Air pressure only builds to 70 PSI?

[Jeff Pettit]

We developed a new problem... we went for a road trip and made it back home without any issues. The next day went out to start the coach the air pressure would only build to 70PSI. The only thing we did after we got home was replace the front driving lights with LED light using the existing wiring, one shouldn't have anything to do with the other but you never know..... I don't hear any leaks and we just replaced the purge value lastweek.  The pressure stays steady at 70PSI and that not high enough to release the breaks or throttle, has always ran around 120PSI. I would appreciate inslight as to what the issue may be.

[Frank Bergamo]

Jeff,

there is an air governor that is hooked up to the air line that comes off engine compressor. Looks like this, https://www.amazon.com/Truck-Governor-Bendix-Midland-Freightliner/dp/B005458FYY/ref=asc_df_B005458FYY/?tag=hyprod-20&linkCode=df0&hvadid=312632042117&hvpos=&hvnetw=g&hvrand=107954182821593921&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9029182&hvtargid=pla-571374496311&psc=1 Also, the air dryer may need servicing. Last, there is an unloader valve that may be sticking or malfunctioning. Start at the governor, and work from there. Hope this helps.

[Jeff Pettit]

Thank you Frank!  Gives me a starting place.

[Lee Welbanks]

Jeff, There is a bunch of videos on Youtube on how to adjust the D2 Governor, you need a 7/16 wrench and a flat blade screwdriver. Turn the screw clockwise for less pressure and counterclockwise for more pressure.

[Jeff Pettit]

 Going to get a replacement tomorrow, this one looks like the original one...  Got it pulled off, wasn't mounted on the air compressor but on top of the engine.

[Jeff Pettit]

Replaced the air governor, no change. Build right up to 70 PSI and stops, could it be some kind of electrical issue... I have had some odd issues with this coach from day one, for example, the air-horn works when parked but not when moving. We may have to cancel our camping trip if I can get it fixed very soon. 

[Eric Maclean]

Jeff
If the governor didn't fix it there are a couple of other places to check
First check for any leaks in the vehicle sometimes shop air to the tanks can help find these ( listen for leaks)
Check with engine running for air escaping from the air purge valve at the bottom of the air drier they can stick and dump all the incoming air ( it should only purge when the system reaches full pressure and the governor shuts down the compressor)
NEXT
1) check the braided air line from compressor to drier for air leaks with engine running ( use soapy water )
2)disconnect the braided feed line front the compressor and check for air flow with engine running ( your looking for good flow)
3)if flow is good in step 2 disconnect the other end and do same ( your looking for obstructed or plugged air line)
4)if air flow in step 2 is low ( suspect unloader valves in the air compressor) they can stick causing the system to not pump

You may want to tap on the top of the compressor unloader valve first and see what happens
Here is a link to a video which sows the unloader sticking and the solution .

https://youtu.be/r5GgEES8Wns

Hope this helps
Eric

[Jeff Pettit]

Eric, Thank you for the detailed troubleshooting info. When I get off work I will start at the compressor and work my way forward.   For some reason, I still think it has something to do with the electrical system not opening a value or something because it shoots right up to 70 PSI and will hold that pressure and not leak down... but working from compressor forward I will know for sure where the air is restricted and will be able to inspect all the components out as I go. 

[Eric Maclean]

Jeff
There should be no electrical in the air system of that vintage vehicle except maybe the low air warning switch .
The air compressor and controls are all mechanical.
However the 70 psi is the pressure at which the priority valve for any accessories like air suspension and or air horns air seats etc
Opens this valve is there to protect the air brake system . It only allow the air to move into the accessories when the system builds above 70 psi that way if there is a catistrophic leak in any accessories like air suspension the system will shut off the supply to the accessories at 70 psi maintaining enough air for brake operation.( The shop air to the tanks should reveal the leak if you have one of that magnitude.)

My bet is on the unloader
Eric

[Jeff Pettit]

More good info Eric! I will check out the unloader along with the complete system. Glad this happened at the house and not on our upcoming road trip.

[Jeff Pettit]

I have exhausted my level of trouble shooting, calling a mobile tech for help. Probably the unloader stuck or something simple but at this point I just want it fixed. We have a camping trip planned for next weekend with my brother and sisters just hope we get to go, the main reason I bought the motorhome.

[Jeff Pettit]

Have a mobile tech coming out tomorrow (8/16/2021), we will see how it goes.

[Jeff Pettit]

FIXED!!!! Found a local mobile tech, he came out and found a small 1/4 " line in the engine bay that had blown out, couldn't hear it leaking due to the engine running. Took him about two hours of troubleshooting, finding the part, and replacing it. Best $170 I have spent!! He is coming back tomorrow to change my oil and all filters said he charge me one hour of labor if I supply oil and all the filters.

[Bill Lampkin]

Glad you got it fixed! Couldn't hear the leak, that's why they're called Clatapillar! Have a safe trip.

[Fred Brooks]

    Jeff, Glad to hear that your Tech found the problem and fixed it. Have a wonderful Trip, Fred