Air Pressure Issue

Hey everyone, nice to see the website working again, you don't realize how much you rely on electronics until they're gone huh?

At any rate I have an air pressure issue and possible cure I wanted to run past you guys. On my 923A2 it suddenly overpressurized the air system to 150psi and the safety valve opened on the primary tank. I shut the truck down, bled down the system, and restarted it. The pressure this time stopped at the normal 120psi and the dryer discharged without issue. Upon shutting the truck down this time I noticed the safety valve still leaking, not at a rapid rate but you could hear it. I replaced the safety valve and again started the truck.

This time the truck failed to stop building pressure and continued close to 150psi again before I shut the truck off to prevent any damage. I tried adjusting the governor which didn't change the problem, I even replaced the governor which still changed nothing, the truck will continue to pressurize the air system, the compressor never cuts out.

I disconnected the feed line to the governor and started the truck to find I had no air at all to the governor, which also means no air to the horns, wipers, and windshield washers. I got under the truck and ended up finding the pressure protection valve is leaking, not a ton but you can hear it and feel it when you put your hand around it. In the TM it says one of the functions on the pressure protection valve is to not allow air past until the truck builds more than 65psi of air pressure for consistant braking. On the other side of the pressure protection valve is the line to the governor and all the accessories I mentioned above.

It seems pretty obvious that the pressure protection valve has failed wouldn't you think? I'm going to replace it and see what I find but I just wanted to run the scenario past you guys to get your thoughts.

Thanks for your time.

You probably have a bad air protective valve the governor is past the valve so if it does not open then the governor cannot tell the compressor to stop making pressure. I bet your wipers and horn wont work also.
Will

Sounds good but one thing you might look to see if it is just clogged and or can be rebuilt as most all of the pieces on these trucks can may save you money.

On the advice of csm davis I removed, cleaned, and replaced the pressure protection valve on the primary tank rather than just replacing it. It was nasty with rust which I was able to clean off, lubricate, and adjust while putting back in place, it functions properly.

I now have air to the governor but not past it, I let the truck build 130 psi with the sense line to the air compressor disconnected and got no air out of it at all. I even adjusted the governor to both extremes of its travel to find no change. I have air to the wipers and horn since they get their air supply directly from the line supplying the governor, but like I mentioned above, no air on the other side of the governor spring. I should also mention I do not get a purge from the air dryer at all since these symptoms began.

seeing how the pressure protection valve looked internally makes me think the air dryer could use a servicing so I plan on overhauling that. Other than that does anyone have any suggestions aside from maybe a second bad governor?

There are check valves that can have issues after sitting with moisture in the air system. The sense line needs to be connected. When you adjust the governor you need to pump the brakes to bring pressure down and then let it build again for a minute to see if there is any change.

Make sure you are hooking governor lines up right. If you are just following where they were when in the service maybe wrong. Is this a new truck to you from gl or private seller? A good way to check governor if you have a air compressor is regulate air to 125 come out of line on compressor either use blow gun or adapt to fitting on governor put air in the right port make sure it come out the other port. If not adjust governor until it does. Good starting place. My 2 cents

A good starting place is to follow the troubleshooting guidelines in the TMs.

I've gone through the TMs several times, I've actually printed all of them out so they are easier to read. There is a condition describing the same symptoms I've mentioned, I've run the entire troubleshooting guide to the end and got to"notify supervisor".

The truck has been in my possession for a few months now and run flawlessly until now, so it's a new issue. Thanks guys for the help, I'll keep looking into it.

Fixed!!! Just to recap: Ended up replacing and adjusting the governor with no change, truck still continued to build air pressure as the "unloader system" never functioned. Replaced the safety valve on the tank and the pressure protection valve both due to leaks which still didn't fix my problem. I even went out and bought a new air dryer assembly, again with no change. Everything pointed to the governor as being bad so I went back to the local fleet supplier and bought a second new governor. Installed the new governor which was already preset at 120psi and it worked as advertised! I guess the first governor I bought was bad, it was a remanufactured unit so it's entirely possible one got out without being correct.