TH400 wont shift to drive

Some details: 69 chevy c20 with a 350 sbc and a th400. Neither the motor nor the transmission are original.

Problem: when starting the truck everything shifts fine from 1st to drive. Say I drive a few miles to home depot. If I spend about 30 minutes or so, and come back to the truck and start it up, it will not shift from 2nd to drive. If I play with the shifter a bunch I can eventually get it to shift into drive, but as soon as I slow down enough for it to down shift again (say a stop light) it will not shift back into drive.

I have checked the fluid levels and they seem to be fine. I know quite a bit about engines and other mechanical stuff, but not as much about transmissions. I am thinking that this may have to do with a vacuum modulator or perhaps the solenoid. But I do not know how to check these, or even if these could be the problem. I would hate to waste the money on parts that were not needed.

Does anyone think it may be one of the two parts I mentioned? If so how would I verify they are broken. Any help would be appreciated.

Thanks Andrew

Reply to
Andrew
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It won't cost much to replace the mod. valve. I'd get an adjustable one--easy to adjust, and this may solve problem; no need to buy high-priced branded mods. BTW: if you pull the flexible vac line hose-connector off the mod valve and fluid seeps out this hose, that indicates it's bad. If no fluid, still may be bad. Ensure no holes/leaks in vacuum line from source, near carb, to the connector hose. If no go, then change fluid and filter. All this is driveway mechanics and won't break the bank. About covers simple service, fixes, testing. That's a good tranny. Easy to make a homemade 'shift kit' while changing fluid and filter--plus this increases fluid-flow to clutches to help it shift into high (and into 2nd) gear much crisper. Do a google search to find how to do this shift kit (where to drill a hole or 2 and finish off job with fluid a quart low), and you just might have a crisp-changing tranny. HTH, sam

Reply to
sdlomi2

You think it's easy to make a shift kit for a TH-400? (Or any tranny?) We tried that back in college with a B&M kit for my friend's 1977 Cadillac Coupe De Ville. That was the last time the car moved until the tow truck picked it up and took it to the garage. Ha!

Reply to
Craig

Well, then you didn't follow the instructions. The B&M kits came with clear instructions and adequate explanation to guide most people.

Once you know what ports to enlarge, the "kit" is largely superfluous.

It's not that hard but you do have to pay attention to detail.

Reply to
anumber1

Check the vacuum line for the shift module. (from the intake to module) If no leaks, pull the line off at the module. You might see tranny fluid. (alot of times the module will leak when bad) I?ve had a ?frozen? governor do this also.

Reply to
thenitedude

It is always good to check for vacuum leaks in and to the modulator. Pull the line at the modulator and look for fluid. If there is fluid in the line, replace the modulator. If you would like a little tuneability, use an adjustable modulator - not all are. That said, the modulator only controls the

1-2, 2-1 shift. It has no influence on the 2-3 sequence.

The 2-3 is controlled by the mechanical governor in the tailshaft housing of the trans. It is also a known problem to get sticky after a while especially in little drive vehicles. You can get to it by removing the rear mount and the tailshaft housing. Free it up and you will probably be OK. You can buy the housing gasket, bushing and rear seal at most good parts stores that handle trans parts. Some shops will sell to you. A shop should also be able to identify this problem through a normal diagnostic. If you happen upon a fly-by-night shop, they may only want to do a complete rebuild even if this is the only problem. If you are a pretty good DIYér, you should bea able to do this yourself. The only thing close to a special tool is a bushing & seal driver of the generic sort.

The passing downshift on your THM400 is controlled by a solenoid which is triggered off the throttle. If passing gear works OK, I would not bother with it.

Reply to
lugnut

Riiiiiiiiiiiiiiiiiiight. Remove this and that ball, do this and that, drill here and there. The instructions were about as easy to follow as a Chinese fire drill.

When I had my C-4 build it was done totally professionally, for dirt cheap, and I was stress free.

Reply to
Craig

I did it on a Fiero and it worked out fine. Nervewracking to take some of those valves to the grinder, but if you work slowly and carefully, I found it went quite easily.

Reply to
HLS

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