90Mustangfan

Ok on the stock tach and on my after market tach....when I shut on my electric fan or push the up or down or shut on my light. my tach needle will move up and down about 100rpm or so..and it stay that 100-200rpm high. So when ever I have the lights on or have my electric fan on my tach will read 100-200rpm high, it like when the window is up and I press the window button up, it will move the tach needle up every time I press the button. What is causing this....could it be because Im missing the ground wires somewhere.....also could this be related to why the O2 sensors arent switch and read lean BTW, they are breand new and they get power...could it all be because of a ground wire im missing?????

88BlueGT

Im not to good with electrical problems (actually I'm horrible) but couldnt that be just from taking power from the alternator causing it to spin slower?

sgt_pepper17

I agree. It's prolly cause you're drawing a lot of power from the alternator. To compensate the motor must rev up. The alternator becomes harder and harder to spin when you increase power requirements. A more powerful alternator will help this. This is the reason why a lot of ricers have capacitors in their cars.... it's to compensate for the draw of their stereos.