A tachometer (as opposed to a rev counter) is a useful diagnostic tool if your engine suddenly cuts out or drops to zero while driving along - look at it before you do anything else including dipping the clutch in preparation to pulling over. If it does suddenly drop to zero, or starts jumping up and down with the misfire (often accompanied by backfiring in the exhaust), then it's a good indication you have an ignition LT problem. But the problem could be loss of ignition voltage to the coil, or loss of the switched earth through the points or electronic trigger. A tach will indicate both problems by dropping to zero (as happened to me when I had a coil with a loose terminal), but a voltmeter can tell you which of those two scenarios you may have. Intermittent problems, that clear rapidly before you can do any testing, can be particularly difficult to track down and you can wire in a voltmeter positioned where you can see it in the cabin, and look at that as soon as the problem occurs. Note an analogue meter is preferable as digital meters can give various results depending on their internal design, and you can also see what's happening at a glance from the angle of the pointer instead of having to 'read' a digital instrument that may not be giving a steady reading. Also note that points systems are easier to diagnose than electronic ignition systems - different systems giving different results under fault conditions.

Connect the voltmeter between the points terminal on the coil (CB or -ve) and earth, and if switchable on its 12v scale. On electronic ignition systems of the type typically available today if there are red and black wires from the ignition module to the coil then it will usually be the black wire.

When the points are open on a stopped engine the meter will display 12v (from ignition voltage coming through the coil) and with them closed it should display 0v (from the earth supplied by the points) - this will be the same for both 12v ignition systems and ballasted systems. On a running engine the voltage will be continually switching between 14v (when charging) and 0v and the meter will display an average of the two that is dependent upon the dwell of the points.

For the 25D4 a dwell of 60 degrees in each 90 degrees of distributor rotation equates to 67% when they are closed (0v measured) and 33% when they are open (system voltage measured). So on a running engine at a theoretical system voltage of 12v you would see an average of 4v displayed, and at a charging voltage of 14v you would see 4.6v.

For a 45D4 the dwell is 51 degrees i.e. 57% closed i.e. 43% open so you would expect to see 6v and 7v. However you can expect to see some variation in that as the revs change, and with throttle opening as with a wider throttle there is a higher cylinder pressure which makes it harder for the spark to jump the plug gap, which means the HT voltage increases, and that is reflected back into the coil primary as a higher voltage. You would see similar voltages on a ballasted system as they still switch between system voltage with the points open and 0v with them closed the same as non-ballasted.

For typical fixed dwell electronic systems installed these days people have told me they have measured a higher dwell than the book value for points, so the voltage shown will be a little lower, and incidentally those coils will run hotter. But variable dwell systems only pass current through the coil for a given length of time largely independent of rpm as the coil doesn't need any more 'charge' than it gets at peak rpm on our systems, so the longer current flow at lower rpms is wasted energy as heat. Fixed dwell systems energise the coil for about 20ms at idle dropping to about 3ms at 6000rpm, whereas with variable dwell it's about 7ms at idle and again about 3ms at 5000rpm. So you will see much higher voltages on a variable dwell system at idle than you will at peak rpm.

But all these variations can be ignored for the purposes of fault diagnosis, as what you are interested in is how the indication varies when a misfire or other problem becomes apparent, compared to when it's running correctly. If you do see a significant change then the implication is that the problem is in the ignition LT circuit although an open-circuit condenser won't give much of a change but will affect running. There is a very easy way of determining if the condenser is the problem and that is by temporarily connecting a known good one also between the coil CB or -ve and earth. If the condenser inside the distributor is the problem the problem will go away. If it isn't then the problem will continue, and having effectively two condensers in parallel with each other will have no noticeable effect.

Basically if the voltage goes up to 12v when the engine cuts out you have lost the circuit to earth through the points or trigger. But if it drops to zero then you have either lost the ignition supply through the coil (test the coil +ve or SW for 12v), or the points or trigger have shorted to earth. In this latter case disconnect the points or trigger wire from the coil -ve and see if you then have 12v on the coil -ve, and if you do the points or trigger are shorting to earth.

I have both conventional multi-meters and an automotive one with dwell and tach as well as voltage, resistance and current ranges - both analogue - and the latter meter has peculiarity that when I'm trying to measure voltage on the coil -ve it displays a much higher value than it should, higher even than the system voltage, because it is actually displaying the dwell value! So that is something to bear in mind.