Air system question

kdaniels

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Picked up a M923A2 a few months back and I have noticed that as I have ran it to build air pressure it makes a loud honking sound , sounds like a pissed off goose, from underneath the truck. Sounds like it comes from the air dryer area . Only does it a few times during a half hour idle....so I'm wondering if this is "normal".

Reason for the question is all has been well with the truck building pressure until yesterday and now it will not build beyond 10psi. I will be checking (replacing) the head gasket on the compressor this week but if said noise isn't normal than the list just got alittle longer.

THANKS

SEMPER FI !!



keith
 
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Artisan

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The best way I found to test the air system, and you might want to do this before you fix anything
is to add a Schrader valve into the system and w/ the engine off simply pump it up w/ your shop compressor and
listen for air leaking. On my M925A1 I just unscrewed that pressure relief valve over the
front rear axle DS and added a brass tee, reinstalled the pressure relief and added
a Schrader valve. It pumps both ways doing it this way.

Make sure all Gladhand's are valved off except the PS front (it has a vent)

Make sure all 4 drain downs that you have been faithfully draining down in proper order are closed.

Mine honks when you release the parking brake.

All of your emergency shut off valves "should" be open.

If all seems tight and still issues I might look at the Govenor, it is like a $20 part,
it looks like a bit of a beeotch to replace but doable.

But first thing you need to do is not listen to any of us, but pull out the TM's
and read them. They are your best friend w/ these trucks.
 
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kdaniels

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I have not serviced anything on the air system yet other than routine air drains. Just finished engine and tranny service last month and the fuel system the month before. I will print off the trouble shooting section of the TM this week for the air system and go from there. Just been reviewing some of the posts from other members and the "common" issues encountered.

Air dryer filter will be on the list to service Porky Thanks..


SEMPER FI !!


keith
 

Recovry4x4

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Is there a logical explanation for the thread title? Nobody will ever find the info in this thread by searching. Thread titles should have the topic of the thread in them.
 

kdaniels

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I have no idea what happen to the title of the post when I submitted it. Sorry...must have been flagged by the "N"SA.



Semper Fi !


keith
 

plode

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Not to threadjack, but since we're on this topic...Are they supposed to honk when you engage the parking brake, or when you release it? Artisan said in the post above that his honks when he releases it, mine honks when I engage it.
 

RustyM923

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Not to threadjack, but since we're on this topic...Are they supposed to honk when you engage the parking brake, or when you release it? Artisan said in the post above that his honks when he releases it, mine honks when I engage it.
Engage the spring brakes = the noise.
 

MtnSnow

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Yep...When you apply (aka "set") the parking brake you should hear air venting (aka honk)

When you release (lift your foot) from the regular service brake pedal you should hear air venting but when stepping on the foot pedal you should not hear air "venting" (you might hear a slight air movement momentarily but no "venting")

If you are unsure of what sounds "normal" get with someone who is AIR BRAKE trained/certified and get trained on what is normal and what is not.
 

RustyM923

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I think setting the parking brake (venting the spring brake chambers) sounds more like a goose being strangled.

The CTIS purging air sounds like a long honk.

The CTIS checking pressure sounds like a regular honk.

The air dryer purging when the tanks hit max pressure sounds like an air dryer purging (PTSHHHHHhhhhhhssssssss <--this)
 

plode

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Yep...When you apply (aka "set") the parking brake you should hear air venting (aka honk)

When you release (lift your foot) from the regular service brake pedal you should hear air venting but when stepping on the foot pedal you should not hear air "venting" (you might hear a slight air movement momentarily but no "venting")

If you are unsure of what sounds "normal" get with someone who is AIR BRAKE trained/certified and get trained on what is normal and what is not.
My friend who has had his CDL for years, drives a propane delivery truck said he has never heard any honking/goose getting strangled sound from any truck he has ever driven before. He told me something must be wrong with my truck(but I knew it was normal from reading on here). And neither have I ever heard it, until I got the M923a2. I've driven fire trucks for 10 years...they only have the typical "woosh" sounds you hear from big rigs/dump trucks etc.
 

quickfarms

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My friend who has had his CDL for years, drives a propane delivery truck said he has never heard any honking/goose getting strangled sound from any truck he has ever driven before. He told me something must be wrong with my truck(but I knew it was normal from reading on here). And neither have I ever heard it, until I got the M923a2. I've driven fire trucks for 10 years...they only have the typical "woosh" sounds you hear from big rigs/dump trucks etc.
Civilian trucks vent the air from multiple locations under the vehicle.

This vehicle has the vent line plumbed to the air intake.

This causes a different sound.
 

kdaniels

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Location
Dothan, Al
Ok...I had a few minutes this morning to go over TM and check lines on the compressor. Manual isn't totally clear BUT it looks like the lines running out of the compressor on the back side are coolant lines as they run back into the block and head of the engine. The small metal line that comes out of the TOP of the compressor head and feeds into the gov on the fire wall had no air coming out of it when the engine was running. This can't be the main air line....it's much smaller than the hard line on my M923.

IF it is...my compressor is shot.


SEMPER FI

keith
 

hklvette

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The air line at the top of the compressor head is the unloader line. It comes from the governor and should ONLY see pressure when the truck's tanks reach 120-130psi.

The best advice is to hook an off-board air supply to the truck while it is off. The front emergency glad-hand (driver's side) is great for this. It makes leaks 100x easier to find when you don't have the sound of the engine running drowning everything out. I have an air line used to supply air tools from the truck and a double-ended connector to allow two air hoses to be joined together to supply the truck. I'll have to get a picture some time.
 

kdaniels

Member
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Location
Dothan, Al
Thanks Henry...I will try that. I have an air kit for tire inflation that I can rig up to my air compressor in the shop.



SEMPER FI !

keith
 

kdaniels

Member
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Location
Dothan, Al
Henry !! I got the time this morning to run a hose from my shop air compressor to the drivers side glad hand. It took about 3-4 minutes to bring the truck tanks up to 120 psi. As I was attaching the glad hand I noticed the valve was in the open postion. Long and the short of it is at some point while I have been working on the vehicle I have opened the valve without noticing it. This is easy to do as the valve opens REALLY easily....and with the seal being toast in the glad hand my air just would'nt build up enough to produce a "hiss" you could track down.

Your suggestion on the trouble shooting method worked !!! THANK YOU SIR ....this one goes in my "don't you feel silly now" file.


SEMPER FI !!


keith
 
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