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Old 08-31-2022, 10:27 AM   #1
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Front tank air gauge malfunction?

Was just wondering if anyone has encountered this problem before?

When applying the brake the front tank air gauge immediately drops down and the low pressure warning comes on and then will go back up and be at normal air level. Does the gauge need to be replaced or is there a problem further down the line? Blue Bird says new gauge is almost $400 so don't want to order until I'm sure. with the engine off and brake applied there are no air leaks

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Old 08-31-2022, 10:32 AM   #2
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Forgot to mention this is happening on a 2009 Blue Bird All American FE
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Old 08-31-2022, 12:46 PM   #3
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Originally Posted by BusMech View Post
Was just wondering if anyone has encountered this problem before?

When applying the brake the front tank air gauge immediately drops down and the low pressure warning comes on and then will go back up and be at normal air level. Does the gauge need to be replaced or is there a problem further down the line? Blue Bird says new gauge is almost $400 so don't want to order until I'm sure. with the engine off and brake applied there are no air leaks

Does the air gauge return to normal air level while your foot is still on the brake pedal, or after you release the brake pedal?

When the engine is running, and brakes are applied, does it leak any air then? You will need a helper to determine the second answer.
Rich
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Old 08-31-2022, 01:04 PM   #4
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so if you hold down the brake and do not release, it will go way down, pressure alarm goes off, then returns back to normal pressure reading. not sure if its leaking when the bus is running. will have to get a helper to determine that. the gauge does this while the bus is running or not though
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Old 08-31-2022, 02:09 PM   #5
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Rich asked good questions...and with more info, you'll find good help here. But I think it's safe to say this isn't just a gauge issue. Yes, the gauge could be bad...that's possible. But the low-pressure sensor on the low air alarm is a separate sensor (it's a switch, not a variable resistance sensor for the gauge). Since the gauge and the low-pressure alarm are separate, and both telling you that you've got low air, I think it's safe to assume the issue is actually a low air/leak situation.
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Old 08-31-2022, 02:38 PM   #6
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so if you hold down the brake and do not release, it will go way down, pressure alarm goes off, then returns back to normal pressure reading. not sure if its leaking when the bus is running. will have to get a helper to determine that. the gauge does this while the bus is running or not though
When you say 'returns back to normal pressure reading'...

1) what exactly do you define as a normal pressure reading, and
2) how long in seconds does it take to get back to it after releasing the brake? Does it instantly pop back up, or build pressure gradually?

I'm guessing it's the latter, that your gauge is working, and it's telling you you've got a problem. But curious to hear your input.

Whatever you do, please don't drive the bus at all until this issue is resolved.
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Old 08-31-2022, 08:00 PM   #7
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Old 08-31-2022, 08:05 PM   #8
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If you press and hold the brake pedal and lose air pressure then you most likely have an air leak somewhere. It's usually the "pancake" in the brake chamber but I've had situations where a fitting corroded and broke off.

If you have someone hold the pedal down (or use a 2x4 or piece of pipe wedged against the seat) and walk around the bus you should hear the hissing.

I'd get a mechanic to look at it before driving anywhere.

-Kevin
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Old 09-01-2022, 07:03 AM   #9
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Do you have an air dryer? What if your tank is full of water, drained it lately?
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Old 09-01-2022, 08:09 AM   #10
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Vision or all american?
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Old 09-01-2022, 09:09 AM   #11
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I was going to try and add a video but can't figure that out. It's an All American. When pressing the brake the pressure drops from 120 to like 35 psi in about 2 seconds. The alarm sounds almost immediately. Not just when it reaches low air pressure on the gauge. Then after a couple seconds it starts to climb back up to about 100 psi or so and takes about 4 seconds to do so. Roughly where the other gauge is at. Not a gradual climb like normal air build up when starting it up, and this all happens while holding the brake down and not releasing by the way
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Old 09-01-2022, 09:31 AM   #12
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Okay , all american has the old dash where the warning light and gauge is separate, I'd be surprised if the gauge was the issue if the light comes on too.

Look for air leaks, and then maybe a restriction in the check valve on the tank. Better yet, install gauges on the actual tanks and verify the issue as as start. If the gauges on the tanks don't mimic what the cab gauges show, then start looking elsewhere. You can remove the hoses from the dash gauges to verify what's going on, before spending 400 to replace them. When was the last time the tanks were drained? If they're full of water they can cause goofy gauge action too.
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Old 09-01-2022, 10:01 AM   #13
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Quote:
Originally Posted by BusMech View Post
I was going to try and add a video but can't figure that out. It's an All American. When pressing the brake the pressure drops from 120 to like 35 psi in about 2 seconds. The alarm sounds almost immediately. Not just when it reaches low air pressure on the gauge. Then after a couple seconds it starts to climb back up to about 100 psi or so and takes about 4 seconds to do so. Roughly where the other gauge is at. Not a gradual climb like normal air build up when starting it up, and this all happens while holding the brake down and not releasing by the way
Thanks for this info. Sounds odd, but if it's accurate, I think it will prove valuable in diagnosing the problem. But words like 'roughly' and 'about' cause me concern. Could you please bring the bus up to full pressure, while running at idle, and then precisely document the following data points (exactly):

1) your full pressure reading (governor cut-out), psi

Now apply full breaking pressure, and keep it held down, and document:

1) the number of seconds passed before the alarm sounds.
2) the number of seconds it takes for the pressure to drop to its lowest point before starting to climb
3) the gauge reading at that lowest point (psi)
4) the number of seconds it takes from the lowest point until the alarm goes off
5) the numbers of seconds it takes from the lowest point until full pressure ( governor cut-out) is reached.

Do that a few times and document if your reading are consistent or if things change each time you try.
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Old 09-18-2022, 12:22 PM   #14
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there

is a few items that may cause this.
1 the air tank is very full of water or ?? reducing the volume of air available.
2 the hose from the air tank to the brake valve is restricted/ kinked/ pinched etc. If the pressure gage and low air pressure sensor are connected into the supply hose from the air tank to the brake valve and down stream of the restriction, then applying the brake will immediately drop the pressure. The pressure will then rebuild in the supply hose if the is no further demand for air.

I doubt that your gage or low pressure sensor is an issue. Don't shoot the messenger!
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Old 09-18-2022, 08:44 PM   #15
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You know I have seen compressors go bad and fill the air system with engine oil, maybe your air tank is full of engine oil, any unusual oil consumption?
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