Compression Test - Best way to gnd coil wire?

[Dshow]

When doing a compression test, what is the best way to ground the coil wire that goes to the distributor? Do people use a special post to ground it out or a clip lead?

Thanks-

 

[ziggy]

Re: Compression Test - Best way to gnd coil wire?

i suppose it's different for every engine. mine. i'm lucky. i got a empty bolt hole the right size within reach of my tower wire....i just plug it into the bolt hole.......

 

[wncrjb]

Re: Compression Test - Best way to gnd coil wire?

on mine, I just remove the power wire on the positive side of the coil....

wncrjb

 

[Robj]

Re: Compression Test - Best way to gnd coil wire?

You can ground the high tension lead going from the coil to the distributor, just disconnect it from the center of the cap and ground it to the block. Or disconnect the power wire going to the coil. Either way is fine.

Have a great day,

Rob.

 

[wire2]

Re: Compression Test - Best way to gnd coil wire?

If the coil to distributer lead won't easily reach or stay grounded, a jumper lead of small wire with aligator clip ends is fine to extend it.

 

[Don S]

Re: Compression Test - Best way to gnd coil wire?

Without knowing the year, make, model, type engine, or ignition system, there is no way of telling you the best way for your system.

 

[Haut Medoc]

Re: Compression Test - Best way to gnd coil wire?

I use a jumper wire with alligators on each end......​

 

[Dshow]

Re: Compression Test - Best way to gnd coil wire?

Sorry, I have a 2001 Mercruiser 5.7L EFI (TBI)...

I have an alligator clip, I will just use that.

Thanks-

 

[SFT2]

Re: Compression Test - Best way to gnd coil wire?

One of the (few) neat things on my previous boat, a Seadoo Speedster, were a pair of lugs on the block that the plug wires plugged onto for compression tests and similar. If I could find something like that with 4 lugs (current motor is distributorless and has 4 coil packs) I'd stick it on there in a minute. Suppose I could just stick some brass rod in the lathe and make something, though.

 

[Dshow]

Re: Compression Test - Best way to gnd coil wire?

Before my present boat, I had a 3 cylinder Waverunner that I would do compression tests on... I thought about breaking 3 old plugs to get the metal spark plug connection out of to make a grounding buss bar. Does anybody make anything like that?

 

[rodbolt]

Re: Compression Test - Best way to gnd coil wire?

with a TBI or an MPI ya gotta do more than ground the coil wire, reading the manual would be a first step.
understanding how the fuel and ign system work is mandatory.
the fuel system does not rely on engine vacum to move fuel into the intake.
Buy a manual before ya burn.
please.

 

[wncrjb]

Re: Compression Test - Best way to gnd coil wire?

yeah, since you have an '01... delete my previous suggestion.........

wncrjb

 

[Don S]

Re: Compression Test - Best way to gnd coil wire?

You can download the OEM manual for free at the link below.
You probably need manual #24 for your engine (Merc SN needed to be sure) and manual #14 for your outdrive.
http://www-alt.mercurymarine.com/mnetdata/sisdata/srvcovr.pdf

yeah, since you have an '01... delete my previous suggestion.........

Actually, that would be the best thing to do with some ignition systems, that or pull the kill switch lanyard.
Without spark, the injectors can not fire and pump gas into the engine.

 

[Dshow]

Re: Compression Test - Best way to gnd coil wire?

So now I'm confused... Is pulling the kill switch lanyard and grounding the coil wire enough to keep from going boom? I did a search on "Compression" in manual 24 and did not see anything on how to perform a compression test. I looked through a service bulletin dated June 99 on how to perform a compression test and it basically mentioned the grounding the coil.

 

[Lakester]

Re: Compression Test - Best way to gnd coil wire?

hello,
i just did one yesterday, monday. pulled plugs. its a 71 85HP v4. evinrude. i used 2 small visegrips set on light and put up inside 2 plug ends and closed. 1 each per side. this made contact with lead high tension end. then i placed end of plier to ground on block. scratched a bit till it got shiny. solid ground there. and then i installed guage, cranked till highest reading etc, etc.
hope this helps some
regards
lakester

 

[Don S]

Re: Compression Test - Best way to gnd coil wire?

Is pulling the kill switch lanyard and grounding the coil wire enough to keep from going boom?

First of all, nothing is going to go boom if you don't have gas fumes in the bilge. Use that gas detector on the front of your face and check. If yours doesn't work, find one that does.
If everything is clear, then go on with your testing.
Pulling the lanyard (if it's actually hooked up and in working order) will kill all power to the ignition system. No power to the ignition means no power to the fuel injectors. At that point, you don't need to ground the coil wire. There is no power getting TO the ignition, so there will be no spark coming from the coil. If you are not sure, check for spark by cranking the engine and holding the coil wire near a good ground. no spark everything is fine.

Lakester.
LOTS of differences between an old outboard and an EFI IO. What works for one, won't work for the other.

 

[Dshow]

Re: Compression Test - Best way to gnd coil wire?

Thanks Don.

 

[rodbolt]

Re: Compression Test - Best way to gnd coil wire?

what DonS is trying to say is over the years different systems required specific procedures.
the one thing specific you need to be careful of when working with any EFI system is the fact of fuel is INJECTED regardless of plugs in or out.
on EFI the ign system needs to be properly disabled and certain operations also require the fuel system to be disabled.
I watched some backyardigans light up a 150 EFI mariner that way.
doing a compression test. I warned them but they knew more than me.
on an EFI motor simply grounding the coil wire WONT shut off the fuel. as the engine cranks fuel is sprayed . this fuel is a chemically correct A/F mix.
it will collect in the bilge as its sprayed out of the empty plug holes.
not to mention it can throw off the compression readings by adding liquid to the cylinder.
having seen a few engine boxs launched over the years and one complete upper deck seperation gas fumes in the bilge i rather scary.