Any TH400 trans experts ?? HELP !
Registered User
Joined: Jan 2010
Posts: 6,105
From: central Indiana
Does the shift points change with additional throttle, or is it delayed and firm at every throttle position?
Make sure the vacuum line isn’t kinked, carboned up, or restricted in any way. The modulator needs to react to changes in vacuum pretty quickly.
Pull the modulator valve from the case, it needs to move freely.
Next thing to check is the kickdown solenoid. Disconnect the wire to the solenoid (orange wire at the case). Check for power, it should only show power at WOT. If so, it’s time to drop the pan for inspection. Make sure the solenoid is intact, bolted securely, and has no gasket (assuming it’s the later model square kickdown solenoid WITH a gasket molded to the mounting surface. The early oval can style used a tin steel shim gasket.
Registered User
Joined: Jan 2010
Posts: 6,105
From: central Indiana
I have tried to copy and paste a article here, for some reason it won’t show up. I’ll try to explain as best as I can, a picture is worth a 1000 words!! I wanted to post a couple hydraulic diagrams.
There is a valve setup in the valve body called the detent regulator valve. It takes line pressure and regulates it to 70psi (iirc) that regulated oil works against the 1-2 and 2-3 valves, holding them in the upshift position. The detent (kickdown) solenoid has a valve that is held closed by a spring. At WOT, detent oil is vented from the detent valves. That allows the valves to go into the downshifted position. There is also govenor oil working against the shift valves. When govonor oil goes high enough (the car is going fast enough) govonor oil will overcome the detent oil pressure, allowing the valves to move into a upshift position.
There are 3 different size detent valves in the 1-2 valvetrain. I’m guessing the different sizes were used for cars with wildly different kickdown needs. A car with 3.91 gears would need a lower upshift speed than one with 2.73. Or a high winding W-31 engine could tolerate much higher upshift speeds than a 6.2 Diesel engine.
As for your problem, if you disconnect the vacuum modulator with the engine running, you should notice a very distinctive change in the way the engine idles from the big vacuum leak. If not, the line might restricted.
Is the line mostly steel? Rubber hose can easily get sucked closed, restricting vacuum.
Do the shift point change at all in relation to throttle position? You said it winds out until about 4000, does it shift at 4000 at WOT?
There is a valve setup in the valve body called the detent regulator valve. It takes line pressure and regulates it to 70psi (iirc) that regulated oil works against the 1-2 and 2-3 valves, holding them in the upshift position. The detent (kickdown) solenoid has a valve that is held closed by a spring. At WOT, detent oil is vented from the detent valves. That allows the valves to go into the downshifted position. There is also govenor oil working against the shift valves. When govonor oil goes high enough (the car is going fast enough) govonor oil will overcome the detent oil pressure, allowing the valves to move into a upshift position.
There are 3 different size detent valves in the 1-2 valvetrain. I’m guessing the different sizes were used for cars with wildly different kickdown needs. A car with 3.91 gears would need a lower upshift speed than one with 2.73. Or a high winding W-31 engine could tolerate much higher upshift speeds than a 6.2 Diesel engine.
As for your problem, if you disconnect the vacuum modulator with the engine running, you should notice a very distinctive change in the way the engine idles from the big vacuum leak. If not, the line might restricted.
Is the line mostly steel? Rubber hose can easily get sucked closed, restricting vacuum.
Do the shift point change at all in relation to throttle position? You said it winds out until about 4000, does it shift at 4000 at WOT?
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