Engine Shutting Down/Not Starting Problem - GM 4108a

[richard5933]

Having a problem starting our 4108. It was an intermittent problem where occasionally the shut-off valve would remain closed and not allow fuel to flow. If I cycled the engine-run switch a few times it worked. I thought the problem was the Skinner valve which controls the fuel shut-off vlave, but I've replaced it and that wasn't the problem.

Then I thought it was the Engine Stop Delay Relay keeping the Skinner valve from getting power, but now that's been replaced and still no luck. When the key is turned and the engine-run switch is in the 'on' position the fuel shut-off will open for a few seconds and then close again. Frustrating! If I put a jumper from the COMM terminal to the NC terminal on the Engine Stop Delay Relay I can force the Skinner valve into letting the fuel shut-off valve to open, so I know that the circuit on that side functions.

Then I disconnected both the low-oil switch and the high-temp switch circuits from the Engine Stop Delay Relay to check, and tried the start procedure. Fuel shut-off opened and engine started. Why? Don't know.

Other symptom that popped up today . . . When I turn the key to start, the oil pressure gauge spikes all the way to the top. It stays there even after starting the engine. It had been working perfectly till this weekend. When I turn things off it slowly falls to zero. I know from the mechanical gauge by the engine that I do in fact have proper oil pressure.

I'm guessing that both the fuel shut-off valve problem and the oil pressure gauge problems are related. Maybe something is grounding out somewhere? Problem is that I haven't a clue where to start the search for the problem.

I've got the manual and the wiring diagrams. There are a few variations on the wiring diagrams, depending on the way the coach was equipped by GM. I don't know specifically which diagram applies, but have a pretty good guess. Problem is that even with that I cannot figure out where to start. The entire harness for the bus systems is largely OEM and doesn't appear to have been messed with much. But, it is 40+ years old and could have a loose connector or such.

I've got a new low-oil switch coming from Luke this week. I'll contact him tomorrow to ask if a oil gauge sending unit would be in order to try and help the gauge issue.

Any suggestions? Any thoughts on where to start looking?

[Dave5Cs]

Sounds like you have some bad grounds. gauges always act that way when there is a ground problem corroded or loose, etc. The starting problem could be also.
When you disconnected the oil switch and the high temp and it started because those might be bad and were shutting it down. Hook them back up and see what happens, then disconnect one at a time and see if there is any difference. Clean all the terminals in your back fuse box and relay box if you have one.

[chessie4905]

Did you check the key switch yet? The problem could be there.

[richard5933]

Checked with an ohm meter. The outer shell of all three sending units checks out with a good ground to the engine block.

Tried to start again. She started and ran for about 5 seconds and then quit. Went back and unplugged the wire which I believe is coming from the low-oil switch (the shortest of the three sending units) and tried to start. The engine started right up and ran just fine. Went back and check terminal on the low-oil switch, and it had a solid ground.

Even with the low-oil switch disconnected, however, the oil pressure gauge was still pegged to the top of the gauge scale. I disconnected the wire on the oil pressure sending unit, and the gauge still pegged out.

Don't know how the key switch would affect this - it really just serves as a way to lock the engine-run switch in the off position by cutting power to it when the key is turned off.

Questions:

1) I know that the purpose of the oil pressure sending unit is to control the ground to the gauge. Which way does it go? Full ground at zero pressure or full ground to peg the needle?

2) If the gauge is pegged with the wire disconnected from the sending unit, is that a sign that the sending unit has failed or the gauge? Or something else like a short to ground in the wire somewhere?

I'm starting to wonder if I've got a short to ground somewhere. If that's the case, problem is finding out where.

[chessie4905]

with key on, engine off, momentary ground wire to sensor and see if gage returns to normal or zero. if that doesnt work, try tapping gage.

[richard5933]

UPDATE:

I got the gauge working. Apparently one of the push-on terminals was corroded under the deceptively shiny brass outer layer. I pulled off all the terminals related to the gauge under the driver's side panel and now the gauge is doing it's thing.

Still trying to figure out why the low-oil shutdown circuit is shutting things down even once the oil pressure comes up (confirmed with mechanical gauge).

The low-pressure switch has a full ground at the terminal end even with the engine at normal oil pressure. I know that this is a problem, so I've got a new one coming from Luke. However, even if I disconnect the wire from the terminal of the low-oil switch the system still shuts down. I've got to disconnect it from the Engine Stop Delay Relay to keep it from shutting things down.

Between the low-oil switch and the Engine Stop Delay Relay is the low-oil relay, and I'm suspecting that it has gone belly up on my. A new one is coming as well.

BTW, I am already going through all the panels I can find to tighten/re-seat/check all connections and grounds.

Any thoughts or suggestions are appreciated.

[luvrbus]

Is that a 2,3 or 4 wire Kysor shutdown system on the bus ?

[richard5933]

Not sure if it's a Kysor shut down system. It actually reminds me of the way a Murphy switch w/delay works.

The relay that controls it is circled. There are a few terminals on this:

Comm - 24v+
NO - empty
NC - goes to Skinner valve controlling fuel shut down air cylinder
Heater - from high-temp switch
Heater - from low-oil switch (this is the solid yellow wire, which if you look close you'll see is not connected at the moment)

Basic operation is: Under normal circumstances the power goes from the Comm terminal to the NC terminal to provide power to the Skinner valve so that the air cylinder open the fuel. When either the high-temp switch or the low-oil switch faults, it sends a ground to the 'heater' circuit in the relay. This heats a tab inside, and when it's hot enough it breaks the circuit to the NC terminal, which in turn shuts down the fuel by activating the air cylinder. Once the heat tab cools the cycle can be restarted.

At startup, the low-oil switch sends a ground to the relay. As long as oil pressure gets above the minimum fast enough the ground breaks and things work as normal. If the pressure doesn't build, then the relay will kill the engine in about 20 seconds. Problem I'm having is the low-oil switch isn't functioning and the ground stays connected to the heat tab which shuts down the engine about 20 seconds after it starts (even with proper oil pressure).

Between the low-oil switch and the Engine Stop Delay Relay is the low-oil relay. It's the rectangular silver box just below the big black relay box in the middle of the photo. I suspect that this relay is also giving me a problem.

[bevans6]

On m bus, which admittedly is an MCI, there was an engine emergency stop relay that would over-ride the run switch to remove power from the skinner valve and stop the engine.  It had three controls - two engine hot switches and one engine oil pressure switch.  Any one could cause the emergency stop relay to stop the engine.  Note that the switches are separate from and different from the engine temp and oil "sensors" which work the gauges.  Mine are all disconnected and I repurposed the emergency stop relay as the transmission neutral relay.  Maybe this is part of your problem.

[richard5933]

On m bus, which admittedly is an MCI, there was an engine emergency stop relay that would over-ride the run switch to remove power from the skinner valve and stop the engine.  It had three controls - two engine hot switches and one engine oil pressure switch.  Any one could cause the emergency stop relay to stop the engine.  Note that the switches are separate from and different from the engine temp and oil "sensors" which work the gauges.  Mine are all disconnected and I repurposed the emergency stop relay as the transmission neutral relay.  Maybe this is part of your problem.

Sounds like you've got a similar but slightly different system. Only two things controlling this stop relay - oil pressure and high temp.

With the one terminal on the stop relay disconnected, the oil pressure tell-tale light appears to function (it comes on at start and quickly turns off once pressure builds.) The gauge is now working after tightening up some corroded connectors. I'd like to have the system fixed to work as intended. I may one day disable it, but first I'd like to be sure that things are doing what they're supposed to do.

[richard5933]

Update...

Spent a lot of time working through this issue, but still have not solved it. By disconnecting one wire from the shut-down relay I can get the engine to run, but right now the thing still is hit and miss otherwise. Here are the things we've done and the results:

1. First I thought that the Skinner valve was at fault, so it was replaced. No help. (will keep the original for a spare)

2. Oil low-pressure switch was sending a ground at all times, even with the engine running and pressure confirmed through a mechanical gauge. Replaced the low-pressure switch. No help - still have a problem.

3. Low oil pressure relay was showing an errant ground. Replaced, but still not working.

4. Checked the function of the high-temp sender (Alarmstat?) It shows no ground at normal temp and appears to be functioning.

5. Wire going to the high-temp sender is showing 55 ohms to ground. Not a full short to ground, but enough that I'm concerned this may be part of the problem.

The engine will start some of the time with no problems. It may do it for 5 or 10 starts. Then for no apparent reason the shut-down relay gets a ground signal and shuts things down/won't let the engine start. Once this happens I can pull one of the wires going to the heater terminal (heat tab that acts as timer) of the shut-down relay and then things will work fine again once the heat tab cools back down.

My suspicion is that I've got an intermittent short to ground somewhere. Problem is finding it. I've checked and double checked everything I can see. All the harness jackets look good. No apparent rubbing or abrasion. Remember, this bus only has about 40K miles, so things are pretty much as pristine as can be and it's generally easy to see things. That said, I'm guessing that this problem has reared its head before, as I see that someone has unwrapped some of the black electrical tape covering on a couple of spots of the harness just below the engine bay electrical panel. Don't see that they made any changes though, as everything they unwrapped is still factory and untouched.

For now, as long as I've got a working oil pressure gauge and temp gauge I'm not going to spend any more time killing myself over this. I will continue to examine the harness throughout the bus as I'm working on other things - maybe I'll stumble upon the problem.

There a couple of electrical demons in this bus, so I'm thinking that somewhere is a pinched or rubbed piece of harness causing the problems. The other issue is that the start button doesn't always work, even if the bus is in neutral. I had one neutral safety switch that was sticking. I replaced it but still have the issue. I can start with no problem if I use the over-ride switch so I don't think there is anything wrong with the starting system itself. However, the normal start switch doesn't always crank, and I cannot get the rear start button to crank it either. My guess is that rear switch is having the same problem as the front one, but there is no over-ride in the rear (it only works if the neutral safety system is functioning.)

If anyone has suggestions I'm all ears.

[richard5933]

I might have an update for this with a possible solution. I'll know soon...

During our trip over the past two weeks we did some night driving, which oddly we have not done since the problem in this thread first came up. As soon as we started the bus the first night, I noticed that the low water tell-tale was glowing faintly. At first I thought it was just a reflection from street lights, but it was in fact glowing at about 5-10% normal. Bright enough to see in the dark, but not at all during the day.

While the bus was at US Coach last week, we changed out the low water probe in the surge tank. As soon as we took the rubber boot off the terminal on the probe it was clear that the corrosion was preventing a solid connection from being made. We tried to remove the nut and clean things up, but the center shaft just spun so we installed a new one. The tell-tale is now totally off when operating.

So, the new working theory is this...

The shut down system was kicking in sporadically and sometimes slowly. With the low water probe being in the shut down system, having the poor connection in the circuit caused the thing to shut down when it shouldn't have.

In the coming days I hope to have time to reconnect the shut down system and see how things work. Hopefully everything will be okay.

[chessie4905]

I thought it was just a low coolant warning; not a shutdown system like overheat or low oil.

[dtcerrato]

Going back to what Dave said about bad grounds - that's where I'm at. Sounds like a bad ground somewhere.

[richard5933]

Going back to what Dave said about bad grounds - that's where I'm at. Sounds like a bad ground somewhere.

The wiring to the low water probe is a ground wire. The water in the tank completes the circuit, best I can tell. So yes, this was a bad ground.

I thought it was just a low coolant warning; not a shutdown system like overheat or low oil.

Not entirely sure yet, but my initial reading of the wiring diagram shows that this is in the same circuit somewhere. The one time that I was actually low on coolant (huge air pocket from a bad fill job) the engine ran for about 20 seconds. The low-coolant light was on the whole time, and then it shut down. Went through this a couple of time till I realized what was going on. Once the water was filled things worked okay. Since I now know that the low-water probe was not making a full ground due to the poor connection I'm assuming that it contributing to the intermittent shut down problems.