Tachometer issue

[Mat7783]

Replaced my broken Tachometer/hour meter with a alternator source version based on schematics o copied off the forum.it is a my2000 Thomas bus MVP ER and the gauge gets 12vdc and needle moves to zero but no tach when running.my BlueFire app has a good running tach at 700RPM idle.ordered another tach (VDO)with any source compatibility thru terminal wiring and this gauge sweeps but no running tach again.I am getting 12vdc from source wire to ground upon ignition.I am at a complete loss why any tach is not registering whil my BlueFire app is getting said data from diagnostics port?anyone have an idea where to look??thanks, Michael Troitino

[flattracker]

Could you provide some details about the make/model of the tach you installed. It would be easier to help troubleshoot your problem that way.

[Mat7783]

VDO 333-363 multi source Tach with hour meter.here are some pics.the new VDO connections and the original damaged tach.there was only 1 source wire(green on left post)so it is either alternator or magnetic sensor pulsed and new tach wired for either but not registering tach RPMs.

[flattracker]

Could it be that the tach reading you are getting comes from a different source than the alternator connection used for the tach? Does the alternator have a connection for the tach? Check to see if the terminal you connected to on the alternator is labeled "I". You don't connect the sense wire for the tach to the same place as the battery.

[Mat7783]

These are good questions.the alternator on my cat3126B engine has 5 wires including battery leads connected behind but hard to see unless I remove it from bracket.