Dwell Meter

Dwell Meter

One of the most important settings on a car is the contact breaker gap, as wrongly gapped points can result in difficult starting, a loss of power and an increase in fuel consumption. It is obviously desirable, therefore, to be as accurate as possible when checking this gap. The use of feeler gauges has a major drawback in that they will measure the gap between the high points on the two contact faces, so any wear or pitting on the points will result in an inaccurate measurement. This problem can be overcome by using a dwell meter to measure the dwell angle.

The dwell angle is the number of degrees of cam rotation that occur while the points are closed; thus the wider the contact breaker gap, the longer it takes for the points to close and the smaller the dwell angle. Similarly, a small gap means that the points will close more quickly and the dwell angle will, therefore, be greater. Using a dwell meter will give an accurate measurement of this angle so that you can adjust the points gap until the angle corresponds exactly with the vehicle manufacturer's recommendations.

There are a number of dwell meters on sale suitable for the DIY market and you must follow any instructions provided with your meter, but the general principles will be the same. Most meters will give a reading for four, six or eight-cylinder engines, whether they are negative or positive-earth. The wiring instructions given in this article are for a negative-earth ignition system but if your car has a positive-earth system, all you have to do is simply reverse the two leads on the dwell meter.

Some meters will have to be set to zero before you can take an accurate reading. If your meter has such an adjuster, wire up the meter as shown in fig. 10. Disconnect the LT lead between the coil and the distributor and connect the appropriate lead on your meter (normally coloured red) to the live coil terminal. The other lead has to be connected to a good earth. Turn the ignition key to crank the engine with the starter. This will give a reading on the meter. Turn the adjusting screw (fig. 12) until the needle is set according to the manufacturer's instructions. The dwell meter is now ready for use.

Connect the lead which was previously connected to the coil mounting bolt (earth) to the LT lead from the distributor (fig. 11). The live (red) lead remains connected to the live coil terminal. Crank the engine with the starter for a few seconds, take a reading on the dwell meter, and compare this with the dwell angle recommended for your car. To reduce the dwell angle, the contact breaker gap should be increased and to increase the dwell angle the points gap should be reduced. After any adjustment of the points, re-check the dwell angle until you are satisfied that it is correct.