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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