On my DS4510, there are 3 wires going to the eng. shutoff solenoid. One is the momentary power to help pull the pin back against the spring and then the other two hold it back. When you cut the power (key off) then the spring drives the pin into the shut off plate in the injector assy. I had a similar problem and it turned out the controlling module under the dash (also controls some of dash functions) was bad. My experience so far.
Tom
Agreed that is how the fuel shutoff solenoid works, a large current thru the fuel shut off relay, this is called the pull current. and a small holding current on the second coil of the fuel shutoff solenoid of less than an amp. This holds the pin back. When the key is shut off the holding voltage/current is removed.
The tach is not mechanical, there is no cable.
The electronic pickup sensor of engine is located in
the gear case close to the gear teeth of the fuel injection
cam shaft. As the gear rotates, frequency occurs
between the teeth of the fuel cam shaft gear and the tip
of the sensor and this is sent to the display unit. Then,
the display unit sends a signal to the tachometer in the
instrument panel.
The output voltage of the display unit differs depending
on the engine speed. When the engine speed is low,
the output voltage is low.
Reversely, the output voltage increases as the engine
speed increases. The tachometer reads this voltage
signal and displays it on the instrument panel in rpm.
At least this is how the modern DK35 and DK40 DK45 work.