Testing an internally regulated alternator

[DanielC]

This is how I tested two internally regulated Nissan alternators.   The alternators with "L" and "S" marks by the "T" plug.

 

First, connect negative from a battery to to the case, or frame of the alternator.  Then connect a test light to the positive of the battery, and touch the positive output post on the alternator.

If the light lights up, the diodes in the alternator are bad.  Get another alternator.

 

 

If the alternator passes this test, go on.

Disconnect the negative battery cable from the battery.

Put the alternator on an engine.  Again, connect a ground wire to the frame of the alternator.  

Connect the positive output post to an amp gauge, and connect the other post on the amp gauge to the positive battery terminal, or reconnect the wire you took off the alternator positive post.  The engine was running when I took this photo. 

Connect the "L" connection in series with a test light.  Connect the other end of the test light to the positive battery terminal.

Connect the "S" connection directly to the positive battery terminal.

I made this test wire harness with a light.

The dark red terminal goes to the positive battery cable clamp.  The white plug goes into the alternator

 

Now, reconnect the negative battery cable.  The light should light up.

 

This is a picture with a voltmeter, and the engine not running.

 

Start the engine.  The light should go out.  Measure the voltage at the battery terminals.

 

Charging voltage should be 13.5 to 14.5.  I know the second picture up had the light on, and the battery voltage is 13.59.  If you just recently charged a car battery, it is normal for the voltage to be slightly high, with the engine off. the voltage will quickly decay down to less than 13.5 volts.

It is also normal for the amp gauge to show a high current for a little while, as the alternator replaces the electricity that was just taken out of the battery by cranking the engine.  I know that is not a good scientific explanation, but it gets the general idea across.

 

This was done with this alternator, from a Nissan pathfinder, on a L-18, in a 521 pickup.

 

Front view.

 

Belt alignment view

 

The pathfinder alternator has 10MM holes on the bottom mount.  I used 8MM ID, 10MM OD sleeves to make up the difference.

[flatcat19]

Good stuff. 

[MicroMachinery]

Thank you for the great write-up!

[datzenmike]

I've pinned this. Now you can direct others here when there are charge questions.

[Beer Snob]

I'll take it if the light stays on and the needle has no movement at all then it's off to the auto parts store?! Cool test. I just performed on a questionable alt (Shucks said bad, NAPA said good 😔). Had a phenomenon yesterday where the "L" and "S" wires had made contact with each other near the plug on the back of the alternator. (Sloppy connection under heat shrink), and when started the motor, I was reading 14.6 at the battery term. Previously had not been charging. However, when I turned the key off the engine keep running. Thats when I discovered the wiring making contact with each other and I assume "back feeding" to the coil somehow? Anyway, that's when the doubt started creeping in weather I had connected the alt. correctly (I've been doing a rewire of sorts). So, PLEASE, ANYONE, this should be simple..."L" is Lamp, correct? And "S" should be my 12volt switched power supply to excite the alt, Correct? I had it hooked up that way and there were no major issues other than no 14+/- off the post that returns to the battery.  This alt. has "E" witch I assume is Earth for ground. What's better, ground to frame or battery? Same thing, right? Thanks all, back to the garage for a few🍻

[datzenmike]

If you mistakenly replace an externally regulated alternator with an internally regulated alternator it won't shut off with the key.

 

To ground return through the frame it has to also have a ground wire from the frame to the battery negative. Don't forget that the body is isolated from the frame with rubber mounts. Engine and transmission also have rubber mounts. The negative terminal is grounded to the block and the body. Or is from the factory. If the negative cable has been replaced it may not have the extra body ground wire on it.

[Beer Snob]

Thanks Mike, I think I'll give another look and make sure that all the grounds are there. Everything worked 10 years ago when it got stripped down but that was 10 years ago. It's so tempting to just replace the alt. but "it ran when parked" and as we all have said before, "There's a difference between a mechanic and a parts installer"

 Also, I'm assuming "won't shut off with the key" would be with an internally reg. alt. still running through a voltage regulator or not doing the proper work to convert? I've done that conversion on my 620 (with your help if I remember right), and the truck I'm working on now never had any external reg. stuff left on it, so I don't think it's a mix's up, just curious as always🍻

[Beer Snob]

Update.... so this is a thank you to everyone (and Mike) who replies to my endless questions. Found some missing body ground(s) and got redundant grounding motor, frame, body, and grounded the alt straight to battery neg. Started it up, and only 12.3v coming out. BUT, then gave a quick rev, 14.6v after that!?! Turned it off, restart same thing. Anyway seems to charge with that initial throttle blip and I'm going to move on to the next fun thing for now. Thanks to everyone for all the help, OH, and to everyone who asks for help and advice with a problem only to get a flood of knowledge and help on this site, let us know WHAT HAPPENED! So many threads I've read and people are offered great advice and ideas, only to drop into obscurity and leave us all in suspense! What happened? Did you get it fixed? Was it what they told you? Did you learn anything? Peace out 🍻

[datzenmike]

The red charge light comes on when ignition is turned on?

 

Is this an original Nissan alternator or a 'rebuilt' one from Auto Zone or something?

[Crashtd420]

11 hours ago, Beer Snob said:

Update.... so this is a thank you to everyone (and Mike) who replies to my endless questions. Found some missing body ground(s) and got redundant grounding motor, frame, body, and grounded the alt straight to battery neg. Started it up, and only 12.3v coming out. BUT, then gave a quick rev, 14.6v after that!?! Turned it off, restart same thing. Anyway seems to charge with that initial throttle blip and I'm going to move on to the next fun thing for now. Thanks to everyone for all the help, OH, and to everyone who asks for help and advice with a problem only to get a flood of knowledge and help on this site, let us know WHAT HAPPENED! So many threads I've read and people are offered great advice and ideas, only to drop into obscurity and leave us all in suspense! What happened? Did you get it fixed? Was it what they told you? Did you learn anything? Peace out 🍻

Any chance there is an led bulb in the cluster for the charge light?

I had that problem of having to blip the throttle to get mine to start charging.. went back to a regular bulb and that problem went away...

[Beer Snob]

No lights are currently working. It's a Mitsubishi brand alt. in my 320. I remember hearing that with the changeover to neg. ground system there was issues with ever getting idiot lights to work so I just dismissed it. I do remember that red light staying on 10+ years ago......🍻

[banzai510(hainz)]

I have heard of self exicting alternators where it takes a rev up to get going.

[datzenmike]

The red light supplies a very small current to the electro magnet inside to get it going. Once charging light goes out and it's self supplying. The reason it starts with a rev is that there's a small residual magnetic field in the iron core but it needs some revs to really start the process.

[Beer Snob]

The very small current from the light, (The "L" side of the plug), is in addition to the 12v. switched supply (The "S" side of the plug)? I thought I understood that the 12v. power supply (S") was the "exciter wire". Am I still all f*$ked up in the head!?!🍻

[iceman510]

Actually in most older vehicles where the alternator is not ECU controlled, the alternator or charge light has system voltage (one side battery, one side alternator output) applied from both sides at normal operating condition, leaving the light extinguished.  If the supply side from the alternator is not supplying voltage, the battery side potential is now greater and turns the light on.  

 

In the stock 510 wiring (not sure which vehicle you are working on here), one side of the light (green wire) is supplied from the fuse block when ignition is on (system voltage from battery) and the other is from the voltage regulator (white/red wire), alternator output.  When both are the same as they should be when a functioning alternator is connected to the battery (big cable), then the light should be out.

[Beer Snob]

So, in simple terms, the light, has power feed from battery on one side, alternator (once engine is running) on the other.  A stalemate if you will of power supply to the light with no winner when all is good. Should the alt fail, the battery side wins, flowing power now illuminates the lamp. Would also be observed with the key on and the alternator not spinning as in initial startup. Am I at least closer? 

[iceman510]

I think you've got it. 😉

[datzenmike]

All Datsun and Nissan internal or external alternators are this way. I put a 2002 Altima 100 amp alternator in my '76 and it's this way. Actually had the S and L wires reversed for 3 years before I noticed. Corrected this and there was no ill effects. Never noticed that the charge light didn't come on with the key. Car would run normally till battery voltage dropped below a certain amount, that it started charging. It was only after I installed a volt gauge that I noticed

[Beer Snob]

Thanks again guys for all the patience and knowledge, it is always very much appreciated and valued! 🍻