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My Lawn Mower Won’t Start: A Troubleshooting Guide – Part 1.

January 20, 2010

You need to mow the grass, but the lawn mower won’t start. Here’s a step-by-step guide for finding the problem.

Troubleshooting Lawn Mower Start Issues

Before we figure out why your lawn mower won’t start, is the engine a recoil start smaller horsepower engine, or is it an electric start higher horsepower engine?

There are a couple of things that are common for either engine type. Does the engine turn over smooth or does it have “tight” places in the rotation? Let’s say that everything moves correctly. Recoil start engines on smaller equipment have a very simple “kill” circuit which could prevent the engine’s ignition from firing. Electric start, larger horsepower engines usually have a “safety interlock” circuit in addition to the primary kill circuit. Either of these circuits can prevent the ignition from firing.

Does the Lawn Mower Have “Fire?”

The next step is to check the engine for “Fire”. The easiest way is with a “Spark Checker” tool (our part # DOP42-031), but a spark plug will also work, it is just a little more difficult. Using a Spark tester tool, you ground the tool’s ground clamp to the engine and insert the other end of the tool into the spark plug boot. Crank the engine over, bearing in mind that it has to turn over 300 rpm to energize the ignition coil. This is common for most small engines. In the case of a recoil start engine, it is imperative that you make sure the engine cranks at the appropriate speed. Looking at the spark checker tool, do you see spark? A well functioning ignition should show bright blue spark. Weak yellow spark is a sign of a weak ignition system.

What If You Have No Fire?

If you don’t see any spark, the next step is to isolate the ignition system from the rest of the machine. On any piece of outdoor power equipment there is a “kill” circuit and on larger equipment there is also a safety circuit. You must isolate these circuits to do the next check. There is a small wire, usually black, that attaches to the throttle control plate or to the operator presence control assembly (on smaller engines this is the flywheel brake assembly bracket) which is the “kill” circuit wire. Unhook the wire and perform the spark check again. If the spark test shows good then your ignition system is good. So you have to check somewhere else.

If you don’t have a spark checker tool, you can perform the test using a spark plug. Insert the spark plug into the boot and ground the threaded area of the spark plug against the engine. Carefully using a pair of insulated pliers to hold the spark plug boot/spark plug, crank the engine. Look at the electrode end of the spark plug and watch for spark. Again if the spark is present and bright blue, your ignition system is functioning correctly. Your problem is elsewhere. Before going any further, since you had fire, re-attach the safety or kill wires.

Did Your Flywheel Key Shear Off?

Before going any further check your engine’s flywheel key. The flywheel key keeps the flywheel keyway and the crankshaft keyway aligned and in “time.” If you have hit something with the blade, it may cause the flywheel key to shear. On larger engines occasionally the flywheel key will shear due to low torque on the flywheel nut allowing the flywheel to shear the key on initial turnover or shutdown. If the flywheel key is sheared, it allows the engine to get “out of time.” An engine will sometimes refuse to start simply because it’s out of time. However, you can sometimes see spark in the spark checking tests. Although the ignition is working, it is sparking at the incorrect time in the cycle and won’t allow the engine to run.

Is There a Problem With the “Kill” Circuit?

If you had no fire with the “kill” circuit hooked up, but after unhooking it you did have fire, then your problem lies in the “kill” or safety circuits. You have to isolate those circuits and test them for problems.

To check the kill wire, you must isolate it. Starting from the throttle control plate or flywheel brake assemble bracket, follow the wire back to its terminator at the ignition coil. Check it for bare insulation areas that could possibly ground against the engine. If the wire doesn’t show any visible problems, then check it with a volt-ohm-multimeter for continuity using the resistance scale of the meter. If this wire checks good, then you have to go on to the safety interlock system or the coil.

Malfunctioning Operator Presence Control?

On a walk-behind mower there is the operator presence control (that extra handle you must grip when operating the machine). Check to make sure the brake is releasing and the switch is opening on the bracket. After making sure the kill circuit is operating, check fire again. If you have fire, then you know that there was/is a problem in this circuit. If you have isolated a problem in this circuit, you have to repair it before going any further. Once you have repaired the problem you should have fire. If everything in this circuit is correct and operating like it should but you still don’t have any fire, most likely your ignition coil is defective and should be replaced.

On a riding mower the safety circuit runs to the clutch pedal, blade control, transmission and seat. The circuit uses switches at each of these places to ground the ignition system. You know the reason your mower’s engine quits if you get off the seat or try to start it in gear or with the blades engaged. You have to isolate each switch and bypass it (one at a time) to find the problem. Once you have isolated the problem, repair or replace the defective switch. If you have done everything correctly, you’re mower engine should have fire and run correctly. If everything has been checked and no problems were found, then the ignition coil is likely defective and should be replaced.

More My Lawn Mower Won’t Start in part 2.

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