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Old 08-09-2001, 12:50 AM
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slepr1 slepr1 is offline
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Default Tach probs

Hi,
In my Cuda, I have a ralley dash tach and also a aftermarket tach under the dash. They are both hooked up properly. My factory tach registers almost 1000 rpm more than the aftermarket tach. I am sure the aftermarket tach is correct. Why is the factory tach showing an incorrect reading? ( the factory tach does rest at zero when engine turned off)
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Old 08-09-2001, 06:35 AM
kingjr kingjr is offline
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I've been trying to solve the same problem on my Challenger. I installed a VDO top eliminator 5" playback tach. We checked it w/timing light and its 500rpm off (low) throughout the throttle pull. After researching it I found that these tachs came w/sheilded cable for signal hookup. This cuts down on engine noise and radio frequencies interfering w/signal. I'm going to replace my sending wire w/sheilded to see if it helps! These tachs have 4 switches that can be configured to work on all types of vehicles. If yours has them, be sure they are set correctly.
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Old 08-09-2001, 06:53 AM
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SpeedSleaze SpeedSleaze is offline
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First off, some tachs have a provision for 4 cyl, 6 cyl, 8 cyl engines, usually a sliding switch. Check the Aftermarket to mak e sure it's set to 8cyls.
They say if you have a watch, you know what time it is. But if you have 2 watches, you're really not sure. Even so, I'd try my Tune-up Tach and get a 3rd opinon on the matter. If it agrees with one of the tachs, you'll know which is right. Then leave the Tune up tach connected and disconnect one of the others. You really need to record readings at several different RPM, say 2000, 2500,3000, etc. Assume at 2k, the Tune-up tach and aftermarket read 2k, but the factory says 1000. If you shut off, disconnect the aftermarket tach then bring it up to 2k again, does the factory still read 1000? The current draw from the after market tach might be messing up the factory tach. That, or technology's gotten better. Or, the factory tach may have been set to go in a 6 cylinder car, but then I think it would read higher, not lower. The factory Tach may have a potentiometer or pot, often a plastic screw that you use to calibrate. It changes the resistance of that circuit. Like a Rheostat or Holley Electric choke.

You probably would need to take the car to a qualified and reputable Speedometer/Tachometer shop to calibrate the factory tach. Good Luck
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Old 08-09-2001, 09:57 PM
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pishta pishta is offline
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The old factory tachs sucked, more for looks than use. I could bounce the needle off the 6000 stop just by reving it up and down, and the sweep time was pathetic. I would trust an autometer that doesnt reset after the key is off anyday. If it were a 4 speed, you could really dial it in by doing some math with the rear end ratio and tire diameter and 2 speedos, drive next to someone at 60.
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