Charge Circuit + Bulb Check Relay: How does it work

[KoukiMonsta]

I am in the process of wiring my S13, from the ground up. It is a bare essentials car so I basically have zero OEM wiring. [no fuse box, relays, etc]

I've been staring at the FSM for charging and bulb check relay circuits and I am not understanding the bulb check relay. It should be simple - relay gets signal and ground to coil it turns on, remove on of those it 'clicks' off..but This is getting +12v from multiple places?

1) The W/R wire going to the alternator - what does the alternator 'do' with this wire? I am under the impression it is a switch, meaning goes to ground when engine is not running and it open when running. Is this correct?

2) Diode 'box' - i don't think I have this thing, i ditched a ton of wiring previously...anyone have a picture? [chassis connector M42 goes to it]

Anyone done a simplified version of the bulb check relay? The alternator dummy light side of things is easy enough to wire but again, without full understanding how the bulb check works I can't determine if it is necessary for proper voltage regulation.

[KoukiMonsta]

This is a pretty good explanation, I think I understand now.

I don't see any need for bulb check relay - does anyone know why I NEED it?
The charge light will still illuminate without it and I dont need to worry about any other lights coming on..

https://www.nissanforums.com/b13-91-94-chassis/108499-charging-system-faqs.html

[tuzzio]

I don't know exactly what the white and red wire does or how it actually works. But I can tell you this.

I have ditched my stock cluster and built a cluster with speedhut gauges. I took the OEM subharness and gutted out the unused wires. (W/R being one of them) and never had an issue.

Until one day my trans harness came unplugged from the fusebox and my alternator stopped charging. Considering the wire goes to NOTHING, It doesnt make sense as to how it works. Maybe it just needs to see power from the fusebox? The fuck if I know.

https://i.imgur.com/pgLN5Ra.jpg?2

[KoukiMonsta]

I don't know exactly what the white and red wire does or how it actually works. But I can tell you this.

I have ditched my stock cluster and built a cluster with speedhut gauges. I took the OEM subharness and gutted out the unused wires. (W/R being one of them) and never had an issue.

Until one day my trans harness came unplugged from the fusebox and my alternator stopped charging. Considering the wire goes to NOTHING, It doesnt make sense as to how it works. Maybe it just needs to see power from the fusebox? The fuck if I know.

https://i.imgur.com/pgLN5Ra.jpg?2

theres 2 wires in a plug on alternator.

one used for the indicator light and one for voltage regulator. You need the regulator and can live without the indicator, as you have proven.

[dbeiler]

This is a pretty good explanation, I think I understand now.

I don't see any need for bulb check relay - does anyone know why I NEED it?
The charge light will still illuminate without it and I dont need to worry about any other lights coming on..

https://www.nissanforums.com/b13-91-94-chassis/108499-charging-system-faqs.html

I don't think you NEED it. From my understanding, it appears to be used to provide power for the instrument light bulb checks. The relay piggybacks the ground signal off the alternator L wire to perform the brake light bulb check. When you think about, it's downright laughable, but hey, electrical engineering wasn't that great in the '80s. I can't find a decent schematic for the diode block; I'd need to remove and test it to see how the diodes are arranged internally.

[dbeiler]

I have ditched my stock cluster and built a cluster with speedhut gauges. I took the OEM subharness and gutted out the unused wires. (W/R being one of them) and never had an issue.

It is impossible for your alternator charge circuit to be fully functional without a working charge warning bulb and that L terminal wire being hooked up to the alternator. I've said this many times on this forum. That L terminal wire provides the startup power for the alternator field coil. Without power on that wire, the alternator WILL NOT charge until you rev the engine at least once after starting the car.


Your car is not charging after you start it. I guarantee you, your vehicle voltage is 12.66 volts or less when it sits there idling. It is NOT rising above 12.66 volts until you rev the engine at least once.


Here is a paragraph from Nissan TSB NTB94-011 (page 4) to prove my point:

Charging System Operation When the ignition switch is in ON or START, current from the battery flows from the L terminal to the E terminal of the alternator, causing the CHARGE lamp to light up. Then when the engine is started, the voltage output increases as the alternator rpm increases. When the voltage output becomes greater than the battery voltage, current for recharging flows from the B terminal. Simultaneously, voltage at the L terminal disappears, causing the CHARGE lamp to go out. Note: It is possible for the alternator to charge, with the "L" circuit in operation by accelerating the engine so the RPMs are above 3,600. The current will raise high enough to complete the field current. The alternator will continue to charge until the engine is turned off. Alternator will not charge below 3,600 RPM because the field current has not been turned on by current induced by residual magnetism. Checking Terminal Voltage When diagnosing the charging circuit the same precautions and test methods as used for the starting circuit should be followed. Visually checking for wires and connections that are loose, corroded, chaffed, etc. can aid in the diagnosis process. The voltage drop method can also be used to isolate poor connections and unwanted resistance just as in the starting circuit.

[KoukiMonsta]

I don't think you NEED it. From my understanding, it appears to be used to provide power for the instrument light bulb checks. The relay piggybacks the ground signal off the alternator L wire to perform the brake light bulb check. When you think about, it's downright laughable, but hey, electrical engineering wasn't that great in the '80s. I can't find a decent schematic for the diode block; I'd need to remove and test it to see how the diodes are arranged internally.

I am with you, except for the fact that I understand why it was used. Cars today still use the same concept.

You want to be able to 'check the bulbs' to ensure that a bulb isn't burn't out. otherwise you might have a CEL or ALT light, meaning you have a problem, but you wouldn't know since the bulb is no good.

For me I ONLY care about the alt charge light. Sure I've got a bare bones 'race car', but why not use something simple as a single wire circuit? The more I think about it I don't need the diode box or check relay to do this.

+12v with key on goes to the ALT dummy light, from there is goes to the L wire on ALT. that means the light comes on when not running and I send a signal to the coil. The relay is used to check the other bulbs...which I could care less about...I'll check my own brake fluid.

Am i correct in this thought process?

[tuzzio]

Your car is not charging after you start it. I guarantee you, your vehicle voltage is 12.66 volts or less when it sits there idling. It is NOT rising above 12.66 volts until you rev the engine at least once.


Actually, you're quite wrong. Its been like this for 2 or so years now, I'll actually provide video evidence if you'd like. Multimeter or volts gauge, your pick.

[KoukiMonsta]

Actually, you're quite wrong. Its been like this for 2 or so years now, I'll actually provide video evidence if you'd like. Multimeter or volts gauge, your pick.

I am curious about this.

The thing is, we know exactly how the circuitry works. it's simple conceptually.

you NEED +12v to the coil in order to energize it. this is absolute fact, if coil is not energized you dont get magnetic field and you dont create AC current 'inside' the ALT.

The ALT does provide its own, internal, +12v to do this job - but the key point is that it requires the RPM's to increase in order to do so. Hence his comment about revving the engine.

If you do not have +12v to the 'L' terminal on alternator [W/R] you simply cannot be producing energy without the engine increasing RPM.

So yes, your car will still work. while you drive the battery will charge, life will be OK. But at idle, if the revs dont jump up when you start you shouldn't be seeing anything above battery voltage.

If you are, the coil is being energized - the question then is HOW?
You do have the sense wire on the alternator, right? [other small one in same connector as the W/R one]

[tuzzio]

Here is my video. Multimeter, volt gauge, and aem v2 standalone check. The only thing I can think of is that since I have no IACV, the initial rev on start up triggers it. However, unless something fails (alternator or the one time the plug from my harness to my fuse box came unplugged) the charging system works as it should without the bulb. I'm not here to argue and say you're wrong, i'm here to say this shouldn't work, and it does. I made a thread not long ago where someone else stated the same thing. Anyways, heres my video. Unsure why imgur doesn't have sound, If you're going to cry about it i'll post to youtube.

https://imgur.com/gallery/CTmf4pL

[KoukiMonsta]

Here is my video. Multimeter, volt gauge, and aem v2 standalone check. The only thing I can think of is that since I have no IACV, the initial rev on start up triggers it. However, unless something fails (alternator or the one time the plug from my harness to my fuse box came unplugged) the charging system works as it should without the bulb. I'm not here to argue and say you're wrong, i'm here to say this shouldn't work, and it does. I made a thread not long ago where someone else stated the same thing. Anyways, heres my video. Unsure why imgur doesn't have sound, If you're going to cry about it i'll post to youtube.

https://imgur.com/gallery/CTmf4pL

no crying here my man, you got nothing to prove to me.

after a few years away from Zilvia and S chassis its fun to have an intelligent conversation.

The thing about IACV and revs jumping is that the book says something like 3500 RPM's

I cant even watch imgur at work, ill check it later sometime.

you got the sense wire on the alternator tho right?

[tuzzio]

I do. If I recall, one wire has power with key on ignition (with the car off, of course).

When the 8 or 10 pin plug from my lower harness to my fusebox (the one with the W/R wire) came unplugged a while ago, the car did stop charging. Plugged it back in, and everything was well.

For science, I'd be willing to unplug and remove my cluster and while the car is running unplug that connector to prove this out (if you and dbeiler really want me to)

I'm wondering if the subharness and bulb sort of have their own circuit, maybe the bulb acts as some sort of a resistor. So if the bulb/circuit isn't present the W/R wire is just happy all the time?

Like I said, I'd also like to know WHY this shouldn't work and does anyways.

Here are my findings from a previous thread:
http://zilvia.net/f/showpost.php?p=6277548&postcount=4

[KoukiMonsta]

I do. If I recall, one wire has power with key on ignition (with the car off, of course). they both should

When the 8 or 10 pin plug from my lower harness to my fusebox (the one with the W/R wire) came unplugged a while ago, the car did stop charging. Plugged it back in, and everything was well.
I don't know my OEM plugs very well, since I re did my harness, but I can make some assumptions...or look at the FSM...about the plug your talking about. I know where it is from seeing wiring conversions and shit. 90% sure that connect the body eng engine harnesses together. which makes sense it connects alternator and fuse box basically.


For science, I'd be willing to unplug and remove my cluster and while the car is running unplug that connector to prove this out (if you and dbeiler really want me to) nah man too much work.

I'm wondering if the subharness and bulb sort of have their own circuit, maybe the bulb acts as some sort of a resistor. So if the bulb/circuit isn't present the W/R wire is just happy all the time? I dont think so. check those screen grabs of the FSM i posted. the 'L' or W/R wire is simple. It sends 12v to energize the coil. the coil does wichcraft magic bullshit when the rotor[or stator or whatever those internal parts are called] is spinning and boom we are charging.

that other wire is so the alternator can sense the system voltage. without it you dont get any charging because it doesn't know what to do. so that explains ur harness coming out and shit not working.

BUT, the fact you state in ur other thread you had voltage with key OFF is not OK. FSM clearly states key ON or START.

Like I said, I'd also like to know WHY this shouldn't work and does anyways.
Here are my findings from a previous thread:
http://zilvia.net/f/showpost.php?p=6277548&postcount=4

I watched ur video, yea I have no clue...shit is working properly. I am GUESSING it revs high enough to kick the ALT into 'self sufficient' mode. Clearly the voltage goes up so its working.

What the word on the voltage gauge? I was looking for something like that last week, but everything looked like complete jun. I like where you put it might do something like that.

car looks cool from the little I can see.

EDIT: re read your other thread post. did you ditch the W/R wire after making that post?

[dbeiler]

I'm wondering if the subharness and bulb sort of have their own circuit, maybe the bulb acts as some sort of a resistor. So if the bulb/circuit isn't present the W/R wire is just happy all the time?


I should edit my previous responses. By "charge warning bulb", I meant "charge warning bulb circuit". The bulb itself completes the circuit. The bulb would also be a resistor (a load) for when the voltage regulator is grounding the circuit.

I'm confused on the position of your W/R wire. Did you remove it completely? Is it connected to something on only one end? Is there nothing connected to the L terminal of your alternator?

There have been some reported cases of aftermarket voltage regulators turning on current to the field coil without anything connected to the L terminal. Shouldn't be the case with an OEM Nissan alt, though.

[tuzzio]

In the case of my alternator wiring, everything is OEM. Except of course, the cluster subharness.

Also, koukimonsta: volts gauge is from amazon. it was like $6. I also saw the other day they have positive battery terminals that read out current voltage. By the time your charging system stops working, you're likely going to be stranded anyways. So I might swap out later for a cleaner look, again. Although, I don't hate the current location.

And thanks, I have a build thread on here if you're more interested in the shitbox.

[KoukiMonsta]

In the case of my alternator wiring, everything is OEM. Except of course, the cluster subharness.

Also, koukimonsta: volts gauge is from amazon. it was like $6. I also saw the other day they have positive battery terminals that read out current voltage. By the time your charging system stops working, you're likely going to be stranded anyways. So I might swap out later for a cleaner look, again. Although, I don't hate the current location.

And thanks, I have a build thread on here if you're more interested in the shitbox.

if you re-did the cluster subharness, then the W/R wire goes to NOWHERE. Correct?

[tuzzio]

That is correct.

[KoukiMonsta]

Fuck if I know.