Torque Converter Rebuild Process

February 2019 Transmission, Feature

Between the engine and the transmission the crankshaft spins a flex plate with a torque converter to start the process of converting engine horsepower to truck wheels rotating at a controllable rate.

Inside the torque converter there is a pump that contains a number of blades which act like a  propeller pushing transmission fluid and creating hydraulic pressure. The shape, space and angle of the blades are an important factor in creating the desired hydraulic pressure.

The torque converter goes hand-in-hand in the performance of the transmission. They work as a team—good or bad, your vehicle can only convert the engine horsepower to the wheels when these two functions work together. A weak link (failure) in one will likely create a domino effect failure in the other. In other words, if one goes down, it will likely take the other one down with it.

The editors of Diesel Tech Magazine got the opportunity to see what happens in a transmission shop when a vehicle comes in with tranny failure. The crew at Ultimate Transmission took the time to walk us through the process that most diesel owners hope to avoid—having to repair or replace a transmission.

When a bad transmission is brought to Ultimate Transmission, there are two teams working in tandem to get it repaired (building it back to better than stock) and the vehicle back on the road as quick as possible. While the front of the shop is rebuilding the transmission, the middle part of the shop is doing the same with the converter. Both are working together to create a better, more durable transmission built specifically for the workload expectation of the vehicle.

Russell Soroka, torque converter specialist for Ultimate Transmission, walked us through the process of rebuilding a torque converter.

“Most of the time it's usually the transmission that fails, and because transmission fluid is shared between the transmission and the torque converter, they basically contaminate each other so both need to be rebuilt,” Soroka explained. “So the torque converter needs to be cut, cleaned and upgraded to the appropriate power level.”

So the first step is to take the torque converter and cut it open and examine the parts to see what needs to be repaired or replaced. Then the parts are run through a washer and inspected more closely for wear and failure.

If the torque converter is fairly common, it’s likely that Ultimate Transmission has a rebuilt one ready for install. (They will take the failed converter and rebuild it, paint it and put it in the bin for replacement of the next failed torque converter that matches those specs.)

But the concept of rebuilding a stock torque converter doesn’t mean using the same stock parts that have already failed once in that particular vehicle. Soroka said Ultimate Transmission will use higher quality parts so the torque converter is better than stock—stronger and more durable.

“Transmissions will get overheated from too much power or a failure of the valve body to maintain a good lockup pressure and the clutch gets wiped out,” he explained. “That will wipe the clutch surface on the front cover. So we bound the new clutch disk on the pistons, buy new pistons from our supplier, drill out the old rivets and upgrade the springs for your desired power level. Then we'll usually upgrade the front covers to a billet unit.”

Soroka said he likes to go to billet front covers because they’re a lot stronger; there’s less flex.

“So as you're in a high power situation, the flywheel will actually pull and flex, creating high spots in the area where the clutch rides,” he said. “This makes an uneven surface and it doesn't give you a 100 percent clamping area. They start slipping and this is where your clutch kind of starts failing.”

So when Soroka rebuilds a converter, he first finds out the application expected of the vehicle and then beefs up the parts that tend to fail. “We just do the necessary upgrades to give that three-year, 100,000-mile warranty,” he said. “From there, after we get the turban done, we'll move on to the stator.”

Soroka said Ultimate Transmission can take a factory stator and machine it for different stall speeds—from low stall ratings to high stall ratings. The angle of the fin is going to determine how the stall speed works and how efficient the torque converter's going to work. He then spends time with the impeller, usually putting new hubs in and extending the fins to increase its efficiency.

“We put new bearings and new seals on just about everything we do,” he explained. “We do new billet hubs if the OEMs aren’t billet. The pistons get inspected, the springs get replaced if they’re not up to par.”

Soroka said he usually likes to use a dampened piston for vehicles that are used on the highway.

 “A solid billet piston is great on the race track … but when you're driving your big power truck on the street those intermediate shifts with the converter locked puts a lot of stress on the shafts of the transmission. So we try to incorporate the dampened style piston and this gives a little play as the converter engages; the clutch engages and as you have a locked converter while you're going through your shifts it's not so jarring on the input shaft. As you can imagine there's a lot of torque that goes through these parts and we were doing data logs you can actually see the torque spike difference in a billet piston versus a dampened piston.”

Once all the parts are reassembled and properly stacked in place, it’s time to weld the torque converter back together. Ultimate Transmission has a torque converter welder that can weld anything from a nine-inch converter all the way to the 13-inch converters.

“We try to keep them down to one weld, just to save on weight,” he said. It is critical to make certain you stay within the proper tolerance—which is about the size of a hair. “You have to keep the hub in line with the pilot. Otherwise it's not going to be very effective in the truck,” he said.

After it’s passed all the tests, the converter will be balanced and painted. Then it’s good to be re-united with the transmission and delivered to the customer.

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