Took the lowboy to a job the other day, parked it while I worked, and then when I started it up, it wouldn't build air past 40 psi. Bled off the little air it had built, started it up again, and it built quick to 40 and then stopped. I grabbed a hammer out of the crane and gave the governor a little love tap or two (I'm that kind of mechanic) and one more try of bleeding off the air and building back up and it was fine, took off for 125. I got around to looking at it this afternoon, and replaced the governor and it didn't change, did the unloader valves in the compressor next and it hasn't changed. I put shop air to the tanks and then it was fine, would build air up okay. I bled it off a couple times and it built back up fine on its own. I'm now thinking I've got a check valve somewhere or the air dryer is somehow messed up. Not sure where to start looking next though? It does sound like the compressor is making air, and it is fast to 40 then just stalls out.
40.... on the dash gauge?? only...
Yes- 40-50psi on the dash gauge.
You sure it's not a faulty gauge...
Put shop air to the compressor discharge line and work back from there. if a governor and unloader kit didn't do it, more than likely the air dryer has an issue.
Air dryer or a relay valve pissing air....take a Union and bypass the dryer and see whatcha got.
No- I'm only optional on some gauges . I like to fun around a little, but really the only gauge on it that's a little finicky is the tach, it mostly works but sometimes disappears.
What type of dryer?
I think its like a AD SP? Its about 3" smaller (smaller dia. and shorter) all around than the old AD-9 that I'm used to, it looks like its a spin on top dessicant cartridge.
Like Junkyard says put a union between the Compressor discharge line and AD discharge line. That's probably a Wabco air dryer {strictly a guess} Those were problematic. Truck Shop
Next time you don't build pressure reach under the bottom of the drier and see if the purge valve is bypassing air. The purge valve orings fail and will prevent complete sealing. We see it all the time when it starts to get colder out.
Try stepping on the brake peddle while the tractor is building air .
That's actually another good test. Still, I just prefer to get them in the shop and hookup to the compressor discharge line and look for leaks.
Was the air dryer. Its a Wabco (some guys just get to be right all the time ) . New one is in, hopefully hauling tomorrow. Thanks, guys.
That Star is the right age to come from factory with a Wabco Turbo 2000 model. Truck Shop