TransGo reprogramming kit for the GM TH400 that firms up the shifts and holds 1st and 2nd to any RPM when the shifter is in those ranges.
If your Turbo 400 shifts lazy under load, flares into second when you tow, or keeps upshifting out of low when you want it held there on the drag strip or a trail, the TransGo 400-1&2 is the kit built for that job. It is TransGo's heavy duty and high performance calibration for every GM TH400 (3L80) from 1965 on, as used in Chevrolet, GMC, Buick, Oldsmobile, Pontiac and Cadillac cars, trucks, vans and motorhomes. Part number TGO400-1-2.
The TH400 has no shift solenoids. Governor pressure tells the shift valves when to move, the vacuum modulator sets line pressure, and accumulators decide how long each clutch takes to apply. A stock unit is tuned for a quiet, long shift, and that overlap is heat that wears the second and direct clutches. This kit recalibrates the valve body and separator plate so each shift is shorter, and it lets you pick how short. TransGo publishes four ranges: passenger cars and light trucks, motorhomes and heavy trucks on big tires, hot rods, and race only. Each range is set by the size of the 2nd and 3rd feed holes plus the 1-2 accumulator spring combination, so a tow rig and a 10-second car get very different shift feel from the same box.
The other half of the kit is manual hold. A new 1-2 shift valve and plug stop the transmission from upshifting in manual 1, and it holds 2nd in manual 2 to any RPM, with back-shifts to 2nd and 1st at any RPM. In Drive it still shifts automatically.
The TH400 is non-electronic, so none of this sets a trouble code. Diagnosis is by road test and pressure gauge.
A shift kit firms a healthy transmission; it does not repair one that slips from burnt clutches or a worn pump. Before you order, check the fluid: dark, burnt-smelling ATF with friction material in the pan means a rebuild comes first. Pull the vacuum line at the modulator, and if it is wet with ATF, the diaphragm is leaking. Then put a gauge on the line pressure port; GM's spec is about 60 to 90 psi in Drive at 1000 rpm. Our TH400 harsh shift guide and valve body testing guide walk through both ends of the shift-quality problem.
TransGo lists the 400-1&2 for all TH400 / 3L80 units 1965 and later. It does not fit the 1964-only first-design TH400 (TransGo uses a different kit there) and it is not a 4L80E kit, even though that unit grew from the 400. If you are unsure which transmission you have, the TH400 has a 13-bolt pan and a vacuum modulator on the right rear of the case. Send us your year and VIN if you want a second check, and see every part we carry for this unit on our TH400 parts page.
If the transmission is apart, the kit's 16 direct clutch return springs and heavy duty 2nd snap ring go in during assembly. Pair it with fresh frictions such as the TH400 friction module or the Coan 22107 master overhaul kit.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. TransGo lists this kit for TH400 units from 1965 on. The 1964 first-design TH400 uses a different TransGo kit, and the 4L80E is a different transmission altogether.
No. In Drive it still upshifts on its own. In manual 1 it will not upshift, and in manual 2 it holds second, both to any RPM. It also downshifts to 2nd or 1st when you pull the lever, so watch engine speed when you do.
Yes. The valve body work is done through the pan opening. The direct clutch return springs and the heavy duty 2nd snap ring in the kit are optional extras that only go in when the transmission is apart for a rebuild.
You choose. TransGo gives four feed hole ranges from passenger and light truck to race only, plus accumulator spring combinations to tune the 1-2. Start conservative, road test, then go firmer; extra gaskets are included for that.
Dexron ATF, the factory fluid for the TH400. Use a fresh filter, and TransGo suggests an Allison-type filter with a bronze screen for hard-launching drag cars.
No. If the clutches are burnt or the pump is worn, rebuild first and install the kit during the rebuild. The kit firms and protects a healthy unit.
