Tomos no spark? Learn how to fix it!

The ignition is the heart of your Tomos. If there’s no spark, the moped won’t start. In this guide we clearly explain what each component does, how to recognise the most common faults, and which steps you can take with a multimeter to find the problem. Suitable for every Tomos enthusiast.

Contents

How a Tomos ignition works

The ignition system on your Tomos starts with the ignition, which consists of a flywheel and coils. The flywheel with magnets generates voltage in the stator. These pulses go to the CDI unit which processes the energy and sends an ignition pulse to the coil.
The coil converts this into high voltage, which travels through the spark plug lead to the plug and generates the spark. If any link in this chain fails, your Tomos won’t get a spark.

Check Tomos kill switch and wiring harness

What can fail

A faulty kill switch can permanently short to ground, causing no spark. Loose or damaged wires and poor connectors often result in no spark or an intermittent spark. Wire breaks are most common in areas with a lot of friction and movement, like at the steering head.

How to spot a faulty kill switch or broken wire

If the moped has no spark while the kill switch is off, or the spark disappears when moving the wires, the issue is likely in the wiring harness or the connections between components. Visual damage and corrosion are also clear signs. Check the connections.

How to test a wiring harness with a multimeter

Set the multimeter to continuity or low ohms.
Hold one probe on a chassis ground point and the other on the kill switch output.
If the kill switch is in the OFF position, the meter should show contact with ground.
If the kill switch is in the ON position, there should be no contact with ground.

Check the wires: measure continuity between the start and end points of the harness.
If you don’t hear a beep or the meter shows no continuity, you have a broken wire.

Check all our kill switches here.

Testing a Tomos spark plug

Recognising spark plug issues

A worn or fouled spark plug gives no or a weak spark. A gap that is too large or too small can also cause a poor spark. A wet plug indicates too much fuel or oil in the cylinder that doesn’t fully ignite due to no or weak spark.

Inspect the plug. If the plug is black and sooty, your setup is running rich. If it’s white, it’s likely too lean. A wet plug points to excessive fuel or oil. If the plug shows no spark in a test and the plug is known good, the problem lies elsewhere in the ignition system.

How to test a spark plug for spark

Remove the plug and check the gap (around 0.45 mm).
Place the plug in the cap and hold the plug thread against a ground point.
Kick the engine yourself or have someone kickstart it. Do you see a clear blue spark? Good. No spark? Repeat the test with another plug to check if the plug is the issue.
If that plug also doesn’t spark, other parts of the ignition circuit are faulty and need further testing.

Check all our spark plugs here.

Want to know more about the different plugs? Read our blog: Explanation of different plugs, resistance, short or long shaft, etc.

Spark plug lead and cap

Why these often fail

Poor contact, moisture or internal breaks in the lead cause a weak or no spark. The insulation can also be damaged, causing the spark to jump to the frame instead of the plug. Visually check the spark plug lead and cap for damage.

Problems with the plug cap

Set the meter to resistance (Ω).
Measure between the copper core of the spark plug lead and the inner contact of the plug cap. No reading? The cap is broken.
Also check for leakage to ground by measuring between the plug cap and chassis. Continuity to ground means the insulation is compromised and the spark lead and/or cap should be replaced.

Testing Tomos CDI unit and coil with a multimeter

What the CDI and coil do

The CDI controls ignition timing. It briefly stores voltage in a capacitor and releases it at the exact moment into the coil. The coil converts the CDI’s low voltage into high voltage.
That high voltage is needed to jump the electrodes of the spark plug. If the CDI is faulty, you’ll get no or an irregular spark. On the Tomos A35, the CDI and coil are one combined unit.

How to recognise a faulty coil/CDI unit

A faulty coil/CDI is noticeable by no or intermittent spark even though the ignition is generating enough voltage.

How to test the coil/CDI unit

Visually inspect for moisture, corrosion or burn marks.
Set your multimeter to Ohms (Ω) to measure resistance.
Ensure the CDI/coil combo is disconnected from the wiring harness.
Measure between the CDI input (wire from the stator) and ground. You should see a low resistance, typically 0.3 - 2 Ω.
Measure between the coil’s spark output (connection to spark plug lead) and ground. This should be much higher, often in the kilo-ohm (kΩ) range.
Infinite (OL) reading likely indicates an open winding (broken wire).
0 Ω reading indicates a short inside the coil.

Check all our coil/CDI units here.

Ignition and flywheel problems

What the ignition does on your Tomos

On a Tomos, current is generated by the flywheel with built-in magnets spinning around the ignition coils. As the magnets pass the copper windings, AC voltage is produced by electromagnetic induction. The faster the flywheel spins, the more voltage is generated.

If a component like a coil or magnet fails, less or no voltage will be produced. Ignition timing can also become incorrect or erratic due to a loose stator plate, loose flywheel, or worn key.

Measuring the ignition with a multimeter

Set the multimeter to AC voltage. Measure between the ignition wire and ground while rotating the flywheel by kicking. You should see a fluctuating voltage. If not, one or more coils may be damaged.
Check that the flywheel is securely fastened. A loose flywheel nut can cause irregular timing or even fly off while riding.
The stator plate must be firmly mounted and in the correct snor or brom position. It is fixed with two small screws. If not tight, the ignition can shift and your Tomos may run irregularly or not start at all.
Check that the flywheel/shaft key is in good condition and not bent or damaged.