75 TH400 leaks with a vengence
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Instructor
Joined: Apr 2007
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75 TH400 leaks with a vengence
MY '75 TH400 had a drip earlier in the spring.......then it became more noticable..then it started to leave small puddles. I tightened the pan and found one screw striped.
I just took a 300 mile trip to Northern Wisconsin and everything went well untill I got 70 miles from home.........smoke was rolling out the back and I pulled over. The bottom of the TH400 was dripping w/ trans fluid, I added a quart and finally got home and found the pan dripping and the passenger side exhaust pipe had burnt on fluid.
There is a vent tube that comes down from the trans vent to an area near the modulator....this area was soaked.....so where is the culprit? Why would there be t/f coming out of the vent? I have an aftermarket modulator because the oem part won't fit...I have Hooker Headers.
My neighbor who is an amazing mechanic suggested that the trans overheated and that's what the vent is for.
Got any ideas? Thx.
I just took a 300 mile trip to Northern Wisconsin and everything went well untill I got 70 miles from home.........smoke was rolling out the back and I pulled over. The bottom of the TH400 was dripping w/ trans fluid, I added a quart and finally got home and found the pan dripping and the passenger side exhaust pipe had burnt on fluid.
There is a vent tube that comes down from the trans vent to an area near the modulator....this area was soaked.....so where is the culprit? Why would there be t/f coming out of the vent? I have an aftermarket modulator because the oem part won't fit...I have Hooker Headers.
My neighbor who is an amazing mechanic suggested that the trans overheated and that's what the vent is for.
Got any ideas? Thx.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
The vent on a 400 is near center top of the case just behind the pump. Usually it's just a plastic tube with a floating cap. Some used a short metal tube that is hooked to the right side of the car. when the trans gets hot, fluid expands and could get to the point it'd dump out the vent- mostly it pukes out the fill tube. (Why GM started using locking dipsticks with the 700's). Look at your trans cooler lines- they are right front on the trans, and look and see if theres a TCS switch in a case pressure tap.
Thread Starter
Instructor
Joined: Apr 2007
Posts: 148
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The vent on a 400 is near center top of the case just behind the pump. Usually it's just a plastic tube with a floating cap. Some used a short metal tube that is hooked to the right side of the car. when the trans gets hot, fluid expands and could get to the point it'd dump out the vent- mostly it pukes out the fill tube. (Why GM started using locking dipsticks with the 700's). Look at your trans cooler lines- they are right front on the trans, and look and see if theres a TCS switch in a case pressure tap.
What should I be looking for? thanks Tim.......Larry
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
If I'm correct, the TCS tap discussed is mounted on the drivers side near the rear of the tranny just above the pan lip. It is a heavy duty plastic fitting with an "O" ring that pops into the tranny. It has either one or two spade terminals - one for the TCS (pollution control) system and one for the kickdown solonoid. Also, check the modulator. It too has an "O" ring and can leak if the hold-down bolt has loosened or the "O" ring is damaged. Good luck.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Some cars had a TCS switch- literally a pressure switch that retards timing when it's in 3rd (Transmission Controlled Spark). It looks like a plain old oil pressure switch with 2 terminals. If that switch gives up, it pumps fluid out in a hurry.
JimT has another good point-- the connector for the kickdown solenoid is on the drivers side- plastic connecter that has an o-ring for a seal. Bad thing about that one is you have to drop the pan to change the seal. I slide a wrench over the tabs on the inside end to compress them and the push it out so I don't break the lock tabs.
JimT has another good point-- the connector for the kickdown solenoid is on the drivers side- plastic connecter that has an o-ring for a seal. Bad thing about that one is you have to drop the pan to change the seal. I slide a wrench over the tabs on the inside end to compress them and the push it out so I don't break the lock tabs.
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Thanks to both of you Tim and Jim, these are things that are new to me. I plan on replacing the pan gasket tomorrow anyway, at least it's a starting point and I have a box of "O" rings as well. I'll let you know how successful "we" are...and thanks a bunch. Larry
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
You may want to torque the tranny pan bolt up to spec limit before you remove the pan. If you have difficulty getting some of the bolts to full torque, you may need to Heli-coil some of the threads in the tranny main case. Better to know that when you take it apart rather than to find out when you put it all together. Heli-coil kits are not cheap (about $50), but they are a lot less expensive than changing a tranny out!
Also, you may want to consider installing a drain plug at the rear of the pan. NAPA has units that bolt in place after you drill a hole in the pan. If you do this, make sure that you place the drain where it does not create any hardware interferrence with valve body parts. I put mine at the rear, on the back side of the pan...rather than below [which would stick out like a 'sore thumb' at judged car shows]. I can't drain ALL of the tranny oil with it, but can lower it enough that it doesn't splash out when I remove it.
Also, you may want to consider installing a drain plug at the rear of the pan. NAPA has units that bolt in place after you drill a hole in the pan. If you do this, make sure that you place the drain where it does not create any hardware interferrence with valve body parts. I put mine at the rear, on the back side of the pan...rather than below [which would stick out like a 'sore thumb' at judged car shows]. I can't drain ALL of the tranny oil with it, but can lower it enough that it doesn't splash out when I remove it.
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Thread Starter
Instructor
Joined: Apr 2007
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I replaced the pan gasket on the TH400 and the seals and "o" rings on the bottom and that did the trick. I think I might have created my own problems to start with.
Before I went on my roadtrip I checked the torque on my tranny pan and found a few loose bolts so I cranked them to be sure of a good seal. During this time I found 2 stripped threads in the bottom side. What must have happened was that I over torqued the pan gasket and it all started from there. I put heli coils in the stripped threads and used a WIX filter and gasket using 13 inch pounds of torque.
Thanks for all the help.....your advice was priceless and now I can pass it on to others.
HAPPY TRAILS! Larry
Before I went on my roadtrip I checked the torque on my tranny pan and found a few loose bolts so I cranked them to be sure of a good seal. During this time I found 2 stripped threads in the bottom side. What must have happened was that I over torqued the pan gasket and it all started from there. I put heli coils in the stripped threads and used a WIX filter and gasket using 13 inch pounds of torque.
Thanks for all the help.....your advice was priceless and now I can pass it on to others.
HAPPY TRAILS! Larry
