Tranny won't shift

steven1213 · 04-01-2008, 08:35 AM

I just got finished installing another engine in my 1976 chevy pickup and now the automatic transmission will not shift out of first. Reverse low1 and low 2 work ok but Drive does not function properly. The accelerator line is hooked up on the carb that comes from the transmission and the tube from the manifold is hooked up but I had to bend it around a bit when re installing it. Short of low transmission fluid, is there anything else that can be causing this? I did not remove the torque converter when I took the old engine out. I just left it in the tranny and secured it so it would not fall out. The tranny was just rebuilt with only about 1000 miles on it. Any help is appreciated.---------------Steve

75K30 · 04-01-2008, 09:46 AM

Steven:
What type of transmission are we working with here?
You mentioned "accelerator line", and this has me a little confused. Are you refering to the kickdown cable? This might indicate a TH350.
If this is too tight, then you may have the source of your trouble. You may disconnect this to verify operation. The trans wil never downshift into passing gear, until it is connected properly, but this could prevent the thing from shifting.
Second gear would try to go back into first, and you may never ever see third if you dont go fast enough, but this is where I would start.
Next would be the vacuum modulator. This could have a restriction, and not permitting the proper amount of vacuum signal (manifold vacuum here not port vacuum, or refered to as metered vacuum) to tell the modulator to shift.
I suspect that if you bent the hard line, you may have kinked the line, and you will have a restriction that is preventing the proper vacuum signal.
If you have a vacuum gauge, you can verify that he vacuum at the source is the same as the vacuum way down at the modulator. Any differential vacuum would indicate a problem.
I would start here since this is the easiest to troubleshoot, and more than likely the problem.
By the way, you do want to verify that you are using a steel line for this vacuum line. If you are trying to use vacuum tubing it will, (or can) colapse and restrict vacuum. Long vacuum runs should be made from steel tubing.
This should get you started, good luck.

steven1213 · 04-02-2008, 12:43 AM

Thanks 75k30. It is a TH350 and it turned out to be the vacuum line coming from the manifold.There is a 2 inch piece of rubber hose which connects the tube to the barbed fitting from the manifold and the steel tube to the vacuum modulator. I replaced each piece with a new piece of tubing. The one at the manifold had a cut in it and I bent the tube down at the transmission so the rubber tube will not be kinked as much. I took the truck out and it shifted normal again. But man, getting that steel tube relaligned again is a major bear.
How tight should the kickdown cable be adjusted? It does not fit on the carb with any real tension on it but there is not much play before the linkage on the carb pull the cable. ----Thanks again, Steve

75K30 · 04-02-2008, 01:22 AM

Congrats! Glad to hear that you fixed it.
As far as your cable tension, this varies from vehicle to vehicle, and the best way to determine operation is simply to drop the hammer while in high gear. In other words, find a nice long stretch of road, and while cruising in high gear (3rd) simply depress the accelerator, and determine if the trans downshifts at the proper time. If it is too delayed, or does not downshift, then there is too much slack in the cable, and it requires a little tightening. If it downshifts too soon, like when you barely depress the throttle, then it is too tight, and requires a little adjustment.
Depending on the bracket that you use, you are probably dialed right in, and making any adjustments may not even be necessary. There will be some slack in the cable.
Again, good job on your find, and repair.

gmctrucks · 04-02-2008, 05:54 AM

That happen to my 70 GTO at one time the hose had come loose glad to hear that 75K30 helped you on this one im thinking if you wouldn't have taking it to a trans shop they would have said your trans is messed up and charged you a good penny just to connect the rubber hose.

04-01-2008, 08:35 AM	#1
steven1213 Full Member Join Date: Mar 2008 Posts: 16	Tranny won't shift I just got finished installing another engine in my 1976 chevy pickup and now the automatic transmission will not shift out of first. Reverse low1 and low 2 work ok but Drive does not function properly. The accelerator line is hooked up on the carb that comes from the transmission and the tube from the manifold is hooked up but I had to bend it around a bit when re installing it. Short of low transmission fluid, is there anything else that can be causing this? I did not remove the torque converter when I took the old engine out. I just left it in the tranny and secured it so it would not fall out. The tranny was just rebuilt with only about 1000 miles on it. Any help is appreciated.---------------Steve

04-02-2008, 12:43 AM	#3
steven1213 Full Member Join Date: Mar 2008 Posts: 16	I fixed it!!!!! Thanks 75k30. It is a TH350 and it turned out to be the vacuum line coming from the manifold.There is a 2 inch piece of rubber hose which connects the tube to the barbed fitting from the manifold and the steel tube to the vacuum modulator. I replaced each piece with a new piece of tubing. The one at the manifold had a cut in it and I bent the tube down at the transmission so the rubber tube will not be kinked as much. I took the truck out and it shifted normal again. But man, getting that steel tube relaligned again is a major bear. How tight should the kickdown cable be adjusted? It does not fit on the carb with any real tension on it but there is not much play before the linkage on the carb pull the cable. ----Thanks again, Steve

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04-01-2008, 09:46 AM	#2
75K30 Senior Member Join Date: Jul 2006 Posts: 2,627	Steven: What type of transmission are we working with here? You mentioned "accelerator line", and this has me a little confused. Are you refering to the kickdown cable? This might indicate a TH350. If this is too tight, then you may have the source of your trouble. You may disconnect this to verify operation. The trans wil never downshift into passing gear, until it is connected properly, but this could prevent the thing from shifting. Second gear would try to go back into first, and you may never ever see third if you dont go fast enough, but this is where I would start. Next would be the vacuum modulator. This could have a restriction, and not permitting the proper amount of vacuum signal (manifold vacuum here not port vacuum, or refered to as metered vacuum) to tell the modulator to shift. I suspect that if you bent the hard line, you may have kinked the line, and you will have a restriction that is preventing the proper vacuum signal. If you have a vacuum gauge, you can verify that he vacuum at the source is the same as the vacuum way down at the modulator. Any differential vacuum would indicate a problem. I would start here since this is the easiest to troubleshoot, and more than likely the problem. By the way, you do want to verify that you are using a steel line for this vacuum line. If you are trying to use vacuum tubing it will, (or can) colapse and restrict vacuum. Long vacuum runs should be made from steel tubing. This should get you started, good luck.

04-02-2008, 01:22 AM	#4
75K30 Senior Member Join Date: Jul 2006 Posts: 2,627	Congrats! Glad to hear that you fixed it. As far as your cable tension, this varies from vehicle to vehicle, and the best way to determine operation is simply to drop the hammer while in high gear. In other words, find a nice long stretch of road, and while cruising in high gear (3rd) simply depress the accelerator, and determine if the trans downshifts at the proper time. If it is too delayed, or does not downshift, then there is too much slack in the cable, and it requires a little tightening. If it downshifts too soon, like when you barely depress the throttle, then it is too tight, and requires a little adjustment. Depending on the bracket that you use, you are probably dialed right in, and making any adjustments may not even be necessary. There will be some slack in the cable. Again, good job on your find, and repair.

04-02-2008, 05:54 AM	#5
gmctrucks Senior Member Join Date: Nov 2006 Location: Houston,Texas Posts: 1,959	That happen to my 70 GTO at one time the hose had come loose glad to hear that 75K30 helped you on this one im thinking if you wouldn't have taking it to a trans shop they would have said your trans is messed up and charged you a good penny just to connect the rubber hose.