too much air pressure

[slusrr]

my 1990 5ton 6x6 over pressured and caused the relief valve on air dryer to constantly relieve. do I need a new compressor governer, and how do I reset the relief valve.

 

[74M35A2]

The relief valve should self seal. Maybe you are not hearing an over-pressure. The air dryer does a purge every time the air governor commands the air compressor to go into bypass (target pressure achieved). What indicated pressure are you seeing on your dash gauges? You can adjust the cut off pressure on the air governor on the firewall. Unscrew plastic end cap and adjust stud screw inside. Target 130psi if you have working CTIS. Or, your air dryer could be stuck open. Have you serviced it recently?

 

[Andyrv6av8r]

Sounds like your PPV valve is plugged. Relief valve should reset on it's own.

 

[slusrr]

my pressure pegged out at 6 oclock position past 120 and held! I just recently got the truck and know very little about it. what type of service do I do to the dryer?
I drove it for 25 miles on hiway and had a near constant air release. after parking it for awhile and re-starting it will only go to about 80 lbs. at idle, while constantly
relieving air at the dryer relief. thanks for any info. slusrr

 

[Suprman]

Yup I would replace the ppv. If it's stuck then horn and wipers won't have air.

 

[wheelspinner]

There are two different issues going on here. One is the pegged air pressure. I will assume that the relief you heard was the safety valve popping at its 150 psi relief. Why is the compressor going that high? It is most likely the Pressure protection valve (PPV) freezing (TX maybe not) or gunked up. If the air pressure goes above 130 and climbing to 150 right then, check the horn and wipers. IF they are inop the PPV is stuck. This valve prevents any air from going anywhere besides the primary brake system air. It also prevents the governor from getting a cut out signal, but it does not mean the governor is bad. Tons of threads on this, a quick search will locate them quite easily.

Now the second thing, not getting above 80 psi is completely different. Is the small plastic circle on the bottom of the air dryer where the air is coming from? That is the expelo (spitter) valve and is cleanable and serviceable. Lots of threads discussing it, search a little and it will be easy. I've done it several times.

The governor is not the culprit until after you have thoroughly explored these other issues first.

One more thing, you said you are new to these trucks and don't know much about them. Please do your self a favor and get into the tech manuals, ESPECIALLY the operators for a lot of basic knowledge. Then you can work into the lower and higher level maintenance manuals which include VERY thorough troubleshooting plans for virtually every imaginable circumstance. They are cumbersome, unruly, difficult and OH-MY-GOD boring but they are full of great information. And remember when in them electronically, Control-F is your friend.

Good luck, most of this is pretty simple if you don't let it over whelm you.