Help with factory alternator harness on '72 240Z

[Zed Head]

1 minute ago, SteveJ said:

Should we trust ZCD?  

I was writing something when you posted.  I'll go ahead and post it now.  It's Reaganesque.

[Zed Head]

Actually you're better off to have that "12V" wire connected out closer to the loads.  Like at the battery.  Connected at the alternator it will deliver lower voltage to the full system, because there's no voltage drop there.  You have yours setup like a "one-wire" system.

But that is all considered in the #14 link.  One of the wires that is being jumpered is the "12V" wire.  That's one of the things that SteveJ suggested you check.

SteveJ has been doing electrical for years, but I was in and out of it and off and on myself so I might have more empathy.  I tried a few of the connect the colors directions when I got in to old cars but always ran in to problems.  Your best path is to consider everything and double and triple check.  Trust but verify.

[RJK]

Ok, here are my readings:

B+ post of alternator to DVM ground, alternator harness "12V" tab to DVM V+                        =7M ohms

B+ post of alternator to DVM ground, alternator harness "ignition key on" tab to DVM V+      =no reading

B+ post of alternator to DVM V+, alternator harness "12V" tab to DVM ground                        =12M ohms

B+ post of alternator to DVM V+, alternator harness "ignition key on" tab to DVM ground      =9M ohms

 

Also, this is going to sound daft, but I really like to eliminate ALL assumptions, in general. My car has two eyelet tabbed wires for the V+ post and ground post of the alternator; the large eyelet reads 0 ohms to chassis, the small eyelet reads about 160 ohms to chassis. This means that the large eyelet wire ties to alternator chassis, and the small eyelet to B+ post of alternator, correct? If so, I had this backwards initially. The small eyelet tab doesnt fit the post on the new alternator, and the positioning of the posts is actually inverted on the new alternator, so I didn't even stop to think to throw a DVM on the wires....

[SteveJ]

The point of making the measurements was to learn how to use a multimeter to discern between the L (lamp or the switched source) & S (sense or battery voltage) terminals on an internally regulated alternator.

With the positive probe on the meter on the S terminal and common on the B terminal, you will see resistance. In this case I'm using a ZX alternator for the demonstration.

However, if you swap the probes, the diodes in voltage regulator will block the resistance reading between the S terminal and B terminal. This meter reads "OL" or open line when the circuit is open.

There are no diodes between the B terminal and the L terminal. So the reading get with the positive probe on the L terminal will be about the same as when the common probe is on the L terminal.

In your second photo, the common (black) probe is on the S terminal.

Frankly, I would connect the wire from the S terminal to the cross of the T terminal on the stock wiring. Connect the vertical on the T connector to the L terminal. Again, you can get the connector from Vintage Connections. Get the jumper plug from Motorsport Auto. Hook them up. You are done.

 

 

[RJK]

ah, gotcha-thanks SteveJ! This is super clear. I've got all the parts on order already; as soon as they show up, I'll implement this. Thanks!

[RJK]

Hey folks, sorry to beat a head horse, but I want to double check my wiring at the stock T harness at the alternator. On the attached image,  would the "ignition key on" terminal be considered the S terminal, and "to 12V post" be the L terminal? Also, just to clarify, the "cross" of the T would be the "top bar" of the T, with the vertical on the T being, well, the vertical pin, with the connector oriented as a capital T? I know this all is pretty elementary, but I need to double check my work. I attempted a start, and got 2-3 cranks, and then right to clicking. My battery read about 12.5V, but on my old alternator/starter rig, the car would slow the cranks down; I never got clicking at the ignition if the battery was low.  Thanks for the help.

 

 

 

 

So, if wired properly, my harness would look like this image? the harness is oriented as the plug on the alternator is, with the green wire being the "top"(S/ignition key on?) pin, and the red wire being the bottom (L/12V post?) pin?

Edited October 25, 2021 by RJK

[Zed Head]

1 hour ago, RJK said:

On the attached image,  would the "ignition key on" terminal be considered the S terminal, and "to 12V post" be the L terminal? Also, just to clarify, the "cross" of the T would be the "top bar" of the T, with the vertical on the T being, well, the vertical pin, with the connector oriented as a capital T

No, 12V post is S, which would be the top of the T in your plug.  Ignition key on is the vertical part of the T.  I don't know which way that plug fits n the alternator so can't say which color is what.  But, by your description, it looks backward.

Here is an image from an 83 ZX Z, which used the S and L terminology. and Nissan used it in theri schematic.  The T is upside down but that should be manageable.

 

[siteunseen]

Much smater replies will follow but I can say this, I hooked mine up wrong after a rebuild/update and one of the fuses under the radio blew. So you're protected at least.

[siteunseen]

Told 'ya! 

 

 

[SteveJ]

You know how to use the tool to give you the answer you seek.

The S or "sense" terminal is to sense the battery voltage. Therefore, with the MSA jumper plug in place, you should see continuity between the sense wire at the T plug and the white wire where the fusible link plugs in. 

Just make sure that you

1. Have the black (common) probe at the T plug.
2. Have the ignition key in OFF.

Conversely, with the key in OFF, you should have an open line on the L or "lamp" wire on the T plug to the white wire, and it should have continuity when the key is ON. That is because it is a switched source.

[RJK]

9 hours ago, SteveJ said:

You know how to use the tool to give you the answer you seek.

The S or "sense" terminal is to sense the battery voltage. Therefore, with the MSA jumper plug in place, you should see continuity between the sense wire at the T plug and the white wire where the fusible link plugs in. 

Just make sure that you

1. Have the black (common) probe at the T plug.
2. Have the ignition key in OFF.

Conversely, with the key in OFF, you should have an open line on the L or "lamp" wire on the T plug to the white wire, and it should have continuity when the key is ON. That is because it is a switched source.

Thanks Steve. I can diagnose the wiring as you described. The last thing I need confirmation on is the terminology used on the labeling of the alternator itself.

 

Is the S or "sense" terminal of the alternator going to be the labeled "ignition key on", and L terminal labeled "12V post"? Or do I have that inverted?

[SteveJ]

29 minutes ago, RJK said:

Thanks Steve. I can diagnose the wiring as you described. The last thing I need confirmation on is the terminology used on the labeling of the alternator itself.

 

Is the S or "sense" terminal of the alternator going to be the labeled "ignition key on", and L terminal labeled "12V post"? Or do I have that inverted?

Stop and focus. I'm guessing that you're worried about making another mistake, and that's keeping you from reading carefully.

From my previous post, "with the key in OFF, you should have an open line on the L or "lamp" wire on the T plug to the white wire, and it should have continuity when the key is ON. That is because it is a switched source."

S - Sensing battery voltage. When the battery voltage drops, the voltage regulator excites the field to raise the output voltage of the alternator.

L - Lamp: On a lot of old cars, there is a light to indicate a charging failure. Under normal conditions, the voltage is the same on both sides of the light. When the alternator starts to fail, the voltage drops on the alternator side, and the bulb lights up. When the car is off, the alternator is not producing any power (and therefore no voltage), so if you are not using a switched source, the charging light would be lit all of the time.

By the way, these are the alternators I'm used to playing with. The alternator is inside the red rectangle. This one is a 16 cylinder engine with quad turbos. It will put out over 2 megawatts of power. The usual voltage output is 4.16kV to 14.4kV.