Automatic Transmission Shifting Problems
Thread Starter
Drifting
Joined: Feb 2003
Posts: 1,461
Likes: 2
From: Allen TX
Automatic Transmission Shifting Problems
Background: 1977 Corvette, assuming its a TH350 transmission. I swapped the intake manifold yesterday for a Performer intake. Transmission shifted fine before changing intake.
Today: At first the transmission wouldnt shift in to 3rd at all, only when I would manually hold it in 2nd and then put it into 3rd and it would only go into 3rd when I had reached high rpms. I have done the basics, checking fluid, checking vacuum lines, reset the downshift mechanism. The vacuum line to the manifold is fine. I replaced the rubber vacuum line that connects the hard line to the modulator down at the transmission because it was cracked. That helped a little bit but I it still won't shift automatically into 3rd until I reach about 3000-3500rpm while cruising. When it does shift into 3rd it is a very hard shift. I can manually shift through the automatic with no problem. It shifts where I want it to when I do it manually. I assume it would be a vacuum leak but there is only one line to the transmission that is a vacuum line and I have checked it. Any suggestions would be appreciated.
Happy 4th!
Today: At first the transmission wouldnt shift in to 3rd at all, only when I would manually hold it in 2nd and then put it into 3rd and it would only go into 3rd when I had reached high rpms. I have done the basics, checking fluid, checking vacuum lines, reset the downshift mechanism. The vacuum line to the manifold is fine. I replaced the rubber vacuum line that connects the hard line to the modulator down at the transmission because it was cracked. That helped a little bit but I it still won't shift automatically into 3rd until I reach about 3000-3500rpm while cruising. When it does shift into 3rd it is a very hard shift. I can manually shift through the automatic with no problem. It shifts where I want it to when I do it manually. I assume it would be a vacuum leak but there is only one line to the transmission that is a vacuum line and I have checked it. Any suggestions would be appreciated.
Happy 4th!
Last edited by CrossedFlags77; Jul 4, 2006 at 05:44 PM.
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
For sure it has something to do with your vacumn modulator because on that transmission the shifting is controlled thru it. Make sure you are getting enough vacumn from the port on your new manifold to allow the transmission to shift. Since it worked normally before you changed intakes I would suspect that is the problem. If that all checks ok then spend around $10-$15 dollars and buy a new modulator. Does the engine still idle alright? If not you may have a intake leak somewhere. Good luck
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
I am not exactly sure how many inches of vacumn it takes to operate the modulator but it will vary because the differant inches of vacumn will vary by engine speed allowing shifting ranges. You did tee off the vacumn modulator line off of the manifold and not the carburator? Good luck and let us know what you find.
Drifting
Joined: Oct 2004
Posts: 1,496
Likes: 1
From: Lafayette Louisiana
I had the same exact problem with an 80. I found a vacuum line disconnected from the check valve on the drivers side firewall area. Sorry I can't remember exactly which line. I reconnected it and everything shifted fine so I did not bother tracing the line. I had the exact same symptoms so look for a vacuum leak. Good luck.
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
Originally Posted by kevinator80
I had the same exact problem with an 80. I found a vacuum line disconnected from the check valve on the drivers side firewall area. Sorry I can't remember exactly which line. I reconnected it and everything shifted fine so I did not bother tracing the line. I had the exact same symptoms so look for a vacuum leak. Good luck.
Thread Starter
Drifting
Joined: Feb 2003
Posts: 1,461
Likes: 2
From: Allen TX
Turns out I had a small leak somewhere between the manifold and the modulator. I replaced the hard line with a regular rubber vacuum line for testing and it shifts perfectly. I knew it had to be something simple. Thanks for the help.
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