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Why doesn't my tach pointer rest at zero when the ignition is off?
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We get a lot of questions about this. Here is the reason, along with some specifics for each product. Most of the electric instruments that we offer use what is called an air core meter. In layman's terms, this means there is no physical part to hold the pointer where it sits, or to force it back to zero when power is removed. It uses an electrical charge to return to zero when power is applied, and when a signal is fed to the instrument, the pointer will indicate the proper reading. This is normal, and no cause for alarm. The unit is not damaged or broken, it just doesn't look like most other instruments out there. For Example: The pointers, on most modern OE (original equipment) gauges, just hang there when power is removed. We use air core movement due to its rapid response characteristics, so when you stomp on the accelerator, the pointer is capable of showing you exact engine RPM.
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On some other gauges, such as the pyrometers or EGT gauges, sit off zero, too. But, even after you connect them and turn the power on, the pointer still doesn't go back to zero. What gives? Well, provided the gauge is connected to its probe, it will show the ambient air temperature. So, unless it is zero degrees F outside, it will not read zero. For example, if your vehicle has sat for a long period of time (allowing the engine to completely cool) the gauge will accurately reflect the outdoor temperature when power is applied. If the outdoor temperature is 70 degrees, the gauge will read 70 degress.
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