scotty305

iTrader: (2)

Hey all, when changing my wires today, one of them was very very corroded, and broke off at the ignition coil.

I've heard people talk about checking the resistance of the ignition coils (this thread:
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/some-good-info-those-running-jacobs-rotary-propack-ign-amp-race-coil-install-131667/ )

but I haven't found how to actually go about checking it.



Help would be appreciated.

Here's the part that was left at the coil:



Here's the wire side:



-s-

superior_force

the spec is right in the service manual...where are you looking, a JC Penny catalog?

be forewarned, however...my coils passed the resistance test with flying colors and they were still toast

scotty305

iTrader: (2)

Update: found the testing prodecure.

scotty305

iTrader: (2)

Update #2:
Inspection Point A-B measured 0.7 ohm , barely within spec.

L1-L2 measured 12.3 k ohm , also close to failing.


I re-installed the original coil, and have sourced a spare leading coil from an FC. The car starts and idles fine; we'll see how it drives tomorrow.


Superior Force: how could you tell your coils were toast? Simply grounding the wire to the chassis and looking for a spark?


-s-

scotty305

iTrader: (2)

Update: car drives fine; pulls a bit harder.

Wires installed were Mazda P/N 0000-18-130A (plug wire set)


Measured resistances (in ohms)

Front trailing: (short blue)
new: 1.54k / old 3.88k

Rear trailing: (long blue)
new: 2.06k / old 5.03k


Front Leading: (short black)
new: 2.57k / old 6.60k

Rear Leading: (long black)
new: 2.27k / old 4.70k

7racer

Hey nice! Thats good information

Xeros

Wait how do you measure the resistance of the coils though? Do you just take a voltometer and set it in the ohms position and just stick the prods in the sockets? Do you set it to 20 ohms? I'm just a bit confused and I think one of my coils might be bad....

Brent 94

iTrader: (1)

How much do new coils cost?? just curious

scotty305

iTrader: (2)

Xeros: to test the resistance, you set the multimeter to ohms. For the A-B reading, the lowest setting (200 ohms on my multimeter) should work. For measuring between L1-L2 , I set mine to 20k.



Brent 94 , prices change, but the primary coil was over $150 the last time I checked. Call Ray at Malloy Mazda (search for "ray malloy phone" if you don't know the number ) and he should be able to give you current pricing.

-s-

tcb100

Zeros, if your coils have too much resistance you know to replace tham but like Superior Force said, they can meet the resistance specs and still be failing under load.

Ask a shop to put an ignition scope on the ignition.

dpeak

Had the same problem, coils spec'd out and gave me a lot of high end break up after warm up. Also, it souded like 1 rotor would get no spark if I let the car sit still and idle for over 10 minutes. Remember that an increase in temp=>increase in resistance. I was lazy and devised an easy way to check them without removing much. Put a paper bag with dry ice in it on the coils and went for a spin, problem disappeared until dry ice evaporated. To double check this when the coils were out of the car I heated them to about 200 degrees in the oven and checked resistance. 1 was completely burnt internally but did not show up until it was hot.

scotty305

iTrader: (2)

Good idea, dpeak. I like how you're taking temperature into account by changing it two different ways: off the car and on the car.

So far I haven't had any problems at all. I'm getting a definite increase in fuel mileage, but it is only a slight amount: 1-2 mpg better than the previous month. The weather is about to start getting hot soon, we'll see how well the coil does in 100 degree weather.

-s-

jbz12

dpeak...Where did you learn the dry ice trick?

Just curious...it's simple...but ingenious!

j

gcthree

Had the same problem years ago with my heavily modified VW bug (2000cc/downdraft 40IDA's/180hp). I used the stock coil, and it ran just fine until the motor heated-up and the car would just die. Let it sit for half an hour, and it would fire right up. Back then, you replaced the stocker with a larger, higher-capacity Bosch blue coil, and the problem went away.

dpeak

Thanks J! Just dreamed it up out of pure laziness. I didn't want to remove everything off the car just to check it. Dry ice is fun stuff to have around too, we use it sometimes to shrink bearings so they will be easier to press in, drop in peoples drinks, put some in a coke bottle with a little water and put the cap on it (boom!), torture small animals and the list goes on......

David Beale

And, it (dry ice) fireproofs the engine.

Speed of light

iTrader: (8)

Back to your coil tower. I've had this happen too. Note that It is usually the result of a lose contact and/or the presence of moisture, and is not indicative of a bad coil per se. In most cases, the coil is entirely serviceable, unless the phenolic insulating tower is severely burned away. I've restored 2 of these on my FD coils that have been working fine for 3 or so years now.

First, clean the inside of the tower our thoroughly by scraping it with something such as a dental pick, get all of the oxide (white powdery stuff) out as possible. Blow it out and wipe down inside. Second, repair or replace the spark plug wires so that the wire's contact seats firmly in the tower and is NOT loose. Now, coat the inside of the coil tower contact with white grease*--this will help prevent the formation of oxides around the aluminum that leads to this problem. *(aka DE-OX, it is available from electrical supply stores for use on aluminum conductors)

Note that resistance measurements are not a very accurate manner in which to check a coil. A resistance check is useful if it is dead shorted or open, but not much in between. They are suggested because that's what most are able to measure, which I suppose is better than nothing, so weigh your results accordingly. A better check would be for inductance and Q against a known good sample. Additionally, certain failure modes in magnetics, especially those involving high voltage, are not always easy to detect with any test; they can check fine and not work at all.

Finally, if the coil's primary input current is similar to a known good unit (under the same conditions) and it seems to be running OK, then it is probably good.

Also, be aware that Mazda uses resistance wire in some parts of the primary ignition harness.

gracer7-rx7

iTrader: (16)

Bumping an old thread as I'm trying to test a spare set of low mileage coils.

I have the coils off the car. When I try and test the Trailing coils, I place the probes on the A and B terminals as per the factory service manual instructions. I get an initial value and after a couple of seconds it starts to change. The intial value is usually .7 or .9 Ohms then it will either vary downward or upward.

Is this change of values over time (a few seconds of contact of the probes against the terminals) correct?

Thanks.