Through years of experience in the transmission industry, our lessons in the field often help us get to the problem quickly, fix it, and get it out the door. But, unfortunately, sometimes, that experience can cause us to jump the gun, make the seat of the pants diagnosis, and cost us time when we should have followed standard diagnostic procedures. As an ATRA Hotline Technician, I’ve seen thousands of examples of missed diagnoses or jumping to the conclusion that wasted time, energy, and money. Unfortunately, we don’t always realize it when we’re deep into a problem, but we sure see it clearly when it’s all over. Here’s my story.
I recently rebuilt a 4R70W in a 2004 Ford Econoline. The shop installed it, and the transmission worked great for about fifteen minutes. Everything in the forward ranges still worked fine, but Reverse started having a delay. And the warmer it got, the worse it got. If you put it in Reverse, it would sit there without engagement until you stepped on the gas. Then it would have a very short engagement. So I put it back into Drive and then Reverse; the same thing.
My first question was, “What is the pressure?” We installed a pressure gauge on the main-line pressure tap (Figure 1). We noticed that in Drive, the pressure responded according to specifications. But in Reverse, it would drop to 30 psi. With a bit of gas, it would jump up and respond normally. We reviewed possible causes like the reverse boost valve, low servo, low servo cover, and the reverse drum. We also considered a command issue; however, we ruled that out since the pressure responded correctly in all forward ranges. Big mistake!
The Range sensor displayed correctly, but we had one on hand, so we tried it to cover our bases. When the transmission came to the bench, the Reverse drum air checked okay through the pump, and the reverse boost valve was free and clean, so I assumed we might have a defective servo or cover.
We removed the valve body to access the servo and cover and found nothing wrong with the servo, cover, or case bore. We replaced the servo anyway for good measure. After installation and another test drive, it was doing the same thing. It worked great cold but wouldn’t engage when it warmed up.
With the slight pressure drop when it was warm, we were sure it was in the transmission and thought it might be a hairline crack in the case. We removed the valve body and servo and cleaned the bore and surrounding areas with Brakleen® but could not see anything wrong. At this point, I was slightly befuddled and telling myself, “It’s just a 4R70W. How hard can it be?” Time to back up the bus, do the standard diagnostics, and do what I should have done in the first place.
This time we monitored the computer command with a volt meter, even though we were all convinced that the computer command was not the problem. Next, we hooked up our DVOM to monitor the voltage command from the PCM on the Pressure Control circuit (Figure 2). We recorded the voltage versus pressure when it was cold and operating normally, and as it started, the delayed engagement in Reverse. What we found was totally unexpected. When it was cold and Reverse worked, the voltage was 3.2 at an idle, and the pressure was 130 psi. Once it got warm, the voltage rose to 4.3 volts. Step on the gas, and the voltage would go as low as 1.8 volts, then jump back up to 3.6 volts (Figure 3), and then have a harsh engagement into Reverse.
Though the shop I worked for didn’t get into electrical work, they did notice that the reverse lights were filled with water and not working. We replaced the bulbs, but the lights still did not work. Could this have caused a computer failure?
We took the vehicle to a local shop that specializes in electrical repairs. They did their routine inspections, checking the wire harness for shorts or damage. In the end, they replaced the computer, which fixed the problem. So, the transmission failed because water got into the Reverse lights circuit and shorted the computer. We missed it by skipping a simple test. And it cost us dearly.
The lesson here is never to assume and go through the proper tests. So, the next time I get something like this on the Hotline, I can empathize better with what they’re going through!
