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Old 10-12-2008, 02:19 PM   #1
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Air pressure to the gas pedal

If i missed the post covering this I am sorry for reposting I have searched high and low and just cant seem to find it.

With that said here is my problem/issue/holdup.

I have a 1982 GMC 40foot transit bus that I am trying to convert. I drove it from Portland to Seattle with no problems. Except that I have no air pressure to my gas pedal when I turn the headlights on. I can drive with the clearance lights on. I can drive with the interior lights on. I can even drive with both doors open. I cant drive with headlights it kills the air pressure to the pedal whenever I try to do so. Has anyone had this problem or heard of it.

I wanted my first post to be an introduction with pictures and all on my project but I have reached a new level of frustration trying to fix this. If any feels like being an angel today I sure could use some help.
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Old 10-12-2008, 10:12 PM   #2
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Re: Air pressure to the gas pedal

my last bus was a transit style, and the gas pedal lost air pressure when conditions were right....ie: parking brake applied,

anyhow....

to fix your problem, you need to trace the air line from the gas pedal to the electric solenoid that shuts off the air to the pedal. all the solenoids might be together in a fancy box, or it might be mounted by itself either under the bus or in a side panel.

it sounds like a wire from your headlights was somehow inadvertently connected to this gas pedal shutoff solenoid. You really don't even need the solenoid to function, you could just make it always on. it may require 12 volts to stay on, or it may be on all the time in which case you'd be all set by just snipping the wire going to the solenoid.

alternatively you could figure out which air line is the "inlet" for the gas pedal valve, and run a new air line after capping the old one....it might however have a pressure regulator in it so it does not function at full pressure, this would be important to know.......

does that make sense? or should i go into more detail?
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Old 10-15-2008, 05:54 PM   #3
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Re: Air pressure to the gas pedal

I appreciate the time you took to respond to me. But this is my first bus and I hate to say it but i feel like I just attempted to read japanese. But I think I got the just of it.


Umm how would i decipher whether or not it has a pressure regulator in it
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Old 10-15-2008, 11:36 PM   #4
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Re: Air pressure to the gas pedal

if you're not handy with tools and some basic electrical and plumbing, it's probably a good idea to let someone qualified to do the work mess around with critical bus systems like brakes, the throttle etc.....however, the more you work on stuff, the better your understanding becomes of your bus as a whole. I wouldn't be nearly as afraid to try and "fix" a broken throttle as i would the braking system.

if you examine your gas pedal assembly carefully, you'll see that there are 2 hoses that attach to it. They may connect underneath through the floor. Assuming you have an air throttle......

the air line if you need to buy any for stuff in your bus is available at any big truck (semi truck) parts place. every big city has at least one of these places. i think it's just called "dot airline" it's not too pricey. anyplace that sells the airline should also sell the connectors, tee's elbow's etc that you may also need. the hardware store also has fittings and many times at a cheaper price.

The nuts that connect the air line to the throttle assembly, or any other elbow, connector etc is usually standard "npt" thread. This is the same thread used on black (and brass/galvanized...use brass!) pipe from the hardware. If it's not npt, you need to adapt to npt thread so that you can connect your air gauge. it might be compression or flair....but as far as i know, those are the only three common threads used in air systems.

there are prob more than one way to find out if it has a pressure regulator in that particular air circuit......i think the easiest way is to disconnect the air line from the gas pedal assembly, and hook the line to a pressure guage. Make sure you use the air line going "in" to the pedal assmbly, and not the on going "out" you won't hurt anything, but the expierment won't work.

You can find a guage at home depot or any other hardware that sells plumbing stuff...you'll pay less money if you can go to harbor freight or some other junk tool store. a gauge like this:



It may be a cheaper alternative to put a shrader? valve (tire valve stem)



on the end of it, then check the pressure with a tire gauge that will read up to at least 120 psi.

connect the valve, start the bus and let it build up to max air pressure, then see if the guage you just installed reads fairly close to the gauges on the dash. If the stock gauge on the dash says 120 psi, and the gas pedal gauge reads 40 psi ( or anythign vastly different from the other gauge) then there is a regulator in the circuit.

you could skip the above process:

it may just be easier to find the solenoid valve that turns on/off the air pressure going to the gas pedal. you can shut the bus off, chock the drive wheels, then crawl around under neath and listen carefully while someone inside the bus actuates the switch that seems to be killing your gas pedal. probably the ignition has to be on, the parking brake might have to be off, the doors closed etc. whatever it usually takes for the gas pedal to work. then have your buddy turn on/off the headlight switch over and over again until you locate the thing that has air lines and electric wires running to it and "clunks" or "clicks" every time the switch is turned on/off. Most solenoid switches make a fair bit of noise when the turn on/off. Once you locate this valve, you need to figure out a couple of things....get a volt meter, and find out wich of the 2 wires have 12 volts + going to it when the switch is either on/off. If the wire has 12 v+ when the switch is turned on, all you need to do it cut the wire and tape the ends. Gas pedal should work all the time. The other possibility is that it has 12 volts when the switch is turned off. in which case you'll still need to cut the wire, but also run a 12 volt + wire from your ignition to the part of the wire you just cut that connects to the soleniod.

does that make it just about as clear as mud?
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Old 10-16-2008, 09:26 AM   #5
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Re: Air pressure to the gas pedal

lapeer is on the money with the troubleshooting, just to clear up abreviations,
npt=national pipe thread, it is tapered and wedges together as you tighten it up, you should use a thread sealant on the threads.
dot=department of transportation, dot fittings and plumbing parts are engineered to prevent vibration damage to safety related components in mobile applications such as truck and bus air systems, it is highly advisable to pay the price and use the correctly labeled/speced parts in your critical air systems. if you choose to go with generic hardware on noncritical systems, ie,suspension,horn,plumbing etc. no one elses safety is in jeopardy.
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Old 10-16-2008, 09:34 AM   #6
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Re: Air pressure to the gas pedal

Awesome info guys! Even though I don't have air throttle, I still learned some stuff I expect to be useful!

Thanks!
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Old 10-17-2008, 03:47 PM   #7
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Re: Air pressure to the gas pedal

Wow most times you find a forum like this conceived for whatever topic out there. But whenver you post you rarely get ppl actually trying to help you. Thank you very much skoolie net for for being genuine. And thanks guys for in depth help on my problem. Appreciate it.
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Old 11-12-2008, 10:52 PM   #8
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Re: Air pressure to the gas pedal

This problem sounds like it is most likely a bad ground on one of the lights that comes on in the headlight position, are any of the lights inoperative or appearing dim?
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