The car:
S4 87 TII, alt checked good (by Advance Auto), less than 1yr old Optima Red Top. Apexi turbo timer.

FYI: I have searched for "low voltage". I see a few people saying they have the same problem, I see suggestions, but I don't see answers. Idle is at 750RPM. All voltage readings are from Apexi Timer.

Problem:
Well, as the title says I got low voltage at idle. A "drive" that produces the problem would be; Start cold car. Slow to turn over. Car starts finally. Turn on headlights, radio, blower fan on high. Drive on street, voltage is 13.6. Come to stoplight, voltage reads 12.7, w/ breaks on. Drive until car is norm op temp, voltage is 13.6. Come to stoplight, voltage dive down to under 10v (all lights, fan, break lights are on). If at idle, after car has warmed up I turn all lights and blower off, no break lights or radio, voltage is around 10.5 - 11.5V. Still very low.

Very close to this thread, https://www.rx7club.com/showthread.p...t=test+voltage

But my voltage is 1-2v less! And at the end of the thread the owner ended up stripping the car, not much help to me.

Any help, thanks.

 

the alternators in the series 4's just suck ***** even if its working properly

if you want a good alternator check the stickys for one of the FD alternator kits. theyre worth the money if you need an alternator.

 

if your voltage drops to 10, then something called the alternator isnt working right.

if it just dropped to 12 at idle its prolly a poor connection somewhere.

 

what do u call it when the battery voltage shoots up to 15.1volt when u start the car in the morning?....then drops back down to 14.4 after its warmed up?

 

What kind of load was it tested under at Advance Auto?

Normally I'd say check your connections and wiring terminations first... 13.6v above idle isn't all that great either...That's just barely enough to produce a charging current, if the battery is good...

Is the alt warning light coming on when the voltage drops? If not, I'd say double check the accuracy of the timer...

Get her hot, turn on everything at idle again (reproduce everything above in your post), pop the hood, and check the batt terminals with a good voltmeter, observing polarity. What do you get?

 

at idle any vehicle should not have have a voltage lower than 13.2, and a vehicle should not have a voltage higher than 15v (being put out by the alternator)

most alternator's work best at/above 2000 rpm's, because it's spinning faster than at idle (duh)

do you have any extra electrical loads on that would make your car's voltage drop a little? like a stereo, some sort of lighting, etc. etc.

check the output of your alternator, and check the voltage on your battery

it seems like you might just have a shitty battery though, test it after the car has been warmed up, then turn her off, you should have at least 12.4v in the battery (75% charge) anything less and you might wanna get a new battery

also, if you've got all that extra **** on, like the blower, lights and radio, and the brakes, that crappy 75 or 80 (correct me if i'm wrong, i don't remember) amp alternator isn't gonna be able to hold all those loads for too long, a few minutes at most before your voltage would drop down to 12...mine does it

if i'm at a light with everything blasting, heat on, rear defroster on, lights, stereo...my voltage will drop to right around 12v...if i turn that **** off and rev her up a little bit (1500-2000 rpm) i can watch the voltage spike back up to 14

any more questions PM me, hopefully this will help though

 

I wasn't really watching, they put it on the "machine", ran the test. Said "tests good, puts out 13.8 volts. Have a nice day".

The alt light does not come on.

Same as the timer voltage, at least with in .1v.

 

The first thing I'd check is the condition of the wire running off the top terminal on the alternator.

My voltages were really shitty until one day my alt. light came on and I barely made it home running off the battery. I started checking connections, and sure enough the alternator wire was corroded and had a horrible connection.

I cut off the nasty portion, crimped a new ring terminal on and it has been working well since. I'm still planning on hacking the whole chunk out and running it straight to the fuse panel rather then having it wrap around down by the starter and up the frame like it does stock.

 

The connections at the alt look pretty good. I might try swaping alts from my 88 N/A to the 87 TII, see if that does anything. The N/A has more electrical load devices and good idle voltage. Now that I think about it. I don't think the test at Advance Auto puts much of a load on the alt, if any. Wish I had a multimeter with more than a 10A max rating.

 

Just swapped my alt from the N/A to the TII. No change. Still VERY low idle voltage. 9.5-10.5v (w/ headlights, heater blower, radio, brakes applied). I even disconnected the ABS system as a troubleshooting step, no change.

 

Wizz, are you using an underdrive pully?

I have the same exact issue which I'm starting to diagnose. I have also swapped my alternator w/o much help..

I have the same symptoms. at idle the voltage is all over the place (from 11.5 to 14) @ 1000 rpm. It seems to be dependant on load, but causes major issues with the fuel pressure, since the pump slows to a crawl...

Alot of people seem to think this is the alternator.. I know I have extra load than a stock FC, but how about you? I have a feeling it wouldn't hurt to remove the underdrive pulley and install a FD alternator.. This is mostly an idle problem for me, when I start revving it pretty much sticks to 13.8v, which still feels low. I think TEC II and the 1000hp aeromotive are hogging power.

I'm on my 3rd optima red top. fwew those things are craap! It could be a weak battery in addition, have that tested at the parts store for free..


hope we can figure this one out.

 

keep in mind a TII alternator only puts out 75 amps. A FD alternator puts out 100 amps..

I found this, might help:

I noticed that my voltage gauge reads 13.6+ going down the road, but when I am at a stop or just idling, the voltage drops to 12.5V. Why?


This could be caused by several things. First, the pulley ratio may cause the alternator to spin too slow for these driving conditions. Using underdrive or power pulleys on a street application can cause this problem because the pulley ratio becomes less than the typical street ratio of 3:1. If the pulley ratio IS 3:1, another possibility is that the alternator is too small or not powerful enough at slow speed for the amp load of the vehicle. Also, the charge wire could be too small or the ground path may have high resistance. The gauge could be out of calibration. Check the voltage directly at the alternator with electrical loads on to determine if the problem is the alternator or the path to the battery.

 

Today I started looking at the starter. Took off the starter, cleaned it up. Tested per the FSM. Checks out good. Checked the connectors, all good. Cleaned them very well, didn't really need to, but did anyway. Go for a drive, no change. Still 10.5 volts with all loads on. Going to take my Optima Red Top back under warrenty.

FYI:
I am not using an underdrive pulley. Unless the ones from hIGGI are.
http://rx7cz.net/pics/pulleys/pulleyjohn1.jpg
I don't think I have that big of an after market electrical load. Even with the raido off it may go up to 11v.

I test the engine ground to battery neg, using a set of jumper cables. No change. I'm going to try the charge/postive/B terminal jumper tomarrow. Before I try to get a new battery.

 

I had the exact same aforementioned problems, now I am running a 3rd gen alt & optima red top, and aside from a slight voltage drop @ idle (13.0 v, not bad) everything is cool. The headlights & tails don't dim anymore at stoplights, and my power windows are a bit quicker now.

 

I tried the B-term to battery positive. No change. So I went ahead and ordered an FD alt from silverrotor. Coming soon. I the mean time I took my Optima Red Top back to Advance to have it tested. It FAILED. Got a new battery, now idle voltage only drops to 11.5 - 12.5 or so with all loads on. I'll post again when I get and install the FD alt.

 

The alternator is eithier bad or just the voltage reg inside the alt however it could also be a bad battery if it turns slow when starting your bat might just be really old and need replacing as everyone says the alt is **** in these cars which is true but all your power doesn't come from the alt you need the amperage from the battery to keep things working prop so have your battery tested or just go buy a 1000amp cold cranking bat for it

 

Wizz, providing your former Alternator(s) are the culprit - I guarantee you that you will be more than pleased with the FD Alternator that I have send you.

I'll bookmark this thread to see your outcome!

 

When you get that alternator.. . Since you have an S4... . you will need to run the TOP terminal on the FD plug to the battery.... (as the alt sits in the car looking at the terminal as in is plugged into the socket) The bottom wire from the plug goes to the wite/black stripewire on the s4 harness (the other wire on the s4 harness gets snipped and taped and thats it)

The FD Alt (and S5 Alt) need to see voltage from the battery when the car is off or it will go into charge mode and slowly drain your battery. S4 the black/white wire is hooked to the ignition switch... with the switch off.. there is no voltage.... in the s5-s6 with the ignition off it sees 12v from the battery

This was all hashed out over on the other forum.. I think I saw Sliverrotor over there too, so, hopefully he's sending out instructions to clear it up now.. (I bought my fd alt from him over a year ago, before the problem was solved.. )

 

whats considered a normal voltage? on idle and cruising i see mine at 13.2-13.5 ish but on boost its just around 12.8-12.95 (especially at night with headlights on)

i run a FD alternator and everstart battery from walmart. do u think its the battery?

and i have a MSD 6A ignition module on my leading coils

 

On a good alternator your voltage should never change whether your engine is scream or just sittin idle always same

 

According to the "Book", alt voltage should be between 14.4 and 15v with the alt rotating above 2000 rpm (about 900 rpm engine speed). They must mean at the alt itself, because once the wiring drops the voltage before the gauge ever reads it, most of us are around 13.5 to 14v...

The temps of the voltage reg have a lot to do with it, too...

I've run with the Wal-mart batts as long as I can remember- never had a problem with them

 

err ok , so do u guys think its the battery or the alternator problem?

 

Clean your terminals and solder them, oxidized ends can cut voltage. Also keep in mind the grounds are the same way if the electricity can't get back to the battery. Try running new wires (try two thick wires instead of one). I gained 1.5 volts at idle.

 

bad alt

 

Well, I just got back from my test drive with the new FD alt. Idle voltage is a little better. With headlights, radio, brakelights, and heater fan on high I'm seeing about 12.3v - 12.5v. (At idle) On a cold engine and crusing with said load I see 14.2v. On a hot engine and crusing with load I see 13.7v. So yes, the FD alt did help.

And in regards to the FD install, I didn't like the idea of an added wire running from the alt to the POS battery post. So I took out the starting/charging harness, stripped the harness of it's dried out tape and added the wire into the harness. Over all the harness wasn't in too bad a shape. Little bit of oil in the area of the oil filter but thats about it. Cleaned up the harness and re-taped it.

The FD alt just barely fits under my strut bar too. Only problem I had was the part store only had one belt in the size I needed. :-(