I have a Delco ignition coil I bought years back and the guy I bought it from "thought" it was 12v. Is there any way I can test it on the bench to find out??

Use a VOM or DVM to measure the coil primary resistance (measure across the two stud terminals).

A 12 volt coil will show somewhere between about 3.2 and 4 ohms. A 6 volt coil's resistance will be half of that (about 1.6 to 2 ohms)

When I was a used tractor dealer I accumulated a ton of coils, heres what I suggest that ALWAYS worked for me.

Stick an ohm meter on the Rx1 scale across its primary, the little + and - terminals.

If it reads around 1.25 up to 2 ohms (many average around 1.5 ohms) it can be used at 6 volts nominal and produce a decent spark when starting drags the battery voltage down to say 5 volts or its under a charge and up to 7 volts I CALL THOSE 6 VOLT COILS and they are usually labeled "6 Volts" and what NAPA may give you if you ask for a 6 volt coil as opposed to a 12 volt coil. Of course, you can still use it on a 12 volt tractor simply by adding an external series voltage dropping (12 to 6) Ballast Resistor.

If it reads more like 2.5 to 4 ohms (many average around 3 ohms) it can be used unballasted at 12 volts nominal and produce a spark if starting drags the battery voltage down to 10 or under and still if under a charge at 14 volts. I CALL THEM 12 VOLT COILS and they are labeled "12 Volts" or perhaps "12 volts NOT for use with external Ballast" and are what NAPA may give you if you ask for a 12 volt versus a 6 volt coil. Some call those coils Internally Ballasted.....

Many tractors used the same coil for 6 or 12 volt tractors, its just that if on a 12 volt they added the external voltage dropping (12 to 6) Ballast Resistor.

If you put 12 volts on a coil designed for 6 (and no ballast) it will overheat.....If you only put 6 on one designed for 12, the spark is weak. THEY ARE NOT ALL THE SAME due to current ratings and their capacity to expel the heat generated to their surroundings.

Hope this helps

1.5 ohms likely a 6 volt coil, 3 ohms likely a 12 volt

John T

Thanks Bob and John, an ohm meter I have!!

Just adding to John T's comments with a few specs.

Coils need to make good spark at 9 volts in a 12 volt system, not 12 volts. The vast majority (90 %) of coils used in OEM 12 volt systems are what many here call "6 volt."

Ignition coils I have checked:

1992 Subaru 12 volt breakerless: 2.4 ohms primary

NAPA IC-12 Echlin 2.1 ohms primary, 8.9 ohms secondary made for no external resistor

NAPA IC-14 Echlin 3.25 ohms primary resistance No external resistor used.

NAPA MPEIC14SB same as IC-14, just cheap Chinese. $18.69

NAPA IC-27 Echlin 2.2 ohms primary, 6.38 ohms secondary - no external resistor.

A500 12 volt 2.6 ohms primary, 9.8 ohms secondary
No external resistor.

Deere 6 volt (as used on 420,430,1010, 1020, 350 crawler, etc. 1.6 ohms prim, 7.12 secondary

Hot Spark brand - HS06HEC - HEI, CDI and electronic ignition systems - .6 Ohms primary resistance. 11.6 K ohms secondary.

Mallory 29219, 12 volt coil - primary 1.4 ohms, secondary 9.8K ohms. 58K volts. Use with external resistor.

Mallory 29217 12 volt breaker-point coil, primary 1.4 ohms, secondary 9.8K ohsm, 58K. Use with external resistor.

Lucas coils:
DLB101............standard 12 volt coil (primary resistance 3 ohms). No external resistor.

DLB102...........standard 12 volt coil for use with 1.6 ohm ballast resistor (coil primary resistance is 1.5 ohms). Also used on 6 volt systems.

DLB105...........high performance 12 volt coil (primary resistance 3 ohms). Use external resistor.

DLB110...........high performance 12 volt coil for use with 1.6 ohm ballast resistor (primary resistance 1.5 ohms). Also with 6 volt system.

Great, Im sort of old fashioned and still use my old trusty Simpson 260 woooooo hooooooooo.

The values Bob and I discussed are more for the old tractor coils NOT new fangled electronic ignitions some of which operate more on a solid state pulse applied to their primary then the points system where curent flowed through the coil but was then interrupted by the opening of the points.

John T

NICE SET OF SPECS...... Heres even more info, I LOVE these techy sparky chats lol

When they started using more of a solid state switched voltage "pulse" to what Id call "firing" the coil their (coils) primary resistance values varied according to the peak voltage and pulse width the control units produced. The earlier soild state control units acted similar to a set of points opening and closing, it was just that a transistor turned on and off instead to make then interrupt primary current flow.

I used to use the early Chrysler control modules to convert Mercury Marine Distributors so the control unit switched coil current flow versus those old points as that Chrysler unit was so easy to adapt. Then they had those Capacitive Discharge units and remember the boxes (I still have one) that still used the points as a low current switch to control a solid sate switch for the 4 amps or so of coil current, they acted something like a solid state relay with the points carrying a very small current versus the normal 4 amps.

The values Bob M and I had in mind are more for the older tractors of course, whose to say what the poster has laying around though???

Take care and happy new year, Im gonna be in and out of here some while on the road South

John T (just an old sparky kinda guy)

Coils are not just one size fits all! Most coils will work on any application at least for a short time. During the days of 12 volt, point type, Kettering ignition(roughly 1955 through 1974 with many exceptions) the automotive industry used what would best be described as a 6 volt coil with external ballast resistor in their 12 volt systems. However, these coils are different in respect to manufacturer. Ford, GM, and Chrysler all had different primary resistances in their coils, along with brand specific ballast resistors that were built to match specific coils. There were also matching condensors with various micro-farad capacities to match a specific ignition system. For optimum performance coupled with long life, the correct parts MUST be used.

Deere used a six volt coil on the B's six volt system. The A and the 60 used an unballasted 12 volt coil. The 620 and 630 use a ballasted six volt coil with a resistor bypass for easier starting. Putting a coil from an A on a 620 will cause weak spark and poor performance if at all. Putting a coil from a 620 on an A will work great for a little while until the points burn up because of twice the specified current flow.

John Deere combines such as the 55 and 95 use a 12 volt unballasted coil like the A and 60. I presume since combines usually run in temperate weather, Deere felt they didn't need the extra expense of a bypass system.

I have experienced countless ignition problems caused by people mismatching coils! The combine or tractor stops and a coil off a junk car, pickup, or tractor sitting in a fence row is substituted. As jdemaris points out, 90% of coils are of the 6 volt type with external ballast. However, when you put one of these coils on an A, 60, or 95 combine it will work just fine for a couple of days and then new points are needed. It is very hard to convince the customer he needs a new coil when it works good for awhile. There is nothing wrong with the coil, it is just using the wrong part for the job.

Bear with me, I"m not a sparky guy. With my meter I get .002 with a K in front of the ohm sign. I am assuming that this is 2 ohms and the K signifies "thousand"??

2 ohms sounds right, Id take it for a 6 volt coil YES K = 1,000

John T

M Mayer

2 ohms does not sound quite right.

Did you check your meter by touching the ends and checking to see that it zeros ? if it does not just take away the error from the reading.

I would expect to see 1.1 to 1.5 ohms (with resistor or six volts)

or 2.4 to 2.9 ohms (no resistor or 12 volt)

2ohms seems in between the norms.

cheers
George

To get an accurate reading when measuring resistance below 10 ohms, first short the leads to each other to measure the lead resistance. Then measure the coil and subtract the two readings.