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Old 03-06-2016, 04:40 PM   #1
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bus will not shut down


I have a 88 crown with a 6/71 and she will not shut off when I turn the key to the off position, can someone explain to me how I can shut her down ? . thank you in advance
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Old 03-06-2016, 04:50 PM   #2
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i dont know the details of your motor but mine has a fuel shut off solenoid that goes bad. if you can find that solenoid, you should be able to pull it out and shutoff the fuel.

i've heard of throwing a towel into the air cleaner as well. never tried that one.

good luck
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Old 03-06-2016, 05:01 PM   #3
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Originally Posted by turf View Post
i dont know the details of your motor but mine has a fuel shut off solenoid that goes bad. if you can find that solenoid, you should be able to pull it out and shutoff the fuel.

i've heard of throwing a towel into the air cleaner as well. never tried that one.

good luck
thank you for your quick response. she shut off about 5 min ago ,but I do not know why, or where this fuel solenoid is .
Would they all be the same on this engine?
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Old 03-06-2016, 06:12 PM   #4
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It was her way of saying I want to keep moving..good luck
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Old 03-06-2016, 07:57 PM   #5
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Probably a mechanical lever or something. You gotta shut off the fuel on diesels to shut em off.
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Old 03-06-2016, 08:18 PM   #6
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Hello there

Hi. I have a 1976 Crown with 671 5 speed. Mine did that one time. I understand there is some sore of magneto of some sort in the fuel pump that can get stuck and it will not shut off. I put it in third and dumped the clutch to stall it out. My 76 hasn't done it again. I don't know if this is much help but stalling it seems to work.....
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Old 03-06-2016, 09:32 PM   #7
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Whatever you do, dont ever try to dump the clutch to stall out a 2 stroke Detroit!
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Old 03-07-2016, 06:37 AM   #8
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All 6-71 powered Crowns have an air powered piston actuator mounted on the top of the governor housing accessible from the left (driver) side engine door. The actuator pushes against the fuel shutoff lever in order to shut the engine down. That lever internally moves the fuel rack to zero fuel thereby shutting down the engine.

The governor is located just above the water manifold at the forward end of the engine. You'll see two levers mounted on shafts coming out toward you on the left side of the bus, when looking at the engine through the door. You'll notice the throttle linkage on one of the shafts, the other shaft has the fuel shutoff lever attached. A (usually red in the later builds) flexible colored plastic air line comes down to the air piston cylinder that pushes on the fuel shutoff lever mounted on the second shaft. The red plastic air line goes up to an electrically operated "Skinner" valve which is how the electric system controls the air shutoff piston.

The air logic requires that the bus air system be aired up in order to provide enough air pressure to operate and push the spring loaded piston against the shutoff lever. That means that you CANNOT shut the engine down until enough air pressure has built up(with the electrics). This may take as much as 5 minutes. That could be exactly what happened to you when you couldn't turn it off. If you tried to stop it right after starting it without any air in the system, nothing will happen until the air builds up. Just for fun, try doing that and see what happens. Start the bus with no air at all in it and turn the key off once it's running. It will run normally until enough air builds up to move the piston and then the shutoff lever. It may die slowly, or not, but it will shut off eventually. You should probably wait a week to make sure all the air is gone from all air tanks.

You can also reach in and manually move the shutoff lever by hand in order to shut it off. Clockwise as indicated by the direction that the air piston is pushing it. You can also move the throttle linkage to speed up the engine from here as well.

I'll mention a couple other bits of information regarding the Crowns and how to use the mechanics control panel located by the fan. The "safety" switch will disable the starter so that the bus won't start from the drivers seat. This of course to protect any mechanic working around rotating engine parts. This could sometimes cause you to think that the bus won't start, if the safety switch has been pulled down below. Another thing that will cause the bus not to start is if the neutral safety interlock switch on the transmission linkage is maladjusted, and thinks the bus is in gear. This will prevent a start as well, the way around this is to push and hold down the dashboard mounted "neutral safety overrule" button while cranking the engine.

The "ign" switch on the mechanics panel will turn on the ignition and allow for an engine start right from the mechanic panel. This way you don't need a key to start the engine, just do it from the mechanic panel. One caveat about running the bus using the under floor start panel, the Alternator will NOT be operating and no current will flow into the batteries. Take heed. Only use it for emergencies and don't run big electric loads. And the engine must be stopped from the mechanics panel, it overrides the key ignition switch.

One other thing that will definitely cause trouble starting normally is if the engine air damper is tripped, which causes a metal plate to drop down and seal off the engine air intake. This is tripped from the dash board by pushing the "emergency (stop)" button. All it takes is to do it once. It pulls a pawl out of a notch with an electric solenoid and allows the spring loaded damper shaft to slam shut. In order to reset the damper you need to open the left side engine door, and you'll see a button, little handle, or something like that, it depends on the build year, right through the left frame rail, and you give it a strong PULL, toward you, the left side of the bus. No need for rags in the air intake with this damper designed in. You can't really reach the air intake anyway, the blower is mounted on top of the engine, between the frame rails, just under the floor.

One other thing regarding stalling out the engine with the clutch while stopped. Besides being rather hard on your nice expensive 6-71 there is a more valid reason to not do that. Most people don't know it but a 2-stroke Detroit Diesel can actually run backwards. It's certainly not good for it and there isn't any oil pressure or fuel pressure for long either, but it can be done. It's usually initiated by a specific sequence of events whereby the drive-line components store up the stopping torque (much like a rubber band) and then release it back into the engine in the opposite direction which can start the engine off going backwards. The clutch will need to be depressed again of course for it to continue, but if the events are all done correctly it can be done. I know this from personal experience. I did it once many years ago with a GMC 4106 that was not very well maintained and all the drive-line and clutch assemblies were very loosey goosey. I was trying to pull away from the curb on a slight incline, and managed to stall the engine out, immediately the drive-line snapped it in the other direction and the engine took off backwards. It was very weird to look back and see engine smoke coming out of the air intakes and finally realize what was going on when the bus wouldn't pull forward. I stalled it with the clutch and left the clutch up to make sure it stopped for sure. It then re-started just fine, I took it to the shop and told them but it seemed to be OK. I've never seen or heard of any Crowns doing this, mostly because they have a very tight closely coupled drive-line configuration. But all Detroits can be made to run in reverse if things are done just exactly right. They were actually designed to run in either clockwise or counter-clockwise direction for Marine applications. They only need the appropriate camshaft, and one of the intermediate gears in the gear train are put into another position and they can be run in either direction. Maybe the oil pump too. Very nice design so you don't need a large inventory of spare parts on-board to work on both right and left hand running engines. Mostly all the exact same parts in each one.
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Old 03-07-2016, 09:30 PM   #9
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Thanks Crown Guy for the detailed and thorough explanation. I had a feeling the first few times she wouldn't shut down immediately it was some sort of cycle that needed to be completed first. But this last time she ran for almost an hour and I was starting to get concerned.
So glad to hear that it's all ok and part of the system design.
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Old 03-08-2016, 07:49 PM   #10
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An hour wait for it to shutdown isn't normal and indicates some kind of trouble. It could be any number of electrical issues where the circuit is not being made to the Skinner valve to activate and open the port to the shutdown air actuator. The air actuator itself may be gummed up and sticking and not moving freely when air is applied. This is definitely not normal behavior and should be investigated and the reason for it determined and resolved. Once air pressure is up there should not be any delay in shutdown. If there is, it is not normal, and means something is not working correctly. It may not be serious, and even easy to repair, but it needs to be looked into and resolved. One test is to give a listen when you turn the key to start the engine. You should hear a very definite air release "pop" from the Skinner valve as it opens the exhaust port and retracts the shutoff actuator piston. If you don't hear this (assuming air pressure is already up) that's a good place to start looking. The electrical connectors to the Skinner valve sometimes get dirty and causes poor connections. There's several places to start looking, but anyone with basic troubleshooting skills should be able to find the problem. At least you can be pretty sure the fix isn't going to be too hard or expensive. There just isn't that much that can be going wrong, a very simple and straightforward system with easily available replacement parts.
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