To diagnose or to measure the performance and service of engine, requires the use of various test instruments and guages.
1) Tachometer:
The tachometer measures engine rpm; the tach may be electric, optical, or magnetic. It counts the electric pulses in the ignition system and reports the result as engine speed. This tells the driver how fast the engine crankshaft is turning. Electrical or optical tachometers are used in the shop. The electric tachometer connects to the ignition primary circuit. The teach counts the number of times per second the primary circuit opens and closes. The optical tachometer has a light beam focused on a rotating part such as engine crankshaft pulley. The teach counts how many times per second a mark on the pulley passes by.
2) Dwellmeter:
The dwellmeter elctrically measures how long the contact points remain closed during each ignition cycle of a contact point ignition system. The average of all cylinders is then displayed in degrees of distributor-cam rotation. The technician can also use the dwellmeter to set contact point gap and to check for unwanted dwell variation as engine speed increases. Excessive variation indicates mechanical trouble in the distributer. In the electronic control system, the ECM controls dwell and it is not adjustable. The dwellmeter is used to check the duty cycle of the mixture control solenoid in a feedback carburetor. A dwell tach meter is a single meter is a single meter that serves as both a dwellmeter and tachometer. This is possible because both meters have two leads and require same connections. Pushing a button or turning a knob on the meter switches the reading from rpm to dwell.
3) Cylinder compression tester:
The cylinder compression tester measures the ability of the cylinders to hold the compression while the starting motor cranks the engine. The compression tester is a pressure guage that measures the amount of pressure or compression, built up in the cylinder during the compression stroke. How well a cylinder holds compression is an indication of the condition of the piston, piston rings, cylinder wall, and valves and head gasket.
The spark plugs must be removed to use the compression tester. Before removing the spark plugs, be sure that engine is at normal operating temperature and the battery is fully charged. Use compressed air to blow out dirt or dust through spark-plug wells. Another way is to disconnect the cables, loosen spark plugs to one turn and reconnect the cables. Then start engine, run the engine for few seconds at 1000rpm. The leaking combustion gases will blow the dirt out of plug wells.
Remove the spark plugs block the throttle wide open so that the maximum amount of air enters the cylinders. Thread the compression tester adapter into spark plug hole of number 1 cylinder. Disable ignition to prevent coil damage from high voltage arcing. Disable the electric fuel pump; operate the starting motor to crank the engine through four compression strokes. The needle on the compression guage will move to show the maximum compression pressure in the cylinder.
