If you have a rev counter instead of a tach then positioning a voltmeter in the cabin can be as useful a diagnostic tool as a tach and in one case better. Note an analogue meter is preferable as digital meters can give various results depending on their internal design.

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 and with them closed it should display 0v - 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. 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.

Other than the condenser I would expect to see such changes on a tach as well - and have done so with a loose coil terminal - as the problem will be changing either the current or the voltage at the tach as well, typically with wild swings or having dropped suddenly to zero while the engine is still spinning and the ignition is on. The only time it may not is if the points are still operating but the dwell has become so short that the coil can't charge up enough at higher rpms and there isn't enough HT to fire the plugs so you get a high rev or wide throttle opening misfire. In that case a voltmeter would reveal it as a much higher voltage than normal, as would a dwell meter of course if the short dwell is apparent all the time i.e. at idle with the bonnet up.

I have both conventional multi-meters and an automotive one with dwell and tach as well as voltage, resistance and current ranges 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.