How to ground wires when doing compression test?

[hkeiner]

How do you properly ground the spark plug wires when doing a compresson test?

I am planning to do a compression test for the first time on my 1996 Mercury 150hp 2-stroke outboard. I have read many posts in this forum on compression testing and they mostly agree that it is important to properly ground the wires to avoid damaging the electrical system. It is not clear to me, however, how to "properly ground" them.

1) Do I unplug and ground all 6 wires at the same time or only ground the wire on the cylinder being tested?
2) How do you physically ground the wires?
3) Is there a kill switch or fuse that could be removed in lieu of grounding each spark plug wire?

My guess is that i would remove all six plugs at the same time, put the removed plugs into the wire boots, wrap unshielded wire around the threads of each plug, and then connect the wire end to a part of the motor block)

 

[Laddies]

Re: How to ground wires when doing compression test?

The most common way is to hook plug to them and ground the plugs we use a spark gap some people use jumper wires to ground the plugs, if you jumper the wire dirrectly to ground the it won't run a timing lite but you can do a compression ck

 

[Barnacle_Bill]

Re: How to ground wires when doing compression test?


I use a push button switch I made. Solder a wire to each connection on the switch and terminate them with alligator clamps. Insulate the switch with tape or whatever, Now you can connect one alligator clamp to the post on the solenoid coming from the battery and the other to the post that actuates the solenoid. Push the switch and it will actuate the starter. You can do this with the key off so you don't have to worry about grounding the plugs.

 

[JB]

Re: How to ground wires when doing compression test?

bhile's method is the safest way to do it.

1. The ignition system is dead, so no spark to worry about.

2. You have the starter control where you are, at the back of the engine, so you don't need a helper and there is no possibility of miscommunication.

 

[hkeiner]

Re: How to ground wires when doing compression test?

Thanks for the advice. I found a remote starter control at a local auto parts shop. It has two leads (with large alligator clips) and a hand held button switch. It cost about $10 and worked great!

I think using a remote starter control with the ingition off was definately the best way to do a compression test. Using the boats's key starter switch and properly grounding the 6 spark plugs at the same time seemed much more awkward and risky.

 

[saumon]

Re: How to ground wires when doing compression test?

Or simply activate the kill switch that shut off the ignition and use the remote starter, much simplier!