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  #1  
Old June 16th, 2017, 11:55
md21722 md21722 is offline
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How to test an ignition coil? Is the coil or DMM bad?

I am wondering if I have a failing ignition coil. It seems like I had decent power and now the engine is louder, tranny shifts more, and Jeep seems to have less power. I put my DMM on the 200 Ohm scale and measured across the coil and got 1.8 ohms. After it cooled down I got 2.1 ohms. A couple of times I have gotten a 3-4 ohm reading but if I test it again it will be about 2 ohms. Any of these values are well above spec of .85-1.15 Ohms.

The secondary side is better, typically 13.5-14.5 k Ohms.

I went down to NAPA and got a Echlin coil and strangely enough it tests the same 2.1 ohms.

So I wonder if my DMM is wrong or both coils are bad.

Last edited by md21722; June 16th, 2017 at 12:04.
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  #2  
Old June 16th, 2017, 12:04
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Re: How to test an ignition coil? Is the coil or DMM bad?

In order to get readings that low, you have to make a good connection to the coil. It is not uncommon to have corrosion on the connectors that would add an ohm or two to the reading. Your numbers actually look pretty good.
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Old June 16th, 2017, 12:23
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

I went and looked again, and touching the leads together on the 200 scale yielded 3.2-3.5 Ohms across the leads. Then I connected the leads to the coil and it was measuring 4.2-4.5 which would mean the coil tests fine with this test. It is quite strange, as last night when I touched the leads together I was getting much lower values with just the leads together. I guess I should upgrade to a nicer meter.
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Old June 16th, 2017, 12:37
trippled trippled is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

Checking resistance on a coil imo is worthless. They can check completely fine and be bad. I just wouldn't put a lot faith into the readings. It's a valid check but really doesn't tell you how it performs under load. It almost sounds like your exhaust is plugging up.
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Old June 16th, 2017, 12:41
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?





I've read that cracks in the housing can be indicative of problems. Would these condemn the coil?

I agree that resistance tests are not the total story. I don't think the exhaust is plugging up because the cat and muffler only have about 40,000 miles on them. I could run a back pressure test but odds are it'd be fine.
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Old June 16th, 2017, 12:59
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Re: How to test an ignition coil? Is the coil or DMM bad?

If the coil is cracked, I would worry, but it may not be the problem you are looking for.

Loss of power over time, coupled with worse mileage, is a common malady. I am fighting the same thing but I now have 200K+ on my stroker. I have checked everything, compression, vacuum, plugs wires, rotor, cap, etc. I am worried about the high lift cam I am running having gotten worn down due to the new oils. I am going to put a dial indicator on the rocker and check the lift.

Good luck.
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Old June 16th, 2017, 13:08
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

A bit of tracing the back story may have given me another idea. Basically its my Red Jeep, 97 XJ 242, 30" tires, 2.5" lift. Always seemed under itself a bit compared to my stock 01 XJ. I figured it was the lift and slightly bigger and wider tires. When I was doing some axle work I swapped out to 3.73. When I first started driving it, it was wonderful. The engine seemed quieter, and more pep with less throttle. On the last few trips its seemed to be dogged down again. But, looking back through my notes I see that it has only been the last few trips where the front driveshaft has been installed. On the initial runs I hadn't put it back in it. Maybe I'll try removing it and see if there's any improvement. It basically ran fine with the front D/S from the Springs up to Castle Rock, but on the way back it was poopy and hasn't gotten any better. Though I find it hard to believe the driveshaft would be causing significant drag???
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Old June 16th, 2017, 17:21
trippled trippled is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

I always notice a difference driving north from the springs and then back down. I always assumed it was the slight grade between the 2. Might be something to consider.

If it starting to crack I'd say the coil is definitely in your future. When mine up and crapped out one morning. The resistance tested perfect, but it had a ton of cracks in the housing.

If you have a quick way to test back pressure I'd do it. At least if it's good you can rule that out.
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Old June 16th, 2017, 17:53
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

I already ordered a new coil from the dealer. It will be here tomorrow.

I bench tested a NAPA Echlin which tested bad out of the box. Primary resistance was fine, but secondary didn't break 10k Ohm. Spec is 11.5-13.5k Ohm. It is back at the store. Out of curiosity looking for something else, I stopped at O'Reilly's which had a BWD and MasterPro. The MasterPro looked exactly like the Echlin. The BWD was similar but different. They are all made by Standard so...

The setup I have to measure back pressure is to remove/disconnect the front O2 and connect that to a standard vacuum/pressure gauge, but I doubt its the problem. The CAT and muffler are your standard Walker Ultraflow types from Advance, I think with less than 20,000 miles. I had the Jeep for a bit before I put them on.

I drive I-25 quite a bit so I know Southbound is a bit harder. In simple terms, on grades, like Eastbound Monarch, Vail, or Eisenhower Tunnel, the 01 XJ can do it in 3rd, whereas the Red one will be in 2nd near redline. Driving on US 24 I can't pass as often in the Red one unless its downhill because it takes so much longer.... The transmission shifts a lot more in the Red one.

Last edited by md21722; June 16th, 2017 at 18:02.
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Old June 17th, 2017, 08:20
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

Alright, so I put the old coil back in and decided to take the night off from looking at this thing anymore. This morning I made the same run as last time before making any changes. It had tons of power, no extra shifts, and MPG's on the overhead were up 2-3 MPG compared to last time. The engine was also a bit "quieter". Now I'm stumped. Whatever it is, it's intermittent. I'm suspecting the coil.
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Old June 17th, 2017, 14:55
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

I picked up the dealer coil today. It is Made in China and looks like the same style as the other Standard ones like the NAPA Echlin. However, it does pass specs. Measured 1 ohm primary side and 13.5 k ohm secondary.

As a side note, it seems like the big 3 are getting a ton of parts from China whereas Toyota and Honda are using a ton of U.S.A. parts.
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Old June 18th, 2017, 16:44
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

Alright, I put the new coil in and took it for a test drive. Jeep seems to idle smoother. One time I looked at the tach to make sure the engine was running. But on some grades it seems to run out of steam. For those of you familiar with the area, on I-25 just south of Colorado City there is a 6% grade where it would not go faster than 68 MPH in 3rd gear. The Cat has 25,000 miles on it. I will remove the O2 sensor and put in a vacuum/pressure gauge there and see what it reads. At idle in the garage LTFT was 5.5% and STFT was -0.8%. For this era that may be good enough but I thought they were supposed to zero out. I do have a vacuum leak through one of my heater box actuators. On grades downhill I can hear it whistle & at WOT it's not uncommon for the heater box to temporarily go to defrost mode and come back on the downhill. Not sure that would contribute to this type of performance problem though. I can plug it and see.
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Old June 18th, 2017, 17:18
trippled trippled is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

Fuel pressure gauge? Or can you watch the o2 and fuel trims when it's running out of power?
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Old June 18th, 2017, 17:23
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

I don't have a suitable fuel pressure gauge to watch while trying to drive. I suppose I could buy one. I could try to record some PIDs with my cheap scan tool.

For grins, I removed the HVAC line at the manifold and hooked up my vacuum gauge.

This was with the engine hot.

750 rpm (idle) 11-12. MAP sensor on scan tool reads 10.
2500 rpm 15. MAP sensor on scan tool reads 7.
When I snap the throttle it goes to zero.

Is the MAP sensor incorrect?

EDIT: My 01 XJ MAP sensor does the same thing when you rev it up.

Last edited by md21722; June 18th, 2017 at 17:50.
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Old June 18th, 2017, 18:06
md21722 md21722 is offline
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Re: How to test an ignition coil? Is the coil or DMM bad?

Quote:
Originally Posted by trippled View Post
Fuel pressure gauge? Or can you watch the o2 and fuel trims when it's running out of power?
I know that assuming recent parts are good is in itself a problem, but the fuel pump is a Bosch changed about 35,000-40,000 miles ago. I have limited diagnostic equipment so I'm trying to test what I can before I have to deal with those items I can't easily...

Likewise, the crank sensor, cam sensor, TPS, & coolant sensors were all changed with Mopar ones at purchase 3 years ago. The 'good' originals (never noticed a problem with them) were put it a bag under the driver's seat in the event road side spares are needed.
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