Voltage Drop

[Zed Head]

He smoked an OE link the first time, I think.  I'd use the meter and see if one side is shorted to ground before going too far.  Easy to do.

[Zed Head]

Forgot to say, his VR could still be bad.  He had charge, maybe backfeeding, then it goes away.  I'm guessing he has a short, it fried his VR, and he's been getting backfeeding through the ignition circuit.  Just playing the solve-the-problem game...

[siteunseen]

When my alternator was back feeding it popped the IGN fuse as soon as I cranked the car, killed all my dash gauges too.  Just saying, my experience. 

[Zed Head]

72 or 77?  I've noticed that Nissan seemed to do different wiring schemes over the years.  On my 76, they run power directly from the alternator to the fusible link and to the fuse box.  If the diagram I'm looking at is right (it shows the a red fusible link as the main one and black as a secondary - I think Saridout got the colors backward). 75 only has two links at the terminal blocks, they went to four in 76.  So if the main one blows, the fuse box is all that gets power.

[DoubleYOOHZ]

@Dave WM - Sorry. The battery with the engine off read as 12.67v. I'll be honest with you, the gauge and the insulation is something I dont know about. I picked up the insulation and gauge from oriellys and put them together as i thought it seemed fit. The light from the dash does light up. Originally, the first fuse that burnt was the OEM fusible link. I dont understand when you are asking if it is 50% rated current of the fusible link? If you can elaborate, Ill do my best to answer. 
@Zed Head - You just made me realize something. I think I have done this wrong. When I did the F to A (bat), I measured at the battery terminals. I completely did it wrong. I wont be able to do the test again until tomorrow. 

 

 

 

[Dave WM]

don't worry about my ramblings about current rating, I was thinking out loud about how the fuse link works. You do need to make sure you are using the correct OE fuse links, including the correct color coding. When testing the alt by jumping the F to the bat make sure you are only idling, over 16v sounds excessive, adj the idle to 800 rpm and start there, turn on the lights and fan (max). Advance the throttle just a bit stop at 14v (if it goes that high on voltage the alt is fine). Compare he volt meter in the car to the digital volt meter reading at the battery terminal to see if they are close.

Edited July 15, 2016 by Dave WM

[Zed Head]

Not really, what you did is still informative.  I was trying to figure out how you got voltage from the alternator to the battery with your main link blown.  No need to do it over, you've shown that the alternator works correctly.  So if that link is blown then you must have been backfeeding through the other link or through other stuff wired in to the harness.  According to the 1975 diagram the only other way in to the positive terminal is through the EFI circuit.  Not really sure how it backfeeds in that way. 

I would measure resistance to ground on both sides of the main link that blew.  If you have a short to ground it will just blow again.

[siteunseen]

1 hour ago, Zed Head said:

72 or 77? 

'72, I apologize, I didn't realize what the difference would be.  It does not run through a fusible link like the 280s, my bad. 

[Zed Head]

Good example though, of how Nissan kept modifying their wiring.

[DoubleYOOHZ]

@Dave WM@Zed Head- I just ordered the correct OEM fusible links. However, after cleaning the female and male terminals of the fusible links and battery, I have been able to sustain charging levels. Lets say, at the battery level readings when I did the F to A link, wouldnt the data tell you that the voltage regulator is doing its job? If the max output is at ~16v (when the F is connected to A), and I am getting a voltage reading of ~14.5v (when FN is connected to battery) shouldn't that be an indicator of functionality? 

Before start up: 12.6v
Car in idle (900rpm) : 14.45v
Car above 1200 RPM :  14.68v
Car at idle with headlights on: 12.68v
Car w/ headlights on above 1400 rpm: 14.55v

In this case, it makes me wonder about the back feeding of electricity to cause my fuse to overheat/burn. If it happened twice, it will happen again. Zed head- When I measure the ground on both sides of the main link, which main link are you referring to? The Alternator/Relay link? 

[DoubleYOOHZ]

Hello Z community,

I am coming back to this thread again because this problem is now persisting. Before, my car was charging. Now it is barely charging. I took the car for a drive two days ago and when I started the car today, my car would not start. It was a dead battery. From the dash tachometer, it read 8V. I charged the battery, and it started up flawlessly. 

Below, I posted some results from my volt readings. It is not the same as before. It seems to barely charge the vehicle. What I dont understand is how my battery dropped from 12.4V to 8V from just two days of just sitting the garage. 

Before start up: 12.41v
Car in idle (900rpm) : 12.83v
Car above 1200 RPM :  13.88v
Car at idle with headlights on: N/A
Car w/ headlights on above 1400 rpm:  N/A

Would it be the voltage regulator/alternator?  

[Dave WM]

bad battery/ bad cables/bad battery terminals. I would not use a battery that had gone thru a deep discharge cycle for what ever reason. You should get it load tested at least and make sure you have good quality cables/terminals and everything is clean. Check to see if there is a current draw with the key off.

Edited September 25, 2016 by Dave WM