I started the Z this morning and noticed it was a bit weak when firing it up but it ended up starting. I drove for about 20 minutes and pulled into my garage. After about 2 minutes, I tried to start the car but it won’t start. I was trying to figure out what the issue could be because I just had gotten back from a drive. Wouldn’t that have charged the battery? 

 

The battery is about 3 years old. It’s showing 12v on the battery gauge in the Z. The charge light is off on the “on” position.

Edited August 6, 20195 yr by BayAreaZ650

With the key in the ON or Run position,  and the car not running, the red charge light should be On.

Check the battery cables for looseness or corrosion inhibiting current draw,  start car with jump from another car battery, see if it starts normally.

Seems like mine show around 13.5 volts while idling. Go to a auto parts store and get a free load test. 

Check the tension on your alternator belt too. Could be a little loose?

 Your battery should read around 12.75 volts with the engine off. 12 volts is too low, it needs charging.

Charge the battery, reinstall it and verify the alternator is charging the system. If not it is more than likely a bad alternator or regulator. 

Or just a bad battery, like a dead cell.

Edited August 6, 20195 yr by rcb280z
added content

3 hours ago, BayAreaZ650 said:

The charge light is off on the “on” position.

This could be a bad connection at the T plug on the back of the alternator.  Seems like you partially realized that something is wrong there.

Or, and I think this is rare, and I can't remember if there's not a second L circuit, the bulb in the voltmeter could be blown.  That will also kill the charging circuit, assuming that it is the only L wire power source.

Looks like they added a second L circuit in 78, with the internally regulated alternator.  What year is your car?

Thanks for the replies guys. It's a 1977.

I replaced the battery with a new one. I also cleaned the T plug with contact cleaner and a file. Everything seems to be working just fine.

I thought it was just strange that the old battery wouldn't start after 20 minutes of driving. 

 Check the battery voltage with the key off, the engine idling and again at around 3000 rpms. Battery voltage should be around 12.75 with the key off. As Cliff noted around 13.5 volts at idle and 14+ volts at 3000 rpms. As far as the old battery not starting the car after 20 mins. of driving, there's a few possibilities. 12 volts Instead of 12.75 (discharged battery) , poor (corroded) electrical connections, a tired old starter, a faulty alternator, loose alternator belt or any combination them.