Scag / Kohler has me stumped

Discussion in 'Mechanic and Repair' started by AlohaMowing, Sep 9, 2011.

  1. AlohaMowing

    AlohaMowing LawnSite Member
    Messages: 58

    I have a Scag Turf Tiger ZTR with the 27 h.p. Kohler Command series CH740 engine that has died and defies troubleshooting. The grass is growing and I am desperate for suggestions.

    Quick summary: The spark plugs fire with the ignition switch in the start position, but stop firing as soon as the switch goes to the run position.

    The Scag has 560 hours and has been completely reliable up until now. I had just started mowing, and about 5 minutes after having started the engine the engine backfired very loudly and immediately died. (I had just started a right turn after having mowed a straight line for about 2 minutes when it happened, probably just a coincidence, but it did happen at the instant I started the turn.) When I looked at the engine the air hose from the air cleaner was off the carburetor, presumably blown off by the force of the backfire.

    When I crank the engine it fires and sounds as if it is going to run, but when I release the key from the start position the engine it dies immediately. I have used a spark plug tester that shows both cylinders are sparking with the ignition in the start position, and the spark stops instantaneously on putting the switch in the run position.

    I have confirmed fuel is being pumped and the carburetor fuel shutoff solenoid is operating correctly. I replaced the spark plugs. When I pull plugs right after attempting to start the engine they are damp and have a gas odor, consistent with the engine turning over a few turns after it lost spark. Visual inspection of the carburetor shows nothing amiss, linkages are working correctly, although the throttle valve moves little from the full open position, which is where I normally run it. I can see fuel pulse into the carburetor throat when I look down it with the air intake hose disconnected while cranking the engine. It appears that the fuel supply is not the cause. To confirm there was no problem with the gas quality I drew gas directly from a jug with my freshest gas with a good dose of Sea Foam fuel treatment; there was no change.

    I have checked that there are no obstructions to the air flow and attempted starting with the air cleaner removed. I find no problem with the air flow.

    The local Scag guy said he has seen similar symptoms with a broken valve push rod. I pulled the valve covers and all push rods appear sound and straight. Pushing on the rocker arms showed all valves moving freely. Compression is good--143 psi in both cylinders.

    I have gone through the troubleshooting steps in the engine service manual, http://www.kohler-engine-parts.opeen..._REV1010EN.pdf, and find no clues in those things I can do without special tools and which seem even remotely possible causes.

    I have checked the safety switches, all seem good.

    I disconnected the kill circuit wire where the Scag wiring meets the Kohler wiring, it made no difference.

    When I disconnected the kill circuit wires directly from the ignition modules I had no spark at all, even with the ignition in the start position.

    I checked out the ignition switch as best I could. There were no obvious defects, but a new one did not cost much so I replaced it. That made no difference.

    One of the possible causes listed in the Kohler manual is a bad diode. My multimeter reported a bad diode which is incorporated in a relay. I replaced the relay/diode. It made no difference.

    I have checked wiring connections. All seem okay, there is no apparent damage to the wiring harness.

    There is an electronic module that does some magic involving safety switches and ignition. I can find no information on testing it, and it is too expensive to replace just on the off chance it has gone bad.

    I am well and truly stumped, and desperate for suggestions. In addition to suggestions for troubleshooting and repair, I would welcome information how to jury rig it so it might run well enough to do some desperately needed mowing pending a full repair. The local professional mechanics are backed up 2 to 3 weeks on repairs, and I cannot wait for them.

    Thank you for any and all comments and suggestions.
  2. djagusch

    djagusch LawnSite Platinum Member
    from MN
    Messages: 4,358

    On my 2002 TT the module went bad. Dealer pulled the white wire I believe to get it to run, all safety switchs didn't work though.

    The S/N would be helpfull to look at a diagram. Guessing most years are the same but never know.
  3. AlohaMowing

    AlohaMowing LawnSite Member
    Messages: 58

    Thanks, djagusch. Serial number is B6200388

    Any idea how the problem in the electronics module was determined?
  4. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    I have a couple ideas but I need the SPEC number off the engine before I get started......
  5. AlohaMowing

    AlohaMowing LawnSite Member
    Messages: 58

    Engine Spec Number is CH740-0041
  6. djagusch

    djagusch LawnSite Platinum Member
    from MN
    Messages: 4,358

    all the safety switches checked out ok. How remember it explained was the switches output goes into the module and the white wire was the out put from the module to stop spark or maybe the key on function.
  7. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    That ^^^^^^^ shows both ignition modules are bad......

    This is what appears to have happened;

    A diode is a one way valve so to speak, It lets current flow in one direction only. When this diode blew for whatever reason it shot positive (battery) current to both modules through the kill (ground) lead blowing both.

    You can verify by testing both modules with your multimeter as shown below, The part number will be stamped on your modules to ID which specs to use.

    ALSO; Test for battery voltage on the white kill lead wire with the ignition switch turned to ALL positions BEFORE hooking them up to newly installed modules, You don't want voltage to blow them again......

    Last edited: Sep 9, 2011
  8. AlohaMowing

    AlohaMowing LawnSite Member
    Messages: 58

    Thanks, Restrorob. I had thought about the ignition modules, but since there seems to be a good spark with the ignition switch in the start position I ended up ruling them out. I did the resistance test per the manual, however. My ignition modules are the 24 584 36-S. My results are as shown below:
    Measured ohms . . . . . Specified
    Right . . . . Left . . . . . . . Range . . . . . . . Measurement
    575 . . . . . 574 . . . . . . 590 - 616 . . . . . . . . 1 to 4 . . kill circuit
    197 . . . . . 197 . . . . . . 183 - 208 . . . . . . . . 2 to 4 . . trigger circuit
    9700 .. . . 15,700 . . . 8000 - 40,000 . . . . .. 3 to 4 . . spark plug

    So while the 1 to 4 measurement is slightly low on both, the others are in the specified range. The 1 to 4 measurement is for the kill circuit, assuming the diagram in the Kohler manual on page 8.20 has the correct layout.

    I'm sure you know these engines better that I do. Would I get the spark with the ignition switch in the start position if the ignition modules are bad? Does it indicate a fatal defect that the 1 to 4 measurement is slightly low?

    And I suspect the diode may not have been blown. The new one is giving the same, defective, reading with my fluke multimeter. Since the diode is combined with a relay I suspect there is some circuitry there which fools the meter into thinking it is bad.
  9. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    Do'n more looking.....
    Last edited: Sep 10, 2011
  10. piston slapper

    piston slapper LawnSite Platinum Member
    Messages: 4,337

    Check the diodes from the engine plug in to the sam module. Start and run circuits.
    Posted via Mobile Device

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