C3 AT Problem TH400
Thread Starter
8th Gear
Joined: Apr 2020
Posts: 8
Likes: 1
C3 AT Problem TH400
I have a 75 that will not go into gear with the engine running. The selector moves through the positions and the linkage is connected rotating the arm.
Would a bad vacuum modulator or governor gear cause this? or probably an internal part inside the transmission?
Would a bad vacuum modulator or governor gear cause this? or probably an internal part inside the transmission?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Is this something that has been sitting all winter? Sometimes the torque convertor will drain itself internally. Just wait a couple minutes in Park and see if fluid pressure builds up again, and grabs in gear.
The Modulator is for controlling shift points.
The Modulator is for controlling shift points.
Last edited by HeadsU.P.; Apr 27, 2020 at 02:30 PM.
Thread Starter
8th Gear
Joined: Apr 2020
Posts: 8
Likes: 1
The back story is this car has been sitting for awhile and I am getting it back up and running. The fluid level was barely showing on the dip stick and I put 2 quarts in. I just got the engine running a few days ago.
With the engine on (not to operating temp) the dip stick shows it is at the "add a quart" mark.
With the engine on (not to operating temp) the dip stick shows it is at the "add a quart" mark.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
If and when a torque converter drains itself, the fluid drains into the pan. Just to let you know, it doesn't go on the floor. However, as the pan "overfills" from sitting the dipstick tube has an "O" ring at the pan that will be tested for a good seal. Some fail from the extra capacity.
As the dipstick states on one side: Check while running in park, warm & level. That makes sure the converter is truly full. For you to add 2 Qts tells me the pan gasket is likely failed or tailshaft housing seal.
Next thought? Wouldn't that be funny if the shift cable came unhooked at the tranny? (Things that make you go Whew! )
As the dipstick states on one side: Check while running in park, warm & level. That makes sure the converter is truly full. For you to add 2 Qts tells me the pan gasket is likely failed or tailshaft housing seal.
Next thought? Wouldn't that be funny if the shift cable came unhooked at the tranny? (Things that make you go Whew! )
Thread Starter
8th Gear
Joined: Apr 2020
Posts: 8
Likes: 1
Well... I got up to temp and checked the dipstick. It was just below the add a quart. So added a quart. Turned off and checked few other non trans items. I started up again got to temp and tried it out. Moved the selector from park to reverse nothing. Then to neutral and drive nada!
Fun fun.
Guess next move is to drop the pan (like suggested). Feel free to add any other facts of wisdom. Thanks
Fun fun.
Guess next move is to drop the pan (like suggested). Feel free to add any other facts of wisdom. Thanks
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Le Mans Master
Joined: Nov 2016
Posts: 9,256
Likes: 947
From: Lake Tahoe, CA
A couple of notes on the TH400 here; https://www.chevyhardcore.com/tech-s...transmissions/
http://constructionmanuals.tpub.com/...g-List-188.htm
Wish you luck.
http://constructionmanuals.tpub.com/...g-List-188.htm
Wish you luck.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Either there is not enough oil in the transmission to engage any clutches, or the shift shaft inside the transmission has come disconnected to the shift lever linkage [on the outside]. My bet is on very low oil level in the transmission.
And "no", a bad vacuum modulator or sticky/bad governor would not cause such a problem.
And "no", a bad vacuum modulator or sticky/bad governor would not cause such a problem.
Last edited by 7T1vette; Apr 28, 2020 at 12:28 AM.
Thread Starter
8th Gear
Joined: Apr 2020
Posts: 8
Likes: 1
FYI update:
The pump was missing a valve. Their had been a rebuilt at some point in it's life and it was missed. I pulled the trans and sent it to a shop.
The fluid was not being pushed through the system. All good now once a valve was installed.
Thanks for the responses.
The pump was missing a valve. Their had been a rebuilt at some point in it's life and it was missed. I pulled the trans and sent it to a shop.
The fluid was not being pushed through the system. All good now once a valve was installed.
Thanks for the responses.
Last edited by Bentwaters; May 3, 2022 at 10:26 AM.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
FYI update:
The pump was missing a valve. Their had been a rebuilt at some point in it's life and it was missed. I pulled the trans and sent it to a shop.
The fluid was not being pushed through the system. All good now that the missing valve was actually in place.
Thanks for the responses.
The pump was missing a valve. Their had been a rebuilt at some point in it's life and it was missed. I pulled the trans and sent it to a shop.
The fluid was not being pushed through the system. All good now that the missing valve was actually in place.
Thanks for the responses.
Jebby
