The most amazing thing happened. My 944 Turbo’s engine cut out on the freeway! As I drifted to the side of the road, my gut was initially filled with panic. But then my brain kicked in and I realized I had an OBD+ Sport DME at my disposal! Using the OBD+ Sport DME, it took just a few minutes to determine what was wrong and fix the issue to get back on the road. As I was triumphantly driving home with my pride running high, I realized something important. I spend countless hours of time developing OBD+ features but very little time explaining how to use them. So, I decided to make a case study out of this and walk you through my thought process.

Me broken down on the side of the freeway 1/26/2024

The first thing I needed to do was calm down. Being stranded on the side of the road is not a pleasant experience. Lots of negative thoughts were running through my mind, but nothing I was thinking or feeling was going to solve this problem. I took a few moments just to breathe and push these distractions out of my mind.

So, first things first, I had a no start scenario. This meant I need to check the flywheel sensors because the DME will not fire the injectors or spark plugs unless it sees BOTH the Mark and RPM flywheel sensor signals. The only exception is the engine will continue to run if you lose the Mark signal after startup. Checking this is extremely easy on the FocusOBD’s “Data Grid” tab. I looked at the “Flywheel Tooth Count” parameter while cranking the engine. The count was 0 teeth, so right off the bat, I knew I was dealing with a bad flywheel sensor signal(s). However, it was odd the error count not increasing on the Mark and RPM sensors. This happens if both the Mark and RPM signals were not present. The RPM error count works by reading the RPM flywheel sensor as it passes over the starter ring teeth. The OBD+ Sport DME counts the teeth until it sees the flywheel Mark signal. That count is expected to be exactly 132 teeth for a 944 Turbo and exactly 130 teeth for a 944 NA. A normal flywheel tooth count on a 944 Turbo is shown below on the FocusOBD Data Grid.

FocusOBD Data Grid Tab (Flywheel Sensor Tooth Count)

If the RPM tooth count is higher or lower than the expected 132/130 when the Mark signal is detected, then the RPM error count is increased by one. If the RPM tooth count exceeds twice the expected tooth count, then the Mark error count is increased by one. This verification occurs on every rotation of the engine.

The next step I took was to graph the flywheel sensors using the Mark and RPM flywheel sensor “Increment” parameters. These parameters are perhaps the most powerful and least understood features available with the OBD+ Sport DME. You cannot view these parameters in the Data Grid, nor can they be recorded in the data log, because the data updates too fast. These parameters are quite simple, they are separate counts that increases by one on every signal pulse (Mark or RPM) generated by the flywheel sensors. The count automatically resets to 0 when it reaches the top of the graph creating a saw-tooth shaped waveform. In this case, the Mark and RPM increments did not increase while the engine was cranking. This indicated both flywheel sensor signals were not present for some reason. Below is a screenshot of what normal RPM and Mark Flywheel sensor increments look like during startup.

FocusOBD Graphing Tab (Flywheel Sensor “Increments”)

This confirmed the DME was not getting the Mark and RPM flywheel sensor signals. At the time, this did not make a lot of sense to me. Usually you lose one or the other signal but not both. When I run across something that doesn’t make sense, I generally use a process of elimination to figure out which way is up. I started by disconnecting both flywheel sensors. As expected, cranking the engine did not result in Mark or RPM sensor increments. Next, I reconnected the Mark sensor, again no Mark or RPM increments occurred while cranking. I then swapped the Mark and RPM sensors on the Mark plug. Again, no Mark or RPM increments occurred while cranking. Finally, I swapped the RPM sensor with the Mark sensor on the RPM plug. And eureka, I got RPM sensor increments while cranking the engine! I then swapped back the RPM sensor on the RPM plug and also got RPM sensor increments!

So, what does this data mean? Well, I determined both flywheel sensors are good, but connecting either sensor to the Mark sensor’s plug kills both sensor signals. At this point, I strongly suspected a short of some kind, and on a 944 Turbo, that kind of thing usually happens behind the rubber boot on a JPT connector. The rubber boot for the Mark sensor plug was badly degraded. I pulled off what remained of the boot from the harness. Below is a picture of what I saw…

Bad Flywheel Mark Sensor Wiring (Boot Removed)

The Mark sensor wire insulation had failed, and the exposed copper wire was shorting to adjacent wire(s). I temporarily separated the wires to prevent contact and the engine fired right up! My next step will be to remove the exposed wire and replace the connectors.

Porsche 944 Freeway Breakdown: Using OBD+ Sport DME to get back on the road!