Scag Cougar troubles

[Boneyard]

Hi, hoping somebody can help us. We (little league baseball field) have a Scag Cougar riding mower with Kohler motor It was serviced a month or so ago and has been working great. On Saturday it was in operation and then started to lope and not sound "right". The operator shut it down immediately. We are not gearheads by any stretch but it seemed to be electrical or fuel related.

Once it was shut down it would not start again. We checked fuel lines and filter, they were fine. We put fresh plugs in. With the plug boot on the rear cylinder plug and a new plug in the front cylinder, we put an old plug in the front plug boot and hit the starter (to see if we were getting a spark). To our surprise, the motor actually started. We killed the motor and put the boot back on the plug. It would not start.

We pulled the boot from the front cylinder and it fired right up.

Any insight on what this problem might be and how to diagnose/solve it? Any advice is appreciated.

Jack

 

[Mad Mackie]

Hi boneyard,
Newer type engines make their own spark for ignition just by the engine cranking over and the coil primary winding is grounded to shut the engine down. On an engine with twin cylinders, the wire harness on the engine that is used to shut the engine down is a "Y" harness so it can connect to both ignition coils. Incorporated into this harness in the Y are diodes that prevent backfeeding back and forth between coils so the engine can be shut down by a single terminal in the key switch. The symptoms that you have stated indicate that one of the diodes in the Y harness has either opened or shorted. This harness is an engine part and the engine harness connects to the main harness in a connector close to the engine. The engine blower housing needs to be removed to gain access to the ignition coils where the Y harness is connected to each coil in order to test it with an ohmmeter. Disconnect the engine harness connector, remove the Y harness terminals from the ignition coils and check each leg with an ohmmeter and reverse the leads and recheck each leg to determine which diode(s) are bad.
Mad Mackie in CT

 

[Boneyard]

Thank you, sir, I appreciate your wisdom!