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"SAFETY INTERLOCKS"

5/29/00

I get lots of questions about riding mowers that quit with no apparent reason. Often it is a "Safety Interlock" that has either failed or accurately sensed a safety hazzard and shut the engine off.

These systems have switches tied together, often with a brain module at the core. Switches can be located at the following locations.
1. Shifter. May be anywhere from the shift lever, linkage or in the transmission.
2. Blade engagement. Again, these will be anywhere from lever, linkage, blade spindle, or if an electric clutch somewhere in the wiring.
3. Seat. This can be a switch right in the bottom seat padding or part of a seat hinge. 4. Clutch/brake pedal. Anywhere from the lever to the brake.

Some systems will have others, such as an additional reverse switch. They have different methods but this is the norm. The shifter must be in neutral and the clutch/brake pedal must be engaged. When they aren't the sytem typically will not allow the engine to be cranked over. Some will shut off the engine ignition system instead.

The seat switch, if not engaged (someone sitting in the seat), will shut off the ignition system when the blade is engaged or the tranny put in gear. On some units the seat switch may also disable the starter. Note: Some seat switches are overly sensitive and a light weight person may not have enough weight for the switch to sense someone is there.

Now, since the shifter, clutch/brake or blade switch may have a function of disabling the igniton at start up, these type systems must have a brain module to sense when the engine is already running to block the switch from cutting the engine off when they are allowed to at start up, but still allow them to work when the seat sensor senses the operater leaves the seat. After all, you want the blade running at times, right? This module is prone to failure as much as the switches.

What this all comes back to is this. When a riding mower engine quits or won't start, after checking the fuel and oil, the first thing to check is ignition to the plug(s). If none is found, the ignition sysem needs to be isolated from the equipment by disconnecting the kill wire to the coil, preferably at the coil. Then with plug(s) removed to prevent accidental start up with the kill disconnected, checking for spark. At this time you'll know whether the ignition problem is an engine or equipment one.

Let's not forget, many engines also have a low oil sensor that shuts off the ignition as well.

Happy Mowing
LMRM;Bob :<=

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