Testing the relief valve is really pretty easy. Just release all 4 of the CTIS valves (at the hubs), and turn the CTIS pump on for a few seconds. (On pre-96 models, be sure to turn the selector switch to some position other than "off".) The dash-mounted pressure guage for the CTIS should read between about 40 and 50, and you should hear the valve working (sort of a rapid "puffing" or "chuffing" shound) if the engine is not running.

If the pressure is not within this range, time to repair or replace the valve.

On my truck, this process took less than 5 seconds of compressor operation to reach 50lbs on the guage. Running longer did not increase the pressure - my valve is working correctly.

Dave Breggin
'95 Diesel Wagon

Excellent post Dave. Just a few suggestions...

First, identify whether it is only one axle, or both. (For those of us with older Hummers, set the axle selector switch to "off".)

If the leak goes away with the selector in the off position, the leak is between the compressor and selector. If it does not go away, it is between the selector and the appropriate axle.

If it is both axles, the leak is somewhere between the axle selector and the compressor.

This is true only if the selector is in the both position.

I am not as familiar with the newer trucks, but I belive that the following would help. Set the switch to front for a while, repeat for rear. If it leaks in both, I would guess compressor - air block leak. If only in one of the positions, probably in that specific system.

If it is one axle, release both of the CTIS valves at the hubs. If the guage still shows a leak, it is in the lines between the selector and the hubs. (The guage will show a very rapid leak in this situation.)

You have to have the axle isolated for this - selector in off.

If the leak stops, it is most likely one of the valves, or a tire. Re-connect them (one at a time) to determine which one.

Just a note here. Check the tire pressure too at the quick disconnect. If it shows lower pressure, the wheel tubing is leaking. If the CTIS lost pressure, the inner plumbing is faulty.

Gerald