I've got a '95 f150 with 302, and I am in the process of trying to order parts for a tune-up (sixlitre). But I didn't want to just throw money at it. I tested the components first, and the primary coil resistance readings move between .3-.4, while sometimes bouncing down to .2. The book says .3-1.0 is normal.

The secondary reading is 7.5K ohm. The book says 8-11K ohm is normal. So, do you think this is still just fine within range and I won't benefit whatsoever from a new coil, OR is there going to be a slightly worse spark from the slightly low resistance?

Appreciate any helpful advice from you gurus.

 

First off, welcome!

I commend you for checking the resistance values before simply throwing parts at your rig. I have tested many, many ignition coils and found that it is common for them to deviate slightly from the published resistance ranges (temperature can affect these readings as well). The small differences in your readings vs. the published specifications will not cause any issues.

From the numbers you have posted, I would not replace the coil. However, doing a low-voltage resistance check does not mean that the coil is OK - you can also have high-voltage breakdown, and this can be determined by doing other testing (which requires an automotive oscilloscope or similar instrument that can measure your ignition system's secondary (or high) voltages).

If you don't have the means of doing secondary voltage tests, again, not a big deal, as you essentially are doing the same test whenever you run your vehicle at a full load, such as up a hill. That's 'seat of the pants' testing at its finest!

 

Thanks for the advice. I don't have the equipment to test the high-end voltage. However, gas mileage has been pretty good at up to 18+ highway. So, I don't feel the ignition coil is in real rough shape. I do think it's original now with around 170,000 miles on the truck.

For the cost of the new coill, I might just leave it alone. I do need to replace the wires, though, as one of them was showing 12K ohm for 1.5 feet of wire.

 

Yes, your plug wires are a wear item, and if they are original, need to be replaced. Don't go cheap on the replacement wires, however - get OEM quality wires. I have learned that the hard way (and one set of OEM wires is usually less expensive then 2-3 sets of cheapo wires, not counting the time to redo the job).

Apply a light film of silicone grease (or "dielectric grease") to the interiors of the boots at each end of the wires before installing them. This seals the boots from moisture intrusion, prevents high-voltage leaks, and helps keep the boots from permanently sticking to either the plugs or distributor cap.

 

I'll probably go with the 8mm Motorcraft wires. I've been thinking about the 9mm Ford Racing wires as well.

 

If your trucks engine is nearly stock, I would just give it a tune up with quality OE type parts. I ad basically the six liter tune up parts on mine for a while. They didn't hurt anything, but they didn't do anything either. By the way, the Ford Racing wires are so thick and long they don't for right are all.

 

I ended up ordering the Ford Racing 9mm even though I wanted the 8mm ones. The 8mm wires weren't in stock, so now I'll have to deal with the hassle of the thicker longer ones. Oh well. Could be worse. I'm leaving the coil alone at this point.

I don't expect any noticeable change in performace since I had already advanced timing approximately 4 degrees or so. I say "approximate" because the harmonic balancer had turned somehow and the timing marks are nowhere to be seen when I put the timing light on it. I did get 1-2 mpg out of that timing bump. I've got another order with Rock Auto, and a new harmonic balancer is on its way.

This 302 engine is not full strength, yet it still gets 18+ mpg straight highway and 13-13.5 (without OD engaged) pulling a single axle 5x10 trailer with 30" sides loaded down with 2000-4000 lbs of wood. Not bad.

 

You sound on top of your situation, and congratulations on your fuel mileage, it is excellent! Glad you are replacing that harmonic balancer, you don't want to run an engine with a bad one (can lead to disasterous results, such as a broken crankshaft like on the GM 6.2l and 6.5l diesels).

You may have to buy new (aftermarket) spark plug wire separator clips and brackets which will handle the 9mm wires - they may not properly fit into the 8mm ones (just like the 8mm wires never fit into the 7mm ones back in the day, showing my age, heh!).

Spark plug wire routing is really important for maximum vehicle reliability and wire life, and it's something that so many "mechanics" seem to completely ignore. Don't be afraid to spend some time to do the job right.

 

I'm reviving this thread because I am in a very similar situation. My pickup is a 1993 F250 with a 460 engine. My primary is reading around .3 ohms, and my secondary is reading around 7.4k ohms.

The reason I decided to test my coil is because my pickup is showing signs of incomplete combustion: Poor gas mileage, feels down on power, exhaust smells rich, and coolant doesn't get hot enough to keep the windows clear.

I would also like to add that the Chilton manual states, "Secondary resistance must be 8,000-11,500 ohms." To me, the use of the word "must" implies anything outside that range is unacceptable and should be replaced.

Any other thoughts on the matter?

Thanks.
- Joe

 

The primary resistance should read between .4 and 2 ohms on your Ford coil.
The secondary resistance should read between 6,000 and 15,000 ohms for your Ford.

Some reading:
https://itstillruns.com/test-ford-ig...l-5941137.html

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You also need to check to see what codes you have before changing parts.

 

Thank you for the link.

Step 2 on that article is a give-away that they are working on an older pickup with a different-style coil. It says,

"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."

I have been trying (unsuccessfully) to find Motorcraft's resistance specs, as I have encountered misinformation in Chilton manuals before...

Thanks.
- Joe