Hey guys, I have a 75 vette with an TH400 auto transmission and need some assistance troubleshooting an issue. I have searched the forums and have gained some insight but I am new to the tranny and confused on which way best to proceed.

Symptoms: the other day after driving to the store, put the car in reverse, backed out and then went into drive. At this point the transmission did not seem to "engage" all the way. It would move forward but when I gave it gas it just seemed to rev and barely go. I dropped it into a lower gear, felt the engagement and got some more momentum but still rev'ved high into the rpm's and had a hard time moving. At this point I stopped, parked, turned the car off and restarted, etc. Same results so I limped home and parked and started reseaching. Reverse has no problems.

Up to this point I have had no problems with the trans other than a little "hard" engagement when shifting between gears (1st, 2nd, etc.). The kickdown (or up) has never worked so this was a going concern. I recently had the pan gasket replaced and had the oil checked, some flakes but no burnt oil. I checked the level both hot and cold and it seemd fine. I checked the vacuum and line, they both seemed fine and the vacuum seemed new. There are no major leaks to address.

I have been reading about the govenor but I am still unclear as to how to check it properly. Are there any other immediate things I should look at? I have ordered a ATSG repair manual so I can get educated and not ask stupid questions.

I could use some guidance as to what to check next. I am reluctant to take my car to the shop as most of them just screw it up and I have to refix it anyway. I am not much in the mood to listen to someone immediately try to sell me a new transmission. If that's what it takes then ok, but I want to be educated before I talk to bubba.

Am I looking at a total rebuild here or is this a symptom of something commonly identifable and straight forward to address? The car is all original and has 39K on the odometer (acurate?).

I appreciate any advice.

 

Since you've checked fluid level, possible causes =

#1 Sprag or reverse roller clutch
#2 Forward clutches
#3 Pump Stator
#4 Oil sealing rings

Check Pump Stator where rings ride,Ck clutch drum where oil rings ride. Most of the time a overhaul kit and a bushing kit will take care of the problem. Alot depends on wear inside. I know a trustworthy shop in Lawton Oklahoma but no one in Texas.

Jim

 

The most likely cause for your problem is low oil level. A defective governor will cause late shifts or NO shifts, but it won't cause slipping clutches. Other causes of slipping at 'light' throttle positions are: worn out clutch pack/plates; low main pressure from the charging pump; bad/cut seals on that apply piston.

 

Where do I check the pressure on the trans? I have a gauge just have never tested one before.

I pulled the pan and there was nothing in the oil, no metal material, and the oil was not burnt. Also changed out the vacuum modulator and checked the valve to make sure it was not stuck. Refilled to proper leve, tested again. Same issue, solid engagement in reverse, little if nothing in drive and low.

IF one of the clutchs had gone bad would there not be some evidence of this in the pan? Or do these just go bad regardless with no warning?

When I have the car on stands and test reverse/drive the trans engages and rotates the tires as expected (engine still revs high when you hit the gas). Could this be a pressure problem?

Just trying to track anything/everything before I drop the trans for a rebuild.

Thanks to all.