Re:Re:Scope help please- 2009 Subaru Outback Sensor Bus Voltage Spike

Hello,
Thanks in advance for any advice. I've been chasing an intermittent map sensor value spike. (-3psi to 28-32psi) for a little while now. The vehicle still runs but just tosses an error code on nearly every trip. This map sensor is fed 5v directly from the ecu and when scoped it is very noisy. I think I've associated the spikes with ignition events in the images below but could use some help in the solution.

Initial diagnosis and review of the wiring diagram showed the ecu is fed voltage directly through a single fuse shared with the fuel pump relay. When I replaced the alternator the frequency of noise was lessened but the peaks are now higher when it does spike.

I've gone through and verified grounds. And relocated the pcm grounds directly to the engine block and have less than .1 ohm variance to other wires.

The noise is measured all the way back to the b+ terminal on the alternator when grounded to the battery.

Is it possible that my ignition coil insulation is failing or that there is a diode or filter missing?

Help!









Image of alternator voltage and current with lots of spiking.

Hello? Curious if anyone has seen 20v feedback spikes on a sensor bus like the first image shown? Is it worth relocating stacked grounds?


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Posting some additional images of my map sensor voltage while cruising in vacuum on the freeway. Spikes from -3psi to 26psi. Graph shows equivalent -3volts to 2.5volts on the 5 volt sensor this is the first time it showed repetitive nature like this. Seems to only react in closed loop and at 1500-1800 rpm with throttle position between 15-18% if anyone may have a reason why there would be correlation



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All of your pico images look normal to me, you are using battery ground for your tests, so you are going to see all kinds of noise.
You're updated post with the new picture (I'm assuming this is scan data?). What are the we looking at and what are the scales?
I'd be looking for an intermittent OPEN in the signal wire of the MAP or an open in the MAP sensor ground. Most likely this sensor uses a bias voltage so opens will drive the signal voltage high

Sorry for the lack of explanation of the second pic. The data log shows the rpm (steady), the throttle position (steady) and the erratic map signal value into the ecu (-10.9psi peaking to 26.11 psi). I calculated that to about 2.5ish volts. I numbered the signal wire to the bulkhead and it still showed the spiking which is why I measured the feed voltage to the sensor on the initial Picoscope image. Here is shows that I'm getting the high spike values. Just to be clear you're thinking I've got an intermittent break on the signal or ground to the sensor?


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The high spikes, yes, intermittent open, but the low spikes contradict that.