TH350, wont shift into second - Chevelle Tech

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post #1 of 12 (permalink) Old Aug 16th, 11, 8:34 PM Thread Starter
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TH350, wont shift into second

This is the TH350 in my '70 El Camino, I just put in the new 355" motor, and tried to take it out for a drive, but my transmission wont shift into 2nd gear, but first and reverse work no problem.

I checked the shift linkage, and everything shifts fine, and the transmission is full of fluid (at the "full" on the dipstick with the motor running).

I thought that maybe during the removal and installation of the engine, I messed with the kickdown cable, so I disconnected it from the carb, with no changes.

The transmission worked totally fine about 6 weeks ago when I first pulled the motor out, so I find it hard to believe anything major went wrong/ broke internally from just sitting for 6 weeks. I left the radiator in the car, and did not remove the hard lines to the transmission cooler at all, and I left the transmission in the car during the motor build/ swap.

Also, when driving the car and rolling at about 2000 rpm, I can roll into the throttle and the car wont accelerate at all, the motor just free revs to about 3000 rpm when all of a sudden it "grabs" and takes off. However if I do the same thing from 1500 it pulls right away as it should, but then hits the "dead spot" between 2000-3000.

I basically have no idea other than what I have already checked and tried, have any of you ever encountered this?

Thanks in advance guys

25 years old
70 El Camino-355, 4l80 w/ freakshow 3500, S60, 4.10
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post #2 of 12 (permalink) Old Aug 16th, 11, 9:17 PM
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Re: TH350, wont shift into second

Is your vacuum modulator hooked up? I don't think you generally hear about them not shifting into 2nd but I know a faulty one, or an unhooked one will cause the car to shift very late or sometimes not at all. The slipping/free revving does not sound modulator related though so you might have other problems. Are you sure the torque converter was fully seated when you bolted the motor back in. I wouldn't expect it to have come loose but I suppose it could happen accidentally.

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post #3 of 12 (permalink) Old Aug 17th, 11, 7:01 AM Thread Starter
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Re: TH350, wont shift into second

In all honesty, I have no idea about the vacuum modulator, how do i check it?

When I reinstalled the motor, I made sure that the converter was seated back all the way.

25 years old
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post #4 of 12 (permalink) Old Aug 17th, 11, 7:52 AM
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Re: TH350, wont shift into second

This is the vacuum modulator,


It should have a rubber line that runs from the canister a few inches and then connects to a hard line which runs up the firewall to another rubber line that then connects to manifold vacuum. Make sure the rubber lines do not have any kinks in them and most importantly that the modulator is hooked up to vacuum.

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post #5 of 12 (permalink) Old Aug 17th, 11, 3:46 PM Thread Starter
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Re: TH350, wont shift into second

Okay thanks a lot, I had not even thought of this until now. When we were putting the motor back in, the vacuum line from the intake got crushed a little bit by the motor hitting it..

But now that you mention that, I bet I know where to start looking for a kink in the hose

Thanks!

25 years old
70 El Camino-355, 4l80 w/ freakshow 3500, S60, 4.10
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post #6 of 12 (permalink) Old Aug 17th, 11, 3:58 PM
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Re: TH350, wont shift into second

I can testify...I forgot to hook mine up when the engine went back in. A light bulb lit above my head and low and behold, I never plugged that vaccuum hose into the modulator.

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post #7 of 12 (permalink) Old Aug 17th, 11, 4:41 PM Thread Starter
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Re: TH350, wont shift into second

Did yours do the same that mine is? When I get off work tonight I will pull the car into the garage and inspect the line closer, hopefully it is as simple as that.

Right now I have hardline right up the manifold, would it be okay to cut below the crushed portion and use a flexible vacuum hose from there to the manifold?

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post #8 of 12 (permalink) Old Aug 18th, 11, 1:14 AM
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Re: TH350, wont shift into second

yes, but try to find some quality stuff, thick wall made by Goodyear or similar. it won't work so hot if it collapses.

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post #9 of 12 (permalink) Old Aug 18th, 11, 6:58 AM Thread Starter
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Re: TH350, wont shift into second

Okay cool, thanks a lot, I really appreciate it!

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post #10 of 12 (permalink) Old Aug 18th, 11, 8:48 AM
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Re: TH350, wont shift into second

[QUOTE=steve_sutherland;3281436]Did yours do the same that mine is? When I get off work tonight I will pull the car into the garage and inspect the line closer, hopefully it is as simple as that. [QUOTE]

Steve,

It's been a few years ago, so I'm trying to remember the exact symptom. I'm kinda thinking that it shifted into 2nd, but not beyond that. Reverse worked fine. I'm not a trans expert by a long shot, so I can't even preach that that's possible without the VM hooked up, but I'm pretty sure that's how it acted.

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post #11 of 12 (permalink) Old Aug 18th, 11, 4:31 PM Thread Starter
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Re: TH350, wont shift into second

Alright thanks, and well, honestly, you guys are just way too good!

I got off work late last night and didn't have time, but I just took at look it now to see how bad the section I pinched is, and figured I would inspect the rest of the line, and found it was totally off at the vacuum modulator! So I obviously just re-attached that, and I still plan on replacing the crushed section of the piping, but I'm guessing that will take car of it!

Thanks a lot!

I will definitely report back if I take the car for a drive and it doesn't work.

25 years old
70 El Camino-355, 4l80 w/ freakshow 3500, S60, 4.10
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post #12 of 12 (permalink) Old Aug 18th, 11, 4:47 PM
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Re: TH350, wont shift into second


70 Chevelle
496 cu. in. "cruiser"
Assembled & Tuned by Mo's Speed Shop Dallas, GA
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