Voltage drop at idle w/ full lights...
 

Ok guys. I was running a miata dry cell battery in the rear bin and thought it may have not been enough for what I am running on this car. So I put in a optima yellow-top and there was no change.I have a voltage indicator ( microtech dash ) and have a drop of voltage at idle from 13.5 to 11.6 to 11.75. I have idle set @ 1000. I have an aftermarket stereo but playing it I only see about a .25 drop. With everything on (stereo , foglights, parking lights, fuel pump, etc.) its goes from 13.75 volts at idle to 12.85 or so. Then twist the knob to full lighting (pop -ups) BAM! 11. 75-barely 12.0.. With no load it does the same when just turning on the lights all by themselves. It seems the pop-ups are drawing a hell of alot. Of course when driving (above idle) my volts are fine. The alternator tested fine. Is there a prob. with my pop -up wiring or should I invest in higher output alternator? Thanks for any input G..

Hey Garret,
Aren't you running under drive pullies on both the main crank and the Alt. ? ? ? I'm running the ALT. under drive pulley on my car with the idle at 750 and see this same drop, goes back to full voltage above 1100rpm's just kinda figured it went with the under drive, I think there 33% reduction, (have to recheck my numbers) but if your running both this may be it.

I run the gel cell miata battery and underdrive everything. I've been running this way for 10k miles...with 11.7 on start-up.

When you hit the brake do the dashlights ever dim for a split second? I've had electrical problems ever since i bought the 7 earlier this year and after replacing the alternator and battery they were better, but not gone. All the dash lighting and voltage was fluxuating, but the rest of the car seemed fine. Next time you notice it happening check the voltage at the battery terminals and see if it's reading the same...or mebbe it's just doing that at the dash power source. That should point you in the right direction.

Yes jack.. I am now believing it is caused by the underdrive pulley. It makes perfect sense. I tested some of the wiring and it was all checking out ok. Oh well ..

Garrett, I get that problem sometimes too. I am also running under drive pulleys as you know, but I have my idle set at about 1100rpm, and my voltage doesn't drop past the middle 13.x v. If you rev the motor does your voltage read higher? If it doesn't your alternator may be the problem. I know we've gone back and forth about your altnernator ;)