Testing coils & spark plug wires - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


Trouble shooting + Test Procedures Need to trouble shoot whats wrong and need to know the best way to test for it? Here it is.

 
 
 
 
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Old 01-13-2009, 04:24 PM   #1
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Default Testing coils & spark plug wires

So, you car is sluggish, hesitates, stumbles, has weak spark, or just does not seem to be running right. Could it be your coils? Could be, so lets test them. All you need it a multi tester. You can get these for under $10 and there is no reason to not have one. Even if your only going to use it this one time. It will cost more to have it checked at a shop and they will do nothing more then we will be doing here now.

This tester was only $19, no reason to get a real big fancy one. Even a small dial one is fine.


First thing we will test it your secondaries. This is easy to do and you do not need to remove them from the car.

Here are your coils. You will want to pull the two spark plug wires off. Give them a twist and pull at the boot, not the wire. Only do one coil at a time to avoid mixing up the wires.


Now set your multi meter to Ohm and to the correct scale.


Now put one probe each on the post of the coil like this and give the meter a moment to settle in. On this 97 I'm getting 5.5 on the 20k scale. A reading of infinity [open] or no resistance [short], the coil is junk.


Put a little dab of dielectric grease on both posts before putting plug wires back on.

Different brands of coils will have different readings. As long as your between 5.0k and 8.0k ohms your coils are fine. You also want your coils to all be about the same reading.

Now we will test the primaries. For this you will need to remove the coils from the car. Again, do this one coil at a time to avoid mixing up your plug wires. You will need a 5.5mm socket, a 7/32 socket will work also. There are two bolts holding the coil to the ICM. Remove both of these. You may have to wiggle the coil a bit to get it loose.


You will need to get some wire to insert in to the slots on the bottom of the coil. Make sure you have good contact.


Now inspect the contacts on the ICM for corrosion, clean if needed. Then add a small amount of dielectric grease to the terminals and put coil back on ICM and bolt in place. No need to tighten the bolts to much, just snug.

Again, different coils will have different readings. Generally you want between .5 and .8 ohms If you have zero, or infinity, the coil needs to be replaced. Also, if your getting a large variation between your coils it should be replaced.


Now would be a good time to test the plug wires as well.

Quote:
For cables less than 25 inches in length, resistance generally should not exceed 30,000 ohms.
For those over 25 inches in length, the maximum acceptable reading is 50,000 ohms.
They say stock wires can last 100k miles. But in reality they should be changed between 30k and 50k. I tested my stock 7mm wires with about 70k and they tested out as having 10x more resistance compared to my new 8.5mm MSDs of the same length.


Quote:
If you have a hard start problem when the engine is cold, remove the coils and place them in your freezer for a god hour. then test them again. The numbers should be the same. If not, you have a bad coil.
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