Day before yesterday I notice the 350 trans stuttering when it shifted gears. No problem accelerating, etc. Checked the fluid, was a quart low (or so I thought. May have misread the dipstick.. more on this later.) Added a quart to it and was on my way. The next day I get into it and it's slipping pretty bad. Check the fluid.. a quart over. Took it to the garage, put it on the hoist and drain a quart out, but there's no difference in it. Today I pulled the pan, let it all drain out (had new fluid in it, drove it 1400 miles last week from Texas to Michigan without a problem. Fluid was dark, but I didnt see any shavings) and replaced the pan gasket along with the transmission filter. Fresh dexron fluid in it.. still slipping badly. My question is, is there anything else I can check? Or is the transmission shot? It seems to be shifting fine, but just not catching. Reverse gear works perfectly still, just none of the forward gears. Was planning on parking it for the winter next week, but I wanted to take it to the labor day drags here before I did. :ball:
Check the vacuum modulator. The vacuum supply must be leak free, and the modulator should hold vacuum.
If I remember right the modulator only controls when the car shifts? (Modulator on the right rear of the transmission, on the exterior?) About two weeks ago the hose had come loose, causing the car to rev higher than normal before shifting. A new hose fixed this, and the car returned to normal shifting.. The car still shifts fine, just slipping badly now. I dont understand why it would suddenly start slipping this bad in all gears without a warning, but I am no transmission expert. I had thought overfilling it would cause the clutches to slip, but shouldnt draining the transmission have fixed that? Any other problems to look for?
Does the problem begin to show when it's up to operating temp? If it's ok when cold, but slips when up to temp, it's a pressure problem of some kind being masked by the colder fluid viscosity. Devon
Operating temp doesnt seem to matter. Slipping all the time no matter if the trans is warm or cold. Had to drive it aprox 2 miles to the garage, speed between here and there is between 45 and 25 mph. Figured it should be warm by the time it gets there.
The modulator provides the engine load information to the valve body. If the modulator is bad, it can cause the transmission to slip. If you drive a slipping transmission, it will self destruct in short order.
Talked with a transmission specialist today. Told him the details, he responded that the dark fluid indicates that the drive clutches are fried. I've got a master overhaul kit for this allready, will tear it apart in my spare time and toy with it. Any way to test the modulator when it's off the car? Also, any recommended places to purchase a ST300 master overhaul kit from? Looking for steel clutches, etc.. All the strongest parts possible for that transmission. Will keep the TH350 as a backup.
after driving the car with a th350 you won't like having a st300 in it. especially a non switch pitch version. the car will feel sluggish with that tall first gear
The vacuum modulator can be tested with a piece of vacuum hose and a hand held vacuum pump. Same vacuum pump used for drawing brake fluid from the master cyl. during a bleeding operation. Hook the piece of hose up to the modulator and vacuum pump and see if it holds vacuum. It's that simple. You'll know if it's bad. John Biggs.
Thanks, will test it out. Should put my mind at ease knowing why the trans went out, it's been bugging me not knowing why it happened suddenly instead of gradually. The ST300 is a switch pitch version. Took it out of a 68 GS, but I dont know who put it in the GS or the history on it. Slipps pretty bad at the moment, and when it was in the skylark you could hear something rubbing inside the trans while going down the road. Sounded like metal against metal.. My guess is something rubbing the inside of the case? Wont know till I tear it apart and see how bad it is.
