Back on the Bench - July - 2021

Is It the Computer?

We recently had a Ford 6F35 with a lock-up malfunction code come into the shop. The torque converter clutch seemed to be working on the test drive, but the fluid was black and had a foul odor, so we were sure there was some internal damage. David Whitt, the shop owner, had me build a stock unit for the vehicle so we could swap out the transmission and keep from tying up a lift.

Dave had me replace the solenoid body and install a Transgo kit during the rebuild, along with clutches, pistons, and an overhaul kit. He replaced the converter with a reman. We have had really good luck with the 6F35 transmission. We usually just follow the procedures, and they go away, never to be seen again. After the rebuild, everything seemed to work fine, so we sent it on its way. But this one came back and paid us a visit with the same code set in it. The customer stated they had driven it for several days without any problems, then the day before they came in, they noticed a jerking sensation a few times throughout the day while driving on the highway, and the check engine light came on.

At first, I wondered what I messed up or missed. I was told that this was the same problem it came in for originally. Since this was a completely different transmission doing the same thing, it was more than likely not the transmission causing the issue. Even though there is always a possibility, we were leaning towards an external problem.

We needed to verify the customer complaint and duplicate the problem. Danny Whitt drove the vehicle several miles and multiple times throughout the day without any issues. After several attempts, he finally got the problem to occur. It felt like the converter started to slip a couple of times and then set a code.

We discussed the possible causes. This could be a solenoid command problem from the computer, a solenoid malfunction, or a valve sticking. Our starting point was checking the command to the solenoid on the scan tool and back probe the solenoid command wire at the computer. Being an intermittent problem, we wanted to cover all our bases before trying to get it to act up again. Since this was a different transmission with the exact same problem, we were leaning towards a computer-related problem, more than likely a solenoid command issue. Any time we see a problem with solenoid command from the computer, we want to check the powers and grounds of the computer. Any kind of power or ground-related issue can affect the computer’s ability to operate a solenoid since it uses these to provide power for the solenoid circuit and provide ground to control it. And an intermittent problem with powers or grounds can cause an intermittent problem in command.

Danny connected his scan tool to the vehicle to record the data for solenoid command and sensor input. He also connected his oscilloscope to monitor the duty cycle for solenoid command from the computer (figure 1), as well as the powers and grounds to the computer. It took some extensive driving to get the problem to reoccur. Once it did, Danny hit the brake pedal twice to give himself a marker to look for in the scan tool movie to go to and monitor solenoid command during the time the problem occurred. After coming back and reviewing the data on the scan tool and the oscilloscope, he found the computer command did not show any signs of an issue on the scan tool data. On the other hand, the oscilloscope showed the command out of the computer had a short glitch (figure 2), while the powers and grounds remained steady during the event.

We felt confident this was an internal computer malfunction, so we replaced and programmed the computer. We got the customer’s permission to drive the vehicle home so we could put some more driving time on it to confirm the fix. Danny could not get the problem to reoccur, so they returned the vehicle to the customer to drive for a few weeks and let us know if the problem came back. A couple of days ago, the customer dropped by to let us know the transmission has been working like a new one, and no lights on the dash are coming on. Always good news to our ears.

Computer-related issues can usually be intimidating. Unlike a transmission, a computer can rarely be open to identifying the problem. Occasionally, you might find a burnt circuit board, loose solder joints, or damaged components. When it comes to computers, the limits of diagnostics for most shops is checking the information the scan tool provides compared to the voltages we see on the wires, checking the voltage coming in, the five-volt reference, and the grounds. All of which need to be checked during the event to prevent replacing a computer over power or ground-related issue, or the transmission landing back on the bench for a computer malfunction.

There you have it. Another computer problem that seemed more like a transmission problem.