Author Topic: th350 detent valve travel question  (Read 109 times)

Offline stevef

  • Garage guy
  • **
  • Posts: 50
th350 detent valve travel question
« on: March 12, 2019, 06:16:48 PM »
I bought a rebuilt th350 for my 79 Trans Am with an Olds 403. The detent cable on these connects to the carb throttle linkage. So, I put in a new detent cable and adjusted it at the carb as I've done with my other cars before. When I first drove it, it didn't shift right. It didn't feel like it was hitting all gears. Also the kick-down only worked sporadically when I tried it. So, I disconnected the detent cable and it's been shifting in all gears fine now, without the cable connected. But, I want the kick-down to work, so I decided to drop the pan to see if I could see any reason that the detent wouldn't work right. The cable and connecting wire works freely, no sticking or binding. The detent valve isn't sticking. Before I put it back together and try again, I was wondering about the detent valve travel in relation to the kick-down activation.

I've noticed that detent valves normally push in with light spring pressure most of the way, but in about the last 1/3 of the travel the spring pressure becomes stiffer. My question is how much of the valve has to be pushed in for the kick-down to work? My valve is pushed in 2/3 of the way with the q-jet at WOT. It doesn't push the valve all the way in. Is that enough? Does that last stiffer 1/3 part of the travel play a role or not? Or is the action already done by the first 2/3 of travel? Or is the last 1/3 of travel where it kicks in? If that's so, any ideas how I can get the cable to pull that last part of the travel. Thanks!

Offline tayto

  • Garage guy
  • **
  • Posts: 58
Re: th350 detent valve travel question
« Reply #1 on: March 12, 2019, 06:53:11 PM »
How did you adjust the cable? Also, have you verified that when press the gas pedal all the way that the carb opens all the way? I have seen LOTS of gm cars where this does not happen!

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #2 on: March 12, 2019, 07:05:55 PM »
Adjustment procedure is: With the q-jet off fast idle, un-lock the snap-lock button, move the throttle lever to wide open, and lock the snap-lock button.

It is possible the gas pedal doesn't move the throttle completely wide open. I'll have to look at that. But I haven't found my other cars to be that particular about being fully WOT for the kick-down to work. But, I will check it.

Still wondering how much of the detent valve has to be depressed for it to work though.

Thanks!

Offline Cliff Ruggles

  • Global Moderator
  • Qjet Hero
  • *****
  • Posts: 3872
Re: th350 detent valve travel question
« Reply #3 on: March 13, 2019, 02:53:54 AM »
The cable on a TH350 is not a player in shift function in the "normal" driving range.  It is for forced heavy/full throttle downshifts so shouldn't come into play until pretty late in the travel of the throttle.......Cliff

Offline tayto

  • Garage guy
  • **
  • Posts: 58
Re: th350 detent valve travel question
« Reply #4 on: March 13, 2019, 04:41:35 AM »
Is it aftermarket cable or AC delco? i have not had good luck with aftermarket TV cables for the 2004R/700R4s. Ac delco ones are the one to use IMO.

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #5 on: March 13, 2019, 06:14:58 AM »
The cable on a TH350 is not a player in shift function in the "normal" driving range.  It is for forced heavy/full throttle downshifts so shouldn't come into play until pretty late in the travel of the throttle.......Cliff

Thanks Cliff. Right now, with the q-jet at WOT, the detent valve still has about 3/16" undepresssed, or 3/16" travel left until it would bottom out. It sounds like I might have to make it so that last part of the valve gets pushed in some more at WOT for the kick-down to work.
I just had a idea. I have a 73 El Camino with a th350 that I installed over 20 years ago, and the kick-down has always worked. I'm going to drop the pan on it and see how far the valve is pushed in at WOT. It's been a while since I changed the filter on it anyway  :) . I've put in quite a few th350's over the years and never had this problem, so never really paid attention to the valve position.

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #6 on: March 13, 2019, 06:21:31 AM »
Is it aftermarket cable or AC delco? i have not had good luck with aftermarket TV cables for the 2004R/700R4s. Ac delco ones are the one to use IMO.

It's an aftermarket, but I compared it to the old original cable, and didn't see any difference in length or function. So, I can't see a reason it wouldn't work. The original one is old and ugly, but I might reinstall it to see if there is any difference in how far it actuates the detent valve. Thanks for the thought.

Offline tayto

  • Garage guy
  • **
  • Posts: 58
Re: th350 detent valve travel question
« Reply #7 on: March 13, 2019, 11:56:10 AM »
was the qjet you are using originally from a car with a th350? if it's from an OD unit the "swing" might be wrong. I had a 2004r a long time ago that i used with a th350 carb and the trans only last 2 years. was behind a 305 in a 64 impala that i drove mainly highway miles.

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #8 on: March 13, 2019, 01:50:07 PM »
Good question, but yes, it's actually the original q-jet and engine that came in this car. So, it's correct.

I've found my answer about the travel of the detent valve. I dropped the pan on my El Camino and when it's at WOT, the valve is completely depressed. On my TA, at WOT, the valve still has about 3/16" travel left to go. So, the TA cable is not completely depressing the valve at WOT for some reason.

I've been using a new aftermarket cable that's supposed to fit my application and looks the same as my old one. I looked up the GM part number for my application, but they're no longer available from GM, or anywhere on the internet. So, aftermarket or used original is the only choice.

So, I have a couple options. I still have my original cable, so I think I'll put it back on and see if it makes a difference. Another option is to shorten the front of the cable slot on the new cable, so the throttle lever starts pulling it sooner so it can push the valve in completely at WOT.

I'll let y'all know what happens. Any other thoughts are welcome.

Offline Cliff Ruggles

  • Global Moderator
  • Qjet Hero
  • *****
  • Posts: 3872
Re: th350 detent valve travel question
« Reply #9 on: March 14, 2019, 04:59:13 AM »
For most set-ups and my own car I don't use the transmission kick-down feature.  The last thing I want to happen is the transmission seeking a lower gear during a heavy throttle passing scenario.  I'd rather manually shift it if/when needed....but my engine does make over 550hp and 600ft lbs torque, so it really doesn't need high RPM's for any sort of "normal" driving........Cliff

Offline tayto

  • Garage guy
  • **
  • Posts: 58
Re: th350 detent valve travel question
« Reply #10 on: March 14, 2019, 08:09:11 AM »
Too bad delco doesn't make one. I found this:
https://www.classicindustries.com/shop/1979/pontiac/firebird/parts/transmission/automatic-trans/shift-linkage/

similar to what you have already? you can get small nico press crimps for small "aircraft" cable that should would, ive used them to shorten throttle cables. i think i got them from home depot or acklands grainger up here in canada. see attached image. have you tried setting the cable tighter? I've always set the tv cables ao cable is tight @ wot, never had any luck with the auto adjuster.

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #11 on: March 14, 2019, 08:51:20 PM »
For most set-ups and my own car I don't use the transmission kick-down feature.  The last thing I want to happen is the transmission seeking a lower gear during a heavy throttle passing scenario.  I'd rather manually shift it if/when needed....but my engine does make over 550hp and 600ft lbs torque, so it really doesn't need high RPM's for any sort of "normal" driving........Cliff

I've noticed you've said that before. Since I probably only have around 200 hp and 320 ft/lb torque, AND only 2.41 rear gears, a kick-down is very desirable  :) .

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #12 on: March 14, 2019, 08:54:24 PM »
Too bad delco doesn't make one. I found this:
https://www.classicindustries.com/shop/1979/pontiac/firebird/parts/transmission/automatic-trans/shift-linkage/

similar to what you have already? you can get small nico press crimps for small "aircraft" cable that should would, ive used them to shorten throttle cables. i think i got them from home depot or acklands grainger up here in canada. see attached image. have you tried setting the cable tighter? I've always set the tv cables ao cable is tight @ wot, never had any luck with the auto adjuster.

That is the type of cable, but that one is for Chevy small blocks. Each make of engine uses a different length cable, so the Olds uses a different one. But, I figured out how to solve my problem. My solution is written in the next post.

Offline stevef

  • Garage guy
  • **
  • Posts: 50
Re: th350 detent valve travel question
« Reply #13 on: March 14, 2019, 09:20:30 PM »
So, I figured out how to fix my situation. I re-installed my new cable and took another look at how it was actuating the detent valve in the valve body of the transmission. As I mentioned earlier, when I adjusted it by the book, the valve still had about 3/16" travel left at full throttle, when the valve really needed to be close to fully depressed, and that's how it still was. So, I tried an experiment. Instead of opening the throttle lever on the carb wide open before locking the snap-lock button on the cable for adjustment, as is the factory procedure, I unlocked the cable and rotated the throttle lever only about 3/4 wide open before locking the snap-lock button the cable. Then I opened the throttle to wide open and checked the valve again to see how that affected the valve travel. Now it was fully depressed! This in effect made the cable adjustment shorter, so when it goes to full throttle, it pulls the cable a little further than before and the kick-down valve becomes fully depressed. I also checked it to make sure the valve was being depressed sufficiently by using the gas pedal, and it was (thanks "tayto"). So, I test drove it and it shifted fine and I have a reliable kick-down function now. I just drove it on a 40 mile round trip to a friend's house this evening with no problem. I still don't know why it's not adjusting correctly by the factory method, but I'm glad I found a fairly easy way to make it work. It takes a little trial and error adjusting it this way, but it only took me 2 tries. Now that I know what's supposed to happen, in the future, I'm going check the detent valve actuation in any th350's I use before putting them in use.
Thank you both for responding!

Offline tayto

  • Garage guy
  • **
  • Posts: 58
Re: th350 detent valve travel question
« Reply #14 on: March 14, 2019, 10:01:20 PM »
Like I said the factory method for setting the TV cable on the 700R4/2004R has never worked for me. This is most likely due to the shift kits installed and how they "fix" the TV system with spacers and stiffer springs etc (my theory anyway...). Glad you figured it out and verified your gas pedal!