Driving down the road the tach will pulse very slight at different speed's
My car has 220000 miles.
My car has an automatic transmission.
If the needle on your tachometer is fluctuating slightly at random speeds, it is possible that the sending unit for your tachometer may be going bad. This sending unit/sensor is usually located near the crankshaft pulley or on the back of the transmission.
This sensor should be tested by a qualified mechanic using the manufacturers recommended procedure. This issue may be occurring due to a loose or corroded wire that goes to the sending unit.
Another possibility that is a common issue with these is that a fluid leak of some kind may exist that is fouling out the sensor. This can happen to many different types of sensors as the electronics are negatively affected by the fluid leak.
This diagnosis can be verified by checking for battery voltage at the connector to the sending unit and also doing a continuity test on the sending unit itself. If you need assistance with these checks, consider YourMechanic, as a certified technician can diagnose the issue with your tachometer firsthand and repair it as necessary.
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