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1987 - 1996 F150 & Larger F-Series Trucks 1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks

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Old 01-31-2011, 02:49 PM
cjben cjben is offline
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How many ohms resistance should a coil wire have?

I am diagnosing a no start with fuel and spark condition on my 95 f150 5.0. Quick question: how many ohms is a coil wire supposed to have? the reason I ask is because the coil wire I took off the truck is reading 4.2 ohms and a new one that I can't use because it is too short is reading .72 ohms. I know there will be more resistance the longer the wire,but there shouldn't be that much difference should there? The coil wire I am using is only 3 inches longer than the spare. thanks for the quick answer!!
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Old 01-31-2011, 04:31 PM
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Sounds correct to me. I know my secondary MSD is 3.7k, and I want to think that my primary is much less.
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Old 01-31-2011, 04:40 PM
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I think most coils should have a primary side resistance around an ohm or a little bit lower, so your .72 sounds alright (maybe a low impedance model?). The 4.2 ohms of your old coil is definitely higher than it should be.
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Old 01-31-2011, 05:06 PM
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I think the OP is referring to the coil wire itself, not the coil. The wire impedance is based on many factors. There is a wide variation in the ohms per foot for each manufacturer/model. You would need to consult the manufacturer's documentation for that rating.
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Old 01-31-2011, 06:44 PM
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4.2k sounds right for factory coil. I have a spare one I could test for reference if needed, but I think you readings are fine.
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Old 02-14-2011, 01:54 PM
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Coil Test / Coil Wire Test

1
Remove the negative battery cable from the battery with a wrench. Isolate the battery cable so that it can not contact the battery.
2
Locate the coil on your Ford engine. The coil looks like a black cylinder with a center wire similar to a spark plug wire and two side posts with nuts and wires attached.
3
Remove the large wire from the center of the coil by pulling it straight out of the coil. Remove the retaining nuts and all the wires attached to the side terminals on the coil as well.
4
Place the test leads from the ohmmeter on the side terminals, one on each side. It does not matter which lead goes where in this case. Note the reading on the meter. This is the primary resistance, and it should read between .4 and 2 ohms on your Ford coil.
5
Move the test lead from the negative post on the coil to the center terminal where the large wire was removed. Keep the other lead on the positive post of the coil. Note the resistance reading on the ohmmeter. Your coil should have a reading of between 6,000 and 15,000 ohms for your Ford.
6
Replace the coil with a new one if the readings are outside either of these ranges, as that would indicate a defective coil. If the readings are within range, reinstall the wires and retaining nuts on the side posts, being sure that they go back on in the same position they were in before you removed them.
7
Tighten the nuts with a wrench and reinstall the large coil wire on the center terminal of the coil. It just pushes in place, but make sure it is all the way in or it may come loose when the engine is running.
8
Reinstall the negative battery cable on the negative post of the battery. Tighten the retaining bolt with a wrench to secure the cable end on the battery.


you may also want to test your coil wire
7000 Ohms per foot of coil wire

note if your ohms reading is jumping allover when you test the coil its most likely bad
even if one test good

like sec. 7500 and Pri. 0.1, 0.9, 48.0, 0.3, 4.0, 0.0
if it moves more then about 0.5 on pri. i replace
sec. i allow around 250.

any case if you are jumping out of range replace

Last edited by JLClayton; 02-14-2011 at 01:55 PM. Reason: Add Info
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Old 02-14-2011, 01:54 PM
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