Working around high voltage could be very dangerous. Please use extreme care when following these practices. Neither the N-News nor Chris Britton take responsibility for any damage to person or property. With that said, the statements and instructions in this article are accurate and true to the best of our abilities, and Chris has been using (and modifying) this tester for several years with excellent results. He wanted to share it with other hobbyists and tinkerers.

Whether you are doing a refurb, conducting routine maintenance, or just trying to remedy a tractor broken down in the field, just about anybody who has owned old iron has, at one time or another had to fiddle with the ignition system. While old tractors can have plenty of problem areas to choose from, their electrical systems and associated “phantoms,” are some of the most vexing to the average owner. And some of the commonly available consumer grade multi-meter testers are overly feature-laden, confusing most people without exceptional knowledge of electronics, only adding to their electrical system angst.

Since starting my collection, I've picked up quite a few flavors of tractors, though I have tried to mostly stick with Fords. I have had a few diesels come and go and still have a few parked out back. Most, however, are gasoline-powered. I do have a pile of lab grade metering equipment and a small pile of very nice multi-testers. I enjoy dealing with electrical gremlins and dig deeper than most when I can’t find the problem. They all live safely inside my climate-controlled house and some of them are bulky enough to make them more or less permanent residents on my workbench. Every time I have had an ignition problem come up, I find myself making the long walk back to the house to get a load of gear to take back out to the field or barn.

I try to keep a fairly up-to-date log of tractor repairs, which I review from time to time. I’ve noticed that whenever I do ignition work, I use the same diagnostic routine, checking for four specific things:
1. Do I have power from the key switch?
2. Are the points opening and closing mechanically and completing the circuit properly?
3. Is my coil making spark?
4. Are my spark plugs good or fouled?

One day, while reading my maintenance logs (after having made one of those ignition repair trips back and forth to the house), I thought about making up a tool that would work as well as a high dollar meter, but was cheap, easy to build, and able to tolerate being left out in the barn (eliminating a trip or two back to the house). With those four prime tests in mind, I thought about a simple circuit setup that could do the job, yet used simple and inexpensive components.

I came up with a small handheld unit made from a heavy walled grey plastic dual receptacle junction box. To this core component, I added a household light switch, an ignition condenser and spare spark plug, a small low wattage dc lamp, a few feet of wire, some alligator clip ends and basic wire-connecting hardware. The junction box can be had for about $4 from a hardware store, the lamp switch cost me a whopping 79 cents, and I robbed the ignition condenser from a can of leftover ignition components. A new champion H12 spark plug came from a Ford ignition tune-up kit and the low-wattage lamp was a panel-mount type with spade terminals on the back, probably salvaged from an old dash or piece of equipment I had dismantled for parts. The alligator clips came from old battery chargers.

The first circuit is the ground for the sparkplug.

The second circuit is a bit more involved. The condenser has two connections - the wire with a screw lug and the case with a screw lug. I connected this across the two contacts on the household light switch. I added a medium-sized alligator clamp to a wire and fastened this with a ring terminal to the side of the switch that the case of the condenser was also fastened to. I ran the wire with alligator clip out of the hole in the junction box where a piece of pipe or conduit would have been connected, which keeps the wiring tidy. This is the ground wire, to be hooked to battery or chassis ground when making tests.

The next wire is connected to a smaller alligator clip and has a ring terminal on the other end. The ring terminal will connect to the other side of the switch that has the condenser wire on it. When making tests, this wire will be hooked to the 'exiting' primary wire coming from an ignition coil that would normally go to the distributor, ie., you unhook the jumper wire from the coil that leads to the distributor. We can call this wire B+. This circuit acts like a set of breaker contacts, with the light switch you flick on and off as the 'points'. When the switch is on and you have your tractor’s ignition on, the coil will be powered just as if its points were closed. Once you turn the switch off, this simulates the points opening, thus letting the coil discharge. If the ignition coil is working correctly, you should see a spark at the test plug.

The plastic box and insulated switch will minimize the chance of getting shocked. (How many of us have tried to hold a spark plug to the head or block and turn the engine over to see if we are getting spark?) As fast as you can flip the switch on and off, you should get sparks at the test plug.

You can also use this circuit to test your tractor’s plugs. Remove a plug from the tractor and simply swap it with the test plug and start flipping the switch. My box happens to be tapped for the size of plugs that Ford uses. If you have many different tractor plugs to test, then drill and tap extra holes in the box and make your wire long enough to reach each hole. (Here's a good tip on tapping the box. If you can't find the right size or thread pitch tap, get a plumbers propane torch, a set of good gloves and a spare sparkplug of the size you want to use. Drill your hole the size it would normally need to be if you were using a thread tap and then take that sparkplug and hold the threaded portion over the flame for about 30 seconds. Then use some firm pressure and screw it into the plastic. It will cut nearly perfect threads. Allow it to cool completely before you remove the plug so that it does not stretch or pull the threads out. Deburr the hole if needed. This is way cheaper than going and buying odd size taps if you don't otherwise need them.)

For the third circuit, mount the lamp to the box. One side of the lamp can be tied to the same lug on the lamp switch that has the case of the condenser (this is our ground). The other side of the lamp will need a wire exiting the box and an alligator clip attached to it. This wire and circuit become a test lamp in that you can hook the ground cable up and then use the alligator clip to probe for power in the key switch, resistor connections, or the coil primary. If the lamp lights up, you know you have power. In my unit I use a very low wattage 12vdc lamp. I've found it works fine on 6v, the lamp just burns a bit dimmer.)

You can use this circuit to see if your points are opening and closing electrically. Using the test lamp wire, connect it to power, and then use the ground clamp to hook to the wire that normally runs to the distributor. Turn the engine over. The lamp should blink on and off if the points are opening and closing correctly. If the lamp never lights up, the points are not closing, or have corrosion on them, or are burnt. If the lamp never goes out, the points have closed up or there is a short, perhaps at the condenser.

To ensure I always have the correct jumper connection with me, I keep a short wire with a sparkplug tip connector on one end and a connector on the other end made for the ignition coil tower with the tester.

When I acquired a Ford 2N with front mount ignition, I made a slight modification so that it could be used to check a square coil as well, which involved taking a small piece of spark plug wire and attaching a small alligator clip to one end and then a spark plug tip connector on the other. This jumper wire replaces the other jumper wire that is made for the round can coil tower.

With the modified wire, clip the alligator clip to the small flat tab on the bottom of the square Ford ignition coil and then hook up to the spark plug as usual. The curly connection on the under side of the coil is your outgoing primary wire. As with the round can coil, when you flip the switch, you should get spark at the spark plug as long as the coil is good and has power turned on to it, your ground clamp is hooked up, and the outgoing primary curly pig tail wire is connected to the alligator clipped test wire designated as B+. The test lamp circuit also works at 'sniffing' power to the coil and from the key and resistor.

To check the points opening and closing on a front mount Ford, you have to change a few connections around. Reseat coil on the distributor. Connect the tester’s ground clamp to the top post on the coil; connect the B+ wire to the battery. For safety, pull all four of your spark plugs wires from their plugs and let them rest on the head. Now when you turn the engine over the lamp should blink, albeit dimly, if the points are opening and closing correctly. If not, refer to the section above concerning the status of the lamp in relation to the points issues.

I originally built this tester in about ten minutes, using the stated surplus hardware I scavenged from my barn and garage. The original unit was not in a box and was just a light switch and condenser and a spark plug and some gator clip jumper wires. After getting shocked a few times, I moved it into the box and eliminated the individual jumper wires using the permanent wires with gator clip ends. At that time I still used a separate test lamp with this unit but within a few weeks got the idea to just hardwire the test lamp function into the box since I always needed both tools anyway.

This cheap little box has helped me chase down a variety of electrical and ignition phantoms on my small fleet of old tractors. It's also great on lawnmowers and anything that has a spark plug. It's small enough to tote over to the neighbors house to help him troubleshoot his lawnmower, too. The best part is, if it starts raining and it gets wet or you forget and leave it out overnight or run over it with the tractor or set it on your tailgate and run off down the road, you haven’t lost an expensive tool.

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