A dwell meter serves two purposes: confirming that points have been set properly, and permitting dwell variation measurement at a fast idle speed, when the breaker plate moves with vacuum advance. Dwell variation beyond a few degrees indicates wear in either the breaker plate pivot bushing or in the distributor shaft bushings. The older Delco distributors with the fixed breaker plate, where the whole distributor moves with vacuum advance, rarely showed dwell variation. Also, most dwell meters have a "Distributor Resistance" bar or range on one of the scales. The width of that bar/range represents .2 volt drop in voltage across the distributor, primarily the points and breaker plate connections, but including the primary wire between the coil (-) terminal (the usual point of connection) and the distributor. As Hotrod Lincoln pointed out, dwell meters measure voltage; if the coil ground circuit (the distributor) has resistance beyond an acceptable level, the dwell reading will be inaccurate. So the "Distributor Resistance" indication on the dwell meter also confirms that the coil ground circuit is satisfactory. The distributor resistance check with a dwell meter is done with the key on, points closed, engine stopped. Voltmeter check is done under the same conditions, but voltage should not exceed .2 volt. High distributor resistance will reduce primary current, and if high enough, will lead to misfires at higher engine speeds, or ignition system failure altogether.
Maybe this is better left unsaid, but ignition dwell meters actually measure the "open angle" of the points during each ignition cycle- thats when the meter has system voltage applied to it. The dwell meter internal circuitry handles the measurement so the meter scale represents the "closed angle" or dwell angle of the points. Example: an 8-cylinder distributor has 45° between cam lobes (the lobe angle), points are adjusted to a gap that provides closure for 30° (the "closed angle" or dwell angle, by definition), the reamining 15° of the lobe angle is the "open angle" that applies voltage to a dwell meter- but the meter scale represents the "closed angle"- the dwell angle of the distributor. Its internal trickery, so to speak, but it works.
Dwell meters were also used to measure the operation of mixture control solenoids in the days of computer-controlled carburetors, before electronic fuel injection took over.
Harvester
