Purpose built drift FC is having issues with charging (S4 into S5 alternator)
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Senior Member
Joined: Dec 2002
Posts: 613
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From: North Atlanta, GA
Purpose built drift FC is having issues with charging (S4 into S5 alternator)
Hey guys, recently got a hold of a purpose built '86 FC car and trying to sort out some loose ends on it.
The car has MicroTech LT 10 on it and it has been fully rewired. No factory colors or wiring left on it!
I noticed that the car wasn't getting a charge at idle. Apparently the alternator it came with was from a Nissan Centra according to the parts store guy.
I also had a good and tested s5 alternator from before. So, I decided to just replace the Centra alternator for the good S5 alternator.
Got it to work but still having an issue with it keeping charge at idle. This is what it does: When reved past 2k rpm it starts charging at ~14 volts, then when the rpms drop back down it starts to slowly loose the charge. For example, at 1100 rpm it still has high 13's volts and at 1000 rpm it goes to low 13's volts. Below 1000 rpm it starts dipping into 12's and so on until it bottoms out at battery voltage (12.5 volts Optima battery).
Now, the belt is at proper tension and I do know that some swapped alternators don't start charging until reved pass 2000 rpm. But I think it should still keep the charge at idle.
This is how I got it wired right now:
Terminal B, I did not mess with at all. The previous owner had a 3rd gen s6 alternator that worked right before it went bad. All the wiring tucked away really nice and I didn't see any reason to undo it all.
Terminal S, I got it hooked up straight from battery post. Also, I did try jumping it to B terminal but the voltage was almost 0.5v less.
Terminal L, I have that one also straight from the battery post but with an inline 150 ohm 5% resistor.
The voltage at idle at the terminals is as follows: Both terminals B and S have the same battery voltage of +12volts and terminal L has a fluctuating voltage of f 9-10volts.
Any insight or recommendations from people with experience on this is highly appreciated.
P.S. I'm sure the car is drivable as is right now but it really sucks about not being able to charge at idle because the car's cooling system is designed in a way that it takes a very long time to get it up to temperature. Thus, letting it sit and idle for extended period of time is very desirable.
Thank you for reading
The car has MicroTech LT 10 on it and it has been fully rewired. No factory colors or wiring left on it!
I noticed that the car wasn't getting a charge at idle. Apparently the alternator it came with was from a Nissan Centra according to the parts store guy.
I also had a good and tested s5 alternator from before. So, I decided to just replace the Centra alternator for the good S5 alternator.
Got it to work but still having an issue with it keeping charge at idle. This is what it does: When reved past 2k rpm it starts charging at ~14 volts, then when the rpms drop back down it starts to slowly loose the charge. For example, at 1100 rpm it still has high 13's volts and at 1000 rpm it goes to low 13's volts. Below 1000 rpm it starts dipping into 12's and so on until it bottoms out at battery voltage (12.5 volts Optima battery).
Now, the belt is at proper tension and I do know that some swapped alternators don't start charging until reved pass 2000 rpm. But I think it should still keep the charge at idle.
This is how I got it wired right now:
Terminal B, I did not mess with at all. The previous owner had a 3rd gen s6 alternator that worked right before it went bad. All the wiring tucked away really nice and I didn't see any reason to undo it all.
Terminal S, I got it hooked up straight from battery post. Also, I did try jumping it to B terminal but the voltage was almost 0.5v less.
Terminal L, I have that one also straight from the battery post but with an inline 150 ohm 5% resistor.
The voltage at idle at the terminals is as follows: Both terminals B and S have the same battery voltage of +12volts and terminal L has a fluctuating voltage of f 9-10volts.
Any insight or recommendations from people with experience on this is highly appreciated.
P.S. I'm sure the car is drivable as is right now but it really sucks about not being able to charge at idle because the car's cooling system is designed in a way that it takes a very long time to get it up to temperature. Thus, letting it sit and idle for extended period of time is very desirable.
Thank you for reading
Last edited by 12abridgeport; Mar 14, 2018 at 10:37 PM. Reason: EDIT: Tried to edit the title of the thread but it's not letting me. My mistake, it should read "(S5 into S4 alternator).
Thread Starter
Senior Member
Joined: Dec 2002
Posts: 613
Likes: 2
From: North Atlanta, GA
^Nothing at all as far as electrical loads during this test. The car has bare minimum of everything and all the lights, fan, etc have their own separate switches. The only loads while running are injectors, coils, and LT10. I tried turning on the front lights while doing this and it literally had no effect above 2k rpm or under.
Joined: Jul 2006
Posts: 432
Likes: 2
From: Central Coast of California
I wounder why the previous owner had a Nissan Sentra Alt on the Rx7? The best bet it to check the charging system wiring to see if the wiring is corroded, damaged or just worn out due to old age, then I would have that alternator benched tested.
Full Member
Joined: Apr 2013
Posts: 247
Likes: 7
From: Suffolk, Va
What is your idle set to? Turn the idle up to 1100-1200 rpms and call it good? I would think high 13's at idle is pretty good and would keep a charge. High idling won't hurt your car and it should help it get to temp faster as well.
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