Back on the Bench - October/November - 2020

I’m Stuck On You!

I’ve shared a few stories of very odd problems that took some unique testing to figure out. I have another one to share, but don’t think of this problem, in particular, think of the method of testing. After all, it’s on an old Elantra (2011), which you probably don’t see very often. But, you’re likely to see something similar on any other car. The call went like this.

This car had a condition where it worked fine, and then all of a sudden, it’d start bucking and jerking from a stop. And then, finally, it stopped shifting, and the check engine light came on. Seems pretty straight forward, right?

It had a third-gear ratio code, and the initial thought was that the overdrive clutch was failing. But everything looked fine. The fluid was full, clean, and had no burnt odor. Time to dig a little deeper and find out more about what the customer did to get the problem to occur.

After discussing conditions with her such as weather, city or highway driving, light or heavy throttle, distance, she offered another clue. She had driven the vehicle over an hour, and then got stuck in stop and go traffic.

After trying to duplicate this, it never missed a shift, and the code had not returned. The technician kept driving it in a lot of stop and go traffic, and some more driving in the country before it began to do what the customer was describing. And then it finally went into failsafe. The following morning, it went back to working fine. This is when I got the call.

We went over the scan tool data the technician had saved and found when it felt like it dropped to first, the gear ratio was showing 2.919, which was indeed a first gear ratio. But why did it have engine braking; the sprag should have freewheeled.

I looked over the hydraulics and solenoid application charts to find a possible cause of it bringing on the low/reverse clutches. Pressure control solenoid A (figure 1) is used for the low/reverse clutches in manual low, and the overdrive clutches in third and fourth. An on-off type solenoid is used to control the overdrive low/reverse switch valve (figure 2), to redirect fluid between the low/reverse clutches and the overdrive clutch. And if this valve did not move, when the computer commanded the 2-3 shift, the low reverse clutches would apply and drop the transmission to first gear.

This narrowed it down to either a stuck valve, a solenoid mechanical failure, or a computer command issue. As we discussed the possible causes, we agreed it was more than likely a solenoid mechanical malfunction, but testing was needed to confirm it. I recommended they attach a meter to the solenoid command wire (figure 3), drive it home again, and when the problem occurred, check the computer’s command to the solenoid.

If the command to the solenoid was there, I recommended they heat up the valve body and solenoid in a vat of transmission fluid since the problem only occurred after it was hot. This is where we get into the tricky part. I shared a similar story previously, using the same technique. It turned out that the valves were sticking, but only when hot. They were able to repair the valve body but would never have discovered the problem if they weren’t able to duplicate the hot condition.

After that, the transmission worked great. The vehicle was returned to the customer, and after driving it for a couple weeks, she called in to let them know how happy she was with the repair. Forget the car and the specific problem for a moment. The lesson is to be able to duplicate a hot condition.

Even though taking the time to properly diagnose a problem can take quite a bit of time, it can often be rewarding. Not only by keeping those transmissions from landing back on the bench, but we can also prevent them from landing there in the first place. And happy customers are what makes a business truly grow through positive reviews and referrals.