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No up shifts

These VW owners are trying to avoid an overhaul. Can this shop fix the up shift info?
Wednesday, May 29, 2013 - 13:52
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Diagnosing modern transmissions accurately can be time consuming and messy. However, when charged out properly, diagnostics can be profitable and provide detailed proof regarding specific issues a unit might have. Informed technicians often are able to resolve these issues without an expensive rebuild or replacement of the transmission. It is worth noting, many re-manufacturers have reported a high percentage of units returned as cores that have minor problems and were erroneously replaced. In our example before us we will see how a little diagnostics both confirmed the customer’s complaint and reveals a repair considerably less expensive than a complete rebuild.

This Jetta presented itself as an interesting case study as it is a complaint we weren’t getting much of before this car rolled in. The customers concern was a no up shift into second gear only when cold. Once warmed up, it shifted just fine. Scan data revealed no codes even though the TCM was commanding a shift to occur (eventually the shift did happen, it was just quite delayed. Anywhere from two to five seconds). It would be easy to assume an internal sealing issue at this point as after it warmed up it shifted fine, kind of like we are used to seeing from the Mopar-style family transmissions with hard seals. Also the fluid looked particularly dark but does this mean it’s time for an overhaul as well? How can we confirm either a sealing issue or some other fault?

Figures 1, 2 and 3

Obviously, a pressure test is going to be the best bet as to whether we have pressure building in the 2nd clutch pressure circuit. But, does little or no pressure mean a leak or something else? How should this pressure test be done? Personally, I feel a pressure test in the 21st century should be done with a lab scope and a pressure transducer. This will allow us to build a data base of good and bad pressures and also have physical proof to our customers that yes, we did pressure test your transmission and these are your results.

Another advantage of using a scope in our diagnosis is the ability to graph out over time other signals and commands that can impact the circuit being pressure tested be it mainline or a specific clutch tap. In this case we will watch 2nd clutch (2-4 brake clutch) pressure and shift solenoid commands. Figure 1 shows us the location of the pressure taps.

Determining which commands to watch along with pressure can be determined by looking at a clutch and solenoid application chart like the one seen in Figure 2 from ATSG’s Techtran™ Manual. On this trans, the second clutch is coming on with no other clutches turning off (not a transition shift). There is a shift solenoid turning off called the N92 by VW (usually referred to as Shift Solenoid C – figure 3) which even with the lack of a hydraulic diagrams being available we can assume is switching feed fluid to the second clutch circuit somehow, and a 2-4 brake PWM solenoid as well that is regulating the clutch apply.

Depending on your scope and hook up capabilities you can watch all or just a couple signals at a time. In this case, we are using a Pico 3423 series PC oscilloscope. I chose to start off with the 2-4 clutch pressure tap using the Pico wps500 pressure transducer. I also decided to watch both amperage and voltage to shift solenoid C, AKA N92. Why both? Voltage is a good way to watch the TCM's command but amperage lets us know if the solenoid is working. We will dissect those signals later.

Underneath the windshield wipers is the TCM with just enough room to hook up some probes. This is a far easier spot to get at compared to the main harness connecter at the trans!

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