s5 alternator in s4 (repost, I know)
 

By looking at FSM for s4 and s5 (wiring diagrams where you can kinda see the internals of the voltage regulator), it looks like the s4 alternator's voltage regulator is the same asa the s5 alternator's voltage regulator, but ...

When you look at the schematics in the "electrical" sections, they appear to be different ...

The s4 alt has a voltage control lead named "R" that connects to the battery positive after the ignition switch.
The s5 alt has a voltage control lead named "S" that connects to the battery positive before the ignition switch.

This means that, when ignition is off, the voltage regulator of the s4 sees 0V, while the voltage regulator of the s5 sees battery voltage.

Why the difference ??? and why one is named "R" and the other "S" ???

Does that mean that people that put s5 alts into their s4 should connect the "S" prong of the s5 alt directly to the battery positive ??? (not to the "R" wire of the s4 harness plug)

Of course, this is a problem only when ignition is off. Is this what could cause the drainage that so many pple report ?

Can someone shed some light on this ? I have tried to search the web for differences between "R" and "S" sensing voltage setups but could not find anything.

Hugues -

Of course, I assume the voltage regulator input wires ahve not been switched, meaning:

the s4 harness "L" goes to the "L" of the s5 alt
and
the s4 harness "R" goes to the "S" of the s5 alt

I think we all know how a voltage regulator works, right ...

If the control voltage goes below a certain value, the voltage regulator will flow current through the rotor coil, creating a magnetic field, which the stator will pick up and transform into DC juice to charge the battery until the control voltage reaches a certain threshold.

Now, when ignition is off, the voltage regulator will see 0V at the control voltage because it is not wired directly to the battery, therefore, (thinking the battery is completely discharged), it will energize the rotor coil and current will flow and will drain the battery.

Right ???

hugues -

Hmmm ... nice try but the juice would have to come from somewhere ... Can only come form the output post (B+ terminal) since the ignition is off .. and the only path to the diode trio and therefore the rotor coil is through the output diodes which should prevent any DC flow going that way.

So unless one of the output diodes is busted, I don't see how this would happen.

Hugues -

well well well ...

This is the correct way to connect the s5 alt to the s4 wiring harness (thanks to KYPREO from the teamfc3s forum for pointing it out):

S4 L wire (W/B) to S5 alt L connector
S4 R wire (B/W) to nothing
battery positive to S5 alt S connector

The wrong way (which many think is the right way btw) is:
S4 L wire (W/B) to S5 alt L connector
S4 R wire (B/W) to S5 alt S connector

If you do it the wrong way, you will drain the battery if you don't use the car.

I actually tried to connect the s5 alt both ways mainly for giggles ...

with the wrong way, I got a 130 mA draw which should drain the battery in 2-3 weeks of inactivity.
with the right way, I got a 20 mA draw.

This is confirmed here:
http://www.rx7.org/public/altxref.html

The teamfc3s thread is here (things get kinda sorted out at the end, at the beginning, it's mostly mayhem):
http://www.teamfc3s.org/forum/showth...threadid=24045

and I added the info on my page (at the bottom):
http://www.geocities.com/huguesdc1/charging.html

I know that from looking at the FSM's for both generations, it looks like the alternators are exactly the same but they are not (regulators work differently).

BTW, I believe the FD alternator is similar to the s5 alternator (sensing type), so this would apply.

Hope this helps somebody out there.
Hugues -

This is very straight forward:

S5:
http://www.teamfc3s.org/forum/attach...&postid=317631

S4:
http://www.teamfc3s.org/forum/attach...&postid=317640

as you can see the set up is almost identical internally in the alt. and now you have the plug info.

The only real difference is that the warn circuit goes to the ABS and the idiot lights on the S5 rather than the idiot light warning relay in the CPU on a S4.

What would have to be done to put an s4 alternator in an s5? My s5 alternator went out yesterday and I have an s4 alternator from an s4 parts car of mine.

-Brandon

FC3S-Rx7 :Would not reccomend that. s4 are smaller amperage than s5, and the entire system on both cars is stressed as is. Get a 100amp fd alternator.

ok, thx. What would it take to wire up an fd alt to an s5 n/a? And does anyone in the St. Louis area have one? I need either an s5 alternator or an fd alternator ASAP.

-Brandon

nothing is needed its a straight up swap nothing needs toucheother then the alt

The LS type alternator requires a constant power reference on the S (sense?) line.

S5 alt is LS.

from the rx7.org website:
http://www.rx7.org/public/altxref.html
Check it out.

Hugues-