Back on the Bench - March - 2021

4L60E NO 2nd

It’s hard to be surprised anymore with all the possible things that can go wrong inside a transmission: circuit leaks, solenoid malfunctions, valve body problems, hairline cracks in cases, and so on. There are just so many different things that can cause the same outcome in a transmission’s operations. And with a computer to control the transmission, we have even more possible causes of various shift issues (we are all too familiar with the Toyotas U140 series and their computer failures).

The 4L60E has been around for a while, and most builders have rebuilt so many of these they can do them in their sleep. You would think by now we have seen all the possible causes for the various problems they have, burnt 3-4 clutches, band failure, reverse reaction shell failure, and the list goes on and on. Just build it, fix the common issues, check the engine inputs, fix any codes, and ship it. Most of which we will probably never see again except for the ten-day checkup.

Recently, we had one come in that was burnt, with a no-move condition, so we were unable to do an initial road test. My co-worker Roy Hagerman built the transmission, did all his normal vacuum tests, airchecks, and other common procedures that most builders do to the 4L60E. He installed it, with the expectation of it working and leaving like most, only to find it had no second gear.

If he drove it normally, it would skip second and go into third, but just a little late. He also noticed that if he held it around twenty miles per hour, it would shift into second and hold with no slippage even as he applied more throttle. On the scan tool, all the commands seemed to be there and on time (figure 1).

After discussing it with Danny Witt, one of the shop owners and lead builder, we felt like this had to be a solenoid command issue, solenoid mechanical, solenoid feed problem, or servo-related issue. Roy did vacuum tested the actuator feed limit valve, which is a pressure regulator to the solenoids. Afterward, we were confident that was not the cause. He did his air test on the servo during the rebuild like he has done a thousand times, and it checked out fine. We were left with a solenoid command or solenoid mechanical issue. We felt like it would take less time to swap out the solenoids than to go through extensive electrical testing.

With the solenoids replaced, it was time for another test drive, and still no second, so we struck-out on a quick fix. Now it’s time for some electrical testing. Danny attached his Snap-On® meter to the solenoid command wires and went for a test drive. During the road test, he noticed solenoid B (figure 2) was being commanded on and off like it was supposed to (figure 3). However, the solenoid A command signal rose very slowly. Once the ground wire reached battery voltage, the solenoid turned off, and the transmission made the shift. Now he could see why it would go into second if he kept the speed around twenty miles per hour.

Danny had determined the problem was outside the transmission and was computer-related. He needed to dig a little deeper into it. Realizing computers, just like transmissions, can have different issues that cause the same problem, he started with checking the powers and grounds to the computer while in operation and malfunctioning. He also inspected and load-tested the wires involved and found all of his power wires to the computer showed battery voltage. And the ground wires were showing three millivolts during the event. The wire harness had no visual damage and passed the load test, which only left an internal computer problem.

Some computers can be opened and inspected for possible fixes, such as loose solder joints or damaged components. This one had nothing obvious inside, so a reman computer was ordered for the vehicle and installed. The transmission was now operating as it should, and the vehicle could be delivered to the customer. When the customer returned for the ten-day checkup, he stated the vehicle had not missed one shift the whole time he drove it and could not be happier with the work.

Electrical diagnostics can often be tedious as well as time-consuming. Sometimes swapping out a solenoid or other component as a test procedure is much faster. Other times, those component prices are greater than the time involved in back probing wires and monitoring signals. An important to keep in mind when it comes to electrical diagnostics is that what you see on the scan tool is not always what you will see on the wire. Scan tool data is always useful in giving us some direction; however, it can also be misleading. And often, when we don’t take the time to check what’s coming out of the computer, it can lead to the transmission ending up back on the bench when it doesn’t need to be.