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Old 01-27-2021, 05:56 PM   #26
Alan29
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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
bobxyz
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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):
https://www.mpja.com/200KHz-Handheld...info/35797+TE/
https://www.mpja.com/60MHz-Oscillosc...info/35886+TE/
https://www.mpja.com/9V-Battery-Snap...info/35965+AD/
Read the instructions to understand the non-obvious features.
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Old 03-03-2021, 04:23 PM   #28
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Bit of an update since I sidelined this for a while with other things going on. Decided to bite the bullet and order another 3bar MAP (AC Delco) per DIY's recommendation to try along with sorting the grounds. New MAP in and same story with 7v VREF at idle increasing with RPM. Didn't get a chance to drive it but I am certain I'm going to get the same scenario. So seems my original MAP is good and I shot the parts cannon at it for nothing...but hey at least I have a spare map sensor now.

Going to get a friend over here with a second multimeter to go through all the grounds, diag, etc to get a second pair of eyes on it. Have to start on my kitchen this weekend which will last who knows how long...which will sideline the car for a bit. Still motivated to get this sorted just a bit frustrating with the timeline to get it done. Appreciate the information everyone has provided so far.
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Old 03-03-2021, 04:53 PM   #29
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Alan!
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Speed parts for sale: http://forums.turbobricks.com/showthread.php?t=359797

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Old 03-05-2021, 03:03 PM   #30
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