TH400 Solenoid Kit

A Turbo 400 that will not kick down to second when you floor it, or that shifts soft and early at wide-open throttle while driving fine around town, usually has a detent circuit problem. This kit replaces the electrical end of that circuit in one go: a new TH400 detent (kickdown) solenoid in the 34420 style, a new one-pin case connector of the 34444A type, a metal gasket and an O-ring. It fits the 1965-up TH400 / 3L80 used in GM trucks, vans, motorhomes and big cars.

What the detent solenoid does in a TH400

The TH400 is fully hydraulic. Governor pressure and modulator pressure decide every upshift, and there are no shift solenoids. The detent solenoid is the only electrical part in the valve body. When the throttle-mounted detent switch closes at full throttle, 12 volts reach the solenoid through the case connector, it grounds through its mounting, and it opens the detent circuit. Detent oil pushes the shift valves back toward the downshift side, which forces the downshift or delays the upshift, and it raises line pressure so the full-throttle shift has enough clamping force. With the solenoid unplugged the transmission still shifts through all three gears; it only loses the forced downshift and the full-throttle pressure rise. Our TH400 shift solenoid guide covers the circuit in detail.

Symptoms this kit fixes

• No forced 3-2 or 2-1 downshift when you floor it
• Early upshifts at wide-open throttle even though part-throttle shifts are normal
• Soft or flaring 1-2 and 2-3 shifts only under full throttle, which overheats the intermediate band and direct clutch over time
• Fluid weeping at the case connector on the driver side of the case, from a cracked connector or a hardened O-ring
• An intermittent kickdown that comes and goes with a loose or corroded connector terminal

If the truck has no second or no third, slips at part throttle, or engages late into gear, the detent solenoid is not the cause. Look at the governor, modulator, pump and clutches instead; TH400 common problems and fixes sorts those out.

How to confirm it before you buy

Unplug the wire at the case and check it with a test light while a helper holds the throttle wide open with the key on. No power means the detent switch, its adjustment or the wiring is the problem, not the solenoid. If power is there, check the solenoid itself: Rostra lists the OE-style TH400 detent solenoid at 9.5 ohms, one pin. An open coil or a reading far from that confirms it. A solenoid that tests good but does not click when energized can be stuck, and a valve body problem in the detent valve train can cause the same complaint, which we walk through in TH400 valve body problems and testing.

Fitment

This kit is for the GM Turbo Hydra-Matic 400 (3L80), 1965 and later, in electric-detent form. That covers the GM car, truck, van and motorhome TH400s and the aftermarket and swap units built on the same case. The kit is listed as 1965-up, so a 1964 unit or a transmission with a non-stock or manual valve body should be checked first; match your old solenoid and the single-pin connector before ordering. The metal gasket is only used on some applications, so it is included in case you need it. If you are not sure what is in your vehicle, send us the year, model and VIN or the tag on the case and we will check. Everything else for the unit is on our TH400 parts page.

Install notes

• Drain the fluid and drop the pan. The solenoid is held to the valve body with bolts and its wire runs to the case connector.
• Pop the old connector out of the case from the inside, install the new connector with its fresh O-ring, and clip the solenoid lead onto it.
• Install the new solenoid with the gasket if your old one had one, and torque the bolts evenly so you do not distort the valve body.
• Fill with Dexron ATF, check the level hot, and road test the WOT downshift.
• Check the detent switch on the throttle linkage. It should close only near full throttle.

The pan is already down, so this is the right time for a fresh filter and pan gasket like our TH400 filter and pan gasket set. If the full-throttle shifts are still soft after the solenoid is fixed, a TH400 shift kit firms up the apply.

Does the TH400 have shift solenoids?

No. The TH400 shifts on governor and modulator pressure. The detent solenoid in this kit is the only solenoid in the transmission, and it only controls the full-throttle downshift and the full-throttle line pressure rise.

What is in the kit?

A new detent solenoid, a one-pin case connector, a metal gasket and an O-ring for the connector. The gasket is not used on every application but is included in case yours needs it.

How do I test my old detent solenoid?

Measure across the terminal and the solenoid body with an ohmmeter. The OE-style TH400 detent solenoid is rated about 9.5 ohms. Also confirm 12 volts reaches the case connector at wide-open throttle, since a bad detent switch causes the same no-kickdown complaint.

Can I replace it without pulling the transmission?

Yes. Drop the pan and the solenoid is right on the valve body. The case connector is reached from inside the pan opening.

What fluid does the TH400 use?

Dexron ATF. Fill to the full mark with the fluid hot after the repair.

Will this fix a TH400 that slips in all gears?

No. Slipping at part throttle points to the pump, the modulator, low fluid or worn clutches. The detent solenoid only affects full-throttle behavior.

• TH400 Shift Solenoid Symptoms, Testing and Replacement
• TH400 Common Problems and Fixes
• TH400 Valve Body Problems: Symptoms, How to Test, and Replacement
• TH400 Filter and Pan Service Guide