Back on the Bench - March - 2017

The Problem and the Cause

The key to repairing any problem successfully is twofold: You need to address the problem and the cause. Fix only the problem without addressing the root cause of that problem, and you can bet that the car will be back.

What do we mean by “addressing the root cause”? Let’s take a look at a 6L80 with a no high gear and no lockup. The owner of the truck says the engine seems to run at higher RPM than normal at highway speeds, and the shifts seem to be firmer after it gets warm. During the test drive it shifted hard, had no high gear, no lockup, and the Check Engine light was on.

A check for codes revealed P0741: torque converter clutch stuck off. Line pressure issues can lead to torque converter failure, so, with the scan tool still connected, you’ll want to check line pressure command and MAF (mass airflow) sensor operations.

One problem to consider when it comes to the MAF sensor is when the sensor is reading improperly, but still within specs so it doesn’t trip a code. For example, a MAF that only delivers an airflow reading equivalent to idle speed, so it doesn’t provide the proper input for the computer to adjust the air/fuel mixture for off-idle operation. This can also cause lower line pressure and lead to premature transmission failure.

Here’s a simple check for mass airflow sensors: At idle, in park, with no load, the grams-per-second signal should be right about equal to the engine size in liters, plus or minus about ten percent. At around 2500 RP M, it should be about twenty times the engine size in liters.

So a 5.0-liter engine should be close to 5 grams per second at idle, and around 100 grams per second, plus or minus ten percent, at 2500 RPM. Always check the line pressure command and actual line pressure to verify solenoid and pressure regulator operation.

After inspecting the MAF and determining it was operating properly, we removed the pan. As suspected, there was a lot of clutch material in the sump; more than likely from the torque converter clutch.

Once we had the transmission on the bench and apart, there was very little damage, other than converter clutch material. The stator support shaft and bushings looked good, as was the input shaft.

Dave Hammond at Atlantic Torque Converter cut this converter open so we could get a firsthand look at what happens to these converter clutches (figure 1). Sure enough, it was hammered.

So we know the converter’s shot, but is that the root cause or just the result of another problem? One possible source of a problem that can damage the converter is bore wear in the valve body or pump. So your next step should be to examine a hydraulic diagram to determine which valves control the torque converter.

The TCC regulator valve is inside the valve body (figure 2). This valve controls pressure to the apply side of the converter clutch. If the TCC regulator valve is damaged, sticking, or the valve bore is worn, it may not provide adequate pressure to the converter clutch, which would allow it to slip.

The TCC control valve is in the pump (figure 3). Fluid makes its way from the TCC regulator valve to the TCC control valve, then on to the apply side of the converter clutch.

A common issue here is bore wear at the end of the valve, which prevents pressure from the TCC pressure control solenoid from stroking the valve completely. This can reduce the apply pressure to the converter, once again allowing the clutch to slip (figure 4).

The best way to verify bore wear is to vacuum test the valves and bores. If you don’t have a vacuum test stand, consider buying one, or even building your own. Jarad Warren wrote a very thorough article in June, 2015 issue of GEARS, where he provides part numbers and instructions for making your own vacuum test bench. This is one of those tools where, once you begin use and understand it, you’ll wonder how you ever got by without one.

The valve body passed the vacuum test, with 20 in Hg. But the TCC control valve in the pump failed at just 13 in Hg. For now, no one offers an oversized TCC control valve and reamer, so the only option was to replace the pump body.

With the pump body replaced, we buttoned up the transmission, installed a new torque converter, and reinstalled the unit. During the test drive, the transmission worked great; chances are good that it’ll continue to work properly for years to come.

No one likes working on the same transmission a second time; so, before the one you’re working on today finds its way back on the bench, take the time to check every possibility for the root cause of the failure, instead of focusing on the results.

By addressing the root cause first, you can avoid costly comebacks, keep your transmissions on the road, and keep your customers happy. And isn’t that really how you want all your jobs to work out?