Grounding plug wires while checking compression

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234rick

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The last few days i've seen several post mention doing damage to electrics if you don't ground plug wires during compression checks. What kind of damage can that cause ? My OMC manual for a 1989 25hp doesn't say anything about grounding plug wires. It doesn't even say to unplug kill switch or any other wiring. Or even to have key turned off. This last one may sound stupid but for what these manuals cost I think they should have reminders about simple things.
 
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ShafferNY

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Re: Grounding plug wires while checking compression

I've never heard of grounding plug wires while doing a compression check. It's new to me.

I can't really see where it would damage anything by not grounding them, unless one of the loose plug wire were to arc off something it shouldn't.
 
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Craig-

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Re: Grounding plug wires while checking compression

I learned the hard way on 2001 90hp. It can take out the power pack diodes. I normally pull the kill lanyard before a comp test to crank, forgot to last time. Seems to affect the OIS models the worst. Now my pack double fires, put the timing light on it and see 2 & 4 at the same time and sometimes plugs fire twice real quick on a spark tester. Never had this on any other motor I've owned or worked on. All cylinders are firing enough to run and double firing and misfiring. You would think the motor wouldn't run at all but it does, taking 1500 RPM with it. Hard to diagnose, thinking it was fuel starved.

See my post for Homemade spark tester video
http://forums.iboats.com/showthread.php?t=423333
 
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F_R

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Re: Grounding plug wires while checking compression

234rick said:
The last few days i've seen several post mention doing damage to electrics if you don't ground plug wires during compression checks. What kind of damage can that cause ? My OMC manual for a 1989 25hp doesn't say anything about grounding plug wires. It doesn't even say to unplug kill switch or any other wiring. Or even to have key turned off. This last one may sound stupid but for what these manuals cost I think they should have reminders about simple things.
Click to expand...

You need to understand something about an ignition coil: When it is time to fire, the voltage going to the spark plug rises very rapidly until it is high enough to jump the gap in the plug. As soon as it jumps the gap, all the energy is dissipated and it is all over till the next time. So what happens if there is no plug gap to jump? The voltage rises to as high as is possible with the system before it stops---OR jumps somewhere else. That somewhere else could be through the side of the coil. Once it arcs through the side of the coil, it is a junk coil.

In some cases, as the voltage collapses, it feeds back through the powerpack. That can cause damage too. I suppose that all depends on the design of the system. Maybe you will get away with it, maybe you won't.

There is another reason for grounding the leads. If they are near a grounded object, they can arc to it. When the motor is cranking, fuel is being pumped out the open plug holes. That, in the presence of the arcing can cause a fire. Believe me, I've been there and done that. Fire shot out about ten feet. Dang lucky I was standing to one side.
 
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bigpoppakdog

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Re: Grounding plug wires while checking compression

would unplugging the power pack save you from grounding out the plugs?
 
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Chris1956

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Re: Grounding plug wires while checking compression

Why not jumper the starter solenoid, with the ign key off, to test the compression? It is much easier than anything else, and there is no risk of damage to the ign system.
 
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234rick

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Re: Grounding plug wires while checking compression

Chris1956 said:
Why not jumper the starter solenoid, with the ign key off, to test the compression? It is much easier than anything else, and there is no risk of damage to the ign system.
Click to expand...

Unless you hook the jumper to the wrong solenoid post & then you fry stuff.
 
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Chris1956

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Re: Grounding plug wires while checking compression

You should be jumpering the +12VDC secondary side of the solenoid to the primary side of the solenoid. If you put +12VDC onto the primary ground, you will get a short, and likely burn your hand, or maybe have a small electrical fire. if you jumper to the other primary screw, the starter will spin. Since the ground side of the solenoid is always black, jumpering to it would be dumb.

I don't see how you could fry anything but your hand.......
 
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Fl_Richard

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Re: Grounding plug wires while checking compression

I cant even see a hand burn here.

No potential for damage connecting to anything but ground. It's not hard Big red wire to little connector. Go big red to big red and the cable sparks and the engine cranks. I fyou happen to hit ground you'll know it :)

I used to be a lanyard puller. No more. This is way simpler... I like simple solutions.
 
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F_R

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Re: Grounding plug wires while checking compression

IF--IF you have an OMC with a safety switch, and IF you jump to the correct small terminal, the solenoid will activate and the motor will crank. BUT if you jump to the other small terminal, you WILL instantly destroy the safety switch (read "fry"). They generally have white wires on both small terminals, so it is risky to guess at which is which.
 
jmendoza

jmendoza

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Re: Grounding plug wires while checking compression

This is true of all engines with coils, be them magneto points, or CDI/solid state, you must provide a path for the spark or it will degrade the coils, condensor, and on electronic CDI systems. It damages the Power-Pack, or electronic ignition module as they are not designed to be back fed 20K volts from the coil when there is no spark plug gap to jump. We see condensors on old points systems go bad too, as well as coils from excessive cranking with no plugs...but the electronic pointless ignitions are the most susceptable and easily damaged. The funny thing is they will work intermitantly for some time when damaged and cause the engine to give you random fits that come and go, making troubleshooting often a difficult task.

Recently we have found on older engines using condensors that unless the condensor is bad, don't replace them. Why? Well because we have had more failures of condensors that were replaced during a tune up than any other ignition failure due to them being now made in Taiwan or China, and they don't hold up as well as the old Mallory, and Phelon USA made condensors.
 
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dazk14

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Re: Grounding plug wires while checking compression

OK - Am I reading this correctly...the best/bullet proof way to crank any electric start motor for a compression test is to jump the solenoid from big terminal to big terminal?

Basically, running the battery power directly to the starter, just taking the short cut of end to end on the solenoid.

It would be great if a couple of experienced guy chimed in here.

Thanks!
 
Rscardina

Rscardina

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Re: Grounding plug wires while checking compression

i'm a bit confused on this method..
i understand the theory behind it and that it will crank the starter..however, i was under the impression this would be done by also unplugging the big red harness connector as well becasue the "kill" is still engaged at the key's normal "off" position?????

Or is that basically the point...to have the kill engaged so you do not create the spark to begin with but still get the cranks you need for testing compression..

I think i just answered my own question..:rolleyes:
 
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boobie

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Re: Grounding plug wires while checking compression

Read FR's post above. It all depends on what mtr you have . As FR stated if there is a neutral sw on the mtr you could do damage. What I would do is take both SMALL wires off the solenoid. Then ground one and put 12 volts to to the other terminal. You are only dealing here with the primary circuit of the solenoid.. Jumping across the solenoid sucks. Two many sparks. Leave your red plug connected and make sure your ingition sw is in the OFF position.
 
HighTrim

HighTrim

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Re: Grounding plug wires while checking compression

I like using a remote starter with the plug lead running into an inline multi cylinder spark tester. tests compession and spark in one. Only downfall is inviting the wife outside as an assistant.

Wish I could teach the dog to it the remote starter button ;)
 
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autohead18

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Jul 25, 2010
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Re: Grounding plug wires while checking compression

put a toggle switch on it i did to mine and its graet just dont forget its there you dont want to kill your batt out there in the watter i dont think it will really effect it though i did it all the time with my 85 hp
 
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Craig-

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Re: Grounding plug wires while checking compression

An automotive remote starter is simply a momentary push button switch which clips on any 12v source and S or 12V on solenoid coil. It energizes the coil which pulls the contactor connecting big red to big red turning starter. The same thing that turning the key does, but not switching ignition on.This only requires 3-4amps, using it to connect big red to big red requires much higher amperage switch. If you use this type of switch you will feed 12v to whatever else is connected to switch side of solenoid, the other side is ground. An outboard remote starter plugs into the engine harness bypassing the keyed ignition switch. An automotive remote can be used but remember the 12v you connect to the switch gives 12v to anything else connected to the + side of the solenoid switch.
 
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