: TH400 Not Upshifting

01-22-06, 12:36 AM
My 1964 Sedan With 429/TH400 Isn't Upsifting Except At High Revs. I've Tried A New Modulator And The Finger Over The Line Test To Make Sure It's Getting Signal.

Any Suggestions I Can Perform In The Car? I Don't Have Time To Rebuild The Tranny Right Now. If It Comes To That I'll Have To Park It.

01-22-06, 01:51 AM
3 things, vacuum to modulator/or bad modulator (mine failed once on my 76 Delta 88 Royale) and the electric kickdown. I had the switch wear and start to stick, and that kept it from upshifting until redline. Some moving the switch manually fixed it up. I think you can simply disconnect the electrical connector on the trans and it will not kickdown anymore, so it won't allow it to be a problem, if this is the problem. Good way to isolate it.

Third would be a damaged or stuck governor. This is also fixable from the outside, w/o removing the pan.

THM400's are somewhat sensitive to that vacuum being right. I had messed with something that disturbed normal overall vacuum readings and my trans really reacted to it. So ensure that the metal line is ok (it needs to be metal for safety) and that the vacuum to it is connected to the right source. Gauge it if you can, it needs to be connected directly to manifold vacuum.

Let us know what you find.

01-22-06, 07:11 AM
Exactly what I was going to say. All the 400's I ever had that wouldn't upshift were either worn out governor gears, or something to do with the modulator.

01-26-06, 10:37 PM
Checking My Chilton's From The 70's... It Appears That I Will Have To At The Least Drop The Pan To Check The Switch Dealie...

Is There An External Check? Does That Thing Get 12 Volt, Ground, Normally Open Or Closed? I Have Some Other Electric Things In The Car That Don't Work And This Might Also Be Affected.

01-26-06, 11:04 PM
IIRC, if the connector on the side of the trans is disc, it will default to no kickdown. I haven't had a THM375/400 in 20 years... The switch on Olds was on the gas pedal under the dash, but I think Cadillac kept it underhood.

01-27-06, 01:19 AM
Kickdown switch should be near the carb on the throttle linkage side if it's anything like the 70s Cads.

05-05-06, 09:43 PM
Wow. That Was It. A Firm Twist With Channel Locks And It Popped Out.

It's Quite Corroded Inside And Sticks If You Get Too Heavy On The Throttle Despite A Copious Amount Of WD-40. The Rubber Boot Has Dried Up And Blown Away. Know Where I Can Find A New One?