To Measure Coils or not

Back yard “Resistance” testing of Coils, using voltage.

You need a good VOM (digital is best for accurate readings to .00)

Any voltage source, 12 volt battery, 6 volt battery ETC. (The battery voltage & connections are discounted using this method)

Test Resistor: Go to radio shack and buy a 10 ohm 15 watt resistor (must be > 5 ohms)

Hook one end of the Test resistor directly to one post (top of coil) of the coil such that the resistor is in series with the coil. Connect +ve battery source to the open end of the resistor and –ve battery to the other terminal of coil (pigtail connector).

Measure voltage across the resistor, one probe on each side, record this reading (X)
Then divide (X) by the value of the Test resistor, or if 10 ohms just move the decimal point one place LEFT. Example Voltage = 10.0V/10 or 1.0 A =(Y)

Measure voltage across the coil, one probe on each side, (top post to the pigtail), record this reading (Z). Divide (Z ) reading by the (Y) calculation = Coil Resistance

Examples 12.5 volt battery source: Xvd = 10 volts/R =
1.0A (Y), Zvd = 2.5/Y = 2.5 Ohms coil 6.5 volt battery source: Xvd = 5.2 volts/R =
.52 A (Y), Zvd = 1.3/Y = 2.5 Ohms coil 6.5 volt battery source: Xvd = 5.9 volts/R =
.59 A (Y) , Zvd = .59/Y = 1.0 Ohms coil

Once you have the operating resistance of the coil you can calculate the Ballast & or fixed Resistance needed in the coil circuit. The front mount needs to be limited to 3 amps of current. 14.5 Volts (operating) divided by 3 amps = 4.83 Ohms Total.

If your coil is 2.5 Ohms you need ~2.33 Ohms Ballast (Front Mount)

If your coil is 4.0 Ohms you need ~ .83 Ohms Ballast (Front Mount)

If your coil is 1.0 Ohms you need ~3.83 Ohms Ballast (Front Mount)

Hope this Helps,

One thing I'll add is that you'll need to measure the actual resistance of the test resistor before you start, because the variable (Y) is a function of supply voltage and this resistance value. Radio Crap might call it a 10-ohm resistor, but it may be as much as 10% higher or lower than that.

I'm going to do this on my good-running 2N this weekend to get a baseline for any future trouble.

Thanks! This is a really good idea.

Regards,

Your whole premise of 3.0-amp TOTAL squarecan ignition coil current is valid (might be a little low, vis-a-vis...the original 6 volt coil and mandatory ballast resistor) but THINK about the non-geek's that might read this post.

The specter of electrically challenged shadetree 12-volt converters DEMANDING a 2.33-ohm resistor from their local tractor parts vendor is FRIGHTNING. (grin)

Us technical types know they don't make 2.33-ohm resistors but we know that 2.5-ohms is probably close-enuff (5-ohm resistors in parallel = 2.5-ohm) or the more readily available 2-ohm will also work.

We're all working on providing a reliable way to increase the LIFE of the squarecan ignition coil that keeps melting its internal insulative tars and shorting out the windings and causing weak sparkies. Lower AMPS is definately one-way to reduce coil heating.

BUT... will 3-amps provide enuff sparkies? After all, its the magnetic field that 3-amps produces that induces the ultra hi-voltage in the secondary that sparkles the sparkies..... ...respectfully, Dell

A Thermal PTC Ballast in combination with a fixed
resistance will provide the (near 3 amp) low end
current. (An, off the cuff, estimate is that a .5
amp drop of a fixed R is reduced to .25 amp when a thermal Ballast is used).

NO your not likely to get a Thermal Ballast much bigger than 2 ohms @ 3 amps.. its the HEAT

BTW: The Old 70's Crysler "dual" resistors used a 5.0 Ohm fixed for running. thats only ~2.4 amps.

You take care Dell, you sure help a lot of PLL

NO! I never said 4.8 "AMPS" (4.83 ohms for both coils) The Amps for the front mount is 3 (Three)

The ohms rating is very differant for the side mount and "must" be a fixed resistor. NOT a Ballast thermal.

I would think lots of folks would have jumped on this with replies, but sometimes I think guys like to remain "in the dark" about what really goes on in these simple ignition systems, and keep buying coils!

To add to the coil confusion, I had a few minutes the other day, in TSC, and was looking at front-mount coils.

They had two, lying on the bottom shelf of the display rack, with the staples pulled loose in the "clamshell" packs, so who knows if they had been switched between packs by folks looking at them.

There were NO markings on the actual coils.

The packages were marked as follows:

"0238001 IG COIL 8N to #263843 1939-1950"
(NO Volts listed, NO mention it will also fit 9N and 2N.)

"0236588 PKG IG Coil Ford"
(NO Volts listed, or ANY mention of what it fits, other than Ford.)

I found the hangers where the coils should have been hanging, and here is how they were labelled:

"FD 2N 9N Coil 12V 0238001"
(The PACKAGE only says "8N", the mark on the display hanger says 2N 9N, and OMITS 8N, but at least the Volts are given.)

The other display hanger says:

"CX 6V 8N 2N 9N 0236588"

So, it's about as clear as mud... the display hangers are poorly labelled, the clamshell packs have been opened, and the contents COULD have gotten mixed around, and there are NO markings on the actual coils, to verify this.

So, it's no wonder folks have NO idea of what coil they have just purchased!

I will take a DVM along next time I go there, and, hopefully get the manager's permission toh measure the coils' resistances, to see what they are actually selling these days.

Also, I will get the Standard Ignition ballast resistor specs we talked about a few days ago posted this weekend.

BOB,
I learned, as a field engineer RCA Broadcast, that there are always 2 or more faults. The one that is evidenced by the smoke & damage, and the one that caused the smoke & damage. Its Hand in Glove.

Once I get an assortment of PTC Ballasts, and "Test" them, I will post a "shade tree" method of measurement.

Phil,

My big 'wonder' on this coil testing business is that we aren't looking at time, inductance, ( or impeadance in the circuit )and duty cycle as a factor... just straight dc resistance.

I'm guessing straight non ventilated points, and a non electronic ignition, breaker or breakerless.. etc.

Soundguy

Soundguy

Many of the round coils are smaller, and have a package more capable of disipating heat.. also.. many of them are oil imersed coils.. further helping heat sink them.

Soundguy

I agree with you about the duty cycle "dwell", as the primary current is flowing only about 40-some% of the time.

However, "Blems" are always leaving their ignition switches "ON", and frying their coils and points, so the amount of current that is "safe" with the key "ON", and the engine NOT running IS probably the most important factor here, in choosing resistors for use with a certain coil!!

Soundguy