Herbonius Posted May 19, 2013 Share Posted May 19, 2013 I've been having electrical issues for a while now, my battery going dead after driving with my lights on, occasionally having to jump start the car, etc. I'm new to this so I wanted to get some verification that my testing methods are correct. I'm measuring around 12.25 volts directly out of the alternator with no accessories on and that number drops to around 11.75 volts with my headlights on. That is indicative of an alternator that needs to be replaced, right? I've checked that the belt drive is seated correctly and that the belt is tight. My battery is new and I have also put on new brass battery terminal connections after noticing one was a bit loose. I checked the resistance between the ground of the alternator and the ground of the battery and it was around -0.01 volts, which should be fine too? Forgive me if I'm a little off with my terminology and testing methods here, I just bought by first multimeter this morning and went out and did some testing after watching a few quick YouTube videos. Anyway, am I safe in assuming that I need a new alternator? Quote Link to comment Share on other sites More sharing options...
Scoota G Posted May 19, 2013 Share Posted May 19, 2013 With vehicle running remove the negative lead from the battery. If she stops she's snuffed! Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted May 19, 2013 Share Posted May 19, 2013 With vehicle running remove the negative lead from the battery. If she stops she's snuffed! That's generally not a good practice. While these cars in their stock form likely would not have an issue, because there aren't too many sensitive electronics like more modern cars, what few electronics are in these cars can suffer damage from doing such a thing. The battery not only serves to provide electrical power while starting, but also serves as a filter to suppress spikes and ripple that is on the electrical system. Just disconnecting the battery while the engine is running can cause a spike, and could be damaging to electronics. Just because the car may shut off doing this test does not automatically mean the alternator itself is bad. Back to the topic at hand: Was this at idle only? Did you try revving the engine a bit to see if the voltage increased? When I had my stock alternator on my car, at idle, it would barely charge with no load, and pretty much no charge with a load (head lights, etc.). Once RPM of the engine went over about 1200, then it would charge better than at idle, but still not very well. This seems to be pretty normal for these cars. If you did raise the engine RPM and there was no change in voltage then you have one of two issues, the alternator is not working or the voltage regulator is not allowing a charge, though they usually fail the opposite way, where they don't control voltage very well and you start to over charge. There is also a signal that is used to get the alternator and regulator to charge, grab an FSM for your car from xenons30.com, and go through the trouble shooting procedure. It may be the signal to get the system to charge has an issue. This was no load on my stock alternator: And with a load: The charging system is not to be considered charging until the voltage is a full volt above battery voltage. In other words, the charging system must be 13.6 volts to be considered charging, anything above that and about 14.4 is good. Below that it is not. You may want to consider an alternator upgrade. Quote Link to comment Share on other sites More sharing options...
Herbonius Posted May 19, 2013 Author Share Posted May 19, 2013 Thank you for the info. Running at higher revs has a negligible effect on alternator voltage output. If I'm measuring directly at the alternator, I can safely assume that is the issue and not the regulator, correct? Quote Link to comment Share on other sites More sharing options...
RB30X Posted May 19, 2013 Share Posted May 19, 2013 It does sound like your alternator isn't charging the battery. Is there a thinner wire going to the alternator giving it a battery voltage signal which tells the alternator to 'engage' and start charging? I tried wiring up a few of the spare alternators I had sitting around from this car and the 280zx's but they all needed external regulators. It was too complex for me to merge a 280zx wiring diagram with a 240z's so I asked an auto electrical parts store to recommend a generic external regulator. (You may have read the following from the thread in the electrical section) A BOSCH RE55 was the answer. Now how do they work??, I've drawn up a diagram to help anyone attempting the same job I've just done. Makes sense really, starting from scratch without all these extra wires. The reg has two pins in a tee, like the alternator does (see pic) The D+ pin gets a feed from the battery when the ignition is on only (otherwise it would drain the battery with engine off), this voltage is feed into the regulator which determines how much charge the battery needs, and regulates the voltage coming out of the DF pin. This can "regulate" from 0-12V determined by battery requirements. This "regulated voltage 'excites' the coil in the alternator via the F pin, determining how much charge comes out of the A pin which directly feeds the battery. Hope that makes sense. Quote Link to comment Share on other sites More sharing options...
luvemfast Posted May 19, 2013 Share Posted May 19, 2013 I've had similar problems in the past. I upgraded the alternator, no probs since. Nuff said! Quote Link to comment Share on other sites More sharing options...
Scoota G Posted May 19, 2013 Share Posted May 19, 2013 That's generally not a good practice. Only if you're wet and naked! Quote Link to comment Share on other sites More sharing options...
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