Tachometer Accuracy and Test Procedure

[Sinisterwillys1940]

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? 

[Moses Ludel]

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

[Sinisterwillys1940]

I had a snap on vantage with the rpm pick up hooked up to the cylinder one plug wire. It looks like the tach is accurate. The scanner is off by 400 rpms. Could this signify a bad ecu?

[Moses Ludel]

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