21st century pressure testing

Jan. 1, 2020
The beauty of pressure testing with transducers on a scope allow us to compare pressures, compare pressures to other signals such as EPC commands.

Figure 1

A picture is worth a thousand words, or so they say. That’s a lot considering the time frame of what a picture consists of. Even more interesting, and possibly worth more words (depending on the footage) is a slow motion video, be it a hummingbird in flight or a bullet going through a watermelon. While the single time frame shot of the event can be beautiful or amazing the slow motion time frame by time frame shots show much more detail effects of the before mentioned bird in flight or the sonic effects of a projectile.

When pressure testing transmissions we have for decades been using the traditional needle gauges and watching with the naked eye the effects and reactions to either mainline or specific clutch tap pressures. What if we could take those pressures and plot them out on a graph instead. Then we can scroll through time and see what a certain pressure was and when. That would be way better than starring at a needle and trying to remember a certain pressure and what condition was going on at the time. Most pressure specs give a minimum and a maximum pressure which in the old days was fine and dandy. All you had to do was either remove vacuum to a vacuum modulator or move a TV cable to check for pressure rise or just do a stall test and you had your min and max pressures.

Figure 2

On a modern transmission just checking the pressure itself isn’t enough. All the means of increasing and decreasing pressure, be it mainline or a specific clutch tap are commanded by a PCM/TCM, those commands are reacted upon via electromechanical means, that cause a reaction on a hydraulic valve, that will then cause the reaction to the pressure tap your gauge is connected to. So watching a gauge is no longer enough. Since we now have the ability to graph out our pressure we may as well plot out the electrical commands that are effecting the change on our pressure as well. This will then paint us a complete picture of commands, electrical integrity, and then hydraulic integrity of the transmission being tested. Well worth more than a thousand words!!

Figure 3

This is all possible today with any modern lab scope set up whether it be a PC-based or hand held unit. The beauty of pressure testing with transducers on a scope allow us to compare pressures, compare pressures to other signals such as EPC commands, or watch how pressure is building on a certain clutch that may be having a complaint. By graphing out pressures and signals we can build a database of known good and bad issues to compare to other vehicles or as visual proof of a certain problem to show the customer later, and best of all it can be opened up, viewed, and manipulated similar to a high speed camera’s footage of something that typically happens very fast and replayed again slower to show great detail.

Take for example the case of a high mileage 2001 Suburban our shop took on. The customer complained of having just done some transmission service himself a month or so before bringing us the truck and now it just quit moving. It worked great both before and after and gave very little notice of a slip or any problem before it just quit. The fluid was indeed a newer shade of red but smelled awful! Clearly a quick burn up and clearly it had to come out. While anything that makes it over 200,000 miles can be considered a good life this guy was a bit miffed that he didn’t feel much for symptoms before it just quit moving on him. It was decided a pressure test may be worthwhile before proceeding with a rebuild to make sure the new transmission doesn’t suffer the same quick fate.

Figure 4

In this case a pressure transducer was hooked up to the mainline pressure tap to the old needle gauge sets fittings and adaptors and an inductive amp probe was placed around one of the EPC control wires. Why this extra step? Of course we could just do the pressure test and see what happens in relation to pressure but by watching the pressure control solenoid amperage we can see the computers command being carried out. Just watching the command on the scan tool may not be enough. Just because a PCM commands a change does not necessarily mean that command is carried out as we will see later.

Figure 5

Back to the case of this Suburban. With the gauges hooked up, the pressures looked good on the minimum side of things. When it came time to perform a stall test on the other hand, there was no rise at all as seen in Fig 1. You can see the amperage change reflected in the red trace but there is no rise at all in the pressure (blue) trace. This tells us the PCM is seeing all the right inputs and commanding a pressure increase reflected in the decreasing amperage of the EPC, but no pressure rose at all on the mainline circuit. Using a Tech 2 (or other capable scan tool), we can control the pressure in 0.10 amp increments and this showed no change in pressure as well as seen in Fig 2. In the case of our Suburban, this is good news and the problem is definitely in the transmission and not an external problem that could hurt the rebuild. In this case all the valves were free in the pump and valve body and the sole reason the pressure was not increasing was a failed EPC solenoid.

 A good pressure test should look like the picture in Fig 3 and Fig 4. These captures show a good pressure response on a stall test in reverse and drive, and good responses when controlled with a scanner using bi-directional controls in .10 amp increments.

Figure 6

Our next example comes from a 2001 Saturn SL1 with a TAAT transmission. This particular car was throwing gear ratio codes P0732 and P0733 for 2nd and 3rd gear slippage respectfully. The shifts felt pretty messed up depending on how it was driven. They felt soft and slipping at times and plain jerky at other times.

The pressure tap on these transmissions is beneath the battery and the tap is the TFT sensor next to the spin on filter. Easy enough to get too and the EPC circuit is even easier to tap into at the dedicated fuse in the under hood fuse box. The pressure test can be carried out in a couple ways. We can do the min/max pressures in idle and at a stall respectively, we could drive the car to see the pressures at the shift points, or we can let the scanner do the pressure test in its automated format. Saturn’s, with the aid of the Tech 2 in this case, will run a pressure test all on its own.

Figure 7

Once selected, the PCM will increase the RPMs and control the EPC solenoid on its own in increments. The commands are shown on the scan tool in the form of the following pressures: 57, 72, 87, 101, 116, 131, 146, 160, 175, 190, 205, 220, and finally 221psi. This all takes place at 1400 rpm and all automatically. Other then setting up the lab scope this is the ultimate lazy mans test! Knowing that, how does this pressure test look in Fig 5? It’s hitting a decent min and max but definitely does not seem to make all the changes reflected in what the scanner was showing its commanded pressure to be, does it? In Fig 6 we will add the EPC amperage. Looks like a big blob of red doesn’t it?

The amperage trace is hitting pretty much the same amp peak during the pressure test. When we zoom in on the red trace at a point in time when the pressure is at the minimum we see roughly a 50% duty cycle control strategy in Fig 7. Compare that wave form to the one of the amperage at maximum pressure in Fig 8 and we can see a lot less “on time” in the duty cycle strategy. So what do we know so far? The pressures are not responding to the commands the PCM is sending out. The commands are making it to the transmission as verified by the changing duty cycle.

Figure 8

Taking a closer look at the amperage trace in either of the zoomed in figures though reveals a certain characteristic in the wave form. The solenoid amperage shoots straight up till about the 4.8 amp point then curves up. Typically when a solenoid (or any sort of inductor) is turned on the current trace starts at 0 and “ramps” up to the point of max current flow. This EPC solenoid is shorted. A good amperage trace is shown in Fig 9 and a good TAAT pressure test after the EPC solenoid was replaced looks like the one in Fig 10. You can see in that pressure test even steps in pressure that now match the scanner commands and the amperage has a slight change to it too in correspondence with the duty cycle change in the EPC solenoid. On the test drive the car shifted beautifully every time as well.

Figure 9

Our last example of the power of pressure testing with a scope comes in the form of a 2004 VW Beetle with a 09G transmission. The complaint on this one is a harsh jerky shift when hot. On the initial road test the car shifted great until the TFT came up 80*C and higher. Once hot, the shifts into 3rd and 5th had a bit of a shuddering apply and definitely not as smooth as the shifts were cold.

So what now? Tear down time? Not quite yet. The shifts felt perfect cold so we know the transmission is capable of shifting normally. Granted things like the frictional properties of the fluid and clutches can change with temp but this transmission type does have some known issues with valve body and solenoid wear, so how can we verify if that is a problem or something else? Don’t guess, pressure test!

Figure 10

We will be hooking up the pressure gauge to the K3 clutch pack and hooking up an amp probe around a control wire to solenoid N90. Fig 11 shows why as that is the clutch and solenoid in play for the shift into 3rd and 5th gears.

 As the amperage decreases we should see the pressure increase like the picture in Fig 12. As the temperature increased and the shift quality began to suffer both the 2-3 and 4-5 shift pressures began to show a certain characteristic. Both Fig 13 and Fig psi 14 show some sever pressure oscillations in the k3 clutch circuit. These oscillations match the feel of the shudder as well. Looking at the waveforms we can see a steady and consistent change in solenoid amperage. So we know the TCM and all the electrical circuits are testing well. The pressure follows this trend cold but as it warms up the pressure doesn’t follow the command as smoothly. These transmissions have lots of problems with the linear solenoids sticking when hot and these waveforms confirm this fact.

Figure 11

Those solenoids are directly responsible for the pressure control within the clutch circuits they are assigned. The only ways to service these is either a new valve body complete with new solenoids from VW or rebuild the linear solenoids yourself with special tools and replacement bushings that support the armature of the solenoid. Those bushings are very small and the original ones have a Teflon type coating that starts to deteriorate and once hot causes binding and sticking within the solenoid. In extreme cases they can cause gear ratio codes as well but this VW was not at the extreme end of bad, yet.

Post repair after rebuilding the linear solenoids and going through the valve body checking other known wear spots shows us a much cleaner pressure rise hot in Fig 15 and Fig 16.

Figure 12

Hopefully these few examples show the power of using a lab scope in pressure testing of automatic transmissions over the traditional gauge set up. Just watching pressure alone is not enough and do you trust your eyes or gauge enough to catch the pressure flutter in the last example? As anything technology marches on and is constantly changing. Scan tools are wonderful and necessary but just seeing a command on a scanner does not mean the commands are being carried out. Or then again maybe the commands are and the element being controlled has failed in some weird way that just won’t throw a code.

Good luck on your pressure testing! Building a data base of good and bad can take time and while it’s good to have a known good before you start to diagnose a car sometimes that’s just not available. Some cars the problem may be intermittent so you will see a good and a bad and others you may not get a good waveform till the car is fixed! Either way it’s beneficial to get a good and bad for comparisons and learning sake.

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Figure 13
Figure 14
Figure 15
Figure 16

About the Author

Shaun O'Neill | Contributor

Shaun O'Neill is a third generation transmission rebuilder for his family business, O'Neill's Transmission Service in Grand Rapids, Mich. He is a CMAT with L1 and L2 and has helped co-write a transmission diagnostic training class for Standard Motors Corp.

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