Reopus Ignition

Dedicated to the Lucas OPUS Ignition amplifier originally fitted to Jaguar V12 (pre 1982)
Aston Martin V8, and Cosworth V8 engines.


Coil Duty cycle test


To carry out tests on your ignition system you will require a digital Multi meter. With the following ranges and functions.

Amps: minimum 10 amp range To test coil current

DC Volts: 0-20VDC

Resistance: Low ohms range 0 to 100.0Ω to test pick up wiring

Duty cycle: To test amplifier Duty cycle (ON/OFF time ratio)

Frequency: To test amplifier

I use a DIGITECH QM-1541, All my test information is based on this multi-meter.

Coil Duty cycle

The most common technical call is "the engine missfires above 3000RPM" in most instances this has been due to incorrect pickup gap. The result is Coil OFF time too short.

Duty cycle this measurement was called dwell angle and was used to check points operation in contact based ignition systems. A Duty cycle meter can be used on the 6 cylinder range but the % reading must be halved for a 12 cylinder engine.

I have found Automotive digital meters may often give misleading Duty cycle results with the OPUS/REOPUS ignition amplifier. When checked with my oscilloscope my DIGITECH QM-1541 does give accurate readings

With electronic ignition systems I prefer to use the term Duty cycle as it is easier to understand. Duty cycle is the percentage OFF/ON time for each coil firing cycle the complete ON/OFF cycle =100%. Your ignition coil produces a spark when the coil current is interrupted this is the OFF time it should be between 13% and 23%

With a V12 engine the OFF time should be not more than 23% to give at least 77% coil ON time, otherwise the coil may not be fully charged at high revs and the result is a weak spark. Also when the coil is switched back ON if the OFF time is more than 25% it may produce at revs above 3500, a weak spark and missfiring especially when hot.

When the coil is in the charge mode (current is passing through the coil) this is the coil ON charge time. Should be between 77% and 87%

How to test Duty cycle

With the engine running, and your meter on the duty cycle range connect the meter negative to the engine or a good negative and the meter positive to the coil negative terminal with the engine at about 2000RPM you should have not more than 23% duty cycle. If you have more than 50% you probably have your meter connected the wrong way around.

If you have more than 23% with a REOPUS 3 or 4 amplifier you can open up the pickup gap from the standard 020thou in .005thou steps to reduce the duty cycle. You will know when you have gone too far as the engine will misfire. If you have the early Version 3 REOPUS and it has not been upgraded to version 3+ contact REOPUS Engineering and we will upgrade your amplifier for you to include temperature compensation, plus we change some other components to improve long term performance.

The REOPUS H amplifier and Pickup should have a Duty cycle of 19% to 20% changing the gap will not alter the Duty cycle the REOPUS H is considerably more stable and less sensitive to pickup gap.


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