Charging Systems - 97 Nissan 200sx

Can someone explain this diagram to me? I know that the B terminal is the output from the alternator to the battery. The S terminal is the sensing circuit to tell the alternator whether or not the battery needs charging. But I don't understand the L terminal ran through the low warning indicator lamp.

The car has a new battery, and new alternator but it's still not charging. The ticket was given to a different tech that put the alternator in. I was just looking at the wiring diagram and I didn't fully understand it or know how I would go about troubleshooting it so I thought I would do a little research.

I added a pdf of the wiring diagram. I was trying to put a screenshot of it up instead of a PDF but couldn't figure it out.

Hey Wood! The L terminal is there to control the dash warning light in the event of an alternator problem (and during KOEO bulb prove-out). Just a simple ground side driver - the voltage regulator grounds the L terminal to allow current to flow through the bulb. As you noted, the S post is there as a sense circuit. FYI, this is a very common Nissan field control design that they used for MANY years across their entire lineup.

Diagnosing a not charging condition on these is pretty straightforward. While not charging, voltage drop the B+ circuit and the case ground. Both should be less than 300mV. Make sure the belt is there, in decent condition and reasonably tight.

Then, check the S and L posts. S should be very close to B+. L will depend on if the bulb is lit or not. If the bulb is lit, the voltage at L should be low. Typically around 1.5V. Disconnect the alternator connector and make sure the bulb goes out. If it does, that means it was the alternator that was grounding the L terminal, and that your wiring is good. If the bulb remains lit, the yellow/red has a short to ground. Find it.

If the bulb is NOT lit, the voltage at L should be high. Around 1V less than B+, usually. If it is, use a jumper wire to ground L yourself. Does the bulb come on? If it does, the wiring and the bulb are good. If the voltage at L is low AND the bulb is not lit, you're looking at an open between the bulb and the L terminal, OR an open in the power feed to the Combination Meter (like the #8 fuse in the diagram).

Did that make sense? :silly: Please ask questions as needed. Usually, both the L and S terminals are required for charging. The resistor in parallel to the bulb is there to ensure that the L terminal gets 12V even if the bulb burns out.

If you haven't seen it already, this video is right up your alley. It's a Miata, but the operation is identical.

Tyler, man I really appreciate it. For the detailed response and the link to one of Paul's videos. Thank you.

Chances are I won't see this vehicle again but now I have a much better understanding of how some of these charging systems work and I can diagnose it properly instead of just throwing parts at it.

I thought I was going to have a thousand questions when I saw I got a response to this post but I think you covered it all. Now it's just a matter of putting it into practice.

Thanks again

No worries! If you do happen to see this vehicle again, please let us know what happens? We always appreciate follow up to repair questions.

Will do! I'll keep ya posted

This was the most common style of alternator wiring on most manufacturers for many years until the control of the charging rate was built into the ECM
A couple of things to watch out for:

(1) The Charge Warning Lamp isn't a reliable indicator the alternator is charging it only tells if the voltage at the alternator matches or exceeds battery voltage. So you could have the situation where the battery is sitting at say 11 volts and the alternator can only generate 11.1 volts but the light will appear to behave more or less normally.
(2) The warning light is a vital part of how the alternator operates, the wattage of the warning bulb has to be high enough to provide enough current to kick start the magnetic field in the rotor field coils. This has caught out a lot of custom car builders who used a dash withn an LED warning light. Where a LED is used a shunt resistor in parallel is required to provide the extra current.

A variation on this type of alternator has an extra wire to provide alternator load information back to the ECM to allow the ECM to adjust throttle opening as an anti-stall measure. The signal back to the ECM is usually a duty cycle modulated 5v square wave.

The 3 wire alternators are great for custom car builders and restorers where a the a 1940's or 1950's DC Dynamo is being replaced by an alternator as the wiring is simple to convert.

This is a lot of solid information. Thank you. Especially the piece about the charge indicator not being a good indicator of alternator charge. Definitely something for me to keep in mind.

From Paul's videos I've watched and all the information you guy's have given me, I feel like a have a decent understanding of this type of system. But you mentioned ECM controlled alternators which is something I'm still I'm in the dark about. Do you guys have any of Paul's videos to refer me to? Any tips or advice? I have the ability to access wiring diagrams so if there's a year or model that would be a good one to look at in this case what would it be?

Paul has a couple videos on ECM controlled systems, a.k.a 'smart charging' systems. I'll link them below.



www.scannerdanner.com/scannerdanner-prem...-bad-alternator.html

Remy also has a Technical Bulletins section that covers diagnostics for several different charging system designs. Definitely worth reading.

www.remyautomotive.com/resource/Technical-Bulletins

I have the ability to access wiring diagrams so if there's a year or model that would be a good one to look at in this case what would it be?

One example of the system Andy was talking about is my '04 Scion xA. It has a light circuit, sense, and a 'load' wire that goes to the PCM. Just going off the diagram, you might think the PCM is calling the shots on the charging rate, but it isn't. You can short that circuit to ground, short it to power, open it, whatever you want and it won't impact charging rate one bit. :lol:

Problem is manufacturers are using different methods we had a good thread started by Tyler on the Ford system.
The Honda and BMW now have fancy load and charge monitoring systems that are giving a lot of trouble, resulting in a lot of owners using work arounds to revert them to dumb systems. www.scannerdanner.com/forum/post-your-re...ittent-no-start.html

I'll definitely check out those videos and pull up some wiring diagrams to familiarize myself with those systems. I'm also interested in reading up on Remy's troubleshooting manuals. That's pretty cool that they offer that, probably because they got tired of people sending back perfectly good alternators.

Speaking of which, I do have an update for you guys on that 97 Nissan 200sx with the no charging concern. Turns out the balancer was completely separated from the crankshaft pulley. This car has had SEVEN alternators AND a new battery put on it!! Thanks to people like you though and a little research I don't have to guess at things and be a parts changer. Thanks again!

Wood wrote: Speaking of which, I do have an update for you guys on that 97 Nissan 200sx with the no charging concern. Turns out the balancer was completely separated from the crankshaft pulley. This car has had SEVEN alternators AND a new battery put on it!! Thanks to people like you though and a little research I don't have to guess at things and be a parts changer. Thanks again!

Hey hey, great to hear! Seven alternators. :ohmy:

How did you figure out balancer had separated? Did it have weak power steering, too?

I wasn't the one that diagnosed it in the end. But I was the one that initially pulled it in and I don't remember noticing the power steering being particularly difficult, which is surprising because that pump couldn't have been turning fast enough either. Probably something that I overlooked. The guy told me he ended up suspecting the front balancer so he scribed a mark across it, ran the engine, and watched the mark separate.

Wood wrote: But I was the one that initially pulled it in and I don't remember noticing the power steering being particularly difficult, which is surprising because that pump couldn't have been turning fast enough either.

Eh, don't sweat it. It might be that the 200SX doesn't rely too heavily on its steering assist.

The guy told me he ended up suspecting the front balancer so he scribed a mark across it, ran the engine, and watched the mark separate.

Oooooh, I like this. Fantastic idea.