Air compresser problem?

[John316]

Howdy,

I'm relatively new here, but enjoy the site. We have a MCI DL3 with a DdS60. A little back ground on the air system. It is governed at 120 psi, the compressor was replaced in December, and it usually gets up to pressure shortly after start up. Two weeks ago we drove 800 miles and the system worked great.

While driving today the air pressure hung around 90 to 100 psi, while driving on the interstate for an hour. When near our house on a smaller road (speeds about 40mph) the pressure rapidly came to 120, where it is governed to, the dryer purged (it hadn't on the interstate) and stayed at 120 till shut down at the house. Later tonight we took it out for another run for an hour and similar, 90 to 100 psi until we were on slower roads. Then it went up to 120, purged, and stayed there while we got back on the highway and it maintained 120, like it has in the past, until we returned again to the house. Back at the house I crawled underneath to see if there was any water in the tanks, if that could have an effect, but the tanks were dry.

Any suggestions or ideas why this would perform like this would be appreciated!

Thanks John

[TomC]

Possible dirt in the purge valve of the air/water separator.  Can buy a rebuild kit for the valve or a whole rebuild kit for the air/water separator usually for less than a C note.  Good Luck, TomC

[gumpy]

Hmmm, are you sure you're having problems?  Or could it be that you have a very tight air system, and it's just not leaking down to the cut-in pressure?

If you were running the interstate, you probably weren't using brakes or wipers or other air operated devices. When you got on the smaller road, obviously you probably used your brakes some, which would bring it down even more, to cut-in pressure, and then it built up to 120 as expected, and stayed there while you didn't use the brakes.

So is it possible it's just not reaching the cut-in pressure?  I can't recall exactly where mine cuts in, but I was thinking just under 90 psi.

I know this may be a long shot given that it's a bus air system and all, but if it's a DL3, it has to be relatively new (compared to most of our older buses), so maybe the air system is tight.

[John316]

Tom I will check the purge valve, thanks.

Gumpy, thanks for the idea, normally on the interstate it usually maintains 120, which is the pressure it purges, and purged at regular intervals. This problem is strange because we have put about 15k miles on it so far and haven't had this issue. We really don't have any air leaks except for the right drive brake chamber, which we are thinking is a diaphragm. Also we have electric wipers.

Not sure if it is related but we had it in the shop and they were working on the bus air conditioning. Things like replacing the compressor mounts, etc. Otherwise nothing has been changed.

God bless,
John

[Sam 4106]

Hi John316,
I think Gumpy has a very good thought. The Prevost I used to have would hold air for months and I can't remember the gauges ever reading less than 80 psi. If the air in your system is not leaking down to below the governor set point (approximately 90 psi), and you don't use any air to bring it down, your compressor will not build air. Your system may be working as it should. The next time you use your bus check the air pressure before starting. If the gauge shows above 90 psi, apply and release the brakes a few times to bring the gauge down below 90psi, start the bus, and see if the pressure comes up to 120 psi and the air drier purges. If it does your system is working properly.
Good luck, Sam 4106

[NJT 5573]

If that diaphram is leaking on the emergency side, it is not pushing the spring all the way off and the brake is dragging. It will turn the drum cherry red, blow the tires, and burn your bus to the ground. Fix it NOW.

[gus]

NJT,

That was a really great way to scare a newbie out of his boots!!

If he were going to crash and burn he would have done it long before he made his original post!!

[John316]

We replaced the governor on the compressor, the check valve (on the dryer), purge valve, and since I was at it, I also replaced the dryer's cartridge. The problem appears to be fixed, haven't tested it extensively yet. I had a little trouble installing the copper input line onto the dryer because the compression fitting was a little old, but got it to work.

Also replaced the diagram in the rear drive brake chamber, which had a little tiny hole in it, and that fixed the little leak that we heard.

Our main tank gauge was down this morning so I need to find that leak (wherever it is )

Thanks for the comments and idea.

God bless,

John