Low Air Indicator

[mcichad]

Hello,

Currently working on a '92 MCI conversion, working on the inside and the other day the low air indicator came on.  However the air gauge is showing normal pressure and items such as door work correctly.  The indicator remains in low air, park brakes do not release and there is no more relief pressure.

Since the work we're doing is solely on the inside with no interuption to the air system, I'm not sure why the gauge is showing normal when the low air is on.  It's almost like part of the system such as the air tank isn't running at full pressure, just not sure what to check next.

Your insight would be appreciated and any tips are welcome!

Thank you,

Chad

[Gary '79 5C]

Chad,
Welcome to the board and good luck in all your efforts in the conversion.

I have a 5C '79, duh one look at my name... But my air pressure switch is at the drivers side engine compartment on the bulkhead. Not sure your's is the same. I am not sure, nor have I encountered this whilst working on mine. 

Again, Welcome and Have a Great Day.

Gary

[John316]

Welcome aboard Chad!

I am sorry to hear about your troubles. What model is your bus. If it is a 92 year, then is it a C? I think that would help some, if we find out what model it is.

Otherwise, I don't have much info for you. Do you know where the sensors are (what tanks)? Do you know whether they are supposed to be open circuits on low air or closed (I think that it would be closed on low air)? I would assume that you haven't had this problem before, correct?

Okay, so I just read the rest of your post (I tend to skim things first, my bad). It sounds to me like the air is getting to the tanks where the gauge's read off of, but not to the tanks where the low air sensors read from. In many ways, it is nice that the rest of the system doesn't have air, because you have a bigger problem to troubleshoot.

Here is an idea. I know that this a long shot, in the dark, but lets see. A couple of times shortly after we got our bus, a 95 DL3, we had trouble with it airing up (and it was above freezing, so we didn't have any frozen water in the lines). The difference was that our gauges wouldn't go over 70 PSI. I did a lot of disassembling, including taking the air dryer ALL apart, and that didn't fix it. Finally, after listening to where I could hear air, and where I couldn't, I traced it to the first bay driver side, front wall. It turns out that something in the kneeling system (I still don't know what) went funky. The kneeling system never has worked (and I haven't taken the time to research it). It turns out that in that box, in the front bay, one of the bypass valve had gotten messed up. I changed the position of those shutoff valves, and played with them, when the sound changed (air running through) and we got air to the rest of the system. I know that it is a long shot, but I don't know....

I don't know how mechanical you are. Have you ever been under the bus (of course, block it up)? Are you pretty familiar with the systems, or are you in the dark on a lot of that stuff.

A little more info, and background on you would be helpful.

FWIWHTH

God bless,

John

[Len Silva]

I'm not familiar with your coach but, typically indicator lights have battery applied to one side and get operated by grounding the other side (which is what the low air sensor would do).  If it's not a bad sensor, then most likely a ground on that lead.

[John316]

Len,

I talked myself out of thinking bad sensor. Because he said that his brakes wouldn't come off.

Chad, I had another thought. I would assume that the dryer doesn't purge right? What happens if you air up with shop air? Can you hear air leaking?

God bless,

John

[mcichad]

Hi John,

I appreciate the welcome, it's an exciting project! 

My apologies for the specs, you are correct it's a '92 102-C3 and the dryer does not purge either, haven't applied shop air as of yet but will be shortly.  I like your description, as it describes and points out some areas where this could be presenting the problem.  I do have a decent mechanical understanding of the system, just not sure where all the components are and what in particular could lead to this particular issue.  Thought the sensor, but when the park brakes wouldn't release as you mentioned, lead away from the sensor.

I would say applying shop air will be next and we'll see if there are leaks and proceed from there.

Thanks again,

Chad

[bevans6]

if it's like my MCI, then I "think" (have not yet verifed totally) that the pressure gauge is not read off the main dry tank that supplies air to the service brakes, but is read from either the auxiliary tank or from the parking brake tank.  the low air warning light and buzzer reads from the main dry tank.  If the main dry tank is low, then you will hear and see the low air alarm, you will not be able to release the brakes (takes a 100 psi application).  the parking brake tank and the auxiliary tank are connected to the main dry tank by a one-way valve so that loss of air in the main tank does not deplete air in the other two tanks.  There is a shuttle valve that will connect the parking brake  tank to the main service brakes if the air pressure in the main system falls to a low enough level, but it's possible there is not enough air pressure in it to release the parking brakes.

If you air up the bus with external air, then the air dryer blow off will not work, it needs a signal from the air compressor governor  for that to happen.  Have you run the bus compressor with the main engine to see if it can air up the bus?  If you drain all the tanks, what does the gauge read?  How fast does the bus compressor air up the bus?  Mine airs up from all tanks drained to compressor blow-off at 120 PSI in 5 min 30 seconds, and 85 psi to 100 psi compressor recovery (not initial pump up of all tanks, but deplete down from full to 85 psi and measure the recovery time) in 22 seconds, legal maximum is 2 minutes for the recovery test.

Anyway, i would start by connecting another gauge to the system to see if the dash gauge matches it reasonably closely.  You could do that at the muffler at the compressor end, or at the emergency fill at the aux tank end, and I would do both.

Hope some of this is useful/

edit: your air dryer will not purge until the governor tells it to.  If it's not purging it's a good sign that the compressor is not producing air, or that the air is not going into the system.  the governor gets a signal from the main supply tank (wet or dry I can't remember) to first of all tell it to stop diverting air to atmosphere and start sending it to the tanks (at low pressure governor kick in, typically 95 psi) and then tells it to stop sending air to the tanks, divert it to atmosphere, and tells the air dryer to purge, typically at 120 psi or 25 psi maximum higher than the kick in pressure.  It's important to remember that the compressor always produceds air as long as it is turning.  The governor then either sends the air to the tanks or to the atmosphere, depending on what the signal line from the supply tank tells it to do.

Brian 

Brian

[mcichad]

Hi Gary,

Thanks for the greeting, this one is in the front drivers lower compartment on the bulkhead also....much appreciate your insight to the location, as this was first on the list and really helps for diagnosing.

Thank you,

Chad

[mcichad]

Hi Brian,

Much appreciated, this provides the details required, as it matches the scenario and therefore I'll adapt the gauges as mentioned. 

Thanks again,

Chad

[Airbag]

My guess it is from sitting too long and needs to be aired up and exercised and the problem will likely dissapear. I wish I had a nickle for everytime I brought a bus to the garage with a leak in the morning and the mechanic just ran it up and cycled everthing parking brake and service brake and walla it held air. He said they just need some warm air to get everything acting right.

[bevans6]

just drug out the schematic of my bus, your is  probably similar.  The emergency fill valve in the drivers side lower side compartment feeds only the parking brake tank, it is isolated from the rest of the system by a check valve between the parking tank and the dry tank.  so if you read pressure there you get the parking tank only.  if you adapt a valve to the air discharge muffler in the rear passenger side engine compartment (little thing on the wall with a big hose going to the top of the compressor, you will read the pressure in the wet tank.  There isn't a simple access way to read the actual pressure in the dry tank, which is connected to the wet tank with a one-way check valve.

My schematic does not show either the low pressure warning switch or the pressure gauge tap locations, which irritates me no end...

BTW, what do you meann by 'relief pressure'?  I thought you meant air dryer purge.

Brian

[Tony LEE]

The low air pressure switches on my MC8 are located adjacent to the rear junction box inside the rear LH engine access door. Two circuits - one controlling the alternator charging and the other the low air pressure light and alarm

[John316]

Brian,

Interesting thoughts. On the dryer, ours doesn't cycle until it reaches 120 (ours is very similar to Chads, his is just the model older). It doesn't matter how long the compressor runs, the governor doesn't tell the dryer to purge until it reaches 120. When I had the problem with the kneeling system, the compressor would run continuously, but that is how I discovered the problem. Air was leaking out of the kneeling control box, and the dryer still hadn't purged.

Chad, I would first start by listening and seeing if I can hear any air leaks (with the bus running). Then I would turn it off, and listen for air leaks. After that, I would add shop air. One note of caution with shop air, DON'T LET THE SYSTEM GET ABOVE 120 PSI. It could rupture different components, and cause you a nightmare of a time. So just be careful with shop air.

God bless,

John

[mcichad]

John,

The shop air worked like a charm, after Brian's insight to a secondary air supply for the park brakes, locks...this is where the low air sensor is mounted.  It's measuring this via the holding tank located in the access panel located just below the driver, added a t to the connection and applied air at this point.  Now all components are airing up and I hear air from a connection in the engine bay, along with a constant relief on radiator shutter valve, with the combination I would say there is too much loss and now triggers without the ability to build full pressure.  There is no dryer purge as a result, just as you mentioned the dryer doesn't purge until it see's approx. 120 psi, then it purges with a sweet relief....lol   Oh ya, thanks for the insight to max pressure for shop air, I was keeping it low and testing.

Brian,

You are correct, I was talking about the 'dryer purge'...just didn't use the correct term, sorry......much appreciate your details and getting out the manual, all of the input here helped to provide a path to diagnose and test.  Now I believe once the connection is corrected, fix the leaking shutter valve we'll be all good!  As for sitting, I would totally agree, prior to getting it was sitting for approx. 2 years and now awaiting the conversion with short spurts, but hope to be on the road within the month, anticipating similar items until it's 'excersized', just couldn't get it to hold air on this one.

I'll be trying these items hopefully tommorrow into the weekend, hopefully all goes well and I'll defiently update.

Thanks again,

Chad

[bevans6]

in actual fact the dryer doesn't purge until it's told to by the governor mounted on the compressor.  It reaches cutoff pressure, which it reads via a line from the supply tank, pressure on the piston overpowers the spring preload and the piston shuttles to exhaust incoming air from the compressor.  That sends a signal down a line to the air dryer that tells it to purge.  So that little purge sound is the result of the compressor filling the tank, the tank telling the governor it's full, and then the governor telling the dryer to purge.  Fairly complex system...  I only note this  becuase if the air dryer isn't purging, it might not be the aiir dryer you should check first...


Brian