TH400 Shift Kit, new and ready to install. Fits the TH400. Ships fast from the USA.
This TH400 shift kit is a new TransGo 400-1&2 Reprogramming Kit, the box most builders reach for when a Turbo 400 in a truck, motorhome, muscle car or street rod feels sloppy between gears. If you are searching because your 400 slides into second, takes too long to grab third with a load behind it, or keeps cooking direct clutches, this is the kit that addresses those complaints from the valve body side. It fits every TH400 from 1965 on and installs without removing the transmission.
Every shift in a TH400 is decided by oil, not electronics. The governor on the output side builds pressure with road speed, the modulator reads engine vacuum and sets line pressure, and the shift valves move when one force beats the other. How the shift feels is set after that, by the size of the feed holes in the separator plate and by the 2nd and 3rd accumulators that cushion each apply. The 400-1&2 replaces the plate and gaskets with TransGo's triple-plate setup, changes the modulator valve spring, and supplies new accumulator springs, so the clutches lock up faster with less slip. TransGo also adds its patented dual feed to the 3rd (direct) clutch, which it says triples that clutch's holding power. The kit's other trick is Gear Command: in manual 1 it stays in low, in manual 2 it holds second, and it back-shifts into either gear at any RPM.
There are no scan-tool codes on this transmission, so these show up on a road test, not a code reader.
Rule out the things a shift kit cannot fix. Low or burnt fluid, a modulator that leaks ATF into its vacuum line, a disconnected vacuum hose, or a sticking governor can all cause soft or late shifts on their own. A pressure gauge on the line port tells you whether the pump and regulator are doing their job. Our TH400 common problems guide covers each of these, and the delayed engagement guide helps if the unit is slow to pick up Drive or Reverse, which points to the pump or forward clutch rather than the shift calibration.
This kit is for the GM TH400 / 3L80, 1965 and later, in GM passenger cars, light and heavy trucks, vans and motorhomes. It is not for the 1964 TH400, the TH425 front-drive unit or the electronic 4L80E. Many TH400s have been swapped into other vehicles over the years, so go by the transmission, not the vehicle: 13-bolt pan, vacuum modulator on the passenger side near the rear. If you want us to double check, send your year and VIN or a photo of the pan and case. More parts for this unit are on our TH400 collection page.
With the transmission in the vehicle, the job is a pan-drop: drain, remove the filter, unbolt the valve body and plate, swap in the TransGo parts and gaskets per the step sheet, and reinstall. A few points from the TransGo instructions that trip people up:
Refill with Dexron ATF and check the level hot. If the clutches are worn, do the kit during a rebuild with a TH400 overhaul kit, and install the kit's direct clutch return springs and heavy duty 2nd clutch snap ring while it is apart. Seal up external leaks at the same time with the external reseal kit.
| Fits | TH400 |
| Part type | Shift Kit |
| Condition | New |
| Brand | TransGo |
| Reference | THM400 |
| Ships | From the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
It is the TransGo 400-1&2 Reprogramming Kit with Gear Command, TransGo's heavy duty and high performance kit for the TH400 from 1965 on.
Both. TransGo's instructions give separate settings for passenger cars and light trucks, motorhomes and heavy trucks, hot rods, and race only. Use the motorhome and heavy truck range for a tow vehicle so the shifts are firm without being harsh.
No. Everything that changes shift feel is in the valve body and separator plate, which come out through the pan opening. Only the optional direct clutch springs and 2nd clutch snap ring need the unit disassembled.
No. The 4L80E is electronically controlled and uses a different valve body. This kit is only for the hydraulic TH400 / 3L80.
At minimum a new filter and pan gasket. If the vacuum modulator is original or leaking, replace it too, because the kit depends on a working modulator. If the clutches are worn, rebuild first with an overhaul kit.
