Some days we help shops deal with the most challenging diagnostics, other days we are a wall to bounce ideas off, no matter what we are here to add value to your shop and save you time. One of our regulars called in with a rough idle on a Honda. They just installed a use engine and were convinced something was wrong electrically.
The vehicle started and ran fine. However, once off idle it would surge and stall. The engine RPM would flare and lope up and down. The shop stated when the TPS was tested (via scan tool) voltage did not fluctuate, it stayed steady. They swapped the old throttle body over to the new used engine and just about every other electrical before they gave us a call.
I had then backprobe the TPS with a DVOM. While operating the throttle I monitored the DVOM and their scan tool using our Autologic Live on-screen face to face app. What I noticed was voltage changed at the sensor but was steady at the scan tool. The big giveaway was the voltage was different. It was 2.35 on the DVOM and 0.25v on the scan tool.
I had them check the ground and 5 volt reference, both were good.
As they scrolled through the scanner and what I found next was no surprise. The MAP signal would fluctuate in sequence with the throttle being opened. This was performed KOEO, there is no change in MAP (normally) when engine is OFF.
TPS and MAP connectors were swapped during the engine install. I had them show me the sensors, I could see the harness was stretched out right from their FARSIGHT tablet camera. Once they plugged the correct connectors into the sensor and the vehicle was fixed.
Blue connector: TPS - Should be the MAP
Grey connector: MAP - Should be the TPS