MCI 9 low air pressure switch location?

Paul S · April 02, 2019, 08:36:57 AM

I've noticed some instrument panel light issues, one being no indication or buzzer of low air. I know this could be ground wire related, so I will be checking that, however, where is the low air pressure switch/buzzer located? I need to get all of this stuff working.

chessie4905 · April 03, 2019, 04:00:00 AM

Even after help you'll receive here, you need to get yourself wiring and shop manuals for your coach. As the years pass, they'll come in very handy, especially out on the road with an issue.

daddysgirl · April 03, 2019, 05:22:08 AM

I'm fairly sure the buzzer is down on the left side of the drivers leg below the switches, and there should be a low air rectifier diode and line flasher on the left side of the front junction box. On the right side, there should be an alarm buzzer. Check the connections to the studs in the front box (on my bus, it's stud #13, wire id #38).
Your bus is different from mine, but my wiring diagram lists those components in the front junction box.
Chessie's point is a good one. Without your bus specific books, it will be difficult to diagnose. Once you have a code and location for a circuit, you can trace it all the way through the bus.
And this might be useless to you, but I use my phone and take a picture of everything. Before I start a project, during the work, and when I'm done. Those pictures have saved my butt more than once, especially with electrical work.

Paul S · April 03, 2019, 06:25:12 AM

Thanks I will do that. I have both the parts and shop manuals but neither have been helpful in identifying the location,

Paul S · April 05, 2019, 06:21:56 AM

FYI - I did find what I think is the low air pressure solenoid at the drivers side tank train under the electrical panel. I also found the maintenance manual clearly identifies the wiring. Appears someone at some point cut and spliced the wires at that sensor for an unknown reason, looks like I have my work cut out to rewire it. Nice thing about the bus in general is everything is labeled nicely and it doesn't appear to be over complicated.

daddysgirl · April 05, 2019, 07:23:41 AM

Before you start rewiring, you might want to make sure you label those wires that serve the fast idle and the step light switch. If you cut those on accident, you'll spend hours trying to figure out what you did.
Also, on my bus, the buzzer and light is operated by the Penn Air switch (large square switch in the engine bay, adjacent to the rear junction box). If you have the maintenance manual, look in section 4. Mine is listed as the "Generator Control and Low Air Pressure Switch"

Paul S · April 05, 2019, 09:36:03 AM

I'm not sure I found the low pressure switch - it's mounted right drain for the accessory air tank, if it's not that I don' know what it could be for. Just looks different then the photos of them I have seen. Does anyone have a readable copy of the air line diagram? The ones in my book and online are almost impossible to read.

daddysgirl · April 05, 2019, 10:02:01 AM

It's in the rear. See edited post

Paul S · April 08, 2019, 10:39:15 AM

Awesome, will do after work. Took it on a 1000 mile round trip animal rescue this weekend and all otherwise is well.

Paul S · April 08, 2019, 03:48:09 PM

Do you have a photo of yours and location? searched all over that damn bus and I can't find it. I did find both of the blower adjusting cylinders are disconnected, and whatever airlines run them are missing. I'm actually wondering if the switch was removed at some point, but I have no clue because I can't find where it's supposed to be.

daddysgirl · April 09, 2019, 06:37:36 AM

I noticed something in your first post that mandates a question. Your manual is much better than mine, but it is slightly different.

You mentioned the light and buzzer was having issues with low air. Did you bleed the air out of the system, or is it leaking? If it's leaking, is it after you stop or while in service?

That switch is basically connected to the outside of the rear electrical box, on the top right side...I have to look around my oil bath filter to see it. It's a big square box.

Paul S · April 10, 2019, 05:12:08 AM

I will take a look again and see if I can find it, or at least the wiring for it - I take my class B test soon in the bus and need to get it working as I suspect they won't be to happy if it's not. Both tag axle switches are present and work though. To that note I do not get low air, oil or water lights when the ignition is turned on.

daddysgirl · April 10, 2019, 01:46:44 PM

If it's not flashing when you hit the ignition, I would start under the drivers window. Section 7-11 of your maintenance manual shows the buzzer location. Look at the Front Junction Box Connections on page 7-34 and trace the wires from the Flashers shown on page 7-61 to the dash lights. And then...

There should be a "38" on front stud number 13 that goes to stud "GR" in the rear and then to the switch. The schematic is on page 7-49.

Sometimes, I find it easier to go back to the beginning.

daddysgirl · April 10, 2019, 02:21:09 PM

Addition: The schematic for the Alarm and tell tale wiring is on page 7-47.

Paul S · April 14, 2019, 05:13:23 AM

The odd thing is that the book doesn't match the bus. It is indeed circuit 38, however it goes to a Johnson Controls P70GE-4 switch mounted to the junction box. If I short the circuit to ground I can get the light and the buzzer to come on. At this point all I can think of is to get an after market switch, plumb it in and see if it works. The Nason switch is completely different from what I actually have.