Erratic tachometer reading?

[Hellbore]

I have an Actron "auto troubleshooter" that cost me like 60 bucks at AutoPWN. Unfortunately, the RPM measurement jumps around a lot. Any ideas why this would be? My Duraspark module is new, dizzy rotor and cap are new, plugs are new, plug wires are new. Engine is a rebuilt 360, sounds like it is idling very nicely but the tachometer reading jumps all over the place. Tach is connected as the manual says to connect it. I'm puzzled... the multimeter seems to work fine besides the RPM measurement...

[Putt]

Is it the inductive type pickup(wraps around a plug wire)? It may be that the sensor wire is too close to other plug wires or the coil wire and it is picking up more than one signal. The same thing can happen with the factory tach if the wire is too close to the plug wires.

[Hellbore]

It is the type of tach that connects one lead to the negative coil terminal, and one lead to ground.


I'll try moving spark plug wires away and see if that helps thanks.

[Freightrain]

Sounds like poor connection. It just takes the pulse signal from coil and creates a reading from it. If the coil or ground connection is at all loose it will create intermittent loss of signal thus causing erratic needle movement. Ya, sounds simple huh? LOL!!!

Make sure connections are tight, and when needle is jumping around, wiggle wires to see if that makes it better or worse.

Only the "inductive" type would be erractic if around other wires. This would be the clamp on style using the spark plug wire.

[Krewat]

Try it on another vehicle and see if it still does it ...

[Hellbore]

Hey, I think I figured out what was causing it... It is a multimeter-type tach, and so far I had only used it while my timing light was hooked up. Well, that damn timing light must be a doozy because the tach works perfect when the timing light isn't connected.

I also noticed, by connecting and disconnecting the timing light while the engine was running, that when I connect the timing light to spark plug #1 wire, the engine actually runs worse! Crazy huh? Why would that happen? Does the timing light somehow rob some of the engine's spark?

[Krewat]

I take it your timing light has to be hooked in inline with the wire and plug? Your timing light might have a problem, as in, it's sparking internally.

The "engine running worse" is because #1 is misfiring

[Hellbore]

Well it's funny you mention that, I do think the light was sparking internally because when it was turned off (trigger not pushed) it was making a ticking noise inside.

Would that internal sparking cause the cylinder to misfire or does a timing light always make it misfire?

[Freightrain]

Depends on the type of timing light. The clamp on style WON'T create problems.

The other variation, that makes you insert it between the plug and the plug wire MIGHT create issues.