Timing Light

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Original Submission by T; July 28th 2006.

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Xenon Timing Light. Connect the Red Wire to the Battery +ve. Connect the Black Wire to Battery -ve. Clip the Sensor around the Number 1 Plug Lead. Pull the Trigger on the Handle to make the Light Flash. The Flash will only occur while the Engine is running and generating Sparks. Photo by HZCambridge. Click to Enlargen.

Timing Light:

Timing Lights are used to check and/or set the Ignition Timing on a Petrol Engine.

Timing Light Types:

Static Timing Light:

This is just a 12 volt Globe. This is useful for timing Breaker Point Distributors. Set the Crankshaft to the Firing Point and rotate the Distributor backwards and forwards until the the Light comes on.

Neon Type:

This is a very cheap way of setting the Ignition Timing when the Engine is running. The flash output is weak and the Neon Tube draws excessive current away from the Spark-Plug so that mis-fires can result on Engines with weak ignition. The light output is low and the flash duration is too long to be of any real use in checking or setting the advance.
12 V DC Light. Can be used for Static Timing as well as for Injector Testing. Image by Jacks.

Xenon Bulb Type:

This is the most prevalent type. It has a pair of leads, one red lead the other black, which are connected across the Car's 12 Volt Battery. Another lead from the Timing Light connects to the number 1 Spark Plug Lead.When the unit is connected and the trigger is pulled, you can often hear a high pitched whine emerging from it. This sound is the Electronic Inverter inside the unit. The Electronic Inverter converts the 12 Volts DC to a much higher Voltage, likely 200 volts, which is connected across a Xenon Tube.
Xenon Timing Light with Dwell, Tach, Amps, Volts, Ohm Meter. A Remote Starter is leftmost. Photo by HRAmbo. Click to Enlargen.
When the number 1 Spark Plug is fired by the Coil, the Voltage from the Spark Plug Lead causes the Xenon Tube inside the Timing Light to ionise. At this point the Xenon Tube conducts and the DC Inverter's 200 Volts discharges through the Tube delivering a blinding flash.
These types of Timing Lights are very affordable and well worth owning if you want to keep your car running in top tune.Connect the Red Wire to the Battery +ve.Connect the Black Wire to Battery -ve.Clip the Sensor around the Number 1 Plug Lead.
Pull the Trigger on the Handle to make the Light Flash. The Flash will only occur while the Engine is running and generating Sparks.

The Ignition Voltage vs Compression Phenomenon:

An important electrical phenomenon appears when a Xenon Timing Light is hooked up to a
Spark Plug Lead on a Petrol Engine.
The Timing Light will flash when the Compression Pressure inside the Cylinders is high, and not flash when the Compression Pressure inside the Cylinders is low. Essentially the Spark Plug can short out the Spark Plug Lead when the Compression Pressure is low and doesn't leave enough Voltage to trigger the Timing Lamp.

This is because it takes more Voltage to fire the Plugs when the
Compression Pressure is high and minimal Voltage when the Compression Pressure is low.

When the Engine is decelerating the lamp will stop flashing because
the Compression Pressure inside the Cylinders is very low. It's low at this time because the
Engine is compressing the high vacuum that was created in the inlet manifold with the throttle
closed and the Engine decelerating. This weak spark requirement is the reason why
Engines polluted heavily on the overrun prior to VK Commodores.

The Lamp will flash brightly under acceleration and when there is
enough Compression to allow the Engine to idle.

If you use a very expensive and sensitive Timing Light you may not see the symptom.

The observation explains why Engines with weak Ignition stall on take off. Take off is the time the Ignition needs are greatest and Idle the time when the Ignition requirements are least. Clearly the overrun case needs even less spark, but no spark at all is needed for the overrun case.

In electrical terms the Spark Plugs behave like dynamic Zener Diodes. (A Zener Diode is one that only conducts when it sees a certain Voltage. A dynamic Zener Diode is one where that certain Voltage can change). 
Timing Light. Photo by HRAmbo. Click to Enlargen.

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