Ignition coil grounding

[Slice of Life]

I'm ready to make a compression test on my 1992 Mercruiser 5.7 350 with Thunderbolt IV ignition system, and after reading a lot of posts here regarding making a compression test, I'm still a bit confused on grounding the ignition coil. I've never done a compression test and am new to this. Some say simply remove the coil wire to the distributor cap. Some say remove the safety lanyard wires from the switch. Some say using a jumper wire to all the plug wires and ground them to the block. The simplest seems to be using a grounding jumper wire on the ignition coil to each of the two wires connecting each other. This is what I've done and do not want to damage the ignition system by doing this incorrectly. All plugs are removed, wires moved away from the engine, and I have my test kit ready. I'm assuming having the throttle all the way forward sufficiently sets the carb correctly. Thanks.

 

[Bt Doctur]

Not the way to do it, that way could damage the module. Easiest way is to just remove the purple wires from the + side of the coil

 

[Slice of Life]

So with a purple and grey wire on each side, I need to remove the purple wire on the + side. Should the grey wire remain there on the + side? Thanks.

 

[Bt Doctur]

the grey wire (negative side of the coil)is the tach feed and the module signal lead. they can be left alone

 

[Slice of Life]

Ok, I see both are purple now. Thanks a bunch Bt Doctur, you're the best!

 

[achris]

There shouldn't be a grey on the + side. 2 purples on the + side and 2 greys on the - side. Easiest way to 'ground the coil' is to take the HT lead from the centre post of the distributor cap and make it touch the engine block/heads/manifold.

 

[Slice of Life]

Yeah I tried that first and couldn't get the engine to turn with the key, nothing happened. Battery's fully charged with good connection on terminals.

 

[Bt Doctur]

then you did something wrong, grounding the HT lead has nothing to do with starting a motor or not allowing it to crank

 

[Slice of Life]

Yup, definitely did something wrong as now nothing happens when I turn the key to start. Great.

 

[Slice of Life]

Ok, so apparently having the shift in full forward does not allow the engine to turn now. I still had it in forward when taking the new outdrive on/off.

 

[achris]

Yep, that's the neutral safety switch. doesn't allow cranking in gear.

 

[Bt Doctur]

you need to be in throttle only position ,either push the button in or pull the handel outward

 

[Slice of Life]

Ok, definitely smelling fumes after testing #1. Since there's no chance of a spark with the purple wires off of the coil on the + side then I'm assuming I'm still safe

 

[Bt Doctur]

That is why you disable the spark producing component, a good test is atleast 4 revoloutions on the motor

 

[Slice of Life]

Ok, completed test with 5 revs for each cylinder.
1) 130 psi 2) 125 psi 3) 122 psi 4) 122 psi 5) 125 psi 6) 120 psi 7) 125 psi 8) 122 psi

According to merc service bulletin http://www.boatfix.com/merc/Bullet/97/97_25.pdf these represent acceptable numbers. I'm reading now I need to try this with a bit of oil in the cylinders. Would WD-40 be acceptable? Then a leak down test. That's another day and a bigger challenge it looks like. I want to run these tests due to possibly running the engine without water for 2-3 mins a week ago when I thought the new out drive was pumping water out to the engine only to find no water in the block afterward. Thanks for the help guys, as always.

 

[achris]

As those numbers look good, I wouldn't bother with anything more. The usual result of running without water is a blown head gasket, and that will definitely show in a compression test alone.

Chris.......

 

[flipbro]

Hooking a vacuum gage up can also tell you alot about the health of an engine.

 

[achris]

Hooking a vacuum gage up can also tell you alot about the health of an engine.

Gee, I thought I was the only one old enough to remember using a vac gauge as a diagnostic tool...