Shut off actuator problem

[Jls8177]

Greetings,

New to the bus world. I bought an old MCI Challenger 1972? that is already converted to an RV and I'm having a mechanical issue. It has the 8v71. The bus runs great except it starting randomly shutting off while driving. The shut off actuator is activating on the motor and I can't figure out why.

Here are a couple of things I've noticed recently that I'm not sure how to fix: Something has changed in the air system, It is dumping air out some sort of overflow valve by the rear wheels. I'm assuming this means that whatever controls the air pressure to keep it at 120 psi (or whatever it is supposed to be) and kick back on at 90 psi is not functioning properly.

Could this extra air pressure be causing that air actuator to engage to shut down the motor?

I'm not opposed to having someone look at it, but I really don't know who to take it to and it will only run for about 10 minutes before it shuts off. Then I let it sit for 10 minutes and it will start again and go for a couple of minutes before shutting down again. Wash, rinse repeat until I get home:(

Anyways, any advice is appreciated. I have the service manual but it assumes that I know what I'm looking at I'm mechanically inclined and not afraid to dig in to something but I've never had a diesel with air...

Thanks.
Jason in Baltimore

[Dave5Cs]

Sounds like the Governor is not operating correctly. It is a 12.00 part at NAPA. It will be back on top by your compressor head. It will have three air lines going to it. Also make sure the switches at the back start box are on the correct possition, either front start or rear start depending on what you are starting from.
Dave5Cs

[gumpy]

The air issue is possibly a leveling valve leaking. These would be in front of the rear axle. One on each side.

The shutdown is not related, but probably caused by a faulty sensor in your safety shutdown system. Most likely temp stat in either side of the head manifold. There should be three. Two are temp stats for the safety system. The third is you gauge sensor. Disconnect each one of the temp stats one at a time. If the problem goes away while disconnected, may need to replace that stat. Often they get loose internally and short to ground. However, it could also be caused by a stuck thermostat which is causing an overheat condition and activating a good temp stat. This would play into your statement that you let it sit 10 minutes and it starts again. So you need to make sure that's not the real cause or you could toast an engine. IR thermometer will help here.

It could also be a low oil sensor, or low water level sensor if so equipped.

Of course this assumes your safety shutdown system is still intact and functional. Many were bypassed by "all-knowing" mechanics.

[Jls8177]

Thank you all, I'll check these things. Anyway to test T-stats without temp gun? Not sure if I have water/oil lever or any of these temp sensors. Thanks for the info, I'll let you know what I find.
Jason

[bevans6]

The shut-off skinner valve (electric switch that controls the air to the actual shut-off air cylinder) is normally open (air flows, engine does not run) and only closes when there is both a good ground and positive voltage from the engine run switch and the master on switch.  The skinner valve has both a ground wire and a positive control wire and they are both prone to perishing.  It is mounted on the side of the engine bay just forward of the rear control box, there are two of them and you can figure out which one it is by tracing the air hose from the air cylinder.

Brian

[bevans6]

There are many many things in the rear axle bay that can cause leaks.  I had the parking brake pressure regulator fail and it leaked a bunch.  You just have to look, listen and see which thing it is.  Off the top of my head there are two leveling valves, two or four brake cannisters, the relay valve, the inversion valve, the aforementioned pressure regulator, and a crap-load of hoses that could have failed.

If your skinner valve/switch/ground described above has failed and the skinner valve does not get electricity and ground, a common failure if you also have an air leak is for all the air to leak out so the engine shut down cylinder can't work (it needs air pressure over 60 psi to work) so the engine starts just fine and builds air for 10 minutes then finally gets to 60 psi, the shut down cylinder finally gets air and shuts off the engine.

Brian