I just purchased a M931A1 (2010 RRAD rebuild) and am having flakey air pressure issues. Being new to these vehicles and never having troubleshot Diesel/Air Brakes, I am not sure what "correct" is regarding air building and pressure fuxuation. However I believe what I am experiencing is incorrect and will explain what I have experienced in the hopes of receiving some guidance from the good folks here...
I picked up the truck in the Jacksonville, FL area. I never measured the length of time, but would guess it took about 30 sec. to a minute for the air to build to 90 PSI and not long after make it's way to 120 PSI and hold there unless frequently braking. Both primary and secondary gauges would build at the same rate, which seemed "correct" to me.
I took it on a three mile test drive and the air pressure stayed high the entire. time. Then I drove it five miles to pick up a trailer and noticed no change in the air performance.
Then I began the over 500 mile trip to Cincinnati, about 25 miles down the road I had to stop to better arrange some cargo and left the truck idling while I did so. When I got back into truck about 5 minutes later the air was about 35 PSI on both tanks and wouldn't build.
Checking around the vehicle with it running I noticed air spurting out the opening on the bottom of the air dryer?(coffee can sized object in the center behind the transmission?)
I covered the hole with my hand and low and behold the pressure started to build. Took my hand off and it kept building up like normal. The truck made the entire rest of the trip with seemingly normal air building and pressure maintaining performance. Even through hundreds of miles of mountain grades.
Yesterday I drove it 40 miles to it's long term location. During that drive I watched the gauges closely because my warning box is inoperative and there is no low air alarm. I noticed at one point, with no braking to drain air, the secondary was at 90 PSI and the primary was at 35 PSI!? I was planning to pull over to check for spurting air below, and then both tanks pumped back up to 120 PSI and remained there.
Then about one half mile before the destination, braking downhill to a traffic light, both primary and secondary are at 35 PSI at the light and won't build. I put the parking brake and flashers on, check for spurting below and there is none like before when the air went down on the long trip. Holding my hand over the hole does nothing and I don't feel pressure like before. I walk around, check under the hood and don't hear/see any obvious air leak, however the tanks were basically empty by then.
I hooked up air to a tank on the left where the bleeding line comes in and pressurize the tank so I can get it a half mile down the road. The secondary was at 90 PSI, and I think the primary was zero or not much higher. I start driving it and low and behold it starts building, right up to 120 PSI?! I parked it and walked away scratching my head.
Thoughts?
I picked up the truck in the Jacksonville, FL area. I never measured the length of time, but would guess it took about 30 sec. to a minute for the air to build to 90 PSI and not long after make it's way to 120 PSI and hold there unless frequently braking. Both primary and secondary gauges would build at the same rate, which seemed "correct" to me.
I took it on a three mile test drive and the air pressure stayed high the entire. time. Then I drove it five miles to pick up a trailer and noticed no change in the air performance.
Then I began the over 500 mile trip to Cincinnati, about 25 miles down the road I had to stop to better arrange some cargo and left the truck idling while I did so. When I got back into truck about 5 minutes later the air was about 35 PSI on both tanks and wouldn't build.
Checking around the vehicle with it running I noticed air spurting out the opening on the bottom of the air dryer?(coffee can sized object in the center behind the transmission?)
I covered the hole with my hand and low and behold the pressure started to build. Took my hand off and it kept building up like normal. The truck made the entire rest of the trip with seemingly normal air building and pressure maintaining performance. Even through hundreds of miles of mountain grades.
Yesterday I drove it 40 miles to it's long term location. During that drive I watched the gauges closely because my warning box is inoperative and there is no low air alarm. I noticed at one point, with no braking to drain air, the secondary was at 90 PSI and the primary was at 35 PSI!? I was planning to pull over to check for spurting air below, and then both tanks pumped back up to 120 PSI and remained there.
Then about one half mile before the destination, braking downhill to a traffic light, both primary and secondary are at 35 PSI at the light and won't build. I put the parking brake and flashers on, check for spurting below and there is none like before when the air went down on the long trip. Holding my hand over the hole does nothing and I don't feel pressure like before. I walk around, check under the hood and don't hear/see any obvious air leak, however the tanks were basically empty by then.
I hooked up air to a tank on the left where the bleeding line comes in and pressurize the tank so I can get it a half mile down the road. The secondary was at 90 PSI, and I think the primary was zero or not much higher. I start driving it and low and behold it starts building, right up to 120 PSI?! I parked it and walked away scratching my head.
Thoughts?
