Test light on the coil plug. If the light is on and the AC clutch isn't pulled in you found it as above: gap too big. Weak coil is also a question of grounds and burnt switch contacts like the low side cut out switch. Diesel engine vibration is better than bumps.
I prefer a voltmeter over a test light because 7v, from a bad ground or burnt switch, at the coil or lift pump may light a light, but, it isn't working at 7v.
I prefer a voltmeter over a test light because 7v, from a bad ground or burnt switch, at the coil or lift pump may light a light, but, it isn't working at 7v.