Hey fellas, well i did a search and didnt get the answers i needed, basically i just picked up a 87 S4 13bt RX-7 and its got some teething problems, when i start it up it says its running 12v if the batt has been on charge overnight, and it goes down to 11-10v after abit of driving, faster with the lights on etc, but if i give it **** (say 6000rpm) the guage says 13-14v and stays there, its givin me the ***** coz i dont wanna have to my car **** in the morning (no i dont do this) and im sick of it breaking down, please help me, how do i fix this annoying problem!

Cheers,
Jake.

 

Got a voltmeter?

VOLTAGE TEST
Engine running, headlights & heater fan on high:
Alternator post to alternator frame = 13.5-14 volts (verifies the alternator output)
alternator post to battery (+) = 0 volts (verifies the primary + wire)
alternator frame to battery (-) = 0 volts (verifies the primary ground)
Turn off the motor, but leave the lights on.
Battery (+) to battery (-) = ~12.5 volts and holds steady. (verifies battery)

In your case it could just be the primary + wire from the alternator to the battery.
A bad connection at the eyelet would give those symtoms.
Or, of course, it could be the alternator.

 

i've seen this test posted so many times. do you got that on a quick copy and paste key my friend, hahah?

 

Anytime there is less than 13.5 volts the battery is not being charged, and anytime there is less than 12.6 volts, there is not enough power coming from the alternator to even run the car.

A properlly running and wired alternator will have 14.4 volts at idle with no load, and never drop under 13.5 volts even fully loaded (as long as the electronics in the car have not been meddled with; like adding a 1000 watt amp for the stereo).

 

Icemark, is that 14.4 at idle (750 rpm) with no load?
my idle is a little low (400~500), should it still hold that voltage?
i have noticed that when I am cold, I have better voltage than when the engine bay warms up,
belt slipping? bad voltage regulator?

 

600-650-700 is normal idle so yes with lower than normal idle than you could see lower voltages.

I still would be concerned anytime there was less than 13.5 volts

 

mine does the same thing, on a full load the voltage drops to 8-9 and my radio cuts off at idle but around 3K on full load it is around 12.5-13. i cahnged the alternators thnking that may be the problem but now im guessin it may be a connection problem. make sure all your wires are connected witn no splits or carosion(sp?) on them.

 

hmm, could be the connections, I am going to redo all the grounds and add a few in a few months.

if it was connection it would be low always, but I notice that when my engine bay is cold it is high, but anytime I warm up, even if I gas it up to 3k, my voltage is low.

but then again I am depending on the stock voltmeter,
is it OK to hard wire a voltmeter from the battery and run it through the firewall and stick it somewhere on the dash or the pillar?

I have a new fd alternator (silverrotor) sitting in the garage, but hate to spend time on changing it if the alternator is not the culprit...

 

I'd better revise my test notes.