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Old 01-27-2021, 05:56 PM   #26
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Join Date: Jun 2019
Location: Tampa, FL

Was able to get away from work for a bit to poke around. Disconnected the shield ground and started the car. Not sure if thinking clearly as its been a rough week, but I can't figure out how to measure resistance on that ground lead. Did a quick google and couldn't find anything that relates. I might be over or under thinking at this point. Honestly my electrical diag knowledge is amateur at best.

While car was running with the shield ground disconnected I decided to measure to see if there was an difference from what I've measured previously to see if maybe this is what is causing my issues. I'm still showing about 3-4v at my sensor/logic grounds. Interestingly the same voltage is showing when I check that now disconnected shield ground.

VREF still showing 6-7 at idle and increases with rpm.

I've rechecked my ground for the coils (not the logic) and no voltage there. My ECU ground at battery showing no voltage there.

Is it possible an internal failure in the MAP could cause this feeding voltage into the ground? Still stuck on this all happening after after blowing my charge pipe. Wouldn't think that sudden change from 14psi down to vacuum would cause a MAP to fail...read up on aftermarket 3bar MAP sensors being prone to failure however perhaps the timing is coincidental.
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Old 01-28-2021, 10:18 AM   #27
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Join Date: Aug 2014
Location: Boulder CO

I checked my short MS harness at home and it does NOT have the CAS (VR1,VR2) shields connected to sensor ground within the connector/shrinkwrap. I must have been mistaken, or was thinking of a different harness. The resistance measurement is simple. If the shield is disconnected at both ends, there should be no continuity, aka infinite resistance, between the shield and sensor ground - just put a probe on shield and one on sensor ground to measure.

Change of topics.
Whenever a coil is disconnected, the collapsing magnetic field generates a brief high voltage spike across the coil. For MegaSquirt installs, the coils are the injectors, the ignition coils, the IAC (not present), and small relays. All of these _should_ be connected to +12v power away from the MS (mostly at the main relay), and _should_ be connected to power grounds. None of the parts normally connected to +5Vref and sensor ground can generate a voltage.

That being said, it would be good to verify that something isn't misconnected. Can you look up the pinout on the ignition coils and make sure that power and sensor grounds aren't swapped? Likewise for the WBO2 (it's not a coil, but it does switch heater power rapidly).

If you want to try a cheap single channel oscilloscope, you could get one of these (or ebay equivalent):
Read the instructions to understand the non-obvious features.
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