TH400 Pump 1971-1998, used and ready to install. Fits the TH400. Ships fast from the USA.
From 1971 on, the Turbo Hydra-Matic 400 settled into the fixed-stator, three-speed workhorse that went behind big-block trucks, motorhomes, three-quarter and one-ton pickups and plenty of street and strip builds. If your TH400 now whines with engine speed, takes its time to grab Drive or Reverse once the fluid is hot, or slips in every gear instead of just one, a worn front pump is the usual suspect. This listing is a used TH400 front pump assembly for 1971-1998 units: the pump body with its gears, the cover with the stator shaft pressed in, ready to bolt to the front of the case.
The TH400 pump is an internal gear type. Tangs on the converter hub drive the inner gear at engine speed, and the crescent splits the suction side from the pressure side. The cover holds the pressure regulator and boost valves that set line pressure from the vacuum modulator signal, and its stator shaft anchors the converter stator and carries the sealing rings that feed the forward clutch. The pump also supplies converter charge, cooler flow and the lube oil that returns through the stator shaft. Because one part sources every one of those circuits, a tired pump shows up everywhere at once.
On 1971 and later pumps the regulator valve bore is the common high-mileage wear point. Sonnax lists its drop-in pressure regulator valve kit for '71-later TH400 and TH425 units and describes a worn bore causing high, uncontrollable line pressure, delayed converter charge and poor Reverse engagement, with converter, crankshaft and clutch damage following if it is ignored.
Check the easy things first: fluid level on the dipstick hot, the filter and its intake O-ring, and the modulator vacuum line. A cracked filter neck or rolled O-ring makes the same noise and low pressure as a bad pump. Then tee a gauge into the line pressure port. GM's chart for the TH400 calls for roughly 60 to 90 psi in Park, Neutral and Drive at 1000 rpm with the modulator hooked up. Read it cold and again at full operating temperature; a pump that makes pressure cold and sags hot is worn. If pressure is high and will not come down, suspect the regulator bore. Our TH400 pump failure guide walks through the full pressure table, and the TH400 delayed engagement guide covers the non-pump causes of a slow Drive or Reverse.
This pump is listed for TH400 units from 1971 through 1998. By 1971 the switch-pitch converter that some 1965-1967 Buick, Olds and Cadillac units used was gone, so these pumps run a conventional fixed stator. TH400s still varied by application over that span, and GM changed details such as cover castings and valve parts, so set your old pump next to this one and compare the bolt pattern, stator shaft spline, cover casting and the sealing ring lands before you tear down. If anything looks different, or you are not sure what you have, send us the year, vehicle and VIN or the transmission ID plate code and we will check it. For other years see the TH400 pump listing, and the full lineup is on the TH400 parts page.
If the unit has been slipping on low pressure, the clutches are probably cooked, so rebuild while it is apart with the TH400 overhaul kit for 1965-1998, and stop external leaks with the TH400 external reseal kit. A front pump alignment band makes centering the pump halves straightforward.
| Fits | TH400 1971-1998 |
| Part type | Pump |
| Condition | Used |
| Brand | OEM |
| 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 listed for 1971-1998 TH400 units, which covers 1985. Compare the bolt pattern, stator shaft and cover casting against your old pump, or send us your year, vehicle and VIN and we will confirm before it ships.
No. Switch-pitch converters were used on some 1965-1967 Buick, Olds and Cadillac TH400s. A 1971-1998 pump is a fixed-stator design and should be paired with a conventional converter.
GM's chart calls for about 60 to 90 psi in Park, Neutral and Drive at 1000 rpm with the modulator connected. Test cold and hot; a big drop once the fluid is hot points to pump wear.
A new front seal, pump-to-case O-ring and gasket at minimum. If the fluid is burnt or the transmission was slipping, rebuild it with an overhaul kit while the pump is out, and reset front end play to .003-.024 in.
Yes. Put fluid in the pump before installing and fill the transmission before the first start so the gears do not run dry. Use licensed DEXRON automatic transmission fluid.
