Little backstory: I run an ATS CoPilot, it independently controls transmission line pressure. It's recently had some issues maintaining control over the line pressure, so I eliminated the CoPilot and began some testing.

The TCM was reporting a loss of communication with the line pressure sensor, low open circuit.

I hooked up an oscilloscope and monitored my 5v reference and the signal return at the TCM connector. The 5v reference is rock solid, never deviating beyond 5 volts. The signal return produces a good signal that seems to track appropriately with the change in line pressure (I have a mechanical gauge as well). Ground through the TCM on pin 53 has some resistance on it, anywhere from 10 to 50 ohms, and it fluctuates. I verified the grounds feeding the TCM and they are all good, those are pins 37,39,51, and 57 at the TCM connector. Pin 53 the ground for the line pressure sensor is the one with the high resistance. It's an internal ground through the TCM.

At this point I'm pretty sure I have something failing inside the TCM to be causing ground resistance.

I took all the grounds off and cleaned them to shiny metal and reinstalled. And that's where this post comes from. If the engine is off and I measure resistance through the chassis ground near the TCM to the battery it's essentially 0 ohms. The negative terminal to chassis ground (driver fender) is 0 ohms. Engine off every ground is excellent, 0 ohms right through to the negative terminal of the battery.

When I start the truck and let it idle, suddenly my meter starts flipping between 0hm and open circuit every couple seconds before settling on open circuit. This is between the chassis ground near the TCM to the battery negative terminal. While this is happening and the truck is running if I measure resistance between the negative terminal and the chassis ground at the battery (that small ground wire that hits the fender) it's 8 ohms. If I shut the truck off it returns to 0 ohms. What? I made a new fat ground wire and stretched it from the battery negative to the chassis ground near the TCM. Engine off? 0 ohms. Engine on? It falls out of the circuit and shows open circuit. What's going on?

 

Externally grounded the sensor, it's working again. As far as the ground issue for the chassis, I probably have a test equipment issue.

 

Yeah, when a circuit is in operation, there is voltage there, and they may be upsetting your meter.

 

Educated myself and learned something new... It's not a meter issue it's a physics issue, lol. You're right, the DVOM passes a known value current through conductor and measures the voltage drop between the leads, calculating the resistance of the material in the process. If there's a voltage already on the circuit, you can't measure your meters. I guess that makes sense, knew the continuity / diode test passed some current but I guess it never occurred to me how the resistance test worked as well.

Oh well. At least I was right about losing the sensor ground 😅

 

The other lesson I learned from an appliance tech over a phone assist call, was to use a light bulb to check resistance rather than my fluke dmm. I was showing very little resistance, because it would handle the very little load the meter puts on the circuit. Replace with a bulb test, and it won't pass enough current to light the bulb.

 

Yeah I do know that trick, I've got a few test lights made with various 12v bulbs but I'll never test a ground on a module with a high amp test light, that's a damn sure way of ensuring it's got a bad ground when you burn out the trace lol