In recent times, since I started repairing Porsche control units and later CTB magnetizers, I have often worked on related CTB coil testers. These experiences have drawn the attention of many vintage car and motorcycle DIY enthusiasts, as well as several professional auto electricians.
Questions and requests for information on coil testers have multiplied, so I decided to publish a DIY project: an ignition coil tester complete with a spark gap, developed from the CTB coil tester circuit I had repaired some time ago.
Recovering the Spark Gap
The visual heart of the project is the spark gap, which lets you observe the spark produced by the coil under test. To find it, I started… at the junkyard: I salvaged an old 1950s coil tester, bought for literally a couple of coins because it was nothing more than a rusty wreck. Perfect for my purpose, since I only needed the spark gap.
The only flaw was the black bakelite plate on which everything was mounted: it had become slightly conductive. Measuring with the megger, I found a leakage resistance between the two tips of about 100–110 M?. It may seem high, but at spark-plug voltages that’s enough to drain all the energy from the coil. To fix this, I 3D-printed some insulators and inserted them into the original plate, simply widening the existing holes and reassembling everything. Problem solved.
Power Supply and Circuit
To drive the coils I had a custom transformer wound, model 25S121, and built the circuit on a standard perfboard. The schematic is free: anyone who wants to assemble it can order the 25S121 transformer by contacting me. Click the image to view it in high definition.
Everything is housed in a Gewiss enclosure, easy to find, inexpensive, practical, and sturdy. For cooling, I added a fan salvaged from a PC: even though the transistor dissipates only about 3 W, without ventilation the temperature would rise above 100 °C despite the heatsink. With the fan, the operating temperature stays around 35–40 °C. Remember to make an extra hole for the air to exit.
Key Components and Protections
The main capacitor is a 1.5 µF Audyn Cap MKP 600 V: I used it because I had it on hand, but any 1.5 µF 630 V polyester capacitor will work. To protect both the capacitor and the BUX85G transistor, I added silicon carbide TO-220 diodes, model C3D02060E. They are essential in case of faulty coils that may send high-voltage discharges back to the driver circuit: without these protections you risk damaging the circuit and causing failure.
Essentially, the circuit is a relaxation oscillator that charges and then discharges the 1.5 µF capacitor into the coil with pulses of about 300 V at very high speed, a principle similar to that of Porsche 911/930 ignition control units. This driving method ensures effective testing of any 6 V, 12 V, or 24 V coil.
If You Can’t Find a Spark Gap
You don’t necessarily need to salvage an old one. Anyone with a mechanical workshop and a lathe can easily build one using simple brass rods. I removed mine from an old wreck with an autotransformer, full of terminals that leaked to the chassis: live voltage was everywhere, making it impossible to reuse, and the analog meters were already out of order.
Safety Warnings
Please note that this circuit operates at potentially lethal voltages. Anyone without adequate electrical knowledge should not attempt to build it. This article is for educational purposes only, and I refer to my Disclaimer page for further warnings.
Conclusion
This small project combines creative recycling, vintage technique, and modern practice. If you’re passionate about classic cars or motorcycles and enjoy experimenting, building an ignition coil tester with spark gap can be a fascinating challenge. I can only supply the 25S121 transformer, but I remain available for advice and technical clarifications.
Happy building—and watch out for the sparks!