Gears Magazine - Can You Handle the Pressure? (Part 2) How to Construct an Affordable Electronic Pressure Gauge (How it Works)!

Hopefully, in last month’s article, I piqued your interest in using electronic pressure transducers and scope to measure transmission line pressure. I introduced a variety of sensors ranging from the cheap $25 eBay sensor to a higher quality sensor costing around $130. This article will show you how well those sensors perform on the inexpensive Pico 2204a, the relatively inexpensive Autel MaxiScope, and the pricey Pico 4425. You’ll be surprised at how well all of these sensors perform, even with the $150 Pico 2204a scope.

Using the scope

We need to set the scope up for the custom probe before using it. Once it’s set up, then we can select it from a channel’s drop-down menu whenever we want it. As shown in figure one, to create a custom probe with the Pico 6 software, you’ll select “tools,” “custom probes,” and “new probe.” Then you give the probe a name and unit, like PSI. Now you can use a linear equation or a look-up table. With the Pico, we’ll use a look-up table. Unfortunately, the Autel does not have this option available. Next, you’ll refer to the specs for your sensor. While referring to table one, you can see for my current location in Carbondale, Illinois, if I plug in those voltage-to-pressure relationships, the Pico software will now be able to label the scope screen with the proper pressures. Instead of seeing voltages on the side of the scope screen, the software displays PSI.

This first example shows an SSI Technologies “absolute” sensor. “Absolute” means a full vacuum of about 30inHg (-15psi) will drop my voltage to about 500mV. If you have an AC pump, you can place a vacuum on it, measure the output voltage, and prove this. The sensor outputs about 625mV volts in the free air at my elevation (about 400ft above sea level). Then, since this is a 500-psi sensor, and considering that it’s designed to measure atmospheric pressure starting at 0 psi, we can figure that 485 psi would output the max voltage of 4500mV.

To be clear, this is a 500-psi absolute sensor, and it will measure at sea level between negative 15 psi (30 inHg or full vacuum) and 485 psi, which is a total pressure sweep of 500psi.

Once you’ve created your new probe, you can select it for any channel through the channel drop-down menu (figure two) and choose “probe,” and find it in the “library.” Now all your measurements and readings will be in PSI instead of voltage.
The Autel custom probe creation is similar, but it doesn’t include a lookup table. I’m hoping an update for their PC-based scope software comes soon. It seems like Pico is constantly updating their software, but Autel hasn’t had an update for their scope in over three years!

The custom probe creation application is one area that needs work. To create a custom probe, select “tool” from the top menu, then “custom probes,” then “create,” as shown in figure three. The only option for teaching the Autel how to interpret your transducer is through a “linear equation,” which, if you remember back to middle or maybe high school algebra, is y=mx+c. This equation teaches the scope the voltage slope relative to pressure. It also introduces the scope what voltage equals zero PSI because these sensors output a voltage, usually somewhere between .4 and .6 volts, when measuring the atmospheric pressure at sea level. I know this doesn’t seem very easy, but you’ll be ready to go if you simply enter the data from table one

The Autel also automatically creates the ranges for you. Unfortunately, the ranges don’t match up to the pressure output. The way I have the Autel setup in figure seven, you can see that the decimal is off two places. For example, the lower pressures for channel A are actually 104.7psi, not 1.047psi. Once again, hopefully, this issue will be corrected in a future software update. However, shifting the decimal to the right two places is the workaround for now. It’s a shame that there are a few software quirks with the Autel scope because hardware-wise, the scope is excellent.

Setting up the scope

Now you’re ready to do some pressure testing. I like to put plenty of time on the screen when performing pressure tests. Both scope manufacturers allow you to zoom into the pattern to get a better look when necessary. When monitoring transmission pressures, you can take the vehicle on a test drive and let the pressures record away, then evaluate them closer when you get to the shop. So, for example (figures 4 through 8), on this 2019 Silverado with a 10L transmission, I have the time base set to 2 seconds per division, and since I have ten divisions across the screen, one full scope screen displays 20 seconds of data.

With the Pico and Autel, their software continues to “buffer” screens, so you can collect multiple screens to review after pausing the scope. You can even save these scope traces for review at a different time or send and share them with others. Here’s a comparison of the three scopes while checking line pressure on this 10L transmission. It’s worth noting that the scope software is free to download at picotech. com for the general Pico 6 program, picoauto.com for the automotive versions of Pico 6 and Pico 7, and Autel.com for the MaxiScope software. You do not need a scope to download and use these programs. There are demo modes available that allow you to explore their features.

On a side note, look at how fast the pressure peaked on this 10L transmission in figure twelve. We observed snapping the throttle during a stall test; the line pressure jumped from about 100psi to a boosted pressure of about 220psi in only 90ms. Measurements with a transducer can pick up and record small changes in pressure from solenoids cycling and valves shuttling. In many instances, using an electronic pressure transducer can help you determine if a shift valve is shuttling, even if there’s no pressure port for that particular clutch. Observing these details can help during diagnosis to possibly rule out electrical and hydraulic valve issues.

Conclusion

While the less expensive 2204a doesn’t display the pressure trace quite as nicely as the Autel or the more expensive 4425, it does get the job done, and the Pico software makes up for some of its inadequacies.

The Autel displays a very nice signal, and it has four channels. Hopefully, the custom range options will improve with a much-needed software update. Using a higher-end transducer isn’t necessary to measure relatively slow switching pressures, like transmission line pressure. Still, if you plan to test fast switching pressures, like in-cylinder pressures, you’ll want to upgrade to the better Honeywell or SSI sensors or one with comparable specs.