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While mowing yesterday, my CH740 27hp shut off.

From that point on it would start each time I turned the key, then shut off.
I checked the oil level thinking it may be a safety sensor for low oil. Level was fine.
I check the fuel filter, thinking it may be running out of gas. Filter and fuel flow was fine.

It will start every time you turn the key to START, but as soon as you let go of the key and it goes back to the ON position, it shuts off.

I found that if you hold the key in the start position, it will continue to run as long as you hold the key in the start position.

What do you think is causing this problem?

Thanks,

GW
Sarasota Florida
Scag Turf Tiger STT52V 27CH
2006 Command Pro 27
Model# CH740
Approx 720hrs

Probably the keyswitch is coming apart internally. I would unplug it immediately and test it, as it could damage your ignition coils,
and they are expensive.

Thanks Fish.

I am replacing the ignition key switch. It seems to make sense that this could be the issue. I don't really have a way of testing the old one and a new one is only $14 including shipping.

Is there anything else that you think could be causing it to shut off after it is running? It starts fine every time, but immediately shuts off when you let go of the key and it returns to the ON position.

Are there any sensors that detect unsafe oil or hydraulic pressure that will force the shut down of the motor as soon as it starts?

I have reviewed all of the other safety switches (seat, brake, left and right steering levers, blades) and all appear to be fine. They all stop the mower from starting if they are in an unsafe position. I am not aware of any other safety switches.

Is it possible to temporarily bypass this ignition key without damaging the electrical system or the engine?

Thanks,

GW

Possibly a low oil pressure shutdown switch. On the CH series it is normally located in the valley between the cylinders, kinda under the carb. May try unplugging it and see what happens.

Find and post the Kohler's "spec" numbers. Scag used a lot of Kohler's with a feature called "SmartSpark" and they were nothing but trouble! So hold off from ordering new parts just yet, as the fix may take some work....I prefer testing and making sure before buying expensive parts.

Spec number: CH740-0041
Serial Number: 3622114311

Thanks

G

Here is some more info if it helps.

Family: 6KHKS.725 2GC

Looking up the CH 740-0041 this is a 25HP 725cc engine.

GW

Fish said:

Find and post the Kohler's "spec" numbers. Scag used a lot of Kohler's with a feature called "SmartSpark" and they were nothing but trouble! So hold off from ordering new parts just yet, as the fix may take some work....I prefer testing and making sure before buying expensive parts.

Click to expand...

glwilliams58 said:

Here is some more info if it helps.

Family: 6KHKS.725 2GC

Looking up the CH 740-0041 this is a 25HP 725cc engine.

GW

Click to expand...

Give fish some time, he should be able to get you run down on some tests before you start buying anything, Be patient.

gfp55 said:

Give fish some time, he should be able to get you run down on some tests before you start buying anything, Be patient.

Click to expand...

No, yesterday, I slipped into a beer-coma early after eating some great chili, so my stepping out was unavoidable and certainly
excusable........

Unfortunately, your engine does have SmartSpark, so that may complicate things a bit...

Ignition

On a normal mower's wiring, when you turned the key to the "off" position, or got off the seat while the deck was running, or a few other scenarios, that would send to your coils a connection to "ground" through the switches, which would shut the engine off.

Same as a pushmower, but only the switch is hooked up to the lever, when you let it go, it "grounds" out the coil and shuts the engine down, been that way for a loooonnnggg time......

And that is the beauty of these engines, whenever the flywheel rotates through the coil field, it produces a spark, if the system is working correctly, the only way to stop that is to ground out the coil...

On the circuitry of a keyswitch, it sends voltage to the starter to "crank" when the key is turned to the crank position. It also
does not let the ignition to ground at this position, it also lets voltage go to the fuel solenoid at this position, so the engine
will start and all....

And when the key snaps back to the "run" position, it still does not allow the system to "ground"/shut off... But still allows power to the fuel solenoid, and the rest of the tractor....

And when the key is turned off, the ignition coil is grounded, and the engine shuts down.....

But Kohler, in their great wisdom came up with "SmartSpark", which requires voltage to the SAM unit to run as well, and confuses us poor rednecks all to pieces.......

So I hate to tell you that you might want to be patient with us here, as we are old disgruntled pharts, that don't get paid a dime,
and truth be told, didn't get paid enough before, when we worked on this crud for a living......

So, be patient, one of us here might slip a pearl in this smelly oyster bed..... But if you want to get serious about it, go buy a decent multi-meter, so we can test the keyswitch.

Thanks for the insightful reply Fish. And I can always appreciate a good bowl of chili.

Thank you for the time you have taken to provide this information.

I picked up new key switch on the way home from work today, along with some 20amp fuses.
Replacing the key switch had no immediate effect on the problem. It does exactly what the old switch does. It still starts up nicely and runs until you let the key go back to the ON position, then it dies.

I also have a nice multi-meter. I have checked the two fuses below the console on the left side of the seat and they are both good.

Let me know what additional information will help you diagnose the issue.

Thanks,

GW

ILENGINE said:

Possibly a low oil pressure shutdown switch. On the CH series it is normally located in the valley between the cylinders, kinda under the carb. May try unplugging it and see what happens.

Click to expand...

ILEngine----I have looked through the engine and parts diagrams and do not see a low oil pressure shutdown switch anywhere. I could be wrong, but I don't think this model has one.

GW

Sorry about the delay, our dsl was out county-wide yesterday, which is bad if you make money on e-bay..... So I'll probably be doing some catch-up today. Here is a link to the scag manual, which is very little help. I'll try to scare up a link to the Kohler link.

scag

Here is the manual for the engine, just go ahead and print out page 36 an 37, and maybe the info on testing below.

http://www.kohlerengines.com/onlinecatalog/pdf/sm_24_690_06.pdf

No worries Fish. Thanks for the manual.
Just to bring you up to date on what I know.

1. The key start switch is good (replaced with a new one and it had no impact)
2. All of the safety switches (seat, brake, steering arms, mower) are in the safe position, allowing the motor to start.
3. The fuel solenoid is good (I took it out, grounded it and witnessed it activate when the key is turned to START and then deactivate when the key is turned to ON).

#3 above tells me that the START cycle appears to be fine at all points but that the ON/RUN cycle is shutting everything down.

Are there switches that are associated with only the ON/RUN cycle? As mentioned previously, I do have an oil pressure sensor on this machine.

Thanks,

GW

Sorry again, I now am working on the wiring in the kitchen.

Read the part that deals with the Smart Spark, and check the voltage going to the S.A.M. unit while cranking and when the key is released and see what that tells you.

I have tested the power source to the SAM.

From the red power source that I disconnected from the SAM, while using the grounding screw, as well as the negative battery terminal I get the same results:
9.4v while in the START position.
0.0v while in the ON/RUN position.
0.0v while in the OFF position.

glwilliams58 said:

I have tested the power source to the SAM.

From the red power source that I disconnected from the SAM, while using the grounding screw, as well as the negative battery terminal I get the same results:
9.4v while in the START position.
0.0v while in the ON/RUN position.
0.0v while in the OFF position.

Click to expand...

Sorry, I have been battling the powers of darkness.......

But this is the problem, so we need to run this down!!!!! Needs 12ish volts at the start and run position, so we need to follow the wires!

I am amazed that no one else has stepped in!!!!! At least with a bad guess...... But the SAM unit needs voltage while cranking
and when the key goes back to run, so it should be a straightforward rundown.......

Yes, the immediate guess was the keyswitch, but I tried to get you to hold off, as these S.A.M. units are nothing but trouble....

So we need to look at your mower's wiring to see what could be starving voltage to the S.A.M. unit when in the run position on the keyswitch!!!!!

Fish said:

I am amazed that no one else has stepped in!!!!! At least with a bad guess...... But the SAM unit needs voltage while cranking
and when the key goes back to run, so it should be a straightforward rundown.......

Click to expand...

Fish I can't help with this one, its over my head and you know how bad advice is worse then no advice. Give it some thought and you can help the guy. Sorry I can't help. "But thats just me"

Well the best guess would have been the keyswitch, but since he has run the test, he gets voltage to the SAM while cranking and has spark, but when the key clicks back to the run position, he loses voltage to the SAM and loses spark, so it should be somewhat straightforward........

Re: Command Pro CH740 only runs if you hold the key in START position (SOLVED)

Fish, you nailed it. It was just a matter of working backward from the SAM.

It was the first connector nearest the SAM. It is a black connector and when you separate it, it has one terminal that shows 12v on START, and one that shows 12v on ON/RUN. Once I saw that I knew I had what I was looking for.
I cleaned off the connectors and pushed it back together and it started right up and ran perfect.

It must have worked itself loose. There is no lock on it. I will zip tie it.

Thanks for all of your help!

GW

Re: Command Pro CH740 only runs if you hold the key in START position (SOLVED)

glwilliams58 said:

Fish, you nailed it. It was just a matter of working backward from the SAM.

It was the first connector nearest the SAM. It is a black connector and when you separate it, it has one terminal that shows 12v on START, and one that shows 12v on ON/RUN. Once I saw that I knew I had what I was looking for.
I cleaned off the connectors and pushed it back together and it started right up and ran perfect.

It must have worked itself loose. There is no lock on it. I will zip tie it.

Thanks for all of your help!

GW

Click to expand...

Thats great, I knew Fish could help you out. I'm glad you got you mower going good, also I bet you learned something. Theres some people on this forum that know their stuff and others that don't, me being one that don't. Good Call Fish...... Good Luck glwilliam58 "But thats just me"

Re: Command Pro CH740 only runs if you hold the key in START position (SOLVED)

glwilliams58 said:

Fish, you nailed it. It was just a matter of working backward from the SAM.

It was the first connector nearest the SAM. It is a black connector and when you separate it, it has one terminal that shows 12v on START, and one that shows 12v on ON/RUN. Once I saw that I knew I had what I was looking for.
I cleaned off the connectors and pushed it back together and it started right up and ran perfect.

It must have worked itself loose. There is no lock on it. I will zip tie it.

Thanks for all of your help!

GW

Click to expand...

Glad you found it!!!! Electrical/ignition problems are difficult enough, but that damn SAM module makes the difficulty meter rise
exponentially!!!! And like many electrical issues, it is usually poor/dirty connections... Glad you found it!!!!!