downshift adjustability on THM350 ?????
Thread Starter
Safety Car
Joined: Mar 2000
Posts: 4,078
Likes: 27
From: Northeast South Dakota
downshift adjustability on THM350 ?????
My THM350 3 speed auto tranny in my 81' doesn't downshift like I would like. It doesn't like to downshift. I notice when I drive around town in 2nd gear after I turn a corner it doesn't downshift into 1st, rather it stays in 2nd and pulls with the engine running under 1000 RPM's. I would like it to downshift in situations like this. Am I correct in thinking that the vacuum modulator valve does that and I can just get an adjustable modulator and throw in in there, and that the TV valve just controls the upshift points????? I adjusted the TV cable according to my Haynes GM auto tranny rebuild manual which says to push the lock button in on the TV cable, then pull the carb linkage to full throttle, allowing the cable to ratchet out of the housing, then release the lock button. Upshifts have always seemed good. What do you all think????
-Justin
-Justin
Race Director
Joined: Sep 2002
Posts: 10,362
Likes: 5
From: San Diego - Deep Within The State of CONFUSION!
Re: downshift adjustability on THM350 ????? (Dalannex)
Part throttle downshifts are controlled by vacuum to the modulator. However this will only do a 3-2 downshift and not into first. The procedure you used to adjust the detent cable is close - but not 100 percent. you need somebody in the car flooring the gas pedal. That way you know you're getting full throttle too. You can check it that way. Opening the carb to WOT under the hood and hitting the detent adjustment is OK as long as you are SuRE the pedal is opening it the same amount.
Anyway the downshift into first gear at WOT should happen at any speed under 25 MPH.
Anyway the downshift into first gear at WOT should happen at any speed under 25 MPH.
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: downshift adjustability on THM350 ????? (Dalannex)
You can try more adjusting of the TV cable. Push the lock tab down and move the housing back towards the firewall one click then test drive. Each time you move it back it will raise the pressure at a lower throttle position. For it to be correct it should work like this: Drive off from a stop ever so gentaly and slowly. As soon a you feel the car go into 2nd, punch the throttle. The trans should kick down to 1st. If it does not, move the cable adjustment back another click and test again. When you reach the point where the trans will drop to first at that minimum speed, youve got it right.
Thread Starter
Safety Car
Joined: Mar 2000
Posts: 4,078
Likes: 27
From: Northeast South Dakota
Re: downshift adjustability on THM350 ????? (Jvette73)
Thanks guys. The car has been behaving like this since I rebuilt the tranny 3 years ago. It's liveable and all, it's just that when I don't have anything going in the shop the little things bother me and need to be looked at. :D The 3-2 shift is good. Once it gets to the spot at part throttle where I feel it should shift it shifts. I would just like a 2-1 part throttle shift is all. It does shift down to first when I really tromp on it, but I would like it to have a little bit more tame downshift for "more casual" driving. :cool: What does an adjustable modulator valve do for us anyway and how do you adjust them?
-Justin
-Justin
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: downshift adjustability on THM350 ????? (Dalannex)
Thanks guys. The car has been behaving like this since I rebuilt the tranny 3 years ago. It's liveable and all, it's just that when I don't have anything going in the shop the little things bother me and need to be looked at. :D The 3-2 shift is good. Once it gets to the spot at part throttle where I feel it should shift it shifts. I would just like a 2-1 part throttle shift is all. It does shift down to first when I really tromp on it, but I would like it to have a little bit more tame downshift for "more casual" driving. :cool: What does an adjustable modulator valve do for us anyway and how do you adjust them?
-Justin
-Justin
Thread Starter
Safety Car
Joined: Mar 2000
Posts: 4,078
Likes: 27
From: Northeast South Dakota
Re: downshift adjustability on THM350 ????? (Jvette73)
OK, I'll use your advice on adjusting the TV cable. I thought it might be a TV thing after I considered it a little more, since the throttle valve just puts pressure on the back of the shift valves, helping to hold them closed.
-Justin
-Justin
