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Alternator Wiring From Scratch

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Old 10-12-13, 12:22 PM
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Alternator Wiring From Scratch

Need to wire up an alternator and have no stock alternator wiring for the S or L terminal. Only the top post terminal on the alternator remains in the car. I have it all fucked up apparently.

What I did:

Top post of alternator to stock wire. No problem. Its 12 volts and goes to battery from what I can tell.

L terminal on plug. From what I understand, the L terminal only triggers the idiot lights in the car. Who cares. I left it unhooked.

S terminal on plug. From what I can tell, it is a reference wire or something. I don't know. I'm an alternator tard. I made a small jumper wire and hooked it to the top post.

What happened:

Car will start and drive around fine Shows 14 volts or so on gauge cluster. If you stop the car and immediate restart, no problem.

However, if you leave the car alone overnight, it is dead as a box of hammers in the morning. It is a brand new (1 day old) battery.

I'm guessing here but I think I need only to leave the L terminal unhooked and wire the S terminal to switched power?
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Old 10-12-13, 12:53 PM
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^^^ This is what I used. S5 diagram.

B: To battery (to junction block attached to battery)
S: To L terminal. (hey, it goes to the battery so why not?)
L: Don't care about idiot lights. No hooky uppy.
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Old 10-12-13, 12:54 PM
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S4? The B/W wire is powered by the 15 amp Engine fuse w/key to on. If you jumper that wire to the output wire then wouldn't it pull voltage from the battery w/the engine off? The jumper appears to be a bad idea. The W/B wire actually excites the alternator when running as it has 12 volts. It's not just for the warning light so it needs to hooked up.
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Old 10-12-13, 01:02 PM
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Its an S4 car with an S5 alternator.

There is no wiring anywhere under the hood for the alternator except the top post.

SO - I guess I have to wire from scratch for the plug. From what I can decipher from the diagram, the S terminal needs to be switched, that is why the jumper to the top post doesn't work maybe.

As far as the L terminal - what else does it do except turn the idiot lights on when it isn't charging?
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Old 10-12-13, 01:23 PM
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S5 would be wired differently than an S4 so disregard my comments for now.
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Old 10-12-13, 01:35 PM
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Originally Posted by 7dust View Post
As far as the L terminal - what else does it do except turn the idiot lights on when it isn't charging?
Originally Posted by satch View Post
The W/B wire actually excites the alternator when running as it has 12 volts. It's not just for the warning light so it needs to hooked up.
L on all of them goes to W/B.

The L terminal is part of the field circuit, with no field there is no electricity being generated.

So...
Originally Posted by 7dust View Post
L: Don't care about idiot lights. No hooky uppy.
Hook it up as the diagram says, not just how you feel like, and I bet all your problems will go away.
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Old 10-12-13, 03:00 PM
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^^^ As I have tried to explain, there is no wiring on this car for the L or S terminal.

WHERE does the L terminal it go if you are wiring from scratch? I'm telling you it generates electricity just fine with it unhooked.

I guess what I need to know is does the S terminal require switched power?
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Old 10-12-13, 03:04 PM
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Originally Posted by 7dust View Post
I guess what I need to know is does the S terminal require switched power?
Constant power. W/o constant power the battery would drain. This for an S5 alternator. S4 would have switched and this is the difference between the two.

Last edited by satch; 10-12-13 at 03:13 PM.
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Old 10-12-13, 03:16 PM
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Is there any wiring left back to the cpu? The missing wire comes from the Alt Warnling Light Relay in the CPU, F-04 on page 50-18&19, according to the FSM. If you can, try to trace down that wire, starting at the CPU, and see how far/where it ends. Maybe it's not completely lost on your car, and then you could just patch in a length of wire up to the L terminal.
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Old 10-12-13, 04:09 PM
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No wires in the engine compartment for the S or L terminal. I'm pretty much convinced the L terminal is just for the lights on the cluster.

I messed up what I was trying to convey I did in my first post.

B: To battery
S: Hooked to B terminal
L: Unhooked. Do not need since it seems to only be for idiot lights

When I start car it charges the car runs and charges battery fine. When the car is left off overnight, the battery discharges. Whatever turns the alternator on - must be remaining on since it always has power even when off.

I am going to wire the S terminal to a switched battery source. I still think the L terminal doesn't make a **** but I might try to hook it up later.
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Old 10-12-13, 06:04 PM
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It's your car. And on a side note, the alternator can self excite itself. And are you familiar w/an amperage draw test?
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Old 10-12-13, 07:11 PM
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I wired my race car (s5 alt) without any of the stock harness, getting the alternator to work properly was the biggest pain. Until I basically learned what all the wires do. You need all 3 wires connected. If you are not using the stock harness you need to create these circuits or the alternator will either not work or kill itself.

B terminal is the obvious part. That's the charge wire. It...um...charges the battery...min 4 gauge IMO.

S terminal needs direct battery signal, can't go through a relay or any switch. It can go through a fuse. This is the "sense" wire. It tells the internal alternator voltage regulator what the battery voltage is and thus how much "charge" to put into the system. The closer you connect this to the battery the better your alternator will work. If you just jumper it to the charge post it really wont fluctuate voltage output to adapt to battery state and load conditions.

And last but not least the one that had me thinking like you. The L terminal which is where the excite wire connects. Yes it does power the idiot light. It does this because current flows if there is a substantial difference between the system or batt voltage and the alternator charge voltage. This terminal/wire must be connected because it is in effect what "turns on" the alternator. BUT and this is a big but. YOU CANNOT JUST CONNECT IT TO A HOT WIRE OR THE BATTERY. It must have resistance or you will destroy the alternator voltage regulator and the alternator will need to be rebuilt because it will not regulate voltage, the voltage will just rise with RPM up to 16-17 volts. I don't have to tell you that is bad. SO, the factory dash has two parallel sources of internal resistance. One is a light, the other is a standard resistor. You CAN NOT skip the resistor because if you just use a light and the bulb burns out, guess what, your alternator shuts off. So, you have to wire this terminal back to +12v (again no relays upstream for this one too) but you must also put a roughly 120-150 ohm resistor on this wire. Up to you if you want a charge light in parallel. It appears in some cases (3rd gen rx-7 and other cars) the factory dashes just use multiple idiot lights as the redundant sources of resistance for this. So if one bulb burns out the alternator does not just quit working. If you do put a charge light in parallel you need to put a diode in also or the light will be on all the time. The diode should only allow current flow TO the alternator. It needs to BLOCK current flow back from the L terminal.

I think in your case your alternator may be damaged or isnt charging because it isnt turned on. But your assumption that the alternator "stays on" when the engine is not running is incorrect. This isn't actually possible. Without the little wheel spinning on the front of the alt it's basically a paper weight, it cant be on without being driven. But, if you damaged the voltage regulator it may create a bleed. If i recall correctly when i blew one trying different things to get it to work properly (read just put unresisted +12v to the excite terminal) it self excited after that and would not regulate voltage at all.

Some reading for you: http://bob_skelly.home.comcast.net/~...ternator1.html

This basically explains all this, probably more eloquently than i did.
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Old 10-12-13, 10:54 PM
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I am still unclear on why the car discharges the battery overnight.

My FD racecar was completely burned up and the harness is built from scratch as well. The alternator is hooked up in the exact same way that I currently have the FC hooked up. No L terminal and the S terminal is just jumpered over to the B terminal.

On the FD, the same damn thing happens. Car charges fine and when the car is off, the battery discharges if it isn't started within a few hours. Of course the FD has a master kill switch for the battery since it is a race car. If it is left on, the battery discharges within a few hours.
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Old 10-13-13, 10:21 AM
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Why not just run the S terminal straight to the battery instead of jumping it to the B terminal.maybe there is a voltage difference from the B terminal than being directly to the battery and causing the alternator to bleed off or kill the battery?

I'm having the same issue on one of our rotary trucks,I also jumpered S terminal to B.i need to try running a new wire to battery on S terminal.
I don't think L terminal needs to be plugged in for anything other than idiot light
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Old 10-13-13, 12:34 PM
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^^^ I think you are right. I am seeing a 165 milliamp drain on the car with the S terminal jumpered over to the B terminal.

Going to try running the S terminal further downstream to the battery next.

I think that L terminal is just a bunch of bullshit too.
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Old 10-14-13, 12:45 PM
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I'd be curious to know what your alt is charging at with the car running with the S terminal jumpered like that...
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Old 10-17-13, 08:25 PM
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i saw rocklobster wrote something about burning the alternator up if you run a direct 12 to the L, i did this for 2 weeks with no problem, alternator didnt burn up. i also had to start from scratch, so i took 2 12v leads from the battery and put them to the alt. voltage did not jump around at all. it stayed at 14ish idling/revving... but ended up changing engine harness to a na which had the plug in it, so i do have a plug now and is wired correctly. would i recommend doing that long term? no. its ghetto, but for a band aid like i wanted it didnt hurt
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