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I have an S5 swapped FB and I re-wired my alternator to use a newer S5 alternator, following this diagram where I ran a wire directly to the battery for the left-side connector pin marked S. The L terminal reuses the original wire to the dash.
I am getting a battery drain of approximately 250mA that drops to 10mA when I depin the S terminal on the alternator connector. It seems the alternator itself is causing a battery drain. I previously had this same issue and tracked it to the alternator 6 months ago, so I ordered a replacement reman from Rockauto. Same problem with the new one now... is there an issue in my wiring somewhere or did I get unlucky and wind up with two bad alternators?
While I don't know for certain details concerning the shaft size changes, the reason most people go with an S4 alt is because it doesn't require any electrical changes. Of course for a 79 you'll need to do a few extra things, but that's all covered in the thread tommyeflight89 linked.
For the S5 a change was made from the earlier resistive type to a sensing type (whether those are the correct terms I am not qualified to say). Specifically this change meant that the alternators require different wiring going to them. Some people do this wrong which then leads to battery drain - I recently rewired mine to remove the relay and no, you do not need a diode or relay. I chose to go the S6 route for 100 amps given that both it and an S5 would require rewiring regardless.
Summarized this would be S1-S4 (excluding 79) have the same wiring and are plug-n-play, and S5-S6 require some simple rewiring but are easy to make work. If you're going to be rewiring things anyway I might consider an S6 alt (unless you want to use the pulley you already have) if the prices are similar.
Last edited by KansasCityREPU; 04-19-22 at 06:41 PM.
Specifically you need to stick a diode in the wire going to the S terminal so that power can go from the wire to the S terminal on the alternator. The stock FB's wiring grounds this wire when the ignition is off, and this causes a draw.
I literally scavenged a diode from a control module that had discrete components (think it was the airbag module from an SN95) and spliced it in. I have zero idea what the specs on it are other than it flows electricity in only one direction
Huh, this is interesting. I have an S6 alternator in my car and have never had an issue with my battery draining with the same wiring as you, derSchwamm. Presently it's been sitting for months and the battery is still where I would expect it to be, granted I've also never measured the current draw and checked if anything is getting pulled via the alternator.
Given a old reply of mine was quoted above, I feel a little bad if perhaps I was too quick to dismiss the alternator draw issue and then write about it. I wonder if there is an internal difference between the S6 and S5 alternator that causes this, or perhaps I do have a draw (although I think I would have noticed a 200+ mA draw by now).
I've also noticed the same grounding issue with the stock 12V ignition supply that peejay mentioned, and that's why I suggest against using it, but the thing is that derSchwamm's S terminal isn't using that wire. Huh, this is interesting.
A diode would definitely solve the issue, although perhaps that wasn't the answer you were looking for, and I'm not sure if it points to something in the alternator being askew. I'll check my alternator for draw the next chance I get (probably later next week) and report back. As far as diodes go, any "normal" 1A diode will be more than enough. That reminds me, I still haven't posted the write-up on repairing the headlight lift motor diodes... Crikey I forgot about that. Oops.
Specifically you need to stick a diode in the wire going to the S terminal so that power can go from the wire to the S terminal on the alternator. The stock FB's wiring grounds this wire when the ignition is off, and this causes a draw.
I literally scavenged a diode from a control module that had discrete components (think it was the airbag module from an SN95) and spliced it in. I have zero idea what the specs on it are other than it flows electricity in only one direction
This should only be a problem in the factory wiring right? I actually have a Haltech and made my own wiring harness for the engine, and the S terminal wire is 100% independent and runs directly to the battery positive terminal. Or did you mean the L terminal? That wire still hooks into the original 'front' harness (once it leaves the new engine harness). Perhaps I am not fully understanding how this alternator works...
This should only be a problem in the factory wiring right? I actually have a Haltech and made my own wiring harness for the engine, and the S terminal wire is 100% independent and runs directly to the battery positive terminal. Or did you mean the L terminal? That wire still hooks into the original 'front' harness (once it leaves the new engine harness). Perhaps I am not fully understanding how this alternator works...
In theory the S terminal should be able to be run directly to the battery with only a 5A (or probably less) fuse inline. The grounding issue is purely to do with the factory 12V supply and is irrelevant if said wire is not used. The factory L wire shouldn't be a problem since it's just for the warning light.
As mentioned above, I have an S6 alternator with the S terminal run directly to the battery with an inline fuse and have not experienced it killing my battery. I'm not sure if the S5 alternator is different, although it'd be interesting if it was. I believe I looked at the FD wiring diagram back when I made the swap and was able to trace the S wire to the fuse panel with no diode or anything in between, but I didn't look at the S5 schematic.
I'll test my S terminal for current draw next week when I have a chance, even though it'll probably be too late to be helpful. I'll be interested to see what the results are.
Thanks. I had this problem with a previous used alternator, so I bought a new reman from Rockauto a few months back to try to fix it. I also now have a new optima redtop battery, and the wiring harness is brand new too. Since basically everything is new, I feel like somehow, somewhere, I did something wrong in wiring this thing but the diagram makes it look dead simple. I tried connecting S directly to the battery terminal as well as through an independent fusible link but got the same draw either way. I'm stumped.
I do have an FD parked next to my FB... I'm guessing if I ran longer wires from my FB harness over to the FD's alternator as a test that wouldn't tell me anything since it's grounded to a different engine? If I have to, I can pull the S6 alternator out and put it in the FB for a test but I really don't want to start dismantling my FD
I do have an FD parked next to my FB... I'm guessing if I ran longer wires from my FB harness over to the FD's alternator as a test that wouldn't tell me anything since it's grounded to a different engine? If I have to, I can pull the S6 alternator out and put it in the FB for a test but I really don't want to start dismantling my FD
True that it wouldn't tell you anything since the two systems are grounded and powered separately.
That said, if on the off-chance you have a spare pigtail, perhaps just wire up S terminal pin straight to the battery on the FD and see if it leads to any current draw. The L terminal being disconnected shouldn't impact anything. That way you could still test the S6 alternator but don't need to pull it out of the FD.
I got tired of messing with this issue and let it sit for a long time but started looking into it again today. I removed everything from the battery positive terminal except the battery charge cable and the 'S' terminal wire to the alternator (gator clipped in the picture). I'm still seeing over 110mA of drain! With nothing else even connected, does this confirm the alternator is bad? It's still under warranty for a bit longer
If that is the only positive cable attached then yes.
Is the connector on the back of the alt unplugged also?
Yes nothing else is connected to the alternator either. I actually realized my warranty expires tomorrow so I went ahead and filed to return it. I'll see how another reman unit behaves.
Is there any way that the ground setup could play a role? I have a large cable run from battery negative to the center iron, then from the center iron to firewall.
Grounds can cause issues but usually only when there is enough resistance that it creates a voltage drop. Think of a badly corroded ground with the green crusties. If you grounds are new like I see you've done, that isn't the issue.
I've occasionally had issues with phantom drain and I've also gone through four FD alts in about 8yrs of driving time.
Here are some things I've learned:
1. A bad Choke/Check relay can be a real headache
I've had two Choke/Check relays fail in different ways. One caused a constant drain on the battery, and the other shorted out so bad that it took out the alt with it. So this is a good place to check if you're having issues. There's a testing procedure in the FSM (check foxed.ca). I also very recently figured out how to "make" a new Choke/Check relay out of a generic 12v DPDT Relay (dual-pole, dual-throw) so if anyone is interested in those details I can make a post since old parts are getting harder to find.
2. Why you need a "sense" wire going from the alt to basically the exact same place the charge wire goes and why you can't just loop it.
Some people might already know this but I often saw the "what to do" without the "why". It turns out that the more amps you push through a wire, and the smaller the wire is, the more of a voltage drop you get over a given distance. So for example if you are pushing 50A through 3ft of 8 gauge wire you will lose about a quarter volt. Double the amperage across the same wire and distance and you'll drop more than half a volt! Longer distance, smaller or poorer quality wire, or higher load means you lose more.
So the sense wire is meant to carry no load at all, that way it can let the alternator "see" the voltage at the other end of the charge wire and decide whether it needs to push, say 14.5v in order to be providing 14v.
All this to say, yes a sense wire to the battery or the main fuse block is needed.
Also, I've learned that a blast of current up this wire can do some serious damage, and I found an alternator manual somewhere that said the sense wire should have a 5a fuse inline, so I've added one to try and blow alts less often. You can get inline fuse plugs/holders that accept the usual small bladed fuses off Amazon for just a couple bucks.
3. The FD warning light and the FB warning light are not wired the same.
The FB one goes through the choke/check relay, then to the instrument panel. The FD one goes through a diode then off to the instrument panel (actually the diode might be in the instrument panel, but either way...).
So in an effort to remove further sources of drain or possible alternator damage I'm adding a diode to my warning light wire. There are several ways to do this. The one I liked was to get a similar inline fuse holder to above, but make it for the mini fuses instead of the standard ATO bladed fuses. Then there's a 1a diode you can get that plugs right in since it's in a mini fuse form factor. Not sure which car used it, but I thought that was a cleaner setup than soldering a diode into the wire of the kind you use in breadboard projects.
4. You should upgrade/replace the charge wire.
This may be optional since the stock one is 8 gauge and that can carry 100a about 4.4 ft while only suffering a 2% voltage drop (assuming it's pure copper)... BUT a 38yr old 8ga wire probably has more losses than that so at a minimum replacing it with fresh wire should be on the list if not upgrading it to something like 6ga. The more voltage drop you have, the more voltage the alt tries to put out to compensate, and that could lead to premature wear or damage to the alt.
5. You *can* use the stock ignition 12v wire (with a diode), but should you?
So someone on the forum here cracked the secret of why the stock black-with-white-stripe wire causes a voltage drain when used as the sense wire when the car is off. The alt doesn't *need* 12v all the time, but when the car is off that wire becomes grounded and some power leaks backwards through the alt circuitry causing a drain. To fix this you can add a diode to that wire... But should you?
I'm going to say no. You might have already guessed why. The FD alt has a sense wire because it needs to sense the voltage right at the other end of the charge wire to figure out the voltage drop, and consequently how much extra voltage to provide. The ignition 12v wire isn't coming straight from the other end of the charge wire. It goes through damn near half the wiring harness and is tapped off to power a half dozen things along the way. It carries a load, it runs a long distance, and it's tiny, all adding to a potentially false low voltage reading. So even though you *can* just use the stock wire with a diode, don't be tempted to do it.
A wire straight to the main fuse block (if you've got an extra spot like me because of a 2nd gen fuse box upgrade) or to the battery is best. This doesn't need a diode because there's effectively no chance of it being grounded. If your battery's positive terminal is grounded you've got bigger problems! But as I said earlier a 5a fuse is cheap security here.
6. Check/clean/fix your grounds!
There are several grounds around the car. If they have gotten corroded, disconnected, or otherwise messed up then there will potentially be additional drain from things across the harness or just odd malfunctions. There aren't terribly many, like only half a dozen. So look them up and go check and clean them to save you some hassle.
So I think that with a new charge wire, a 5a fuse on a new sense wire straight to the battery, a diode on the original warning light wire, and a verification of the Choke/Check relay, anyone's FB should be in good shape for running an FD alt.
Hope this helps someone!
Cheers,
Jon
Last edited by vipernicus42; 10-16-22 at 06:30 PM.
Well I got my new alternator in the mail and installed it but am even more confused. I am now getting 2AMPS of draw with nothing else connected. It was 110mA before.
I eventually removed absolutely everything from the battery, then ran a gator clip from battery positive to the S terminal, and measured draw with a multimeter between battery negative and the firewall (not even using any of the ground wiring I added). I also disconnected the charge wire. Still 2AMPS of draw. How is this even possible?
This is now the third alternator I’ve tried in two years and it’s just getting worse.
So uh… which pin on the alternator is the S terminal? The one closest to the drivers side or passenger side?
Based on diagrams in other forum posts I thought S was drivers side. I removed my FDs alternator to swap it in and looked up wiring for it and saw the S terminal was switched from how I had it wired for the S5. That got me thinking maybe the S5 alternator was wired backwards. Looking back at the diagrams the connector is shown on the back of the alternator but in reality mine is on the top, so it didn’t match the diagrams…
So uh… which pin on the alternator is the S terminal? The one closest to the drivers side or passenger side?
Based on diagrams in other forum posts I thought S was drivers side. I removed my FDs alternator to swap it in and looked up wiring for it and saw the S terminal was switched from how I had it wired for the S5. That got me thinking maybe the S5 alternator was wired backwards. Looking back at the diagrams the connector is shown on the back of the alternator but in reality mine is on the top, so it didn’t match the diagrams…
Best way to locate it is not compared to left or right side of alt or car, the best reference is the little clippy part of the connector.
So if the clippy part of the connector is the "top", then on the diagram it looks like on both S5 and S6 alts the S wire is to the left of that clip (on the alternator) even though the orientation of the connector faces in opposite directions.
So check out the diagram, note where the S and L pins are in relation to the clippy retainer thing on the alt, then look at yours and match it up.
Also check through my post above. I'm pretty sure you've got all the bases covered, but just in case...
I started second guessing myself (are those connector pictures representing the alt or the harness, and from which side) so I went over to foxed.ca and started sifting through Factory Ship Manuals and Wiring Diagrams.
The clearest picture I found was on page G-6 of the S5 (89) FSM. It clearly shows the alternator itself, the connector, the orientation of the clip, and labeled terminals.
Sure enough it's Sense wire to the left of the clip, Light wire to the right of the clip.
Edit: KansasCityREPU also confirmed the direction. Too bad I wasn't watching the thread while I was knee-deep in technical literature
Thanks everyone. So yeah, I've had it wired backwards the whole time. I'm kind of amazed it worked at all. I looked at the diagram above, saw that the S terminal was on the left side of the alternator when looking from behind, and wired it up that way. When I went to install an S6 alternator instead, I noticed the diagram's clip orientation didn't match what I was holding in my hand, and realized it was all probably backwards for the S5 alternator as well.
So, there's loads of good information above that should hopefully help anyone install and upgraded alternator 100% correctly but at the end of the day, I just made a dump mistake reading a diagram!! Probably the second dumbest mistake I've made with RX7s after somehow overfilling the oil
...
Probably the second dumbest mistake I've made with RX7s after somehow overfilling the oil
Thanks again!
Don't worry, I made the mistake of jacking the car up from the middle of the door area instead of the jacking point. THAT was a tough one to fix. Misreading a diagram is peanuts by comparison - and I think I still have you beat for number of blown alts haha
04-19-22, 10:22 AM
