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Unread 20-11-2005, 19:26
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DonRotolo DonRotolo is offline
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Re: tachometer mystery?

Quote:
Originally Posted by sciguy125
I noticed that mine likes to bounce. When there is a load, the tach drops. When the control system tries to recover, it overshoots. Then it realizes it went too high and comes back down. The A/C does this when it turns on. If I turn the bass on the radio up, it'll follow the music.

On a side note, I did notice that there's a certain temperature that my engine doesn't like. The problem is that it's a little above ambient for this time of year. When the engine's at this temperature (at start up), it revs really low. Every so often, it's so low that it feels like it's getting ready to stall. It's fine when I give it some gas, but it doesn't like to idle properly. Once the engine warms up, it's fine though.
The reason it follows the music is because those bass notes draw a lot of current, causing the alternator to draw more power from the engine. If you can see an effect from 63 watts of brake lights, 500 watts of bass will do it, too. Note that many A/C compressors draw up to 5 HP off the engine. Modern electronic engine controls should compensate for the load, but not all cars are the same. As for the bounce, what you are seeing is the control system feedback loop frequency response, which is either in need of repair (see below) or wasn't designed right in the first place (which I highly doubt).

For the poor idle regulation at a certain temperature, there is obviously something wrong with the warm-up system. Given enough money, the car will run fine at any temperature, but now it becomes a question of how much you want to spend to make it right. A decent mechanic can tell you which part needs replacement, but I would hazard a guess it is the mass air flow sensor that is fouled a little.

Don
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