

CAUTION:With the selector lever In the "Park" position, the parking pawl should freely engage within the rear (reaction) internal gear lugs or output ring gear lugs and prevent the vehicle from rolling, which could cause personal injury.

• Align the indicator. If needed.
• Release the parking brake.

 

[chengny]

Next check the kickdown switch to make sure it isn't energizing the downshift detent solenoid. If you look at the accelerator pedal bracket, you'll see a plunger type switch bolted to it:

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If properly adjusted, when the accelerator is heavily applied the pedal arm makes contact with the plunger and depresses it which closes the contacts. This sends power down to the detent solenoid on an orange lead. The orange lead comes through the firewall (in it's own grommet),

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runs down along the LH side of the bell housing and then towards the back of the transmission housing. There is a connector on the outside of the TH400 casing that the orange lead plugs onto. You'll see it when you check your gear shift linkage:

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On the other side of the casing is an extension of the orange lead that connects to the detent solenoid:

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Anyway, if the downshift switch isn't adjusted correctly - and is always closed - the downshift detent solenoid will be energized all the time. I am not sure how much that would affect 1st gear operation (if at all). But it would be an easy test to simply unplug the orange lead connector from either the switch on the accelerator bracket or at the connector on the transmission housing.

If disconnecting the lead regains normal 1st gear operation, leave it off and figure out how to fix it properly later. Not having a downshift function for a while is not the end of the world - you probably won't even notice it. Certainly a lot better than having a useless 1st gear.

 

[HotRodPC]

For sure check the shift linkage to make sure you're not getting a bleed between detents and not all the way or part way in one and part way in another.
Once you know that is correct. Try manual first gear. Put the shifter in 1 and see how it does. The transmission makes 1st gear 2 different ways, both in D, and a different way in manual 1st. So process of elimination to find out if a clutch pack is bad or not. What did the fluid look like when you changed it?

 

[87scotty]

What did the fluid look and smell like when changed

 

[QBuff02]

another thing to check and verify, is that the vacuum modulator hose is hooked up to the "manifold" vacuum port up at the carburetor. If the hose that supplies the vacuum modulator on the trans is hooked to the "ported" vacuum port on the carb, the trans will "slip" under acceleration or "light" load.