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Real case: the A/C idle surge was a false speed-sensor pulse

By Borojosoft · Updated 2026-11-12

A 1993 GMC Sonoma (2.8L TBI, 5-speed) surged at idle whenever the A/C came on. No codes. Parts were changed and cleaned with no luck until the live data was recorded.

The symptom

With the A/C on, RPM jumped up and down. Without A/C it was better. The Check Engine light gave no codes, so the A-B jumper didn't help.

What the data showed

Recording idle with the scanner revealed something you can't see by eye: with the truck stopped, the computer registered vehicle speed in 37% of samples. In other words, it thought the truck was moving, so it changed its idle strategy and opened the IAC valve, causing the surge.

The full chain

1. A slightly rough idle (more so under A/C load) produces drivetrain pulsation.
2. The pulsation rattles the transmission gears in neutral (normal on a manual).
3. The rattle shakes the rotor in front of the VSS, a variable-reluctance sensor: it generates voltage from motion, not speed. Result: a false pulse.
4. The computer counts that pulse, "believes" it's moving and reacts.

How it was proven (not guessed)

• With the VSS unplugged, samples with false speed dropped from 37% to 0%, and returned to 36% once reconnected.
• Pressing the clutch made the effect disappear.
• Key ON, engine off: 0 of 59 samples (rules out the A/C electrical system).
• Compressor, fan, harness, alternator, grounds and the sensor itself (new) were ruled out by measurement.

The fix: one resistor

A 1.5 kΩ resistor in parallel across the two VSS wires cuts the signal to about half: rattle noise no longer reaches the threshold, but the real pulse from rolling does. Verified result: 0 false pulses in 318 samples, steady idle with A/C, working speedometer and no code 24. Later confirmed on the road.