Locks the torque converter clutch in 4th gear on GM 700R4 and 200-4R swaps with no computer, using a 3/8 NPT fourth-gear pressure switch.
A 700R4 or 200-4R that went into a carbureted hot rod, a classic C10, a Chevelle or a Camaro will shift into overdrive on its own, because the shifts are hydraulic. What it will not do on its own is lock the torque converter. In the trucks and cars these transmissions came out of, an engine computer fed 12 volts to the converter clutch circuit. Take that computer away and the converter stays unlocked forever: the tach never makes its last drop at cruise, the fluid runs hotter, and you give up highway economy. Part 500412 is a no-computer lockup harness that restores that 12 volt signal using transmission oil pressure as the trigger.
Both transmissions carry a lockup solenoid inside the case, wired to a 4-pin connector on the driver side. When the solenoid gets power it lets the converter clutch apply, which ties the engine directly to the input shaft instead of driving through fluid. This kit does not replace that solenoid. It supplies the power to it, and only at the right time. The oil pressure switch threads into the case at the 4th gear pressure port. That port only sees pressure once the transmission has made the 3-4 shift, so the switch closes, keyed ignition voltage flows through the harness to the case connector, and the converter locks. Downshift out of 4th and the pressure drops, the switch opens, and the converter releases.
There is no vacuum switch, brake switch or manual toggle in this kit. If you want lockup to drop the moment you touch the brake pedal, a normally closed brake switch in the power lead is a common addition.
It will not fix a converter that shudders when it does lock, a slipping 3-4 clutch, or a no-overdrive condition. Those point to the converter clutch, the valve body, or the TV cable, all covered in our 700R4 common problems and fixes guide.
First check the transmission actually reaches 4th. If it does and the tach still never drops, look at what is plugged into the case connector on the driver side. In most swaps it is empty or the old harness is cut, which means nothing ever commands lockup, and that is exactly what this kit solves. If you already have 12 volts there and the converter still will not lock, the problem is inside: a failed solenoid, a pinched internal wire, or worn converter clutch valves. Our 700R4 valve body testing guide covers the hydraulic checks, and a known-good 700R4 TCC solenoid is the usual internal fix.
This kit fits the GM 700R4 (TH700-R4, later called the 4L60) and the 200-4R. The one hard requirement is a 1/4 inch NPT 4th gear pressure port in the case. Not every 700R4 or 200-4R had that port opened from the factory, so pull the plug and look before you order; on a case where the boss is cast but never drilled, the port has to be machined with the pan off and the valve body protected from chips. The 4L60E is electronically shifted and uses a different connector, so it is not a fit. If you are unsure which unit you have, our 4L60E vs 700R4 interchange guide and 200-4R vs 700R4 comparison show how to tell them apart, or send us your transmission tag details. Browse every part for this unit in our 700R4 collection.
Check the fluid level after the install with Dexron ATF; Dexron VI is backward compatible with the older Dexron grades these units shipped with. Lockup works best on a transmission that shifts correctly, so set the TV cable before you judge it. A Lokar KD-2700HT TV cable kit is the usual swap fix when the factory cable will not fit, and our TV cable guide explains why that adjustment matters for clutch life.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. It powers the lockup solenoid that is already inside the 700R4 or 200-4R. If that internal solenoid or its wiring has failed, replace it while the pan is off.
The manufacturer instructions call for a 1/4 inch NPT 4th gear pressure port, and the kit includes a brass bushing to adapt the switch. Some cases were never drilled for this port, so verify yours before buying.
No. The 4L60E is shifted and locked by a computer and uses a different case connector. This kit is for the hydraulically shifted 700R4 and 200-4R.
To a 12 volt source that is hot only with the key on, through the supplied 15 amp fuse. A constant battery feed could leave the circuit powered with the engine off.
It unlocks when the transmission leaves 4th and the pressure switch opens. There is no brake switch in this kit; many builders add a normally closed brake switch in the power lead so lockup drops as soon as the pedal is touched.
