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Hello everyone, I have the best luck finding useful tips and information for things that are over my head. I have noticed a huge difference in what the tachometer reads verses what the scanner says. I looked all every where for a proceedure on how to test go about on figuring out why the readings are so different. Could my ecu have a problem? 

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The tach uses a voltage pulse and wave signal.  Different tachs have different pulse wave calibrations based upon the number of cylinders. On 1987-90 YJ Jeep Wrangler models, there is a different tachometer for the 4-cylinder engine versus 6-cylinder models.  After the TBI/carburetor era (which ends in 1990), the tachometer is the same part number for 4- and 6-cylinder Jeep MPI (1991-up) engines.

Having the wrong tach for the engine type will alter the tachometer reading accordingly.  First, verify actual engine speed with a scan tool or remote tach that picks up a true #1 spark pulse.  Once you know the actual engine speed, see if that matches the tach reading.  If not, make note of the percentage/fraction error in the speed readings.  This could be the difference between a 4-cylinder and 6-cylinder tach.

If the instrument cluster has been changed, it may be from a chassis with a different engine type.  Or the cluster could be from a later model chassis.

Moses

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If the scanner is an induction pickup and the scan tool works okay (verified on other engines), you should get an accurate read.  If the engine runs smoothly, without misfiring, the #1 wire's signal should be accurate for rpm at the scanner. 

This is your 2.5L TBI Jeep YJ?  The ignition is not coil on plug; it has a standalone coil that fires all cylinders based upon the distributor's pulse signals.  You have a distributor, cap and rotor.  Each distributor plug lead feeds to a cylinder/spark plug and only reads that cylinder's spark.  So, an induction pickup on a scope or scan tool should simple pick up the firing pulse for #1 cylinder (like a timing light strobe).  That firing pulse reads as rpm on the scanner tool, which should be accurate.

Moses

 

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