swapping to a th350c

[Jwill41]

I got a tranny for my 68 today. It came from an 84 GMC and has a 4 prong plug on the drivers side so I asume it is a th350c. Does anyone know what I need to do to make it work correctly? I just burnt up a 700r4 and dont want to do that again. I know the drive shaft and cross member will work but how do I wire it?

[LUV2XCLR8]

Well my old tranny has a 4 prong plug, but only 3 wires, My buddy told
me it could be wired up on a simple toggle switch, but I have no idea

[pjmoreland]

This is only a guess, but I wouldn't be surprised if the 350C gets wired the same way as a 700R4 gets wired.

[masterbeavis]

Somebody has to sell a harnass/switch for it. A buddy of mine has a 350c, ran it for years without the lockup hooked up. Bought the switch for it, loves it. Just have to remember to kick it out when you stop. I've heard that if there is not enough pressure to keep tq locked, things get angry. I cant confirm that tho.

[cgrossl]

I have a TH350C in my 72 and one in my 85. I didn't install the tranny in my 72, but whoever did it wired the lockup function to an extra dimmer switch in the floorboard. This works, but it isn't the proper way of doing it. My 85 came from the factory with a TH350C. The lockup function on it is based on engine vacuum. There is a switch on the firewall right next to the brake booster that is hooked to manifold vacuum. There is also an inline restrictor to keep sudden vacuum changes from actuating the switch. When manifold vacuum gets above a certain point, the switch closes and the torque converter locks up.

I am not sure how this is wired, but you should be able to get a wiring diagram for an 85 C10 and use that for reference. It shouldn't be that big of a deal.

[jimfulco]

Check the length of the later trans. It may have a longer tailshaft housing.

[Tx Firefighter]

If it's from an 84 GMC, it should have a 9 inch tailhousing, which makes it the same length as the 700R4.

[Jwill41]

it does have a 9" tail shaft and is the same total length as the 700r4.

[Jwill41]

Ok, I got-er installed and the vaccume line connected and all is well. Tranny shifts great and kicks down like it is supposed to. Now for the lockup part of it. This picture is if you were laying under truck with pan and filter removed. It seems to me that I need to run a hot wire to the plug on the positive side. It looks like it will get the negative when the pressure is right in the tranny. I dont understand why the pressure switch only has one terminal being used and if I hook up a negative to the plug then the solenoid would "always" have a ground. I think the only thing I need is the hot wire and that the negative on the plug is for something else but it just cant be that simple.

[pjmoreland]

That doesn't make any sense to hook up both wires to the same terminal on the vacuum switch. You should connect one wire to each terminal. I would recommend connecting the red wire to one end of a fuse, and connect the other end of the fuse to the "IGN UNFUSED" terminal on your fuse panel. That way the circuit will not be active when your truck is turned off. You may also want to replace your brake light switch with one out of a later model truck that has two connectors on it. You can pass the power for the TCC circuit through the extra connector so that your torque converter clutch will release when you apply the brakes.

[Jwill41]

It doesn't make sense to me either but that is the way it is wired. The two black wires both go to a single crimped spade connector.

Is this what you mean, I would run a hot wire from the fuse box to an inline fuse and then to a switch on the brake pedal, then to the positive on the tranny. When the brake pedal is pushed it breaks the circuit and unlocks.

[pjmoreland]

That's right. That's how I have my 700R4 transmission wired, and it works great.

If you leave the vacuum switch wired that way, the vacuum switch will not work. If the vacuum switch is wired correctly, the torque converter clutch should disengage when you press the throttle enough to lug the engine, but not enough to shift into 2ND.

The brake switch needs to be closed when the brake pedal is not being pressed. This is the opposite of how the brake light switch works.

I would also recommend installing a vacuum delay valve in your vacuum line that runs to the vacuum switch. This will help keep your torque converter clutch from kicking on and off too quickly. Vehicles that were originally equipped with torque converter clutches had these little valves installed. When I first installed my 700R4 I didn't have a vacuum delay valve, and my torque converter clutch would engage and disengage rapidly if I was right on the edge of the lockup point.

[Jwill41]

thanks for all the help. i will start getting parts together in the next few days.