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

PP · May 11, 2009, 07:21:26 PM

Yes Steve, that is my diagram. Unfortunately, the real thing doesn't look like that anymore. Someone has been messing with the wiring to the sensors. There are 3 wires on the #33 terminal, just like the diagram. But there is only 1 rectifier in the console between the lights. Also, the single wire on the sensor reads 24V on the multimeter and the other 3 across from it show ground on the ohmeter when read individually. Plus, I can't get either sensor to close no matter what pressure or lack thereof. Yet, earlier, the one appeared to be working with pressure changes. (Does that make sense?)
Yes, Len, the buzzer also warns if oil psi drops, eng temp rises, and low coolant (I think) But those lights go out when the engine is running so I'm assuming they're working correctly and not setting off the buzzer.
I dug out the pressure switches in my tubs of parts, but they're for the fuel line, not air lines. I really do appreciate all your help and any suggestions, Will

Busted Knuckle · May 12, 2009, 12:12:47 AM

Sure wish Sammy was still around! He'd had you fixed in no time with a little problem like this on a Prevost! Good luck PP

BK

PS sure do miss ya Sammy! BK

stevet903 · May 12, 2009, 06:35:48 AM

Will - are your sensors in the the same place as my '89? On mine, they are in the top middle of the spare tire compartment screwed into a double check valve. They are not round like the ones for the brake lights, but kind of an elongated six sided affair.... One of the mounting screws had come loose and allowed the check valve to twist. If the same thing happened to yours, it may have been able to wear through the wire and ground it....

PP · May 12, 2009, 08:29:00 AM

Steve, As soon as the rain quits I'm going to crawl back in there, but I'm going to feel like a real fool if it turns out that I've been mistaking the brake light switches for the lo air sensors. One of these is plumbed into a manifold mounted just behind the brake pedal valve and the other is mounted in a line that actually runs into the pedal valve (I'm assuming that the manifold looking thing mounted directly beneath the brake pedal is the pedal valve) and yes, they are both round with (2) terminals, one of which is a ground. The only elongated looking thing I can see controls the parking brake light (When I disconnected it the park warning light went out). After I look a little deeper under there, I'll repost my findings.
BTW-JackConrad-Maybe I should have looked a little deeper originally when you said the sensors shouldn't be in the brake lines. My bad.
Thanks guys, Will
PS-Steve, Where do you come up with those schematics? I really appreciate you taking the time to load them and share them.

PP · May 12, 2009, 12:32:31 PM

Yes Steve, I'm an even bigger idiot than I thought. I found the air sensors in the place and description just like you said. I'd been messing with the brake light switches

all this time.
Now, I have to figure out why both front and rear systems are showing no air pressure even when aired up. Correct me if I'm wrong, but if one of the sensors is bad, it wouldn't set off all the lights on the dash, right? So, am I safe to assume the problem must be in the wiring that they have in common? Or a defective rectifier (Diode). FWIW-I found the other 2 diodes mounted in the panel just before the buzzer.
Thanks, Will

stevet903 · May 12, 2009, 06:06:49 PM

You can find the diagrams at http://prevostparts.volvo.com/technicalpublications/en/wiring.asp

Click on the top sections for owners manuals, tech manuals, etc, but they are scarce for the early years...

Here's how I would troubleshoot this:

Disconnect the non-ground wire at each of the sensors, one at a time and see if or which light goes out and if the buzzer goes off.

With air in the system ( greater than 70 PSI), check each sensor for continuity to ground (open or closed).
Each should be open with air, and closed under 60 or so PSI.

If the buzzer is still going off and the lights are on with both sensors disconnected (you may have to run the engine to get the buzzer to go off for the oil pressure), then I would go back to terminal 33 in the driver's compartment. There are likely to be two wires connected to the upper half of the block - these should lead to 13A and 13B in the diagram, through the diodes. Disconnect each in turn and see if the buzzer goes off. Post the results and we'll go from there.

From looking at the wiring diagram, it looks like if one sensor is bad (or air is low in one or the other tank) and closed all of the time, the buzzer will go off and light the lo air light as well as the corresponding front or rear light. If all three are lit, it could mean there are multiple problems - possibly the diode on the far left of the diagram is shorted plus there is a bad sensor or a wire (13A or 13B) shorted to ground.

Post the results - we are close!!
Steve

PP · May 13, 2009, 07:46:40 AM

Steve,
I might not be able to get on it as quickly as I'd like because I'm on Jury duty this week, but I will post my results just as soon as I can. Thanks again, Will

John316 · May 13, 2009, 08:01:32 AM

Quote from: PP on May 13, 2009, 07:46:40 AM
....but I will post my results just as soon as I can. Thanks again, Will

About what, the jury duty, or the bus

.

Sorry that you got called, but it is our duty. I hope things go well for you, and that the trial doesn't drag out forever.

I also hope that you get your bus problem figured out. Those kind of problems are rough. I am chasing some strange ones myself.

God bless,

John

PP · May 15, 2009, 09:15:24 AM

Hey Steve,
I did exactly as you instructed and discovered a shorted wire leading to the secondary system sensor. I corrected that and everything appears to be working correctly again.

The wire appears to have melted together with a ground wire. At first I thought they were stuck together in a little glob of undercoating or asphalt, but no, they'd melted. When I disconnected the wires from one of the sensors, it started leaking air by the wire stud. I think it may have been leaking all along, but I aggravated it

so now I need to replace it. Does anyone have any suggestions where to get these? I checked with NAPA (yeah, the same place that doesn't have a clue what a Prevost is) and they have something 'similar' but not quite. It looks more like my brake switches. I don't have a dealer license so please don't recommend any places that won't sell direct to an end user. Thanks, Will
PS-John316, Jury duty is a real kick in the pants. I've never shirked it, not because I necessarily feel a strong civic duty, but because I really enjoy doing it. (One less tweeker on the street this week LOL)

Len Silva · May 15, 2009, 09:48:50 AM

NAPA low air switch MBI BE13241 $26.29 N

When you go to NAPA, find the old guy who knows how to use the books and not the computer.

Busted Knuckle · May 15, 2009, 10:02:29 AM

Quote from: Len Silva on May 15, 2009, 09:48:50 AM
NAPA low air switch MBI BE13241 $26.29 N

When you go to NAPA, find the old guy who knows how to use the books and not the computer.

Or call JD @ C & J Bus repair in Bloomington, MN! He'll hook you up with what ya need!
C & J Bus Repair
9010 Pillsbury Ave
Bloomington, MN 55420
1-800-228-7349

BK

John316 · May 15, 2009, 01:34:31 PM

Quote from: PP on May 15, 2009, 09:15:24 AM
PS-John316, Jury duty is a real kick in the pants. I've never shirked it, not because I necessarily feel a strong civic duty, but because I really enjoy doing it. (One less tweeker on the street this week LOL)

Good for you. That is good that you enjoy it

. For me, it is tough to miss work for that long (of course their 10 dollars a day goes a long ways

). Glad that the case worked out well for justice.

God bless,

John

PP · May 15, 2009, 06:43:51 PM

Len, what if there isn't an 'old' guy handy, should I just take one with?

When I gave the 'young' guy the Napa # he got a little hostile, but it did motivate him to try a little harder. He used that Napa # and Xrefd it to the genuine part even though the genuine part number meant nothing to him at first. Of course, the real thing is 10 bucks more than the Napa part, but it is identical to the one I took off. There goes my jury duty money LOL. Actually I am going into the big city tomorrow and will check a big truck shop there. If I don't have any luck with that, I'll give BK's source a call Monday. (Thanks for sharing that BK.) I should just get it from Napa, but I really didn't like his attitude (before or after I gave him the part number) What can I say? I'm not desperate enough to have to tolerate it LOL.
Thanks for all the help from everyone, Will

stevet903 · May 15, 2009, 08:24:21 PM

Great news Will, glad you have found the problem. I believe the Prevost part number for this is 640975, for a whopping $20.78. You can call Prevost parts at 800 621 5519 or order online if you have an account. They ship same day and you should get it within 2 days.

Steve

PP · May 16, 2009, 04:37:55 PM

Thanks Steve, I owe you a cold one for all your help.

I found one in the city at a Truck repair shop identical p/n as the one I pulled off. Put it on this afternoon (just finished testing everything) and all is right with the world again

I am putting the Prevost phone number in my Bus Log for future needs. You know the saying, if you're prepared, you'll never be in need LOL.
I don't know where you're at, but if you ever find yourself on the central coast of Oregon, drop me an email (its in my profile) and I'll treat you to a cold one while you school me on buses LOL.
Thanks, Will