Hi folks.....IN my 1992 C1500, 305, my voltometer will pulsate when I put the turn signal on. Not to mention it will drop to about the "11" mark when I turn on the AC, headlights. Weak battery or weak alternator? Alt. reads high 13's, with an occasional low 14. Battery shows 12.5, but cant read CCA with my little $5.00 Harbor Freight tester.

 

make sure you test it right . test the battery with the car off you should have a voltage between 12.2-12.6 v, on a good battery.if not. charge it.then later with the car running test the battery your voltage should be about 14.2 if its around 13 like you said . there's a good chance you alternator isn't charging properly

but charge or replace your battery befor testing

 

You will have to spend some good $$ on a ampmeter to get the CCA readings, I have a Fluke meter that has a clamp to read up to 1000A AC or DC. it was over $400, My truck pulls between 450-550 CCA when starting, its always good to have a good meter with a fast refresh rate when working with DC voltage. Those HF meters are not accurate from my experience.

It might be a loose connection causing the meter to drop, it will drop if something is loose or corroded since the electrical system pushes harder to light up the lights. Check the wire connections, It might just be a weak/loose connection to the alt, battery etc...