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Had an interesting troubleshooting experience today that might be useful to other FMTV owners. For several days the pressure on the air system has been dropping a little at night, requiring to be pumped up again the next morning. I had been unable to find the leak.
Today the leak was worse because when I shut off the truck I could hear air escaping from a point along the inside the left frame rail, about half way back. When I can hear it, it's a bad leak! Looked like a quick release valve was bad, QRV.
As I came out from under the truck it was apparent the right front tire was lower than it should be. I knew it was ok earlier in the day. Careful listening indicated air moving through the wheel valve, and right out the QRV.
When a wheel valve leaks its logical path is out the QRV. That is exactly what is supposed to happen when the CTIS module opens the wheel valve with the 15 psi, takes a pressure measurement, then dumps the QRV to close the wheel valve. If the wheel valve does not seal, the tire continues to vent.
So this became a fix-the-wheel-valve exercise, much easier than fixing a tire or replacing the QRV. Taking apart the wheel valve to clean it, I broke off one of the little bolts in the cover so I swapped out the wheel valve and all was well. The leaking wheel valve appeared just fine inside so I'll never know what was holding it open.
This experience could also serve as a warning that not much of an obstruction is needed to upset a wheel valve, so using any kind of balancing media in the wheel (recent thread) might be a bad idea.
Bob
Today the leak was worse because when I shut off the truck I could hear air escaping from a point along the inside the left frame rail, about half way back. When I can hear it, it's a bad leak! Looked like a quick release valve was bad, QRV.
As I came out from under the truck it was apparent the right front tire was lower than it should be. I knew it was ok earlier in the day. Careful listening indicated air moving through the wheel valve, and right out the QRV.
When a wheel valve leaks its logical path is out the QRV. That is exactly what is supposed to happen when the CTIS module opens the wheel valve with the 15 psi, takes a pressure measurement, then dumps the QRV to close the wheel valve. If the wheel valve does not seal, the tire continues to vent.
So this became a fix-the-wheel-valve exercise, much easier than fixing a tire or replacing the QRV. Taking apart the wheel valve to clean it, I broke off one of the little bolts in the cover so I swapped out the wheel valve and all was well. The leaking wheel valve appeared just fine inside so I'll never know what was holding it open.
This experience could also serve as a warning that not much of an obstruction is needed to upset a wheel valve, so using any kind of balancing media in the wheel (recent thread) might be a bad idea.
Bob
