Q&A

Ask Away! With Jeff Smith: Troubleshooting Lockup Converter Problems on a 700R4 Transmission

four-pole brake light switch

This is the four pole brake-light switch mentioned below. The lower connections are normally closed, which would route power to the 700R4. When the brake pedal is depressed, the switch opens and power to the torque converter clutch is removed and disconnects from the input shaft. Simple and effective. (Image/Jeff Smith)

I recently purchased a’71 Camaro with a small-block and a 700R4 overdrive trans. The guy who I bought it from said the trans has a lockup converter and that everything worked great. But driving it now for a while, I don’t think the converter is really locking up. I have a buddy with a 700R4 with a lockup converter and his rpm drops a lot more than mine does when it shifts into fourth gear. So how do I get the converter to lock up in fourth gear? — T.D.

Jeff Smith: The most popular way to convert a 2004R or 700R4 to lockup is to do so when the transmission shifts into 4th / overdrive.

Several companies, including Painless and TCI, offer kits.

The contents of these kits vary, but before you purchase one, it’s possible the transmission may already have the conversion accomplished and all it needs is a simple 12-volt connection.

The kits add a pressure switch inside the transmission that enables the torque converter clutch to engage when the transmission shifts into fourth. All that’s needed to test this first is to apply 12 volts to the A terminal on the four-pin connector on the side of the transmission. If there is no electrical connection to the transmission, that is a huge indicator that the system won’t work in its current configuration.

If the male plug for the connector is missing, you’ll need to find one. We ran down a Dorman part that will do the trick. The A terminal is the one that needs switched 12 volts while the B terminal will need to be grounded. For a quick check on whether this works, merely connect switched 12 volts from the fuse box to the A terminal on the connector. With the engine running and 12 volts to the transmission, if the trans has already been converted, when the trans shifts into fourth gear this will also engage the torque converter clutch (TCC) to lock up. The rpm drop will be noticeable.

If you try this and the converter doesn’t lock up, then you will need to convert the transmission to one of the above-mentioned kits. The Painless kit costs a little more, but offers an extra feature that you might find worth the higher price.

Most standard kits include the internal components to convert the transmission. The TCI kit for example also offers a relay switch that must be wired into the brake light circuit. This is because when you apply the brakes, this switch disconnects the power to the TCC, which disengages the TCC when you apply the brakes.

If this switch isn’t employed, the TCC remains on until the transmission downshifts out of overdrive. Often this won’t happen until vehicle speed drops below 20 mph. This can begin to lug the engine a bit, which doesn’t hurt anything, but in a panic-stop situation, it’s possible the locked TCC could stall the engine.

That would be bad.

Imagine attempting an aggressive stop only to decide immediately to accelerate (like sitting on the railroad tracks) and then having the engine stall.

This is a very common four-pin brake light switch brake light switch that operates the brake lights normally but also has employs a separate pair of electrical connections. This set is normally closed, which connects 12-volts to the transmission until the brakes are applied. When the brake pedal is depressed, the switch opens and cuts the voltage signal to the transmission and disengages the TCC.

The Painless kit adds a second low-vacuum switch.

This is wired in series with the above wiring and is connected to sense engine vacuum. This switch disconnects the voltage to engage the TCC when the throttle is opened under heavy acceleration (roughly six to eight inches of manifold vacuum).

So when you step on the pedal to perhaps half- to three-quarter throttle, the vacuum switch disconnects the electrical circuit, unlocking the TCC so the converter adds stall speed and allows the engine to accelerate with more power. Assuming you’re still in fourth gear, when the throttle is relaxed, the vacuum rises in the manifold, the TCC re-engages, and you’re back to cruising.

It shouldn’t take much diagnosis to determine if your 700R4 is fitted with one of these kits. If not, it’s probably best to have a professional shop install the kit in the trans since this will require removing the pan and adding some wiring and a pressure switch to the valve body.

This is also a good time to add a new filter and some new fluid.

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3 Comments

  1. what in your opinion would cause (causes) a torque converter to stay locked on a 700r4 causing the engine to stall ? i just replaced the torque converter after replacing transmission pump in the transmission. i also replaced the decant cable and reset. i had perfect operation of the transmission for a couple hundred miles then suddenly stated having an occasion hard shift into second feeling as though it was the torque converter engaging. The converter is still locked as i limped home. By reading your article it might highly suggest the this to be as simple as the brake switch ?? in you opinion is this a mechanical issue or a electrical issue …thank you in advance

  2. Sticking TCC solenoid.

    • steve carlsen says:

      my 700r4 was rebuilt to put in my 31 chev behind a 300 hp 327 cu in evey time I brake the engine slows down and then kills other than at the flywheel thats all that connects the engine and transmission

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