Engine Compression Test Harley Twin Cam Discuss Engine Compression Test in the Motorcycles forums; I was wondering about a step in the compression test. Everything I read on the forums says you should ground both leads of the spark-plugs so the coil does not ...

[BCJ]

I was wondering about a step in the compression test. Everything I read on the forums says you should ground both leads of the spark-plugs so the coil does not get damaged. In the Harley maintenance manual it does not include this step. It just says to remove both plugs to perform the test. What is recommended and why? If grounding the plugs how and where is the best place to do it?

[frisco-rigid]

I have No clue why the HD manual leaves this step out but I just went to re-read & confirm the instructions for all the Elect ign's I've used over the years and they all say Do not run motor without grounding the plugs or wires if the plugs are not installed..
I've read it's to protect the Moduals,, maybe it's to protect the coils too, dunno..
Just to be safe I always ground if spinning a motor..

To ground the plugs (still in the wires) either lay the main body (ground) of the plug on the head or where ever it will sit without falling off,,,, or make/buy a set of alligator clip wires,, one allig clip to securely hold the plug ground strap or main body,,,, then fasten the other end to the sickle frame, heads, barrels, etc.etc. anywhere there is a path to ground..

Have a ball

[unclepsycho]

Without the plugs grounded, doesn/t that send a voltage spike thru the system that can fry things?? maybe thats why the suggestion is to ground the plugs.

[Whopper]

I think the problem is in the modules like Frisco said. Unlike ignition points, which are mechanically open in order to fire the coil, the electronics use a diode like "switch" to stop current to the coil. Without grounding the plug, the energy in the coil tries to find ground through the "switch" and can fry it.

[unclepsycho]

thats good to know,.... thanks.