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Alternator Not Charging RB25DET

Eric0
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Eric0 Enthusiast

Eric0

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Posted

Hello!

I’ve read multiple post on similar issues as I’m having but following their steps hasn’t brought me much closer to solving my issue it feels like. So, I created a new one! 
 

Issue: I installed a brand new alternator after thinking my old alternator was the issue but I’m having the same problems. The alternator is not charging the battery at 14v resulting in constant recharge to be able to drive it for another 5 days before the voltage gets too low that it can be harmful to multiple components.

When car is off:

12.43V

When car is on:

12.51V

Diagnose: 

With a multimeter on VDC 20V, I attached the negative probe to the negative side of the battery. Then, I measured all the grounds on the alternator while car is on.

Negative Side of Alternator 

alternator ground to block: 0.05V

alternator case: 0.05V

alternator bracket: 0.04V

alternator battery power stud: 2.5V

alternator Battery power nut: 2.5V

chasis to block where alternator ground is hooked up to: 0.05V

Grounds around alternator: 0.05V 

With a multimeter on VDC 20V, I attached the positive probe to the positive side of the battery. Then, I measured Voltage drop while car is on.

Positive Side of Alternator 

Alternator Battery Power stud: 9. 47V

Alternator Battery Power Nut: 9. 47V

Rechecked grounds on alternator: 12.42V

The 75Amp fuse is receiving 11.8v on the top pin and 0.01V on the bottom. 
 

Where I’m At: 

With the diagnose I’ve done I’ve determined that the grounds have a prettt good connection, the 75Amp fuse is good, but there is a substantial voltage drop of 9.47V.

Question:

Does anyone know why a voltage drop could occur and how do I fix it? Could the battery be the issue?

 

 

 

 

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tridentt150v Grand Master

tridentt150v

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Posted

Put your lights on high beam for 5-10 minutes and see if they begin to dim.  That will tell you if your battery is Ok or only holding a surface charge.  I have found that AGM batteries esp. will show great volts and fool you into thinking they are OK but then they fail under load.

Also, really, you need to check amps to the battery in series.  IMO that's the only true way of knowing if your alternator is working properly. 

Kinkstaah Grand Master

Kinkstaah

Posted
Posted

It may be the alternator trigger wire?

Given you've already replaced the alternator... I only found out all about trigger wires (or the alternator exciter wire) when I was troubleshooting the exact opposite of this - My alternator always being on. When the car was off!

Eric0 Enthusiast

Eric0

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  • Author
Posted (edited)
55 minutes ago, tridentt150v said:

Put your lights on high beam for 5-10 minutes and see if they begin to dim.  That will tell you if your battery is Ok or only holding a surface charge.  I have found that AGM batteries esp. will show great volts and fool you into thinking they are OK but then they fail under load.

Also, really, you need to check amps to the battery in series.  IMO that's the only true way of knowing if your alternator is working properly. 

I have an EverStart yellow top battery that was purchased last year in August. It hasn’t been a year yet and currently sits at 12.43v. 
 

I also followed this video to test if the alternator is working properly and everything checks out except when I measure for voltage drop on the positive side of the battery, instead of getting something below 0.3V I get 9.47V. Also, the battery does not jump to 14v+ when the car is on. 
 

link: 

Now I’m unsure what to could be since the 75AMP fuse has 11.8v on top and 0.01v on the bottom and the 75Amp fuse isn’t blown. 

Edited by Eric0
Added Link
Eric0 Enthusiast

Eric0

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Posted (edited)
33 minutes ago, Kinkstaah said:

It may be the alternator trigger wire?

Given you've already replaced the alternator... I only found out all about trigger wires (or the alternator exciter wire) when I was troubleshooting the exact opposite of this - My alternator always being on. When the car was off!

Excuse me for my lack of knowledge, but where is the trigger wire located?  How do I test it?
 

I tested the plug to the alternator and while the car is on, the big wire receives 11.8v and the red/white wire I don’t recall. 

Edited by Eric0
Wanted include a question.
Eric0 Enthusiast

Eric0

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Posted
5 hours ago, Kinkstaah said:

Going by google and SAU results, this is the wire that goes to the dash bulb.

Your car says it is a 240SX.

See this thread: http://forums.240sxone.com/showthread.php?t=8702

 

Hello Kinkstaah,

Yes, it is a 240sx. Awesome, I found the red/white wire on the alternator plug. I’ll check over the wiring for the exciter wire and report back. 
 

Thank you! 

Eric0 Enthusiast

Eric0

Posted
  • Author
Posted
6 hours ago, Kinkstaah said:

Going by google and SAU results, this is the wire that goes to the dash bulb.

Your car says it is a 240SX.

See this thread: http://forums.240sxone.com/showthread.php?t=8702

 

I ran some measurements and the exciter wire and this is what I got. 
 

When the car is off:

Big wire: 12:55v

Red/white(exciter): 0v

When car is on 

Big wire: 11.62v

Red/white(exciter): 9.83v

This is what the forum through the link above said about the exciter wire:

”the larger one in the plug goes to pos 12 v, this one i'd add a fuse.

The smaller wire (white/red) goes to the change bulb wire in the guage cluster. It is the exciter wire if you don't hook this up you will need to add a resister to drop the voltage going to the alternator to less than 6 volts. if you power this wire at 6 volts your alt will charge at 15 volts. also if you just plain dont hook this wire up, your alt will not charge.”

I’m getting 0v on the exciter when the car is off but then I get 9.83 volts when the car is on. Could that cause the alternator to not charge since the other forum says it should be 6v? They mention adding a 600ohm resistor to drop voltage, could this fix the problem? 

Eric0 Enthusiast

Eric0

Posted
  • Author
Posted (edited)

I’m going through the dash board wires and I found the red/white wire at 8.88V on the cluster plug. I believe it’s the correct wire. The red/white wire on the alternator plug is 9.88V. Well, I don’t have a dash light or anything going to that wire. I have aftermarket gauges. On the diagram I made back in 2016 indicated the wire as “battery charger.” I never actually drove the car with the new gauge setup until now but I’ve been driving the car for months until just recently the yellow top battery i purchased 8 or so months ago has been dying more and more. 

27F44C78-A442-48D7-9A9B-CF99B94D1101.jpeg

FC8DAD38-2FBA-4B83-9E93-B49915276C42.jpeg

Edited by Eric0
Added image
Kinkstaah Grand Master

Kinkstaah

Posted
Posted

I don't know the characteristics of that wire, seems to be an issue with conversions (as an OEM RB25 car ostensibly has it wired to the dash).

Plenty of "My alternator not charging, turns out it was the bulb in the dash that did it? weird" threads.

Try the resistor and give it a go? I don't have a RB25 anymore so I can't comment further!

DaymoR32 Proficient

DaymoR32

Posted
Posted

i have an rb25det neo converted r32 with a haltech IQ3 dash. my car has a sidelight globe wired in underneath the dashboard (as i dont have a normal dash) when i flick the ignition on the globe is illuminated. once the car is started the globe goes off. 

I pulled this globe out as i was unsure of what it was for, soon found that my alternator wasnt doing anything so i plugged it back in. might be worth sticking a globe in on the wires previously mentioned.

Eric0 Enthusiast

Eric0

Posted
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Posted
On 5/25/2020 at 5:05 PM, Kinkstaah said:

I don't know the characteristics of that wire, seems to be an issue with conversions (as an OEM RB25 car ostensibly has it wired to the dash).

Plenty of "My alternator not charging, turns out it was the bulb in the dash that did it? weird" threads.

Try the resistor and give it a go? I don't have a RB25 anymore so I can't comment further!

 

On 5/25/2020 at 6:58 PM, GTSBoy said:

Just instate the charge light in the circuit? As per the original wiring diagram?

 

On 5/25/2020 at 9:24 PM, DaymoR32 said:

i have an rb25det neo converted r32 with a haltech IQ3 dash. my car has a sidelight globe wired in underneath the dashboard (as i dont have a normal dash) when i flick the ignition on the globe is illuminated. once the car is started the globe goes off. 

I pulled this globe out as i was unsure of what it was for, soon found that my alternator wasnt doing anything so i plugged it back in. might be worth sticking a globe in on the wires previously mentioned.

I tried a combination of the methods mentioned above and within attempting these, I discovered the main battery charge wire off the alternator and into the dashboard had ripped and grounded with the metal and shorted out a bit. My battery is in the trunk for a clean engine bay. Well, I replaced the wire, insulated it, cleaned all my grounds, and now the alternator is charging! I just needed to get under my dash to discover burnt wires. That was a whole parade for me! 
 

Thank you all for your kind help! 

 

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