An aircraft manufacturer was experiencing intermittent problems with one of their fuel tank valves: It would work fine for a while and then randomly give a fail-to-close alarm.


The first day the Control Systems Inspector (CSI) visited the plant, the valve started responding and the error cleared up. He had to wait another three weeks for the valve to act up again, before he could take another stab at the mystery.

He knew from the start which valve was faulty, but had no wiring diagrams to show him which inputs and outputs on the PLC controlled the valve in question. He was looking at the PLC code in an entirely separate building, 100-150? away from the tank, so he had to manually trace out all the wiring.

A visual inspection of the valve showed that it was indeed physically closed, while the code was showing the valve to be NOT closed.

The next step was to examine the microswitch inside the valve control box.


There were two microswitches plugged into the PCB (printed circuit board). Normally one shows valve open and another shows valve closed — but in this case they were using one microswitch to relay both states. The contact was always making, even when the switch wasn’t depressed, so it would give a false reading.

To make things worse, the microswitch plug should snap tightly into place on the PCB, but upon closer investigation the CSI ascertained that the microswitch and terminals were loose.


The CSI wasn’t able to repair the terminals, but there was a spare microswitch in the valve, so he moved the wires over to the second set of contacts. Now the valve opens and closes smoothly and consistently with no alarms.

This particular valve had been intermittently setting off alarms for years — with a factory defect as the probable culprit. ACES sees cases like this all the time, when new equipment isn’t properly commissioned and causes sporadic glitches.

If you have a piece of randomly functioning machinery that needs troubleshooting, give us a call.