tyson Posted July 26, 2013 Share Posted July 26, 2013 Hey everyone, Just encountered a problem with my 240z running a ZX alternator. I went to start it this morning and noticed it was slower to crank. My SDS panel said only about 12.5 volts at the battery idle, only about 12.8 with increased revs. Amp gauge was slightly to the right of midline, but with a little gas it went all the way right. A physical test at the battery showed the same ~12.5 volts at idle. Alternator post showed about 13.5 volts at idle. Alternator is receiving 12v through the plugs with ignition switch on. Battery terminals were freshly cleaned Ground was freshly cleaned and verified good Connections are all clean and tight. I traced the large wire to the amp meter, where I have the ~13.5 volts and traced it back to the battery. Double checked, yup still low voltage at the battery. I went for a little cruise to see if it would somehow resolve, nope. Am I looking at a faulty alternator, battery, or something else? Much appreciated. Quote Link to comment Share on other sites More sharing options...
Nigel Posted July 26, 2013 Share Posted July 26, 2013 Where do you have the voltage sensing wire for the alternator connected? Nigel Quote Link to comment Share on other sites More sharing options...
tyson Posted July 26, 2013 Author Share Posted July 26, 2013 Where do you have the voltage sensing wire for the alternator connected? Nigel The only plugs it has connected are the L plug, the "main post" and the ground. Been that way for the last year that I've owned it. Quote Link to comment Share on other sites More sharing options...
Nigel Posted July 26, 2013 Share Posted July 26, 2013 Connect the S terminal to the positive battery post so the alternator knows what voltage to put out to keep the battery charged. As for the volt you're loosing, after 40 years some of the wire connections in the charging circuit have likely loosened up and/or are corroded. Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted July 27, 2013 Share Posted July 27, 2013 How old is your battery? and is it a regular battery or a gel/drycell? Quote Link to comment Share on other sites More sharing options...
tyson Posted July 27, 2013 Author Share Posted July 27, 2013 Thanks for the tips. I'm unsure how old the battery is, it's an Optima red top, so gel. I've got it on my charger right now, I'm going to see how it is in the morning. Quote Link to comment Share on other sites More sharing options...
tyson Posted July 27, 2013 Author Share Posted July 27, 2013 Must be the battery. After charging it overnight the battery reads like 13.4 volts. Hooked it up and now I'm getting 15v from the alternator post all the way to the battery. Side question, does a lower voltage going into the alternator regulate it's output--hence resulting in a lower voltage out of the alternator? I was always under the impression that they just got a "yes" ~12v signal and put out ~14.5v I know 15v is about .5v too high, anything to be worried about? Quote Link to comment Share on other sites More sharing options...
Nigel Posted July 27, 2013 Share Posted July 27, 2013 A reading of 13.4 volts immediately after charging is not unusual. Since you don't have the S terminal connected, what voltage going into the alternator are you talking about? As I already indicated, the S terminal is what tells the alternator how much voltage to put out. Without connecting that, the alternator has no way of regulating it's output to keep up with demands on the electrical system and keep the battery charged. That's what it's there for. 13.5V out from the alternator is already on the low side for charging, and with the voltage drop to the battery (1V isn't excessive), you're undercharging the battery. Nigel Quote Link to comment Share on other sites More sharing options...
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