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I have an air leak only when the ignition is on under my day cab. I have tried everything, to include soapy water in a spray bottle. I can not seem to figure this out. It only leaks with ignition on and will stop after I shut the truck down a minute and a half later with no substantial loss in pressure.

I am looking for help as this air leak is getting louder but I can not locate. It's only ignition on, what should I look for? Shifter? PTO is not being used? But never leaked before?

Thanks in advance,

QC Driver

P.S. It's been a very frustrating 3 week ordeal.

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basically check all systems that you know that have air/electric to them. The problem area is most likely solenoid controled as the leak stops when you cut electrical power. Even though pto is not being used,would be good idea to check it out also.

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basically check all systems that you know that have air/electric to them. The problem area is most likely solenoid controled as the leak stops when you cut electrical power. Even though pto is not being used,would be good idea to check it out also.

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Ok now think about it if its only hissing when the truck is running then have a look at your charge/wet system. The primary and secondary air tanks have pressure protection valves and check valves to prevent the air from running back into the wet tank. the two gauges on the dash only register pressure from those two tanks, not the wet tank. You might have a leak in the charge system or wet tank.

How about this little test, all air accessories like horns and seats are ran off the wet system. When the hissing stops after you shut the truck down, try to blow the horn or operate the air seat. If nothing works then the charging system has a leak somewhere.

If you have a shop air compressor find the wet tank (which is easy as it should be the only tank hooked to the air dryer and governor.) and get air to it by unscrewing the drain valve or a plugged port and screw a coupling or hose to it. Charge the system using your shop compressor (make sure your compressor only supplies about 100-120 PSI, use a regulator is necessary). This method will make tracing the leak easier as the noisy engine wont be running. Start at the governor/compressor then check around the air dryer. Sometimes those purge ports on the dryers get stuck open. Check all air lines and accessory lines for PTO's, transmission, air suspension, air seats, air horns etc... Maybe the break is in the middle of a run from one air device to another so spray over all the hoses.

Let us know of your progress.

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I dont know what kind or what year truck you got but let me play like it is a late model so when ign. is on it may be a air fan problem.more infro will help alot these other guys may know what you have but i domt glenn

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Sometimes air leaks can be a b___ to find. I would guess the air dryer purge valve is leaking. Could be a faulty O-ring in the purge valve. Or a sticking air governor will cause the purge valve to hang open after it cycles. A leaking fan clutch will stop leaking within 2-3 seconds after the key is off.

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Sometimes air leaks can be a b___ to find. I would guess the air dryer purge valve is leaking. Could be a faulty O-ring in the purge valve. Or a sticking air governor will cause the purge valve to hang open after it cycles. A leaking fan clutch will stop leaking within 2-3 seconds after the key is off.

New Gov and Dryer

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Ok now think about it if its only hissing when the truck is running then have a look at your charge/wet system. The primary and secondary air tanks have pressure protection valves and check valves to prevent the air from running back into the wet tank. the two gauges on the dash only register pressure from those two tanks, not the wet tank. You might have a leak in the charge system or wet tank.

How about this little test, all air accessories like horns and seats are ran off the wet system. When the hissing stops after you shut the truck down, try to blow the horn or operate the air seat. If nothing works then the charging system has a leak somewhere.

If you have a shop air compressor find the wet tank (which is easy as it should be the only tank hooked to the air dryer and governor.) and get air to it by unscrewing the drain valve or a plugged port and screw a coupling or hose to it. Charge the system using your shop compressor (make sure your compressor only supplies about 100-120 PSI, use a regulator is necessary). This method will make tracing the leak easier as the noisy engine wont be running. Start at the governor/compressor then check around the air dryer. Sometimes those purge ports on the dryers get stuck open. Check all air lines and accessory lines for PTO's, transmission, air suspension, air seats, air horns etc... Maybe the break is in the middle of a run from one air device to another so spray over all the hoses.

Let us know of your progress.

This is my shop compressor. Would you just hook up quick conects with 3/8 air line or 1/2? I ask this because I believe it's a small line.?

post-307-1186530891_thumb.jpg

Edited by QCDriver

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The leak is NO WHERE BY THE GOV. OR DRYER, its on the other side. Driver side 2001 Maxi CH613. Meritor Trans, brakes and rears.

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The leak is NO WHERE BY THE GOV. OR DRYER, its on the other side. Driver side 2001 Maxi CH613. Meritor Trans, brakes and rears.

Your saying its inside the engine compartment or next to the cab?

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Your saying its inside the engine compartment or next to the cab?

To help out these guys that are helping you, you may get some one to turn the ign. on and off while you get up close and listen untill you find out if it is inside or out side then once you know that get back on here and tell us but i have found it is hard to tell what the problem is unless you are there and you are there so if i was you and can hear the leak then put your hand in there and start to pull and tough and rub around things untill you hear a change and now you know where it is a little now look with a good light for a oily spot where air may be leaking becuse some times there is a little oil in the air and it is left on the leaking spot and now dirt gets mix on that and it makes a dirty spot. I am not sure but i think you said it leaks with key on but do the engine need be runing? glenn

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the meritor trans may be eletric instead of air on the range now if it is get under there and see if air is coming out of the trans, breather and if so you know where it is now but if so you should have a shifting problem. glenn

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Finally found it. Main compressor line. After installing new comp and line missed 1 tie down. Rubbed a hole in line going thru frame rail, making it sound

like it's coming from left side.

Oh well live and learn. Thanks for help,

Doug

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