Had my perfectly functioning TH400 rebuilt last fall in my ‘68/442 as I needed to install a stall converter and it was leaking. I did the remove/replace myself. Transgo kit was used. Guy that built tranny was very experienced but he retired afterwards. I was only able to take a few rides before winter and just got car out again this week. Tranny definitely has issues shifting into 3rd or even staying in third. Seems like it works 90 percent of the time but the other times I have to manually shift back to 2nd and then up to third (console shifter) when it acts up. I don’t have a kick down installed yet so I manually shift sometimes anyways. Definitely more noticeable when car is hot. Fluid is topped off and new. Is there a trick to adjusting linkage as I just reattached everything how it was prior to removal?

 

I was able to track down the tranny builder and ask a few questions. The tranny has a brand new modulator. He doesn't think linkage adjustment has anything to do with it since the gear selector works well. His suggestion was to get a kickdown installed as that is involved with 3rd gear. Tranny worked well other than upshifting too early before I had it rebuilt, and it didn't have the kickdown switch then either so not sure how that could be the issue.

 

If the TransGo skit kit was installed during the rebuild, is it possible he mixed up the valve body gaskets? The shift kit spacer plate has gaskets with a added hole for the dual feed feature. If you compare the case gasket to the spacer plate you will find a hole in the plate that a stock gasket will cover up.

 

Thanks Matt. Is that something I can verify myself my dropping the pan?

 

Spoke to tranny builder this morning and he said zero chance he used wrong gasket. Used the correct gasket from the Transgo kit. I'll replace the vacuum modulator and triple check the vacuum hose to make sure we're all good there.

 

Does the transmission have a good solid reverse? Next time you drive the car, get it going about 20-25 mph, let off the throttle, pull the shifter into 1st gear. The trans should immediately go into first gear, with lots of engine braking. If so, you can be reasonably sure the direct clutch and reverse band are hydraulically sound. That leaves the valve body as the next suspect.

Once the valvebody is out, you can air check the direct clutch and reverse servo.

 

Matt, no issues with reverse or dropping into 1st. Vacuum modulator appears brand new, which is what tranny guy said, and vacuum line is in good shape. One thing I’m wondering is if tranny is low on fluid. Although it reads full on my aftermarket dipstick I remember it taking less than what rebuilder told me it should take. Is it possible the dipstick is wrong? I’ve seen anywhere from 6 qts to 13 qts as the TH400 fluid capacity posted online. I also have a 1 qt extra capacity cooling pan on my tranny. Didn’t make a note but pretty sure I have 10 qts or less to get to full on my dipstick. Is there a right answer on fluid capacity for the TH400?

 

Ya if it's low it can suck air and slip on hard acceleration.
A stock TH400 takes ~11-12qts total from a dry trans and converter.
6qts in the trans 5 in the converter.
Start with 4qts on a pan/filter drop and sneak up on full.
W/O a stock dipstick its a guess. You need to measure it.
Buy a pan with the drain plug in it. Cheap and makes fluid drops sooo much easier.

 

Drop the pan, the full mark on the dipstick should be at the level of the oil pan sealing surface. Other option, put a deep pan from a later model GM truck. Make sure you get the pan, longer oil filter tube, filter bolt and spacer. Even if the dipstick is wrong, the deeper pan will hold more fluid and still read full on the the dipstick. Maybe someone in readerland would have a verified matching dipstick and tube length for comparison. I can’t recall ever seeing that information anywhere.

 

Great thanks guys. I’ll drop the pan and see where the FULL mark is. The extra capacity pan does have a drain plug so easy peasy.

 

This might sound like a dumb question but....

Are you checking the fluid level with the engine running on level ground?

 

I believe the problem is solved. I have been checking fluid with engine off, which was indicating just over FULL. With engine running it is just under ADD. Took about 3 quarts, believe it or not, to get back to just over FULL with engine running. Wrung her out afterwards and no issues. So apparently there’s no such thing as a dumb question with me. Thanks for all the help fellas.

 

I'm glad it was a simple fix.

 

Matt I’m diving back in as I never fully resolved my shifting issues when hot. I typically drive car 20 minutes each way when I take it out but if I go longer and fluid gets hotter I have return of symptoms: not upshifting automatically, but will always shift if I manually shift. I want to revisit your question about VB gaskets now that I have pan off. Is removing VB and inspecting gaskets relatively easy with tranny installed? Looks easy if tranny is out and inverted, just afraid I’m going to struggle with the check ***** during reassembly. It is a TransGo shift kit as I still have box from the rebuilder.

 

Installing a valve body kit is fairly easy if you have a clean work area. Don’t force anything, make sure the valves move freely and snap back to their resting place with some authority. Use Vasoline to hold the check ***** in position, DO NOT use wheel bearing grease. Bearing grease won’t dissolve in atf and will clog the filter.

Good luck.

 

Get with TransGo, the valve body gaskets for their shift kit are unique. The stock gaskets won’t work. Pay attention to the kickdown solenoid design, the early oval metal can type uses a thin metal shim type gasket, the later open face style does not.

 

Thank you