Mariner DVA readings at ignition coil

[Coxn]

I'm looking at the electrics of a rough running Mariner 60hp. Red stator, trigger, adaptor, switchbox, and coils all check out according to DVA and ohm specs. When I measure the cranking DVA from the switchbox to the ignition coils, I get a steady 275v on all three. When the engine is running, however, the DVA readings at the coil terminal cut in and out and are all over the place. The connections are sound enough. Anyone know why that might be?

 

[Faztbullet]

Using a DVA adapter?

 

[Coxn]

Yes. The DVA doesn't seem to steady out the multimeter when measuring across the terminals and the engine is running. Only when cranking.

 

[Texasmark]

The DVA is a sample and hold circuit. The trigger pulses are narrow with a long rest time. To get a meter to read such, you need a storage element (capacitor) to "hold" the voltage sampled when the trigger fires. With your DVA working best when the rest time is greatest, I'd say it and your meter are working ok but you may be picking up noise from the spark plug firing on your leads.

A solution to try would be to: (1.) Move your meter as far away from the engine as the leads will allow; or (2.) Twist your leads together crossings spaced about 1" such that interference energy strikes both wires equally which cancels the effect; or (3.) Wrap the wires side by side in an outer covering of aluminum foil to shield them.

That's what I'd do if I encountered what you said.

 

[Coxn]

Thanks for the excellent suggestions. They didn't help, but I get where you're coming from I was thinking there might be some kind of fault in the coils themselves which is adding the interference. The old ones meet ohm specs and spark across a 7/16" gap, but it's possible the sparks are intermittent and not hot enough.

I'll try new coils and see what shakes.