In a time crunch I put a used TH400 from a 1990 454 Suburban in my 1987 454 Suburban... I did clean it up and put fresh fluid and filter in before I bolted it up, and it was really clean inside.
After it was in I found that it had the same issue as the one it replaced... ie no lockdown or engine braking in 2nd gear... it just runs away like in N.
Also I noticed it shifted much much softer than the original trans, which maybe had a shift kit. Also it makes a soft whining noise when you engage drive and at low speed.
Ok, so I put up with it until this week, while moving house it started doing odd things...
1/ Drives fine in all fwd gears but at 50 to 60mph will stay in 2nd unless I lift off the throttle almost to coasting.. then it slurrs into 3rd. Any aplication of gas makes it downshifts into 2nd again.
2/ Reverse is all but gone away.. it engages but judders and has no bite..
Fluid is fresh looking and level is good.
I have the original tranny still, and a rebuild kit in a box somewhere, but timing is not right to do it... just moved house, and garage in chaos..
Anyone got input on a short term fix? Maybe the vacuum modulator went bad?
Oh, also the original trans has a large "HD" cast onto the case... does that mean its better or I need different parts?
TH400 experts please help!
After it was in I found that it had the same issue as the one it replaced... ie no lockdown or engine braking in 2nd gear... it just runs away like in N.
Also I noticed it shifted much much softer than the original trans, which maybe had a shift kit. Also it makes a soft whining noise when you engage drive and at low speed.
Ok, so I put up with it until this week, while moving house it started doing odd things...
1/ Drives fine in all fwd gears but at 50 to 60mph will stay in 2nd unless I lift off the throttle almost to coasting.. then it slurrs into 3rd. Any aplication of gas makes it downshifts into 2nd again.
2/ Reverse is all but gone away.. it engages but judders and has no bite..
Fluid is fresh looking and level is good.
I have the original tranny still, and a rebuild kit in a box somewhere, but timing is not right to do it... just moved house, and garage in chaos..
Anyone got input on a short term fix? Maybe the vacuum modulator went bad?
Oh, also the original trans has a large "HD" cast onto the case... does that mean its better or I need different parts?
TH400 experts please help!
Last edited:
It sounds like you are losing the high clutch pack. This pack is also applied in reverse which could explain the condition you have described, unfortunately there is no external fix for this....
