Normal tach behavior?
 

I just had a quick question I had to ask about.. I noticed the other day while sitting at idle at a stop light that if I do something that draws power..for example, I hit my power window switch..I hit it up when the window was already all the way up so the motor didn't actually use the power, but it WAS drawn from the alternator. Now my alternator gauge reads very low. When I hit the button it dipped to the very bottom rung of the normal range. When it did that, I noticed my tachometer went up by about 50-100 rpms. The more powerful the accessory, the higher my tach spiked, but the engine rpms weren't actually increasing. Is this normal behavior, and could this be affecting the accuracy of the tach at upper rpms on the throttle?

that happens with every car i have ever owned, i think its normal. cause the alternator is putting more load on the motor the engine revs up a little to keep the rpms where they should be. mine drops big time when the windows are already down and you push the both buttons down at once. when you push the button up or down when the window is already in the up or down position it is still drawing power, actually more, because the window motor is struggling to push or pull the window up or down

I was just curious about that since my last car didn't have a tach at all so I didn't have the ability to observe it. Or should I say..it DID have a tach, but the lowest it ever read was 5000 rpms...I tapped the gas and it hit the bottom, well past 7000.

I think that you will find that if you install a good quality Auto-Meter tach and leave the stock tach hooked up and watch both function as you drive you will see just how inconsistant and unreliable the stock tachs are, ESPECIALLY above 4,500 rpm's. The stock tach does not even come close to representing what the real rpm's are at the hiogher rev's. I have found this to be true on every 5.0 that I have ever seen, literally hundreds of them.