Help with ignition coil


#1

I have a 1995 Mits Eclipse GSX and I am testing my ignition coil. because my car seems to idle a little rough. According to the Chilton’s guide, it states the primary coil resistance should be 0.70 - 0.86 ohms. I am getting approx 1.5 ohms. The secondary coil resistance is within normal limits. My question is do I have a bad ignition coil or it possible that the Chilton’s guide is wrong? I would hate the replace a part that works fine.


#2

Chiltons manuals are never to be trusted IMHO. They’re very sparse on real info and frequently print generic or flat out wrong information.

There are many reasons for a rough idle so it’s difficult to be concise here.
Low compression, misfire due to plugs, wires, etc., vacuum leak, idle speed too low, incorrect ignition timing, weak valve spring, bad injector spray pattern, or a faulty engine mount.
I would look at the front center mount by the radiator very closely. These are often problematic.


#3

Rough ignition…the LAST place I’d look at is a faulty coil. There are many things I’d check first…

Plugs, Compression, Fuel filter, wires, timing…Check all those first…i’ll bet you’ll find it there.


#4

Have you zeroed out the ohm meter on the low scale. Touch the two leads together. If you get a reading and the DVM does not have a ‘zero’ button, subtract this reading from any subsequent reading on the coil. Also the temperature of the primary has an effect on the resistance reading. The higher the temperature the higher the resistance. Usually when you are looking for trouble in the coil primary you are looking for opens, i.e. infinite resistance, or lower than specified resistance, i.e, shorted winding. Usually a coil problem leads to no start or intermitant dying as if the key were turned off.

As others have said you may have a spark plug, plug wire, fuel or mixture problem. Is the CEL light ‘on’. Have you scanned the computer for codes.


#5

Chilton manuals are always wrong. You can still replace it just because it’s old.


#6

“my car seems to idle a little rough.”

Forget the coil. Check for vacuum leaks or a stuck / dirty EGR valve. Dirty injectors will first show up at idle…

Ignition problems usually show up first at full throttle…