Inductive Pickup Clamps for Timing Lights, Scopes.

[George7941]

Why does it have to be positioned close to the spark plug? The instructions always specify that.

My understanding is that the clamp senses the electrostatic field around the high voltage spark plug cable. The voltage is just as high in the middle of the spark plug cable as it is at the cable end next to the spark plug.

 

[eljefino]

Maybe so it can differentiate one cyl sparking? Wires get bundled together on the way to the coil(s).

 

[JHZR2]

Yeah my vote would be differentiation, back emf from some other plug and/or because the plug is just a friction/butt connection and perhaps is more leaky.

 

[George7941]

Differentiation,I don't know. The clamp does encircle the cable so that there is a closed path around the cable to pick up all of the electrostatic field around the cable and also to reject outside fields. In the case of Timing Lights it either sees a strong field and triggers the light or it sees a weak field and does not trigger the light. In other words, a weak stray field does not affect the trigger signal at all.

Two good guesses but I am not buying either one.

 

[KB2008X]

I'd say it's recommended for accuracy, obviously. And I'd assume it helps eliminate picking up the wrong signal from another wire.

I suppose another possibility is that it takes time for the spark to travel down the wire. You'd want to set the timing based on when the "spark" arrives at the spark plug, versus when it first enters the wire, at the distributor cap for instance. Now, how those few inches (10 to 30 inches ? ), at 800 rpm can make a difference is beyond me. Although, from a performance standpoint it's recommended that you keep the wires as short as possible.

Years ago when all engines had distributor caps and fairly long wires I moved the location of the pick-up from near the plug to near the cap on numerous occasions and don't recall an appreciable a difference.

 

[George7941]

Electricity in a wire travels at the speed of light, so the time it takes to travel the length of the plug wire is simply not an issue.

 

[RF Overlord]

Just for grins, the voltage will take approx 1 quadrillionth of a second to go 1 foot. That's 1 femtosecond, or .000000000000001 of a second.

Not even Toyota can control spark timing that closely...

 

[spackard]

Encourages the slack to lay on the valve cover instead of falling into the radiator fan, on V8 with distributor at front of engine. That's my guess.

 

[George7941]

I checked the Actron website since they sell timing lights and looked at a user manual for one of their lights. Here they do not mention that the clamp has to be close to the plug. In the accompanying diagram the clamp is actually on the middle of the cable.
http://www.actron.com/publish/2007/03/08/pdf_english_16315.pdf

I checked the OTC site but they do not have manuals for their timing lights at the site.

 

[George7941]

Just as an interesting exercise, a question - why is the clamp marked on one side 'THIS SIDE TOWARDS PLUG'?

I know the answer, will reveal after seeing how many know.

 

[JetStar]

I have not tried flipping the clamp on a timing light to see what happens, but I can tell you that on an amp clamp, direction will determine the polarity or phase (if you are using one on a scope) of the indicated current.

 

[George7941]

You are right, it is to ensure the correct polarity of the trigger signal generated by the clamp.