Sensors Weep 2

I pulled out my box of electronic connectors and my scope.  The trans harness wasn’t hard to get to, so I patched right into the circuit and watched the signals.  As the transmission went from normal shifting to “limp mode” the signal was clear and healthy.  No cause for concern.  Here is where I wished The Man wasn’t so stingy and we had some kind of interactive scan tool.   We ended up bidding on a chip for the Snap-Off wonder brick on e-bay.  I must press on until then.  I took a recordings of current, voltage and resistance of the speed sensor circuits for half of the day, finding everything fine.  As I analyzed the data, I notice something Spock would call “peculiar, but logical”.   A very tiny voltage DC was present across the speed sensors.  About .025 volt.  Just for gits and shigles I unhooked the speed sensor and measured just the voltage across the TCM directly.  It went up to .750 volt.  One eyebrow raised.  Could this be what the TCM is using to make an assumption of the condition of the circuit?  I turned the car into a Doc Brown special with meters and probes sticking out of it, and went for a drive.   When all was well, the trans shifted fine, and the little voltage that could stayed in place. At exactly beyond walking distance back to the shop, the fluke meter on the windshield  read 4 zeros strait across, and the trans jumped out of gear, then into limp mode.  Ahah!  Gotcha sucker.  This was filtered DC voltage.  The AC signal of the speed sensor was still clear.  My first thought was the TCM.  It was getting late.  I parked it inside and went home to my pipe, beer and wife.

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