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Alternator wiring fundamentals

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I have done research and I believe I understand the concept. I am still unclear on some smaller things, so I would appreciate your experience.

Denso 4 wire alternator.

B is clear, charges the battery, goes either to battery or in my case to the FIA 6 pole master switch.

L, lamp, exciter wire, connected to a bulb

IG, connected to Ignition switched +12V

S, sense, connected to battery to provide the actual charge voltage

So far so good.

It's a bit more complicated than that though, isn't it? I attached a rough sketch of an alternator wiring diagram. Please have a quick look if you will and answer me these questions.

Things to keep in mind:

This is a ground up build, no existing wiring or fuses.

The whole fuse box is ignition switched because it's all after the master switch which serves as ignition to keep things as simple as possible.

The distance between the battery and the fuse box is approximately 1m of cable.

Regarding S wire. Battery being relatively close to the fuse box, would you take S from the fuse box or still rather run an inline fuse, taking S directly from the battery?

Regarding IG. If I do in fact take S from the fuse box, I can just daisy chain into IG since they come from the same source. Reasons not to? OEM this wire had a 10A fuse but I assume there were other things on the same fuse since this is the "Ignition switched" wire. Do I still go with 10A or is perhaps 7.5A sufficient? Maybe less?

Regarding L. L is a ground wire, correct? I have a light bulb and I understand I need a flyback diode. Please check the polarity of the diode in my diagram. Is there a rule how far from the actual bulb itself the diode should be? This too was on a separate 7.5A fuse OEM. Isn't that excessive for a 3W light bulb?

Attached Files

I've never needed a diode. When the Alternator isn't turning, there will be a small resistance to ground on the Lamp connection, and since the lamp will have 12V power on the other side, it will illuminate. Once the alternator is turning, the Lamp connection voltage will be the same as the alternator output (nominal 12V), so the lamp will go out.

I asked Alternator related questions before and everyone was bringing the Diode up. Even when a resistor replaces the bulb. OEM there was a diode there too. Attached the diagram.

Can you please give the rest of the questions a go?

Attached Files
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It could be drawed like this in the diagram only be because it's a LED in the cluster.

Never had to put a diode either, and I wired a 2JZ Alternator a couple weeks ago which is exactly as your wiring diagram. Connect it as the OEM wiring diagram and you won't have problems.

IG is +12v ignition controlled.

L is +12v ignition controlled with a inline bulb (or a resistor).

S is hot at all time. You could plug it to a kill switch if you have one.

You said you never had to use a Diode amd then you said to do it as OEM. OEM uses a diode though.

Sorry for the misunderstanding. I never had to add a diode when building a circuit from scratch.

I was using the OEM Supra cluster which has a built-in diode inside the cluster. I just checked the OEM wiring diagram and I think they put that diode there because there is many circuit connected to this particular wire (from other diodes) and it prevents from back feeding the ignition circuit.

The "problem" resides in the cluster / the way they designed the circuit and not the alternator itself. I have attached a part of the wiring diagram to show what I mean.

I understand you want a light to monitor if your alternator works. If you are not using the OEM cluster, I would just wire a normal bulb in the circuit. If you don't want a bulb at all, you can use a resistor to put a load on the circuit. I have no idea what wattage the OEM bulb is, but to mimic a 3W bulb, a resistor around 65 ohms would be needed. You would need a resistor that can handle that power, so a 5w resistor would work.

Regarding the rating of the L fuse, Toyota connected other circuits to it. If you would use a dedicated fuse only for the L wire, the L circuit uses less than 1 amp...

Attached Files

I actually did not want a light. At first I didn't realize the L wire is the exciter and I was going to forego it all together. Then I was instructed it's necessary and advised to use a bulb. I bought a bulb with a holder, I am using this now.

I only ever found the simplified view schematic from Toyota (it's for a Jzx100), but it's most likely just like you said, diodes because other things are connected. Which also explains such a big fuse for such a small load.

I got it now.

Do you have any ideas about S and IG fuse sizes? What do you reckon is correct? Those are probably big because other things happen to share the fuse as well.

You have you diode installed incorrectly, in reference to the OEM diagram. You have the diode in parallel, whereas it is in series in the OEM layout. This is probably to stop any possibility of feedback into the cluster from the alternator. I don't know about your particular alternator, but GM alternators that I am familiar with require a lightbulb or equivalent value resistor, otherwise the regulator will be damaged, may or may not be the same with your alternator. I have had to add a diode in some alternator swaps, so that the vehicle would shut off. Without it, the alternator would back feed through the light connection and keep the ignition powered up, so again, this is likely why the OE diagram shows the diode in series like it is.

Another alternator I've used (and currently on one of my cars) is an alternator from a late model Toyota also a Denso IIRC, and I only use the 12V ign input (and sense wire), I don't use the light connection. It charges fine. You might be able to do the same with your alternator, if you're not wanting to run a light.

Last I asked (either here or in the Facebook wiring group) I was instructed to use a bulb (or resistor) because L is the exciter wire. And I was instructed to use it with a diode.

I see my error in the diagram now yes. Are Denso 4 wire alternators known to feed back through the bulb in your experience?

The 12V switched and the L are both exciter inputs, the Denso alternators I use, I only use the 12V swtched input (and they had the L terminals used in the OEM applications, I just depinned that connection). It's easy enough to test whether it's needed or not if the engine is currently in a running state, simply depin the L wire and start it, checking voltage. If it's not in a running state, you can run a wire through the harness, back to a point where it could be accessible, connect the 12V switched wire and test when you get the engine running again. If the L-wire is needed, then you can connect it. It's not going to hurt having an extra wire in the harness that you may find you want to use later anyway. I kinda did this with a GM RVC (Remote Voltage Control) alternator since I wasn't entirely sure what the second pin did, since it wasn't clear in my research and people said that it was needed, while others said that it wasn't... it wasn't needed and turned out to be a feedback wire to the BCM/ECU to know how much load is being put on the alternator, a very cool thing to know if the ECU can actually use that connection (Megasquirt can, and do some cool things with the alternator control). Anyway, The wire is in the harness, if it's ever needed later, say like Haltech adds the ability to monitor alternator load, It's (fairly) easy to access.

Like I said I haven't used the L connection on the Denso alternators, so I can't say for sure that it will backfeed, but it's likely, especially since the OE application has that diode in place. OEs don't add parts if they aren't necessary.

I would like to do it and be done, so I will count on having a bulb in the cockpit, I already purchased it. I have a few spare diodes that came with the ECU, so I will use one for this. It can also share the Fuse with the IG pin the way my car is going to be wired. Thank you for your time.

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