09/28/2026
How do you know a valve is good without guessing? You ask it twice.
An Audi came in with a transmission problem. Inside the hydraulic unit there's a little valve that controls the clutch. Before we blamed anything, we wanted to know if that valve was doing its job. So we put the PicoScope on it and tested it two ways.
TEST 1: the car's computer runs it (images 1-4)
🔵 Blue = voltage the computer sends
🔴 Red = the ground side
🟢 Green = current through the coil
1️⃣ Computer switches the valve on: 13.7 V, then a smooth current climb to 1.2 A in 3 ms
2️⃣ Then it pulses the valve about 8,400 times a second to hold the current where it wants it
3️⃣ Between pulses you see −1.25 V. That's the coil's stored energy draining through a diode. Normal.
4️⃣ Ground side reads 30 mV with 1.3 A flowing. That's a good ground. The spikes are just noise from the switching, not a bad connection.
6️⃣ The current "wiggles" 100 times a second on purpose. It's called dither. It keeps the valve spool from sticking.
TEST 2: we unplug the computer and power the valve straight off the battery (images 5-6)
Why? Under computer control, the valve only ever gets what the computer asks for. Feeding it ourselves shows what the valve can do on its own.
▪️ 11.6 V at the valve, 2.3 A through the coil, dead flat for the whole tap
7️⃣ Zoom in on the turn-on and there's a little hesitation in the current climb (aka the pintle hump). That's the spool physically sliding, showing up as an electrical signature on the screen. A stuck valve doesn't do that.
▪️ On the scan tool, every tap sent the clutch from closed to fully open and right back, and the hydraulic pressure dipped about 145 psi while the pump refilled it.
Verdict: the valve is good, electrically AND mechanically. Same answer from two independent tests.
That's the point of testing. We didn't just clear the valve, we proved it, and now we know exactly where NOT to look. The real problem was somewhere else, and we found it with numbers, not a parts cannon.
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Diagnostics first. Zoom Auto, Lynchburg VA · 434-849-1445
