Transmission problems

JJHStang · May 7, 2022

All of a sudden my automatic 3 speed transmission is not shifting properly.
My 71 Mach 1 came with a 302 2V and I am not sure but may be the original transmission.
The car when I bought it has a 351W and had a 4bbl put in. I have owned approximately 5 yrs with no major problems.
I only drive occasionally, few Times a month and do not drive it hard.
Yesterday I drove it about 6 miles no problems let it set for an hour then drove it back about 5 miles stopped went into store for maybe 10 mins no problems.
Started up the car, put in drive took off. Stopped at a light and when taking off seemed to be in neutral. Shifted into neutral and back into drive. It took off but didn’t seem to shift to 1st or 2nd. It will take off in 1st and I can manually shift to 2nd but not sure if it shifts. It will go in reverse.
I checked the fluid when I got it home in park with engine running. It seemed to be full. Not sure if it is a C4 or 6
I am going to take it out again today and pay more attention to the shifting.
Any ideas

will e · May 7, 2022

The next simple thing to check is the vacuum line to the modulator. Make sure it is connected and doesn't have any leaks.

JJHStang · May 7, 2022

Thank you

Kilgon · May 7, 2022

+1 with will e. Most likely a vacuum issue. Check the line at the manifold and also at the vacuum modulator on the rear of the transmission. Could also be a bad modulator valve. If that is ok than there are numerous things that could be causing it. The next thing I would do is pull the pan and change the fluid and filter. Check the pan and filter both for any metal flakes or cellulose material from the bands and disc. If it still does it then it could be in the valve body, pumps, bands or discs. At that point unless you know enough to tear it apart I would I would to take the car to a trans shop so they can check it.

JJHStang · May 7, 2022

Thank you and everyone. I took my car out this morning and it did not shift at all. I basically had reverse and 1st gear.
My friend agreed that it might be a vacuum hose.
I had him come over. We took off the air filter cover. He shined his flash light behind the carburetor and said there it is.
Rubber hose was disconnected. Hooked it up, car started right up and sounded good.
Took it for a ride, it ran and shifted very good.

Kilgon · May 7, 2022

JJHStang said:
Thank you and everyone. I took my car out this morning and it did not shift at all. I basically had reverse and 1st gear.
My friend agreed that it might be a vacuum hose.
I had him come over. We took off the air filter cover. He shined his flash light behind the carburetor and said there it is.
Rubber hose was disconnected. Hooked it up, car started right up and sounded good.
Took it for a ride, it ran and shifted very good.

Glad you found the problem and it was a simple fix. You might want to see about replacing that line. As they get old and stiffen they come off easier. For a few dollars you can't go wrong.

will e · May 7, 2022

I am glad it worked for ya! Kilgon has a great suggestion but I also want to ask, did you have a backfire? A backfire will sometimes pop the vacuum lines off.

What a relief! Things are working great and the all of the sudden the transmission stops working. I am glad it was a simple fix.

JJHStang · May 7, 2022

I plan on replacing soon. I do not recall a backfire, I did hear something when I started it.

mrgmhale · May 8, 2022

JJHStang said:
All of a sudden my automatic 3 speed transmission is not shifting properly.
My 71 Mach 1 came with a 302 2V and I am not sure but may be the original transmission.
The car when I bought it has a 351W and had a 4bbl put in. I have owned approximately 5 yrs with no major problems.
I only drive occasionally, few Times a month and do not drive it hard.
Yesterday I drove it about 6 miles no problems let it set for an hour then drove it back about 5 miles stopped went into store for maybe 10 mins no problems.
Started up the car, put in drive took off. Stopped at a light and when taking off seemed to be in neutral. Shifted into neutral and back into drive. It took off but didn’t seem to shift to 1st or 2nd. It will take off in 1st and I can manually shift to 2nd but not sure if it shifts. It will go in reverse.
I checked the fluid when I got it home in park with engine running. It seemed to be full. Not sure if it is a C4 or 6
I am going to take it out again today and pay more attention to the shifting.
Any ideas

Two things come to mind. Check and adjust the shifter linkage for the tranny. While there adjust the neutral safety switch. Next, make sure you are getting vacuum to the transmission modulator. If you’d modulator had two vacuum lines going tomorrow the larger hose will be leading to the rear of the modulator is connected directly to the intake manifold. The second smaller hose leading to the side of the modulator leads to the same ported vacuum line that feed the EGR vacuum diagram. It is not a bad idea to perform a tranny service and replace the filter, as well as Adjust the band.

JJHStang · May 8, 2022

Thank you, the problem was vacuum, rubber hose back of carburetor was not connected.
Seemed to be worn out was not tight and came loose. Going to buy a new one.

Transmission problems

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