Next issue: trans shifting
Thread Starter
Intermediate
Joined: Feb 2020
Posts: 29
Likes: 3
From: Rosharon, TX
Next issue: trans shifting
So as I get my 77 barn find moving I find the next problem. On it seems to be the tranny. It’s automatic 3 speed. Shifting from 1 to 2 is good but it doesn’t like to go into 3. I can feather the throttle and it will shift but goes back easy. Is shifting in this tranny an action of vacuum? I did change the trans filter. The old one was collapsed so thought I had it but not. Ideas? ( other than pull and re-build)
Thread Starter
Intermediate
Joined: Feb 2020
Posts: 29
Likes: 3
From: Rosharon, TX
I think I got it. I followed the vac line from back of trans to motor. At the location the vac line goes into the motor it splits 3ways. The headlights and the cruise control. I plugged the others and shifting got good. Now shifts 1-2 around 3200 and 2-3 around 3500. 1-2 is a hard shift but 2-3 is really soft, almost like overdrive.
I’ll leave it this way. Don’t need the cruise and when ever I can afford it I’m going to install elec headlight motors.
thanks for the input.
I’ll leave it this way. Don’t need the cruise and when ever I can afford it I’m going to install elec headlight motors.
thanks for the input.
Race Director
Joined: May 2006
Posts: 18,740
Likes: 2,583
You did check the fluid level on a fully warmed up motor, engine running, shifter momentarily placed in all positions 1,2,3,,N, and then park?
The fluid is clean with no burnt smell?
Remove the vacuum line from the back of the transmission, if you see any fluid the modulator has to be replaced.
The fluid is clean with no burnt smell?
Remove the vacuum line from the back of the transmission, if you see any fluid the modulator has to be replaced.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If you didn't have much throttle on it, those shift points are way high. Sounds like the vacuum modulator (can where the vacuum line connects) is toast. Inexpensive and standard replacements available at any auto parts store. Low throttle shifts should be in the 1500-2000 rpm range.
If the car has been sitting for many years, the valve body may have some sticky valves. Since you have changed tranny oil, just use it a while to see if "things" loosen up.
If the car has been sitting for many years, the valve body may have some sticky valves. Since you have changed tranny oil, just use it a while to see if "things" loosen up.
Thread Starter
Intermediate
Joined: Feb 2020
Posts: 29
Likes: 3
From: Rosharon, TX
So I had always been told that changing out all the fluid was not a good idea. I dropped the pan when I changed the filter . Fluid had a copper look. Added back about 3 quarts. When you say “service” the trans what all do you mean? “All“ fluid, filter and ...?
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
You can't remove the fluid remaining in the torque converter; so you can't do a 'complete fill' of the trans oil system. The 'copper look' to the old oil was probably very small friction plate "fines" that are abraded off the clutch plates as they are used up (100,000 miles or more). They are bronze-surfaced steel plates. As long as that metallic material is very fine particulate, that is absolutely normal. Next time you change trans oil, use a magnet to see if that material is magnetic. It should NOT be, if just normal clutch facing wear. If larger pieces and they are magnetically attracted, time for major overhaul.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
Transmission should take atleast 4 quarts, you need to add until full. If there was maybe a teaspoon of fine like table salt it's normal. If there's tablespoon or its coarse like beach sand transmission probably needs repaired. Since its already changed, drive it and hope for the best.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Actually you can change all the fluid in the trans.
you take off the return line to the transmission ( after it’s gone though the radiator) put that line in a drain pan.
start the engine and let it pump out 3 quarts.
shut off the engine, and refill the trans through the dipstick tube.
start engine again three more quarts , turn it off and fill.
repeat this until you have flushed out whatever the capacity of the transmission is.
reconnect line
you now have all fresh fluid in your transmission.
you take off the return line to the transmission ( after it’s gone though the radiator) put that line in a drain pan.
start the engine and let it pump out 3 quarts.
shut off the engine, and refill the trans through the dipstick tube.
start engine again three more quarts , turn it off and fill.
repeat this until you have flushed out whatever the capacity of the transmission is.
reconnect line
you now have all fresh fluid in your transmission.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Yes. You can do that. You can run the transmission completely dry. At idle speeds, you still won't get all the oil out.....but you will get more of it.
Oil change places (for decades) don't purge the entire system. It's just not necessary--unless you have had a failure inside the trans. Then you need MORE than a complete oil change; you need a full rebuild.
Oil change places (for decades) don't purge the entire system. It's just not necessary--unless you have had a failure inside the trans. Then you need MORE than a complete oil change; you need a full rebuild.
Le Mans Master
Joined: Aug 2006
Posts: 7,427
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
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Don't over fill it!
Your th400 takes 5 PINTS to refill after just dropping the pan to change the filter.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Yes. You can do that. You can run the transmission completely dry. At idle speeds, you still won't get all the oil out.....but you will get more of it.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa