There is a flywheel psoition sensor that is screwed into the side flange of block where it would be pointed back at front face of flywheel as it attached to back flange of crankshaft (WHEW!). This is read by a machine called a Mono-tester that used to be a Volvo dealer tool. The Mono-tester reads the sensor and compares it to a #1 plug wire inductive sensor that you clip onto #1 plug wire. The comparison of flywheel position to spark event will give an analog readout of ignition timing. The Mono-tester also has a voltmeter and dwell meter. Can be outfitted with a piezo sensor device that attaches to diesel injection line and it will count rpm and flash the timing light so injection pump timing advance can be monitored.