Check battery cables for tightness and corrosion also check solenoid trigger wire
It could be that there is a bad connection between the starter solenoid and the starter, or that the starter motor itself has failed. Check the wires first. They're cheaper to replace. This also happens when the battery is very low, so it may be worth charging the battery if you haven't already eliminated that possibility.
start by having the battery checked. If the battery is good, then check the starter relay. If this is bad, repalce the starter. Also check battery connections. Sometimes they can seem tight but just aren't. Take them off the battery terminals and clean them. This will also stop your batter from charging.
most likely the solenoid on the starter or could be a key switch problem doesn't sound like a battery problem
I had a similar problem turns out the Alternator was bad.
If the starter is engaging and the starter is able to turn the engine normally, there is nothing wrong with the solenoid. Instead, I'd look at fuel, ignition and compression.
If the battery tests good and battery cables are clean and tight at both ends and you are jumping the starter solenoid correctly with no results, you probably have a bad starter/solenoid.
Start by cleaning the battery cables. Dead battery. Dirty/loose battery connections. Not fully in park or neutral. Blown fuse. Bad neutral safety switch. Bad starter. Bad starter relay. Bad starter solenoid.
Dead battery? Loose or corroded battery cables? Seized engine? Defective starter solenoid? Defective starter? Defective neutral safety switch?
Nothing, as long as they're both the same voltage. The 18 Ah battery has just slightly more power. It will work perfectly.
It could be that there is a bad connection between the starter solenoid and the starter, or that the starter motor itself has failed. Check the wires first. They're cheaper to replace. This also happens when the battery is very low, so it may be worth charging the battery if you haven't already eliminated that possibility.
Check the starter and your fuses.
Attempt to start-nothing happens or may drag like weak battery-or may grind
take the battery terminals off and clean them and the battery posts real good with a wire brush and reinstall and see what happens.
Seized engine? Bad neutral safety switch? Bad starter or starter solenoid? Loose cable connection at battery or starter?
Neutral switch Starter solenoid Starter Loose or dirty battery cable
Make sure that the battery fuse is not blown. It is a 40 or 70 amp in the fuse box under the hood.
start by having the battery checked. If the battery is good, then check the starter relay. If this is bad, repalce the starter. Also check battery connections. Sometimes they can seem tight but just aren't. Take them off the battery terminals and clean them. This will also stop your batter from charging.