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Old 08-24-2016, 09:19 AM   #1
thorvintage
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Question My Automatic Transmission Isn't That Automatic

My automatic transmission isnt shifting as well as it should be. I am having to manually shift pretty often. All the gears seem to work well once its in that gear it just doesnt seem to be shifting at the right times on its own.

Also as I am going down the road when it is in 3rd it often goes to neutral until I grab the gear shift and move it to neutral and then back to drive and it reengages until the next time. Starting to feel like have a stick shift without a clutch LOL

Back story: just bought the truck and the guy I bought it from owned it for only a couple months (he had a stroke) put a new used transmission in it and he said he thinks the seals may be sticking a bit or the vacuum modulator may need to be cleaned or replaced. The truck was sitting for almost 15 years before he bought it.

Does anyone have an info or links/videos on how to check, service, or replace the vacuum modulator? I am also open for any other ideas or advise on what I could do to solve the issue...thanks for reading!
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Old 08-24-2016, 09:37 AM   #2
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Re: My Automatic Transmission Isn't That Automatic

We need to know what transmission is in it.
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Old 08-24-2016, 11:34 AM   #3
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Re: My Automatic Transmission Isn't That Automatic

Oh thanks! It says hydra matic on the bottom of the pan and my truck is a 1971 C20934...if you need anything else just let me know where to look and I will post it.

I went ahead and replaced the vacuum modulator since it is so easy figure it couldnt hurt. Will post an update after I drive it here in a few...thanks again!
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Old 08-24-2016, 11:55 AM   #4
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Re: My Automatic Transmission Isn't That Automatic

Vacuum Modulator should not make it go from 3rd to neutral but it will not hurt to replace it. My first thoughts are the shifter levers are out of adjustment, low on fluid or something internal.
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Old 08-24-2016, 01:04 PM   #5
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Re: My Automatic Transmission Isn't That Automatic

I think the modulater will make it not shift out of second on a TH350 not sure about the 3rd and neutral part. Could have some junk hanging up the valve body whould be my guess. If the clutches feel ok when it actually is engaged. I think when you manually engage 2 nd if runs on the band instead of the clutch drum so if I remember that correctly maybe 2nd gear is smoked. I worked at a transmission shop about 25 years ago but as an installer not a builder so anything I did know is limited and most likely forgotten. Lol
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Old 08-24-2016, 02:18 PM   #6
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Re: My Automatic Transmission Isn't That Automatic

update: vacuum modulator replaced and it seems to do better than its has for the first few minutes (unless it was in my head) but then started doing the same thing. Wouldnt shift out 1st until I messed with the gear shift once or twice but then fine several other times.

It went from 3rd to neutral a couple of times again but then I move the gear shift to neutral and back into 3rd and would be ok for a bit but eventually do it again.

I was going to see if there are any leaks in the vacuum lines but if that doesn't have anything to do with the issue I am just wasting time.

I am not sure how to adjust the shift levers?

I appreciate you all chiming in! Still open to anything other advise or where to look? THANKS!
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Old 08-24-2016, 03:59 PM   #7
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Re: My Automatic Transmission Isn't That Automatic

Is there a governor on these transmissions? I don't know much about them, but I had a 78 Caprice that seems to have acted the same way. It was the governor.

Note AAMCO-Rip said I needed a new transmission (go figure). Local fixed it for $80, but that was 30 years ago.
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72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Donor, hauler - leaks. Luxuries like gauges, heat, wipers, headlights don't work, I prefer it that way.

How is it frame-off when the frame is the last thing left Tony
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Old 08-24-2016, 04:46 PM   #8
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Re: My Automatic Transmission Isn't That Automatic

Yes a 350 has a governor. I know we replaced some of those but I don't remember what the symptoms were that required that. We still haven't conf if it 350 or 400. Both are hydromatics. I just read an old thread yesterday that showed pictures of all the pan shapes to identify transmissions. I don't know how to post links.
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Old 08-24-2016, 05:15 PM   #9
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Re: My Automatic Transmission Isn't That Automatic

Quote:
Originally Posted by old Chevy guy View Post
Yes a 350 has a governor. I know we replaced some of those but I don't remember what the symptoms were that required that. We still haven't conf if it 350 or 400. Both are hydromatics. I just read an old thread yesterday that showed pictures of all the pan shapes to identify transmissions. I don't know how to post links.
Just highlight (i.e., select) the web address, right click, copy and paste. That or Control C to copy, Control V to paste. That's how it works on windows anyway.
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71 Custom Deluxe, SWB, 2WD, fleet, 402/400TH, A/C money pit. Leaf springs too. Speak of those Looking for rear OEM sport bumper brackets for this. They're uncommon brackets. I need what's basically brackets off a 2WD, SWB, GMC.

72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Donor, hauler - leaks. Luxuries like gauges, heat, wipers, headlights don't work, I prefer it that way.

How is it frame-off when the frame is the last thing left Tony

Last edited by 71CHEVYSHORTBED402; 08-24-2016 at 10:33 PM.
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Old 08-24-2016, 10:21 PM   #10
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Re: My Automatic Transmission Isn't That Automatic

http://67-72chevytrucks.com/vboard/s...d.php?t=278141 If this works I learned something new today.
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Old 08-24-2016, 10:32 PM   #11
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Talking Re: My Automatic Transmission Isn't That Automatic

Hopefully, you didn't get a full manual valve body......
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Old 08-24-2016, 10:37 PM   #12
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Re: My Automatic Transmission Isn't That Automatic

Quote:
Originally Posted by old Chevy guy View Post
http://67-72chevytrucks.com/vboard/s...d.php?t=278141 If this works I learned something new today.
lol I just learned how to do that last week myself
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Old 08-24-2016, 11:02 PM   #13
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Re: My Automatic Transmission Isn't That Automatic

It worked thanks for the link! I have a 400.

Drove it some more this afternoon after making sure there were no vacuum leaks. Next I may check out the carborator to be sure the vacuum pressure coming out of it is accurate for what it is suppose to do to help cause the transmission to shift.

After reading more today about how the vacuum modarator works it sounds like my issue has something to do with the vacuum pressure and its relationship with the transmission.

Getting a little frustrated that I cant figure it out LOL
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Old 08-24-2016, 11:04 PM   #14
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Re: My Automatic Transmission Isn't That Automatic

BTW what does the governor look like?
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Old 08-25-2016, 09:36 AM   #15
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Re: My Automatic Transmission Isn't That Automatic

There is a round steel cap with a spring wire clip,it inside that it's kind of a centrifugal thing that open up when it spins. I'll try and post a picture in a bit.
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Old 08-25-2016, 10:00 AM   #16
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Re: My Automatic Transmission Isn't That Automatic

I was partially right. It's a round steel cap but on a 400 it's held on with 4 bolts. Excuse the sideways picture. This is on the passengers side just before the tail.
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Old 08-25-2016, 10:09 AM   #17
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Re: My Automatic Transmission Isn't That Automatic

Google images of turbo 400 governor and you will see what's in there. I couldn't figure out how to post that link.
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Old 08-25-2016, 11:27 PM   #18
thorvintage
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Re: My Automatic Transmission Isn't That Automatic

Thanks for all the info everyone...I Greatly appreciate it!

Does the governor affect the shifting or what role does it play in the transmission? Thanks!
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Old 08-29-2016, 12:32 PM   #19
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Re: My Automatic Transmission Isn't That Automatic

Quote:
Originally Posted by thorvintage View Post
Thanks for all the info everyone...I Greatly appreciate it!

Does the governor affect the shifting or what role does it play in the transmission? Thanks!
http://mechanics.stackexchange.com/q...is-it-used-for

http://auto.howstuffworks.com/automa...smission12.htm
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72 Cheyenne Super, LWB, fleet, C20, 402/400TH, A/C. Donor, hauler - leaks. Luxuries like gauges, heat, wipers, headlights don't work, I prefer it that way.

How is it frame-off when the frame is the last thing left Tony
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Old 08-29-2016, 11:18 PM   #20
thorvintage
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Re: My Automatic Transmission Isn't That Automatic

Thanks for those links very helpful thanks...that may be the next thing I replace to see if that helps things at all. Appreciate it!
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Old 08-31-2016, 04:27 PM   #21
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Re: My Automatic Transmission Isn't That Automatic

If it is a TH350, the kickdown cable could be the issue not letting it up shift. I would try disconnecting it from the carb. No harm to trans, just won't downshift as expected or hold in lower gears as expected.
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Old 09-08-2016, 01:03 PM   #22
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Re: My Automatic Transmission Isn't That Automatic

I have the TH400 would that be the same advice?
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Old 09-08-2016, 01:08 PM   #23
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Re: My Automatic Transmission Isn't That Automatic

Transmission place says it needs a rebuild $700-$1200

Any advice on rebuilding it myself?
How hard is it to buy a used one and install it?

FYI i have never done any of this before but thats why I bought the truck to challenge myself and learn some new things. Its my therapy for anxiety and stress LOL
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