What is this??

[Scott91370]

Gotta transmission question, I think.
This thing on the firewall, does it control passing gear on the TH350? This cab is an 85 that had a TH350 but now has a 700-r4 with the TV cable. Do I need to worry about it or just remove it?
What the heck does it even do?

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[PrairieDrifter]

I would say that's from the 700r4 the truck probably originally had. That's part of the torque converter lockup systen.

 

[Turbo4whl]

Gotta transmission question, I think.
This thing on the firewall, does it control passing gear on the TH350? This cab is an 85 that had a TH350 but now has a 700-r4 with the TV cable. Do I need to worry about it or just remove it?
What the heck does it even do?

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If your truck originally had a TH350, it would be a TH350C. The "C" stands for locking convertor. The 700R4 also has a locking convertor. Yes you need the convertor lockup working correctly for a 700R4.

 

[StickyLifter]

Sorry to ask this dumb question, but how to you know if your converter is locking up or not? My truck has a 700r4 and drives well on the highway; it's a nice change from my Nova to be honest. I can tell it's using all the gears but I don't know if the converter is locking up.

 

[PrairieDrifter]

Sorry to ask this dumb question, but how to you know if your converter is locking up or not? My truck has a 700r4 and drives well on the highway; it's a nice change from my Nova to be honest. I can tell it's using all the gears but I don't know if the converter is locking up.

The easiest way if the system is working properly is to lightly press on the brakes while cruising on the highway, that should create a noticeable rpm rise.

 

[Trucksareforwork]

Yeah. You want that. It’s the vacuum torque converter lock up control. Basically you unlock your converter two ways: one is pressing the brakes, one is pressing the accelerator. In this era, the vacuum switch monitored for a drop in vacuum and sent a signal to the trans tc to unlock. It’s a newfangled kick down switch but only for the tc.

A lot of people unplug all this stuff “because of emissions” and don’t realize they are disconnecting an important function of the drivetrain.

 

[Scott91370]

If your truck originally had a TH350, it would be a TH350C. The "C" stands for locking convertor. The 700R4 also has a locking convertor. Yes you need the convertor lockup working correctly for a 700R4.

Yeah. You want that. It’s the vacuum torque converter lock up control.

Problem with using it with the transmission in the truck is there is nowhere to plug anything in.

 

[Turbo4whl]

Problem with using it with the transmission in the truck is there is nowhere to plug anything in.

The TH350C has a 2 pin plug for the locking convertor. The 700R4 has a 4 pin plug (uses 3 of the 4). You need to replace the old 2 pin plug with the proper 4 pin 700R4 plug. You will need to add a ground lead to the third pin.

I will hunt for the correct wiring for the new plug unless someone has it handy.... @Matt69olds

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Here is the plug on a 700R4

 

[dusterdude]

Nevermind guys

 

[Grumpy]

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