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Old 11-01-2009, 12:33 AM   #1
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Air Controlled Gas Pedal

I noticed today something that might be normal but I've never experienced it so I wanted to get some feedback on it. Immediately after cranking the bus my air pressure shows good. But my gas/accelerator pedal is not responsive for about 30 seconds. taping the pedal i hear a small release of air...tapping it few times during the 30 seconds does nothing...then it starts working normal.

Usually i have let the bus sit idle for a minute or two before pressing the gas so it may have always done this...i've only had the bus for 2 months. any thoughts.

Marc
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Old 11-01-2009, 12:40 AM   #2
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Re: Air Controlled Gas Pedal

Marc, I have the air pedal on my Cat 3208 Turbo & the pedal will not work until the air pressure builds up to at least 50lbs. If there is air in the system (over 50lbs) it will work at startup. Is this occuring when your pressure is low & the red light or buzzer is sounding after starting it up? I just reread it... What is your pressure at when you say it shows good?

Jonathan
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Old 11-01-2009, 04:29 PM   #3
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Re: Air Controlled Gas Pedal

Quote:
Originally Posted by emberglow
This is just speculation, but I've wondered if there might be a safety valve of some sort that prevents air getting to the "accessories" (non-brakes) until the pressure is built up. That would explain why your air throttles are unresponsive for a little while. The "small release of air" Marc describes could be the last of the pressure in the air throttle line escaping when the valve (pedal) is opened? Do any other air-operated accessories (door, driver's seat, etc.) not work during this time?

)
that is true. I believe it is a DOT standard that only the brakes get air until the system reaches 60 psi.
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Old 11-01-2009, 08:04 PM   #4
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Re: Air Controlled Gas Pedal

The air pressure gauge should have two needles in it or two separate gauges. One for brakes and one for everything else. I wouldn't think anything will work properly until the air pressure is at normal operating pressure.
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Old 11-01-2009, 09:22 PM   #5
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Re: Air Controlled Gas Pedal

that is correct, nothing else that is air driven works until the brakes have reached the minimum pressure...
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Old 11-01-2009, 09:58 PM   #6
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Re: Air Controlled Gas Pedal

I'm pretty sure my pressure was well over 100psi...but i'll have to confirm it tomorrow.
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Old 11-01-2009, 11:05 PM   #7
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Re: Air Controlled Gas Pedal

It's quite possible that it's simply designed to keep you from revving the engine right away in an effort to make sure that oil has reached everywhere it needs to and that the idle has stabilized a little anyway. I've seen this in other places. For instance, when GM sells standalone PCM's to run marine engines they do not program a fuel pump relay enabling circuit for cranking like they do in cars. Instead, they rely on the oil pressure switch to enable the relay during cranking. The reason is that boat guys tend to pin the throttle as soon as things start and they want to make sure it has oil first. Check it out sometime. I bet you'll notice a cold boat has a longer cranking time than the very same engine in a car or truck.
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Old 11-02-2009, 10:16 AM   #8
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Re: Air Controlled Gas Pedal

My Gillig with a CAT 3208 does the same thing; no throttle 'til the air builds up to the proper pressure. This bus has a high idle switch, which ups the RPM's to around 1000 - 1100 using a solenoid. I think the idea is to run at a higher speed to build up air pressure quicker.
I had an old 1957 PD4104 for a short period. These old buses have an even odder low air quirk.....instead of a solenoid to shut off fuel and stop the engine like most of our Skoolies, the PD4104 has an air actuated stop. Guess what...start her up with no air pressure, you can't shut it off until air builds up!!
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