Let me just preface this and say I am pretty new to cars in general. I have been learning fast and one of the reasons I got a project mustang was to learn.

I just purchased a 1976 mustang cobra for a project car and have started working on getting the engine running. The engine ran previously as I was shown it would before I bought it (It is in poor condition and requires starter fluid and some other things to start). I let the seller's take the battery and in turn they let me take some tools I needed. I had a battery at home so it worked out well. Now when I first tried turning the motor on with my battery it cranked but didnt run (didn't have starter fluid on hand so that made sense). But ever sense that the starter solenoid started clicking rapidly. I investigated further by bypassing the solenoid and the starter didn't crank at all. The battery I was using had enough juice to start my truck so I assumed that it wasn't the battery. So I got a new starter motor (DL3132). I tried using my truck's (daily driver) battery again and it barely made the new starter spin and the gear didn't engage forward. I took it back to AutoZone where I purchased it to make sure it wasn't broken and they got it working just fine. So now I am thinking the batteries I have been using haven't been enough. So I bought a new battery and it barely turned the starter as well. I tried using my wife's battery and had the same issues. So I am at a loss right now. Maybe they aren't charged up enough? I just don't understand why my trucks battery can't turn an old starter yet it can start my truck. And all the batteries I have used have had a CCA of over 600. Any ideas?

 

Eliminate the battery first as culprit. Go buy a battery hydrometer for ten bucks or so, get some instructions in it's use, and determine whether a weak cell is preventing starter operation. The battery has 6 cells; any one bad will prevent good operation.

 

That clicking noise is usually caused by a loose or corroded connection or a weak battery.As a test: with the wiring harness disconnected from the ignition coil and multimeter leads connected to the + & - battery posts (not the cable terminals) have an assistant attempt a startup with the key while you use the meter or if an assistant isnt available,you can also lay a screwdriver across the large posts on the solenoid to jump the starter,if your solenoid is mounted in the engine bay with the positive battery cable and starter cable attached to it. Once the starter begins turning over (or even if it doesn't) once the key is in the start position,the meter should register a minimum of 9.6v or higher during crank mode.If it doesn't,the battery doesnt have enough reserve capacity to power the starter so it will have to be recharged or replaced.You can also try connecting another vehicle using jumper cables to see if it aids in starting.If this helps,your battery is likely the issue.If you've tried several batteries and none of them are getting the starter moving,I would possibly suspect a ground or starter cable/wire issue.A loose,coroded,damaged or disconnected cable/wire between the battery and solenoid,battery and engine block,starter and solenoid or another ground elsewhere in the engine bay is a possible cause.The cable between the battery and block is the power ground for your starter and alternator,so a bad ground here can cause starting issues.If theres a short in the starter cable,the starter will usually turn over fine when the engine is cold,but it will drag or not turn over at all when warm.I would advise to hand spin the engine using a torque wrench and socket on the crank bolt,to make sure nothing inside the engine is the cause of the no crank,but if you've already seen it run in person,this isnt necessary. Its also important to note,that even if you get the starter to turn over and it gets the engine running,
you still need to test with the meter to make sure the battery is outputting a minimum of 12 volts 2 amps when the engine is running,otherwise the engine will die once the battery is totally drained because the alternator ceases to function if the voltage and amperage specs from the battery are below these values.This is needed in order for the alternator to create the magnetic field needed for the charging process to begin.