Question: transmission weirdness!

[Tntruckguy]

Any body ever had this happen... I have a 80 L-48 auto/3sp. Recently the tranmission is wanting to shift from first to another gear at about 5-10mph. It sounds like a drive shaft knock but I have recently replaced both half shaft u joints. I have taken off the Cat and smog and that helped alot of the shiftng problems. Is there some kind of module or something I am missing?This is my first vette and learning quickly!

 

[Evolution1980]

Hey TruckGuy! First, welcome to the

As for your trans shifting early...someone else will likely chime in here, but I'll take a stab at it. I'm no transmission guy, but I'll toss these things out there for ya. (no guarantee on their accuracy :L)

1) When is the last time you've had the trans fluid changed or the transmission flushed?

2) I think your problem could be caused by a stuck check-valve in the transmission (or something like that)

3) Possibly a vacuum problem?

I only knew just a little about the TH-transmissions and they little I did know is now rather rusty. :eyerole

Ya can't go wrong with at least trying a fluid change and/or trans flush.

 

[Tntruckguy]

Trans weirdness

Thanks for answering back. As a matter of fact my mechanic did a flush fairly recently, he told me he put some kind of transmission conditioner in it. Not sure what kind. Since I removed the cat I havent had no problems. I think changing the half shaft u joints was just coincidetal. When I checked the fluid this weekend it was a little low, so I topped it off but no change...any other ideas...maybe I should get the conditioner out?

 

[wishuwerehere82]

The module you are looking for is actually a vacuum modulator on the transmission.
The TH350 transmissions use a vacuum line that runs from the intake manifold to the vacuum modulator on the transmission to determine engine load based on manifold vacuum.
The vacuum modulator is a canister that contains a diaphragm and a calibrated spring. The diaphragm senses changes in engine vacuum due to throttle position/load changes and transfers the diaphragm motion to the modulator valve in the transmission. The calibrated spring opposes the diaphragm movement. This provides the means for the transmission to adjust internal pressures based upon torque demand by the engine.
If, for example, the vacuum line is leaking, the transmission would stay in 1st gear and not want to upshift.
You have the reverse problem. The modulator is too sensitive to engine vacuum. Either it has gone out of range or it is going bad. There is a screw in the center of the modulator to adjust the shift points within 2-3 MPH of your desired speed. Most likely, it is going bad.
You can replace the modulator fairly easily. It is a round module on the side of the transmission where the vacuum line is attached to it.
You can get a new one and adjust the shift point to your liking.

 

[wishuwerehere82]

Thanks for answering back. As a matter of fact my mechanic did a flush fairly recently, he told me he put some kind of transmission conditioner in it. Not sure what kind. Since I removed the cat I havent had no problems. I think changing the half shaft u joints was just coincidetal. When I checked the fluid this weekend it was a little low, so I topped it off but no change...any other ideas...maybe I should get the conditioner out?

Most of those conditioners will make the transmission shift extremely hard. They swell the seals and O-rings in the valve bodies but also make them softer causing the valves to actuate faster. Thus the bang when shifting.
If it didn't really need it, it may be better to go back to the straight Dextron/mercon fluid and see if it improves.
Problem is that unless the torque converter is removed and drained, you will never get all of it out.
Then again, the process might not be reversable.

 

[Evolution1980]

The module you are looking for is actually a vacuum modulator on the transmission.
The TH350 transmissions use a vacuum line that runs from the intake manifold to the vacuum modulator on the transmission to determine engine load based on manifold vacuum.

Yeah, that's what I was thinking of. Thanks for chiming in on that!

 

[Tntruckguy]

shifting fix

First of all let me thank those that have "chimed in" I was not expecting quite this response. Thanks guys! I will try changing that vacuum modulator this week end. I assume I can get that at any auto parts supply house. I think I will also change the Trans fluid and drain the converter. As I find out more about my vette, I find out more about the people who have vette's. This CAC is a good thing for guys like me since I am more of a chevy truck guy. Currently restoring a 75 Silverado to a resto-mod condition. It is funny how my car has some of the same sounds as my chevy trucks. Anyways, I will post my results for someone else to learn from.

TnTruckGuy

 

[wishuwerehere82]

First of all let me thank those that have "chimed in" I was not expecting quite this response. Thanks guys! I will try changing that vacuum modulator this week end. I assume I can get that at any auto parts supply house. I think I will also change the Trans fluid and drain the converter. As I find out more about my vette, I find out more about the people who have vette's. This CAC is a good thing for guys like me since I am more of a chevy truck guy. Currently restoring a 75 Silverado to a resto-mod condition. It is funny how my car has some of the same sounds as my chevy trucks. Anyways, I will post my results for someone else to learn from.

TnTruckGuy

Auto zone has a rebuild kit that includes the modulator, gaskets, filter. Advance Auto has the pan gaskets, filter and modulator separately , get the B&M one.
You will have to drop the tranny if you really want to drain the torque converter. No drain plug.

 

[Tntruckguy]

Thanks to all!!!

Well everyone, I did change out the Trans modulator and that has seemed to fix the problem. It does shift hard, especially into third. I will change out the conditioner that was put in and I should be good to go. Thanks to the experts out there. I know for my next project on my car I will consult the experts.(CAC).

Thanks,tntruckguy

PS: how do you put the picture of your car below your name?
I wanna do that to!!!! :cool

 

[Evolution1980]

PS: how do you put the picture of your car below your name?
I wanna do that to!!!! :cool

Well, you're in luck! Here at the CAC we have a forum just for these type of questions! Check out the Site Help & Feedback forum!
Specific to this issue, we also have an answer to your exact question! "How do I get that picture under my username?"