So I have the switch, but now I find there is a gasket for this part of the transmission, so of course I want to make sure I get the right gasket.

1985 Hawk, 1.8

I think I found it at rockauto, would this be known as the "case gasket"?

part #8631979

Looks like 11 bolts, but IIRC, Orlen once mentioned 8 bolts.

Would all the 1st gen Js have the th125c, or did GM like to mess with us and have different variations to look out for?

Wayne I just happen to have the '84 Sunbird at work today!!

I went out and took a good look and felt around - I counted 10 bolts on the side cover. Since I watched the Converter Video I have been interested in getting the TCC up and going again on the 1st gens too. Hope you can keep us posted on your progress and sources - prices. It is my understanding you can remove the side cover with the transaxle in place - just need to remove the inner splash shielding.

I also watched another video last night while eating dinner from the same guy talking about the gearing in an automatic (??gear head?? - Hahaa)) - very informative. The engineers that invented this stuff were really sharp!

Looks to me that automatics have gotten away from the bands and now use clutch packs extensively. It is amazing how they figured out how to use multiple gear sets to create the 4 , 5 , and 6 speed automatics!


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At rock auto it is Pioneer 749046, Valve body side cover gasket. In other brands the number is, Victor W39368tc and Felpro is TOS18661.
Standard makes the solenoid TCS20, it is much easier to install than the original. You can do the whole job in less than a half hour.

Thanks Frank!! (Looks like I missed a bolt during my check too....)

A quick check on ebay shows Felpro and TCS20 at under $25 for both - great pricing!!

Is the TCS20 used on all THM125c 1st and 2nd gen?


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Wow, you guys are amazing, and fast!

Orlen--going under the car and counting bolts while at work...my laugh for the day!

Is the solenoid what I disconnected at the blue clip, or is that something else I should replace as well. I have the thing with the blue clip, but didn't know until yesterday there was a gasket involved.

The solenoid is on the valve body, located under the cover on the side of the transmission. One wire goes to a pressure switch, next to it, and the other to the connector on the front of the transmission. And yes Orlen it is used on 1st and 2nd gen TH125c.

here is the valve body cover that has to be removed.

Once it is removed you will see the valve body with the attached tcc solenoid and pressure switches.

This happens to be a later trans (90 and up), with the only difference being the extra switch on the right and associated wiring (2nd clutch pressure switch). The earlier trans just has the switch with the green connector and it will have two terminals instead of the one on this switch, with one of them connected to the tcc solenoid.

This is the tcc solenoid it is held in with a single screw that takes if I remember correctly a T20 torx bit. If you get the standard TCS20 solenoid you will be cutting the two wires attached to the old solenoid and putting them into the attached connectors on the new solenoid. Then it is just a matter of replacing the valve body cover and reconnecting the plug on the front side of the trans, if it has been unplugged to disable the sticking tcc solenoid. You will find that most of the time in doing this repair is in removing the left front wheel and taking the bolts out of the cover and putting the cover and wheel back.

FrankD wrote:At rock auto it is Pioneer 749046, Valve body side cover gasket. In other brands the number is, Victor W39368tc and Felpro is TOS18661.
Standard makes the solenoid TCS20, it is much easier to install than the original. You can do the whole job in less than a half hour.

ordered, that part number made it so easy to know I had the right one. thanks again.

I saved this topic to my hard drive - going to be very useful next year!

Thanks Frank for taking the time to post up your how-to. I look forward to replacing those solenoids next year when pretty weather graces us once again!! :-)

I personally have a far better understanding what is going on inside these transmissions nowadays and appreciate the lockup feature. Heck I even learned how to get much more out of those 3rd gens too since the '02 has a factory tach in the IC. I used to think a tach was so useless in an automatic - this old dog can still learn!



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The tach has been a lifesaver while I had the switch disconnected so I wouldn't rev over 3 grand

Well wayne cool,but on a unique comment I never look at my tach in my Big truck I drive for work.I shift soley by speed or sound (trucker side) of thoughts.The only time I really use my tach is when getting ready to hit a grade/hill and wait and watch my speed and tach to shift prior to the 1300 rpm point so the truck continue's to pull when I go to shift down 1 gear.Cars are similar but,not in the same scenario with weight pulling in mind.Just a odd thought on rpm's.And I seldomly use the clutch as I float the gears by bumping the throttle and shift.Not related to your subject but sharing some truck driver stuff.

I don't really stare at the tach or anything, but since the car isn't running as normal because of the disconnected switch, here and there I check it to make sure I'm not over-revving it. Often when I look, I'm pegged at 3 anyway. Plus the car is moody with the weather, so sometimes 45mph has it at 3, other times I can get to 50mph.

Back in the day, while driving a Mitsubishi Fuso turbo diesel 4, I always started in 2nd since 1st was useless other than on a hill under load. First time I drove it was interesting as I did two compression skids since I wasn't used to the European shifting pattern. When I had my usual truck, an automatic with a 300 pound liftgate on the back, I got 10% better mileage than anyone else in the fleet, and they all had manual.

When the company got a new truck, automatic but with a Vortec V8, since I got the best mileage and kept the cleanest truck, they gave it to me!

Unlike the 1sr abd 2nd Gens - the 3rd gens with the 4T40's have Pulse Modulated TCC's and it really helps to have a tach handy since lockup occurs in different gears and even different amounts of lockup pressure. The 3rd Gens also have rather numb drivetrain mounts that make it harder to really tell when the shifts are occuring. It is a whole different ball game.

I have been able to see 1500RPMs at around 45MPH if I feather-foot the throttle an let everything engage.

It has been very interesting to be able to see what is going on with the '02. With the '98 we have driven it for years and have had no idea what was going on except when you floor the gas and after several kickdowns the engine roars.

Also seeing Rons discusion of Heavy Truck driving - those super tough transmissions in them are well suited to speed shifting with the throttle and/or jake brake.I found over the years of driving - that the average passenger car or light duty would eventually wear down the syncros if speed shifted frequently. Of course speed shifting a front wheel drive usually involves a whole lotta gear grinding....LoL.



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Orlen Brown wrote: I have been able to see 1500RPMs at around 45MPH if I feather-foot the throttle an let everything engage.

Mine is 45 MPH at a mid throttle , 35 MPH at feather throttle. This is a 94 replacement trans with 2.93 ratio compared with the original trans a with 3.08s. ( plus I have the tallest 14'' tires that fit on the car)

Doug in P.R.


92 Pontiac Sunbird LE, 2.0, AT, Red / Black with Grey 155K miles. Hurricane Maria Survivor ! ( It takes a licking and keeps on ticking ! ).....in Salinas, Puerto Rico!