2002 4runner with 230k miles engine won't crank sometimes

For the last couple of days, my 4runner has been acting up. When turning the key to the battery on and then ignition on, it works normally but sometimes when I try to crank the engine the battery will shut off, and sometimes it still won't start but when I take the key out it triggers the alarm. When I let the battery run for about 20 secs and then crank it works. I recently replaced the head unit with a carplay-capable sony unit and all of the speakers. Could this have anything to do with it? The battery isn't dead because a minute later the battery will turn back on. Could it be my alternator? Any suggestions would help.



https://www.sony.com/lr/electronics/in-car-receivers-players/xav-ax1000

 

check that by simply disconnecting the Head unit's fuse, if that does not help then listen to see if you can hear a solid click of the Starter Solenoid (have someone help with the hood open to listen) if you hear the solenoid click in then everything dies the starter would be the suspected item, I have seen Ignition switches wear out over time also but that in no way means that is what your issue is.

 

It sounds like either battery cable corrosion, bad ignition switch or bad grounds.

I've had a very similar issue on 2 cars, including my 4runner.

Problem was the terminal and the internal copper in the cables was severely corroded. Any corrosion on the battery terminals?

Leaving the lights on, reversed the corrosion in the battery cable just enough to allow the starter relay to activate. Eventually that wouldn't even work. Cutting off 12" of the positive cable to remove the section that had corrosion and soldering in a new section and terminal has fixed it for years now.

I, for one, wouldn't suspect the alternator. I would test to see, if while the car is running, that you get 13.5v+ across the battery terminals. If you get that, then the alternator is fine.

Check your grounds to the engine and body and clean them.

Usually these intermittent starting issues are too much resistance in the starting circuit.

 

A lot of good details here, I was going to run down the list of possible Items but ran out of time doing it from my Phone so I figured that was a start and would come back to it latter.

 

Although the alternator could put out the correct voltage, it does not necessarily mean that the alternator is working OK and is not causing a short. Having checked the voltage with the engine running, turn the engine off. Make sure everything is off (ex: lights,...). Disconnect one of the battery cables (I prefer positive) from its post and check for the presence of current between the cable and its post. You should only see a few milli-amps. This accounts for things that are always on (ex: clock). A bad alternator will show a much larger current flow. I suggest that you do this with a fuse-protected meter, otherwise your meter will be destroyed with a bad alternator.