I purchased a Equus 4" Tachometer to install in my custom dash. I hooked it all up and used the inductive pick-up on the spark plug wire/coil wire.. tried both options. The tachometer would read a proper idle but once the rpm was raised it was jump around and became choppy.. I returned the tach and got another one thinking it was bad and reinstalled... same thing happened. Does anyone have any experience with aftermarket tachometers or maybe a preference on brand to go with. Any info or knowledge is greatly appreciated. Thank you

 

Have you contacted the tach manufacturer?
Thats your best bet!

First thing that comes to mind, is the plug pulses are saturating the tach at higher RPMs.
The inductive pickup and/or tach input circuitry could be too sensitive for the Jeep ignition.
Or,
the inductive pickup or wiring to the tach is picking up noise from other wires, etc.
Try other plugs or reroute the wiring to the tach far away from other plug wires.

Are you running resistor wires or straight spark plug wires?
Can you post a pic of the inductive pickup hooked up to the plug wire and the cable wiring to the inside of the cab?

 

I have contacted the manufacturer. Actually had the first tach tested by them and they said it was bad. I am running 8mm Accel Plug wires that are not resistor type. I actually went and purchased some OEM wires to see if maybe it was an issue with heavier insulation on the Accels but no change. I will hook it all up again and try the rear most plug wire and see if that helps. I will take pics and post once I do that. Thank you

 

The one I bought used the coil lead as the pickup, not induction. There was also a switch on it to set it to 4,6 or 8 cylinders, it was way off until I set it to 4 instead of 8.

 

May I ask what Tach you have and also how did it hook up to your coil?

 

On the Renix era 4.0L, there is a wire coming out of the ICM on the pass fenderwell, that is the tach signal wire. The ICM and Coil are a unit on these systems.

 

I think mine is the 2" Sun tach form Advance Auto. It has a positive and negative wire you hook up to a supply and ground and a green sensor wire you use to tap into I believe the positive wire going to the coil. On mine the coil connector has a terminal labeled Tach, so I just attached it there. Mine is an 85 model so that might be different.

This type counts every time the coil fires, so you have to set the switch on the back to the number of cylinders so it knows what to divide the total fires per minute by.

 

I returned the Equus Tach and actually ordered a Autometer Tach... I spoke with Autometer prior to purchasing the Tach and they stated that they show having installed on my exact model xj.. So we will see once it shows up. I will try the Autometer and update everyone.. pictures this time as well. Thank you