How to use a Compression Gauge

I just got a new compression gauge in the mail and the directions say I need to remove the ignition coil wire from the ignition coil.

Do I really need to do this or can I just pull the plug and put the gauge in?

 

You can just pull the plug caps off the plugs.

 

*palm to forehead*

Man I feel dumb.....

 

Remember to hold your throttle wide open when you crank it over.

 

You can damage your coil firing it without having a path to ground.

To get a good reading all the plugs should be pulled. If you don't want to disconnect the coil, put the caps back on the plugs and let them rest against the engine.

 

So, we should beware of very wide plug gaps & failing plugs then.

 

The factory book says to unplug the TCI unit. I think that pretty much is the same as disconnecing the coils, just from the other end.

 

Disconnecting the TCI and pulling all the plugs: I can see that. But why should you hold the throttle wide open? The starter isn't going to turn the engine any faster..

 

So the closed throttles don't affect the intake, you want the cylinders to pull as much air as they are capable of. It's less hassle than pulling the carbs although that would probably be better yet.

 

Yep. And often much easier to get to.

 

Spark plugs fail by providing an alternate ground than through the electrodes (broken insulator, fouled etc).

The coil works by having the primary windings build up a magnetic field. When the power is switched off the field collapses inducing high voltage in the secondary. That energy has to go somewhere. If it can't dissipate out the plug wire it will dissipate through the coil, potentially damaging the insulation and creating a permanent "leak".

 

I am not spoiling for a fight, Carl, but my research tells me that if the HV is fed back into the coil, it gets converted back to 12 V. Wiz.

 

Wiz, you are right. The returning voltage will be generated. And will FRY components on the upstream side of the TCI as well as bugger the coils.
It cannot be stressed enough that you NEED to disconnect the TCI or ground the plugs when performing a compression test.
Other notes on compression tests:
Pull all of the plugs. This way your starter isn't fighting the compression of the other cylinders to spin the engine. This keeps you from running your battery down more quickly.
DO NOT operate the starter more than 3 or 4 revolutions without a cooling cycle of about 3 or 4 minutes. You will run the risk of overheating the starter and this will increase the current draw of the starter motor. This in turn will run down your battery more quickly.
Open that throttle up as much as you can (you can pull the carbs if you are inclined, this keeps the engine from pulling in fuel and washing the cylinders with unburnt gas). This lets the engine pull as much air in as it can, allowing for greater accuracy on the test.
Write down the numbers. Make a chart and track your readings. Keeping records will help you track your engines health.
Anyone want to chime in?

 

Amen?

 

I disconnect the vacuum line so I'm not pulling gas. That is in addition to the stuff Robert and others have outlined.