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DIY coil tester
Posted to Tool & Equipment Forum on 1/17/2016 19 Replies

Many years ago I found myself diagnosing Ford COP ignition coil units, but I didn't have great equipment (or training) to do this well. One test I did have was to manually stress test the coil- using an adjustable spark tester to check maximum output. This was impractical to do on the car/truck. I didn't find any production tool to bench test coils, so under the unwholesome influence of TV shows like MacGuyver, A-team, Junkyard Wars, & Mythbusters I went & made my own. ;)

Basically it was a window motor that spun the top half of a Ford TFI-IV distributor- all mounted in a large molded case. It worked well enough, but as I gained knowledge & experience, I used my scan tool or scope more, & the spark tester (aka "Sparky") less.

Nowadays I still find occasional use for bench testing a coil. Maybe I want to stress a suspect unit that didn't show anything on a scope (no symptoms in the bay but codes or mode6 points towards it), or check coils that are going back under an intake plenum after a plug change, or just to check functionality of a new part.

I wanted to "update" Sparky to something a bit more compact, so here's what I came up with- Sparky 2.0. [DIY coil tester in use] Connect the coil to the tester & see if it can fire a 3/4-7/8" spark gap. Works on 2-wire COP units like Fords or Chryslers, or most any "hei" type coil. The basic idea is that a square wave signal generator drives an ignition module- that fires the coil. Here's the functional block diagram. [DIY coil tester block diagram] The diode provides reverse polarity protection.

The signal generator is based on the NE555 timer chip, you need one that has two potentiometers to adjust frequency AND duty cycle (Left one changes freq, right one changes duty cycle & freq- the jumpers change freq ranges). [NE555 signal generator] Input spec is 5-15Volts- output voltage is very close to input level- about 200mA max. Find it on ebay, amazon, etc. for $2-4 each. Buy several as it's easy to fry the chip if you're not careful- reverse polarity, short the output, etc= poof. The unit uses a 3-pin "DuPont" 2.54mm pitch connector- it's similar to stuff used in desktop pc's - mine came from an old pc case cooling fan. I adjusted the 555 timer so that it outputs a 12V signal with a 3ms neg/0V pulse(max positive duty cycle) at 8Hz or so.

The ignition module is a Ford TFI-IV "black" remote mount unit (Standard Motor #LX-241T). Special thanks to Albin Moore for his recent (& timely) post on Ford ignition systems. This computer controlled dwell module charges the coil as the SPOUT signal goes from 12V-0, then fires as it goes 0-12V. The module current limits ignition primary at 6.5-7.0 Amps. I didn't have a TFI connector on hand so I used some female terminals that fit the module pins securely then filled the module cavity with hot glue & gave it a wrap of duct tape. Here's everything stuffed into a plastic case I had laying around- I added a couple LED's for power & triggering status. [Inside the coil tester]

DIY stuff isn't for everyone, but I enjoy "tinkering" in my spare time, & if it produces something useful, then it's a real win-win situation in my book.

Chris from New Jersey

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