Coil Voltage

[Matt99XJ]

How many volts should be coming from the wire that goes to from the Ignition coil to the distributor? Will it be AC or DC?

[Rev Den]

A lot, think thousands of volts,....DC.....You will not be able to measure it with a voltmeter, you will need a scope. You can also use a plug tester to see it.

Rev

[Bender]

I'd say between 25k and 30k volts.

[Matt99XJ]

Awesome. I have been trying to figure out why the Jeep won't start. I was driving down the road and it just shut off. Now it just turns over and never starts. I thought it was the CPS so I replaced it and I still had the same symptoms.

I tried to test the coil when trying to turn it over and got like 40 volts.

The manual says to test the coil resistence to see if it is bad and I kep getting really low resistance. If it went bad it should be OL right.

Does it sound like the coil would just go out on me like theat? I only have 50k miles.

Also how much do coils normally run?

[Rev Den]

Did you clean the coil contacts on both the coil and the coil holder?? The resistance should be whatever the FSM says it should be. How did you read the 40 volts? You cannot use a meter to read the coil output.

Rev

[Matt99XJ]

Yes I cleaned all the contacts.

The manual says that I should have 1150-1350 ohms through the coil. I was getting like 24ohms.

I thought if the coild went bad it would read OL

[Rev Den]

Make sure you are reading the correct side, primary or secondary, check them both. Also, an interanl short could cause the resistance to read lower, it would still be bad. BUT......make sure you are reading the correct side.

Rev

[Matt99XJ]

From what i gathered from the manual.

Primary is from the + to - through the plug

Secondary is from + plug side the the + coil on the front.

[AZ Jeff]

There are TWO sets of windings in a coil. (It's a step-up transformer, in reality.)

The "primary" side gets fed something around 12V (maybe less on some systems in certain cars, but at least 8V in all cases). The "secondary" side will generate between 40 and 60 THOUSAND volts. The problem is that the secondary voltage is a pulse of very short duration, NOT a steady level. Thus, capturing this on a meter is impossible.

Measuring the impedance of the coil is the only way to do a home-brew diagnosis of the coil's performance. The primary side is measures across the two small terminals on the coil. The secondary side is measured between the contact in the center of the coil, and the metal outer jacket of the coil, which is ground.

Coils can fail two ways:
1. by becoming open (no connection), and having infinite impedance. (OL)
2. by shorting internally, and having lowered impedance.

Replacement coils can be had for $20 or so at most parts shops, if the impedance measures bad.

HINT: BE sure to DISCONNECT all wires to the coil when measuring impedance, so as to not have other items that are connected thru the wiring influence the readings you get.

[Matt99XJ]

Thanks Jeff. I am pretty sure that is the problem. I am going to get a new coil and out it on tomorrow.

Anyone else have any input?

[Rev Den]

Jeff.
How ya gonna measure impedence?? Impedence is a function of an AC signal thru an inductor. I think what you ment to say was resistance.

Rev

[sternbal]

Originally posted by Rev Den
Jeff.
How ya gonna measure impedence?? Impedence is a function of an AC signal thru an inductor. I think what you ment to say was resistance.

Rev

In the absence of any capacitive or inductive effects, impedence is equal to resistance. The use of impedence in this context is technically correct, however it is probably overkill.

[Rev Den]

A coil works via inductive effects....as such...the term is used incorrectly in this case. While we may debate if the term is technically correct, it is misleading in a real-world enviroment that is represented here.

Rev

[sternbal]

I agree that a coil does work through inductive effects, however they are not present when measuring the coil with a dc meter since di/dt=0.

I also agree that resistance would be a better term to use for this situation, although it probably isn't really important anyhow. I've just that when I'm not spending all my time on my XJ, I'm spending all my time on EE.

Matt99XJ, I hope that wasn't an expensive meter you used to measure the coil output voltage. I'm not so sure I'd trust it to measure resistance after that.

[Rev Den]

"I agree that a coil does work through inductive effects, however they are not present when measuring the coil with a dc meter since di/dt=0."

Exactlly.

Rev

BTW - Check out http://www.thinkgeek.com

[Rev Den]

:D

Rev

[XJEEPER04]

just a though but you might want to check your ignition cotrol module

[Matt99XJ]

Turns out the coil was shorted. Once I found that out I took it to the dealer cause I have a 8/80 warranty. They hooked it up to the computer, verified it and changed in about an hour.

Runs just like new now.

[ChEwBaCcA]

Ahhh Shizlil . I know an EE that DIDNT know how a auto engine works! not to mention didnt know how a DC AC motor works......

Fo Sizzle
Chewy

[Jeremyvnc]

I have a 94 jeep cherokee but cannot find my ignition control module where is it?