Will not turn over

Make sure all the connections are good and clean. Check the alignment of the starter as well.

Roy

Battery must be checked first. Then with engine running, you should have 14 to 14.5 volts at the battery.

Roy

You cannot read a battery while the charger is attached to it.

Nobody knows that. Everything starts with a battery test and then test the charging system.

Check the voltage at the starter solenoid and check for start signal at the smaller wire on the S terminal when you hold the key in crank position when it stalls. We can start here.

Have battery checked for condition including load test first. Auto parts do for free.

If you decide to check the condition of your battery yourself here is a link to helpful information from this site that features written instructions, pictures and a video explaining how to load test your battery yourself.

https://www.2carpros.com/articles/how-to-check-a-car-battery

Have the charging system tested. If the generator is defective, replace the battery at the same time unless it is less than about two years old. GM has had a huge problem with their generators developing harmful voltage spikes. It is the job of the battery to dampen and absorb those spikes and render them less harmful. Batteries lose their ability to do that as they age. Not replacing the battery is the main cause of repeat generator failures, and those voltage spikes can interfere with computer sensor signals.

Could be a weak fuel pump not keeping the system full. You will need to install a manual fuel pressure gauge to verify the pump holds pressure when the car is shut off, otherwise it has to refill the system which causes long cranks to get it started.