TH400 :Holding 1st gear from automatic upshift in low 1

Well, i get this question 3 or 4 times a year. “When i am hauling ass across the track in low 1 the damn thing upshift’s right at the last second to 2nd gear and i lose all power before the hit!”  the other complaint is those of you guys using th400’s still for heavy towing applications, but you don’t see that very often these days because of gas prices, so pretty much the th400 is more of a performance item these days. i digress…….GM designed the th400 to force upshift even in low 1 to prevent engine damage. you usually have to hit 3500-4500 (depending on wear)and be on the throttle to make it upshift, but it does happen.  well, here’s the simple mod’s to hold the low 1 gear at all speeds .


First off, you must drop the oil pan and remove the valve body. you then remove the 2nd gear shift valve exposing the spool and sleeve assembly. there are three valves on this end of the valve body, all of which the pins remove from the case side of the valve body, so therefore the valve body must be removed to do this. there are the 1-2 shift, the 2-3 shift which has a spring at the rear against the retaining pin, and the smaller 3-2 downshift valve to the opposite side. you want the large 1-2 valve assy as shown.

On the spool valve, you then grind 3 small lands in the large area to the rear of the spool, of which there is an arrow pointed to it in the pic. this is to allow oil flow to bypass this area of the valve. you then must plug the exhaust port in the valve body itself, which is the other arrow in the picture. to do this you can either wedge a check ball in place, or what i do is tap it for a 1/4″ set screw and plug it using loctite….being careful not to tap and install it too deep preventing the valve from moving at all. i suppose you could also spot weld it…..but never done it that way since this is an area you do not want to warp otherwise the valves for the shift will bind! remember you must plug the exhaust port AND grind the land for relief on the spool valve otherwise it won’t work.

once you are done with this, re-assemble it and beat on it. it is obvious that this kind of a mod is a pain in the ass for something installed in a car, but it is a piece of cake during a rebuild. i do this for most all serious street/strip builds and derby guys who insist on a normal shifting tranny for derby. this way they can still hold 1st gear without issue.

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