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Old 05-02-2016, 09:55 PM   #1
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Join Date: Jan 2016
Location: Tepme AZ
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Year: 1976
Coachwork: Crown
Engine: Detroit Diesel 6-71
Rated Cap: 52 pax
Air system and brakes

Hello Skoolies! How are you all? While the conversion is coming along slowly, as in no real work has taken place but I do have a garage full of parts, I have been enjoying driving it around town just for fun and to learn how to handle the big girl. Here are a few things I have noticed. During start up at about 55 psi I can here a lot of air movement from under the dash. I have felt around down there and cant feel any obvious leaks. I don't think it makes these sounds on the road but its loud and hard to tell. Its takes a while and while I have never timed it, it feels like forever sitting there waiting for it to finally get to 120 and cut off. after that it never makes it below about 90 psi before it goes back to 120. After shutdown it looses air fast through the air actuated devise that pushes out to shut off the fuel pump. It is a DD 671 by the way. it seems within a minute or two for the air to go from 120 psi to 60 and it seems to hold there. I think the thing I a trying to describe is the culprit. it leaks slowly during operation but could fill a tire after the engine is turned off. what is this device? it has no part numbers on it. Where can I get a new one? Thanks in advance for your help!





P.S. I really hope the pics show up, it will make this whole conversation much easier...
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Old 05-02-2016, 11:05 PM   #2
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I put a lot of faith in Cowlitzcoach he seems to have a great diverse knowledge of the different motors. Hope he sees this soon. I'd like to know too.
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Old 05-03-2016, 10:05 AM   #3
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That looks like an air operated shut off. It's located on the governor of the engine and has air pressure applied to it when you want to turn the engine off. Use a soap and water solution to find your leaks under dash. There are numerous solenoids and valves which can leak under there. The reason it builds up to 55 psi quickly is because all of your accessories will start receiving air pressure about then. It's a fail safe so that you can still brake properly if your accessories develop a leak or you use them too much.
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Old 05-03-2016, 11:34 AM   #4
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To start with, regardless of the make of the bus, the air pressure gauges on the dashboard tell you how much air pressure is in your service brake air tanks. Those gauges do not reflect how much, if any, air pressure is in your accessory tank.

Your Crown should have four tanks in the air system. Buses that do not have air operated accessories probably will not have a fourth tank.

The first tank is the wet tank. It is the first tank in the series and is there to "catch" moisture before it goes onto the other tanks. If you do not have an air dryer and automatic spitter valves there should be an easily reached manual valve that will drain that tank. This valve should be opened at the end of each day and left open over night to allow all of the moisture to drain out. You really do not want moisture of any kind getting into your brake chambers or accessories.

The next two tanks inline provide air to the front and rear brakes.

The last tank provides air to the accessories like the engine shut off, wipers, service door, driver's seat, air horn, suspension, etc.

They are plumbed that way so that the service brakes get the air first. Each tank has a one way valve in so that if there is a leak in any of the circuits it won't bleed the air off of the other circuits.

When you start your engine it fills the tanks in series and the last one to fill is the fourth tank. You may have noticed that the engine will not shut off if you have less than 60 PSI and depending upon the bus it may take 90 PSI before the engine shut off will work. The amount of air pressure it takes is determined by how much pressure has to be in the service brake tanks before the accessory tank starts to fill.

The air operated engine shut off valve needs to not leak. If it leaks it may not have enough "strength" to pull the rack back to shut off the engine.

If your air system drops below 90 PSI in less than five minutes after you shut off the engine then you have a serious leak that needs to be addressed before you do anything else. I seriously doubt you will ever be stopped by a weight cop (unless you happen to go into British Columbia) but that much air loss is generally considered a red tag, don't move it, fix it immediately type of problem.

Many times the culprit will be a loose fitting and can be fixed by retightening.
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Old 05-03-2016, 11:52 AM   #5
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I have been underneath her and my particular Crown has 5 tanks. Three up front at about the front axle and two at the back just in front of the rear axle. I can say yes it leaks down below 90 rapidly after shut down. I figured out how to shut it off without this valve, simply doing it by hand. I have inspected the air lines in the rear of the bus and they seem solid and tight.


@cowlitzcoach, Do you know where I can get the air operated shut off valve? I know this part needs to be replaced I just need to know exactly which one. I have seen themonline I just don't know the correct one to get or from where..


Dose anyone know where this part can be found?


Thanks guys
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Old 05-03-2016, 12:37 PM   #6
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Quote:
Originally Posted by cowlitzcoach View Post
The first tank is the wet tank. It is the first tank in the series and is there to "catch" moisture before it goes onto the other tanks. If you do not have an air dryer and automatic spitter valves there should be an easily reached manual valve that will drain that tank. This valve should be opened at the end of each day and left open over night to allow all of the moisture to drain out. You really do not want moisture of any kind getting into your brake chambers or accessories.

The next two tanks inline provide air to the front and rear brakes.

The last tank provides air to the accessories like the engine shut off, wipers, service door, driver's seat, air horn, suspension, etc.

They are plumbed that way so that the service brakes get the air first. Each tank has a one way valve in so that if there is a leak in any of the circuits it won't bleed the air off of the other circuits.
I've never seen that. Is that a crown thing? Most of what I've seen have had the wet tank, a primary tank, and 1 or more secondary tanks. The last two items being filled with no priority to either. They then usually run the accessories through a protection valve that is mounted to either the secondary tank or the wet tank. Everything around here has a drier so the wet tank is very rarely wet.

Quote:
Originally Posted by cowlitzcoach View Post
If your air system drops below 90 PSI in less than five minutes after you shut off the engine then you have a serious leak that needs to be addressed before you do anything else. I seriously doubt you will ever be stopped by a weight cop (unless you happen to go into British Columbia) but that much air loss is generally considered a red tag, don't move it, fix it immediately type of problem.
I agree, I'm not sure of the exact spec off hand (something like a couple psi over 5 minutes time) but if the needle moves anything significant over a couple minutes, you got a leak that should be fixed.
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Old 05-03-2016, 02:53 PM   #7
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Something guys used to do in the army was to adjust the drain valve on the wet tank so it just barely leaks. It was a way to avoid crawling under the truck each night to drain the tanks and it kept the water out of the system. It's worth checking.
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Old 05-05-2016, 11:32 AM   #8
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Join Date: Feb 2014
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Quote:
Originally Posted by Robin97396 View Post
Something guys used to do in the army was to adjust the drain valve on the wet tank so it just barely leaks. It was a way to avoid crawling under the truck each night to drain the tanks and it kept the water out of the system. It's worth checking.
Or one could simply install an inline dryer...
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Old 05-05-2016, 11:43 AM   #9
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On my Crown I also heard air under the dash, turned out to be the lines going to the windshield wiper motors, their quick disconnects. Take a look there.
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Old 05-05-2016, 09:23 PM   #10
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Location: Tepme AZ
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Year: 1976
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Rated Cap: 52 pax
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Originally Posted by Stu & Filo. T View Post
On my Crown I also heard air under the dash, turned out to be the lines going to the windshield wiper motors, their quick disconnects. Take a look there.
You know I have always wondered about those things. I will be sure to get my soap bottle and check hose and everything under the dash. Also, the part in the pictures is a fuel stop solenoid. Would you be able to look at the one on your crown and see if it has an obvious part number on it? Don't go through too much trouble looking for it but I would be most appreciative if you found something,

I have been able to find some universal engine stop solenoids but they are all electric and just a push button. I cant find any that are air operated for any application. I would have to turn the key off then push a button to shut the bus down. Its not that pushing a button is too much work, but I would like to keep it operating as it did from the factory and not feel like some cheap backyard fix.
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