How to Use a Tach Dwell Meterby Jeffrey Caldwell
A dwell meter is used to check the dwell angle on vehicles with a points-type ignition. Dwell angle is the length of time (measured by the degree of rotation of the distributor cam) the contact points open on a points-type ignition system. Points-type ignition systems were commonly used on vehicles manufactured before the mid-1970s. The dwell angle must be periodically checked on these vehicles to ensure proper operation of the ignition system. In addition, the dwell angle on these vehicles must be checked before setting the ignition timing.
Connecting the Tach Dwell Meter
Run the engine long enough to bring it up to normal operating temperature and then shut it off.
Connect the positive lead on the tach dwell meter to the positive terminal on the ignition coil.
Connect the negative lead on the tach dwell meter to the negative terminal on the ignition coil.
Open the small metal cover on the side of the distributor cap and insert an Allen wrench into the screw behind the cover.
Checking the Dwell Angle
Turn the engine on.
Take the reading on the tach dwell meter and compare it with the figure on the engine tune-up decal in the engine bay or in the vehicle's service manual.
Turn the Allen wrench slowly to adjust the dwell angle to the correct setting.
Turn the engine off.
Remove the Allen wrench from the distributor. Make sure the small metal cover on the side of the distributor is closed.
Remove the electrical leads from the ignition coil.
- Make sure the electrical leads for the tach dwell meter do not interfere with any of the moving parts on the engine such as the engine fan. Also make sure the electrical leads do not contact any part on the engine that develops excessive heat, such as the exhaust manifolds. Some distributor caps do not have a window to insert an Allen wrench to adjust the dwell angle. If yours does not, you will have to stop the engine, remove the distributor cap and incrementally adjust the dwell angle.
- While working on a running engine, keep your hands away from moving parts on the engine, such as the accessory drive belts or any part of the engine that becomes excessively hot.