Back on the Bench - September - 2020

A Sticky Situation

Just when you thought you have seen it all, life and transmissions will throw another curve ball your way. Things like bushing wear, spring breakage, bore wear, are commonplace, and the list goes on. Sticking valves are probably one of the most common things on that list.

We see it all the time, and we have our usual repairs to get the valve body back to working order. Or, at least we think so.

One that got me a few years back, and I’ll always remember it. It was a Honda BYBA, and after the rebuild, it didn’t have reverse; all of the forward gears fine. The unit uses the fifth clutch for reverse, so I checked it for pressure and, sure enough, no pressure in reverse. It had to be a valve body problem.

Upon opening the valve body, I found nothing wrong. The reverse CPC (clutch pressure control) valve would literally drop in the bore and bounce on the bottom.

After beating my head against the wall, I decided to leave the valve out and mount the valve body onto the bell housing. I inserted the valve only to find that it was stuck in the bore. When I loosened the bolts in the area where that valve was located, it would fall in and bottom out. The valve body was warped. I replaced it, and it worked. It’s a short story and from a long time ago but I always knew that I’d see something like that again, someday. I was right.

Recently, I rebuilt an A750 in a 2014 Toyota 4 runner. Like most that land on my bench, I didn’t have any background information such as what it was doing, any codes, or any previous repairs. I did my usual procedures: air checking during the disassembly, checking for wear and damage, and then vacuum testing the valve body. These were just the usual things we all do during an inspection. It was a simple rebuild. Get it back in the vehicle and another job well done. Or so I thought.

The shop installed it, test drove it and then delivered it to the customer. The customer showed up two days later and stated that after driving it for about forty minutes, it would stop shifting into third. After it cooled off, it would start working again. Frank, the owner of the shop, drove the vehicle, which, by this time, was working as it should. Then he gave me a call.

After he told me about the complaint, I asked him for any codes that may have set. It had set a P0973 code (shift solenoid A performance). This code is set when the computer commands third gear by shutting off shift solenoid A (S1) but doesn’t see the expected gear ratio through the speed sensors. This is a performance code and is typically not an electrical issue.

Our next step was to hook up our test equipment and go for a nice long drive in the country. We installed a DVOM to monitor the voltage on the shift solenoid A command wire (figure 1), a scan tool to monitor commands, and a pressure gauge on the C3 pressure tap (figure 2).

Frank left the vehicle running on the lift for about thirty minutes in hopes of reducing the driving time. Then, after about twenty minutes on the road, it happened; no third gear.

We noticed the command was there on the scan tool as well as our meter, this narrowed it down to something inside the transmission. Our gauge remained at zero during the command, so we knew the problem is either with the valve body or shift solenoid.

We swapped out shift solenoid A (figure 3) while it was still hot, and went on another extended test drive. Sure enough, the problem reoccurred. At this point, I was convinced it was in the valve body. We removed it for an inspection.

At this point, the valve body was cooled down, and the 2-3 shift valve (figure 4) would stroke freely in the bore. So, like the Honda I previously mentioned, I left the valve out and reassembled the valve body. The difference here was that I wanted to heat the valve body and then see how it stroked. I’ve had similar heat-related issues in the past, so I test parts in a vat of heated transmission fluid. I use this for all sorts of tests: drums, solenoids, valve bodies, you name it.

I use a hot plate and a large pan. I can vary the temperature, and for this test, I brought the fluid up to 180°. I let the valve body sit in there for about an hour while I worked on other jobs. After removing the valve body from the heated oil, I immediately tried inserting the valve. It was free when it was cold, but this time it was sticking in the bore. This valve body was no good.

Fortunately, I had another valve body. I went through it, as usual, but checked this one while hot, too, just in case. Frank drove it home and kept it overnight. He drove it the next day too, and it kept on working.

He returned it to the customer, and when they returned for the ten-day check, he stated it had not missed a shift since.

It is always helpful to get the customer complaint and any codes set before the transmission hits the bench. Things don’t always go the way we want them too, and lack of information usually leads to more work. Taking the extra steps to heat the valve body would not be worth the time on any other build, it is worth the time when you get one that only acts up after extended driving. Those few extra steps we take, when we need to, can sometimes bring to light problems we don’t normally see, and keep these transmissions from landing back on the bench.