91 s10 ignition firing problem

Hello: Drove my s10 to work ran fine, at quitting time wouldnt start. It would crank but would not try to start, all fuses were good. Today I checked the fie from ignition coil with volt meter. Im not an expert and thought I would have to spin the motor to get a reading but just turning the key on should 13 volts. The way I checked it I attached a clamp with a wire on the post of the coil and wraped the wire around the red probe on the volt meter and grounded the black probe. When I turned on the ignition switch it showed 13 volts. Next I checked the fire from a post on top of the distributer cap the same way. Funny thing is when I tried to crank the engine it started. Put the spark plug wire back on and it sill would start and run. Why would checking the fire from the distributer cap fix the problem. Iam thinking mabe a ground problem but just guessing.

Sounds more like your coil wire may be bad, or it just had a poor connection. And you moving it around got the contact back.

spark plug wires over time fail internally as well as externally. with high resistance and the internal conductors damaged you may have an open which would cause the fire to jump to the ground at another area than in the spark plug.

remove the wires measure resistance and check for exterior wire damage. make sure the wires are properly routed as built.

if you find wires damaged/open then on instaling new wires use dielectric grease inside the boot ends to prevent damage to the internal conductors.

water and electrical currents [fire] create a conductor eating condition. this special grease prevents this.