TH350 Transmission Shifting - Team Camaro Tech
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post #1 of 13 (permalink) Old Jul 28th, 02, 06:46 PM Thread Starter
 
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I have a TH350 in a 68 Camaro. If I let the transmission shift on it's own, it shifts are very lower RPMs, even at WOT. It will rev up if I manually shift it. How should this transmission act on it's own?

Thanks
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post #2 of 13 (permalink) Old Jul 28th, 02, 07:20 PM
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Is your detent cable hooked up at the carb? If it is, then is it pulled completely tight at WOT?

Larry
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post #3 of 13 (permalink) Old Jul 29th, 02, 05:24 AM
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I'm sorry, what is WOT
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post #4 of 13 (permalink) Old Jul 29th, 02, 06:26 AM
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Wide Open Throttle.... that means with the accelerator on the floor.
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post #5 of 13 (permalink) Old Jul 29th, 02, 06:31 AM Thread Starter
 
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Wide Open Throttle
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post #6 of 13 (permalink) Old Jul 29th, 02, 07:33 AM
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As long as this thread is going I have a couple of similar questions.

Is the governer responsible for part throttle upshifts? With light throttle mine shifts real soon. Already went the adjustable modulator route without much change. I wasn't expecting much but it was an easy swap.

What is responsible for part throttle downshifts. Third to 2nd NOT at WOT.

-Funk
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post #7 of 13 (permalink) Old Jul 29th, 02, 09:19 AM
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<BLOCKQUOTE>quote:</font><HR>Originally posted by CFunK:
As long as this thread is going I have a couple of similar questions.

Is the governer responsible for part throttle upshifts?

Yes. It works in conjunction with the vacuum modulator. I previously thought the governor only controlled WOT shifts and the modulator controlled the part throttle shifts - its not that simple. An adjustable modulator is good for only 2 - 4 MPH adjustment, there is a kit you can get that allows you to change the springs and weight in the governor. This affects the WOT and part throttle shift points.

With light throttle mine shifts real soon. Already went the adjustable modulator route without much change. I wasn't expecting much but it was an easy swap.

What is responsible for part throttle downshifts. Third to 2nd NOT at WOT.

At part throttle the detent cable pulls a lever that pushes a valve in the valve body. That is what causes the trans to down shift. The governor will not let it downshift if RPMs are too high.

-Funk
<HR></BLOCKQUOTE>



[This message has been edited by Eric68 (edited 07-29-2002).]
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post #8 of 13 (permalink) Old Jul 29th, 02, 10:18 AM
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I thought I was up on all lingo! That's spanish
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post #9 of 13 (permalink) Old Jul 29th, 02, 10:38 AM
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Ok, let's take the questions one at a time.

How can I get the tranny to hold 1st longer, at light, part throttle when in D. Massage the governor?

-Funk
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post #10 of 13 (permalink) Old Jul 29th, 02, 10:48 AM
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No Habla English.

Yes. "Massage" the governor. You have to buy a kit and change the springs and weights under the round cap. The cap is held in place with a clip on the outside. B&M and TPI Performance sell the kits.

I need to do the same to my tranny. If I hit the gas right off the line it bakes the tires and shifts to 2nd before they grip. If I feather the throttle it shifts to 2nd too quick and long before I can get the pedal to the floor. What a PITA.

[This message has been edited by Eric68 (edited 07-29-2002).]
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post #11 of 13 (permalink) Old Jul 29th, 02, 03:18 PM
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<BLOCKQUOTE>quote:</font><HR>Originally posted by CFunK:
As long as this thread is going I have a couple of similar questions.

Is the governer responsible for part throttle upshifts? With light throttle mine shifts real soon. Already went the adjustable modulator route without much change. I wasn't expecting much but it was an easy swap.

What is responsible for part throttle downshifts. Third to 2nd NOT at WOT.

-Funk
<HR></BLOCKQUOTE>

The govenor is a pressure regulator that varies "govenor pressure" with vehicle speed. The modulator is a pressure regulator that varies "modulator pressure" with engine load (sensed by vacuum). Each shift valve has govenor pressure pushing against one end with modulator pressure and a spring pushing against the other. Various springs in the govenor, modulator, and shift valves set up a balance. When govenor pressure is high enough to overcome the pressure of the shift valve spring and modulator pressure the valve begins to move and eventually toggles moving fully to the shifted position.

Part throttle downshifts occurr when modulator pressure rises sufficiently that it overcomes govenor pressure. Shifting too soon can be caused by improperly installing shift valve springs (too light of spring), missing springs on the govenor causing govenor pressure to be too high, or a messed up modulator circuit causing modulator pressure to be too low.

I can explain a bit more, but I urge you to find out what is causing the transmission not to shift correctly before modifying anytning. It wouldn't hurt to verify that the modulator valve is correctly installed (inside the case). Don't use a magnet to remove the modulator valve since it can magnatize it and cause steel particles to jamb it up. It can be finessed out with a pick probe. Make sure the modulator vacuum line is connected to manifold vacuum.

Is it possible that the transmission came from a car with a lower rear-end ratio and you're running it with a high ratio rear end (i.e. 4.11:1)? If so you may need to reshape the weights, or stiffen the springs in the govenor assembly. There are some other possible govenor problems that cause cause the transmission to shift too soon. Looking at the first port above the weight assembly (govenor in your hand with the weights down and gear up) You'll see the govenor valve partially exposed in the opening. See that a 0.020 feeler guage just slips in between the top of the port and the valve land. If not, you'll have to take the govenor apart and bend the tangs on the inner weights evenly. Ask me and I'll email you some pictures and text explaining any weight reshaping you might have to do.

Generally it is desirable for a transmission to shift soon. If you are racing, manually shift it. Doing so will engage an extra band in 2nd gear and increase apply pressure in first. Generally though shifts should occurr later and firmer the more you accellerate due to the modulator. The detent cable should cause a WOT downshift.

-dnult

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post #12 of 13 (permalink) Old Jul 29th, 02, 08:48 PM
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Dnult,

I like all your answers, not just on this thread but on others as well.

Since you mentioned the manuel shift thing I would like to mention a couple of things. The 350, and 700 both take off in low "D" on shifter only using the center support sprag. The low/ reverse clutches only apply in manuel low. The later 700R4 center support has wider rollers in the sprag, and will fit in the TH350. This is one of those since we are on the subject things, did you know this? Just thought I would add that on to your already good answer.

Larry
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[This message has been edited by lnjstreetrods (edited 07-29-2002).]
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post #13 of 13 (permalink) Old Jul 30th, 02, 02:04 PM
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<BLOCKQUOTE>quote:</font><HR>Originally posted by lnjstreetrods:
Dnult,
The 350, and 700 both take off in low "D" on shifter only using the center support sprag. The low/ reverse clutches only apply in manuel low. The later 700R4 center support has wider rollers in the sprag, and will fit in the TH350. This is one of those since we are on the subject things, did you know this? Just thought I would add that on to your already good answer.

Larry
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[This message has been edited by lnjstreetrods (edited 07-29-2002).]
<HR></BLOCKQUOTE>

Are saying a late model 700R4 sprag will fit in a THM350? I was not aware of that.

-dnult
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