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Ignition Coil Resistance Values, Ohms 1990 XJ

Black1990jeep

NAXJA Forum User
Location
california
I am testing my ignition coil on my 1990 Renix 4.0 liter six cyl. XJ with an ohm meter. Coil is removed from jeep.

What values are specified for resistance across the two primary contacts? What of the Primary contact to high voltage contact?

I haven't found that info in the factory manual, and my Haynes manual is vague, like..."less than infinite" ohms, or "close to zero"

My across the two primary contacts reads 1.3 ohm, which I think should be correct. (close to zero). But I am clueless on primary contact to the high voltage contact.
I get 4.88 K Ohms. (less than infinite) across primary to high voltage contact. What is expected here?

Thanks!
 
Well, my '88 FSM says:

@75 deg Primary 1.13-1.23 Ohms, Secondary 7700-9300 Ohms

@200 Deg Primary 1.5 Ohms, Secondary 12,000 Ohms
 
ok, thanks. Now my question is what is the difference if any between a 1988 and 1990 coil?

Now may be my factory manual does list the values, I just havent found it yet.

What section of the 1988 manual listed the ohms values for the coil?


my primary to high voltage ohms is a bit lower than for the 1988 coil specification. mine was read at near room temp, likely a bit lower than 75F degrees.

thanks for look with data up and posting
 
P.S. my 1990 coil is a not the traditional oil filled can style. Rather it looks like a squarish tranformer you might find in a old T.V. set, the type with laminated steel plates. My squarish coil plugs into a control module.

My manual I found does list the coil ohms as above, but it is for the 4.2 liter YJ motor, and it is the oil filled can type coil, unlike what is in my 4.0 liter Renix 1990.

So the search continues for the ohms specification for this squarish coil found in the 1990 Renix.

Thanks!
 
The way I check the coil is low ohms primary, high ohms secondary. As close as possible to battery voltage at the larger yellow wire going to the module. Near the same voltage at the coil test point as the large yellow wire.

Check the contacts/connector between the module and the coil. They sometimes get hot and relax (get loose) and make poor contact. They can also get covered in corrosion.

I check spark at the plug, sharp blue-white spark that makes a crack sound when it fires. You can also measure your high voltage cable ohms. It is not uncommon for there to be a break in the conductor under the insulation in the spark plug, coil high voltage cables.

If you replace the cables it works better if you keep near-stock ohms for the coil to cap cable. I have no idea why it just works better.

Side note I've tested different Renix coils with four different meters and got different values for each. Maybe the reason coil testing is kind of glossed over in many manuals. I think the inductance of the coil(s) can make ohm testing unreliable. "In electromagnetism and electronics, inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor."
 
The section that had values was in the "Tune Up" section. My FSM is a PDF but lacks pages numbers on the pages. According to Acrobat, it's on page 175.

I also found the "Coil resistance check" section with the "vague" specs.

Generally, I'm with 8Mud.

Test A is checking for a short to ground. So any measurable resistance is bad.

Test B checks the primary. Fewer windings means less resistance. Should be low but not Zero.

Test C checks the secondary. Lots of windings so very high resistance. As long as it's not an open circuit, it should be good.
 
I'm going to ask the stupid question...why are you testing the coil?

I am an electronics engineer by profession so I will tell you that testing the ohmage of the coil only tells you so much. If you are seeing an intermediate misfire or problems at high speeds, it may give you a false sense of security.

If you are able to get ahold of a 'megger' you have a better chance of finding what causes these problems, mainly insulation breakdown at high voltage. In other words, they arc over internally.
 
All the above is great information plus a replacement is "dirt" cheap!
 
ok i will order a coil looks like Rock Auto has them about 1/2 price of the Orielly or the AZone.

Which brands are considered good, which brands to avoid?

I will measure the ohms on the new one and report back


had jeep 2 years. cranks fine but takes a lot of cranking to fire up usually. then had horrible idle, hunting traced to crack spark plug. ohmed out cap and wires, all good, compression test about 140 to 150 psi, smoke test of intake, no leak found. cleaned engine bay relay contacts, main power relay terminal and wire lugs, clean coil contacts, just plain going thru it all cause although now idle is more stable hunting ceased, it is idling a bit fast, and for some reason manifold vacuum is low, only 10 to 12 inches at about1500 rpm.

I want to find why vacuum is so low. it got better after cracked spark plug was replaced as it was as low as 5 inches with the cracked spark plug, actually all six plugs replaced , missing from that cracked plug made her run extremely rich, exhaust smelt of raw gas, now smells like roses, well no, not actually roses, but better anyway! new O2 sender two months ago also, passed smog then.

Any way low manifold vacuum, fast idle are concerns Should be borrowing a fuel pressure gage tonight assuming the looters dont get me, had to pass by 11 squad cars at the freeway overpass this evening. another big protest is planned, hope it stays peaceful. A bit unnerving.
thanks
 
Bad ground to numerous sensors, including MAP?

Go to www.cruiser54.com and complete Tips 1 through 5 before taking any rabbit trails. BTW, Coils in all Renix interchange 87 to 90.

Here's an excerpt from one of the Tips:

The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.

All the components listed below ground at the dipstick tube stud:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition Control Module, Fuel Injectors, ECU main ground (which other engine sensors ground through, including the Oxygen sensor, Knock Sensor, Cruise Control and Transmission Sync signal. All extremely important stuff.
 
Thanks Cruiser54. Last year I per your excellent website fixed the dipstick grounds.

after fixing the dirty connections at the main power relay stud that powers all the fusable links yesterday I found my electrics work a bit better, the nut on the stud could have been tighter, and the ring terminals were a bit dirty.

All sorts of little issues have added up to problems. cleaning the relay pins and coil pins also helped, she starts quicker now. also my power locks have started to work better after cleaning that main relay stud.

will continue on.

thanks all!
 
Bad ground to numerous sensors
All the components listed below ground at the dipstick tube stud:
.

Bad grounds are not limited to Renix-era XJ, none of our XJs are less than 20 years old, rust and corrosion take their toll.
Both my 97 and 2001 XJs have had ground issues traceable back to engine grounds, both on the passenger side.
Periodic ground renewal is a good practice.
 
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