Help - Low Air warning buzzer won't shut up!

PP · May 08, 2009, 01:29:43 PM

Got ready to go today and the low air buzzer keeps buzzing. Everything works and the gauges are reading 125PSI each. Shifts fine, brakes work properly, suspension adjusted itself after releasing the level low but the low air light stays on and keeps buzzing. Any suggestions on what and where I should be looking?
Thanks in advance for any advice you can give me, Will
PS-I haven't read Morse code since mid-70's in Malaysia----NOT GOING THERE AGAIN!

John316 · May 08, 2009, 01:47:31 PM

If I was you I would check to see if the sensor is bad. Otherwise, I would check to see what tank the sensor is in. Does it have the pressure? If it has the pressure I would then look into a new sensor, but then again, I am not a Prevost proud expert.

God bless,

John

JohnEd · May 08, 2009, 05:40:51 PM

STOP DOING THAT.

Its rude.

PP · May 08, 2009, 06:26:34 PM

John316,
Thank you for the advice. I tried checking the only two sensors I could locate, they're up front near the obnoxious buzzer and tapped into the brake lines below the foot pedal valve. I'm not sure if I checked them correctly though, as I don't know if the contacts should be open under pressure, or closed under pressure or if I should be getting a voltage reading between the contacts. I'm so confused

Plus, are there others besides those two? Like I said, everything appears to be working correctly, so I'm assuming it must be a sensor too. Thanks, .-- .. .-.. .-..

John316 · May 09, 2009, 05:50:42 AM

PP,

That sounds like a rough problem! First off, does Prevost have a tech support number (like MCI)? If they do, a quick call to them would probably take care of the problem. It does sound like a sensor, but which

. These kinds of problems are so frustrating. I hope you get it figured out very soon.

God bless,

John

JackConrad · May 10, 2009, 05:22:09 AM

I think the 2 sensors in the brake line are for the brake light switch and brake lioght indicator. The low air sensor will will be in an air line not associated with the brake system. Unfortuanately, I have no idea where that sensor would be located on your bus. On our MC-8 it was part of the Penn switch in the engine compartment. Jack

stevet903 · May 10, 2009, 07:10:14 AM

What is the serial number of your bus - Prevost uses the letter in the 10th position of the VIN plus the last 3 or 4 digits - i.e. H-1635.

Here is a wiring diagram that may help - http://prevostparts.volvo.com/technicalpublications/pdf/D060574a_c.pdf. I don't know if it is the right one without the serial number.

There are two pressure switches, one each for the primary and secondary system. From this diagram it appears that they are open when the air pressure is normal, and closed when low. I don't know where the switches are but this may help - If this is the right diagram for your bus there will be a block in the top drivers electrical panel. Look for number 33 and follow the wires labeled 13A and 13B - these should lead you to the pressure switches. Put a multimeter on each one and whichever one is closed with normal air pressure is bad.

Steve

PP · May 10, 2009, 10:05:01 AM

Thanks a million Steve,
That's the correct diagram and I was looking at the right sensors afterall

. It appears the primary is staying closed even when pressured up. The secondary has both terminals wired to a single terminal on the primary switch which doesn't make any sense to me. I have a feeling that someone made a few wiring mods along the way because it looks like the secondary has been bypassed by doing this. In one of the big boxes of stuff that came with the bus are several new switches, so that will be my project tomorrow. I'm also going to print that schematic for future reference.

Thanks again, Will

John316 · May 10, 2009, 10:16:26 AM

Quote from: JohnEd on May 08, 2009, 05:40:51 PM
STOP DOING THAT. Its rude.

John, Now what are we talking about???

Thanks.

God bless,

John

DaveG · May 11, 2009, 07:42:48 AM

Just so you know, the law requires that your low air pressure indicators work on BOTH the primary and the secondary (for all you with dual air brake systems).

So the test is to build up full air pressure, then drain the primary tank and verify that the low air warning comes on, then build air back up and drain the secondary and verify that the low air warning comes on.

This is what the inspectors do when they check you out.

Be safe.

PP · May 11, 2009, 08:53:37 AM

Update-It's not the sensors. they're both doing what they're supposed to do, even if the secondary isn't wired correctly. With air, they're open. Under 60 PSI they close. I'm backtracking the wiring now. Something tells me I will be making a few changes to the wiring to get both systems back on track. It seems simple and straightforward-obviously the secondary circuit was only relaying what the primary told it being wired the way it was. Thanks for all the input, Will

stevet903 · May 11, 2009, 01:33:03 PM

Are both warning lights on the dash coming on along with the buzzer? Depending on which one, or both can help to narrow it down....
Steve

PP · May 11, 2009, 03:14:06 PM

Steve, there are three lights on the dash. Low Air-Low Air Front-Low Air Rear and all of them are coming on. But looking at the wiring, I'm not surprised since the secondary sensor isn't really connected to anything but itself. The schematic you sent is a little off, but close enough. My guages are piped with 1/4" airlines, not electrical senders, but other than that, everything seems pretty close. (My bus is a little older unit-1983) I'm starting to pull hair out now, because everything I do seems to give me the same result whether aired up or not and the sensor no longer closes when I let the air down, it remains open all the time. Yet, if I throw a jumper across the contacts, nothing changes either

I'm just glad it's still driveable. If push comes to shove, I may be doing a little rewiring. Any suggestions? Thanks, Will

stevet903 · May 11, 2009, 04:59:34 PM

Does this diagram match the wiring better? It's for an '83.

http://prevostparts.volvo.com/technicalpublications/pdf/D060409_f.pdf

Let me know, and I'll take a look at it and see if I can make some sense out of it. At first glance it looks like if there is low oil pressure or an overheat condition it will trigger the buzzer as well....

Len Silva · May 11, 2009, 05:25:14 PM

I'm not a Prevost guy but looking at Steve's diagrams, it looks like everything is at terminal 33 in the front panel. It looks like something is putting a ground on that terminal. There is also a possibility that one or more of the three isolation diodes is shorted.