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FIX FOR AFTERMARKET IGNITION AMPLIFIER WITH K-JET
by Jon Glommen

Most k-jet fueled Volvos will not start, or start with difficulty when using an aftermarket ignition amplifier like MSD, Crane, Jacob's, etc. This article suggests a fix that was given to me by a fellow T-brickster.

The K-jetronic ignition system utilizes a fuel pump relay that detects engine RPMs; at 0 RPMs the relay cuts the power to the fuel pump as a safety precaution. From the diagram below you can see that the fuel pump relay connects to the (-) coil junction (as the tachometer) to pick up the RPM signal from the OEM ignition amplifier. Most aftermarket amplifiers intercept the OEM amplifier signal at this junction before processing it and sending it to the coil. Problem is, the fuel pump relay no longer recognizes the signal from the OEM amplifier and assumes 0 RPMs and cuts fuel pump power --no start. This is because the original coil, as a transformer, lowered the resistance at this junction enabling a higher current to signal the the tachometer, fuel pump relay, etc.

To restore this condition another transformer must be hooked up as a dummy coil to increase the current available at the (-) junction. MSD sells a small transformer, PN8910, to do just that. But the 8910 is not allowed to discharge as a coil and slowly heats up, unable to dissapate the energy it is absorbing. The better countermeasure is to substitute a 12v automotive coil in place of the 8910. Automotive coils (transformers) while encased in oil are better suited to handle heat. You can optionally gound the high tension current of the dummy coil with a wire leading to the car body and fitted with a BIG inline resistor; this allows the dummy coil to discharge. In my application I have not had to do this.

Please refer to the diagram below for generic installation instructions. Good luck!

 

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