These instruments work off of an electrical pulse, usually on the ground. You cannot read this pulse with a regular multimeter, because they only take the average. You need a running multimeter that graphs the voltage with respect to time. Then you have to verify that these pulses match the factory specs.

You could try a few things. Switching to a new style tach to see if it reads the signal correctly, replacing the tach with the old style to see if your current one is broken, buying a good multimeter tool and troubleshooting it yourself, or paying someone with really good equipment to fix it.

Personally, I'd try to get your hands on a new style gauge. They're about $25 I believe, or borrow one from a friend. Then hard wire it and see if it can read the pulse straight from the coil.

 

All the other 4.2 to 4.0 swap threads I've read said you can hook the green wire on 87-91 tachometers straight to coil negative and they'll work fine. I just listed the voltage for reference sake, I used to have an oscilloscope that would have been great for this but I don't know where I put the thing. Though even if it was sending the wrong number of pulses wouldn't the needle still move? The reading would be off but it would still move, right?

If I switched to the newer style tach I would have to change speedometer and cluster lights too, not really looking to do that, I just wanted to rule out my tach before I start buying replacements.

 

With this swap did you put in a new coil? If you're using any existing wiring you could be dealing with a gremlin that was killed by "hard wiring".

 

My 1990 yj also has a 1992 4.0 swap in it.when I first bought it tach was working,a couple of days after having it a fuse that powers the radio blew so I replaced it and since then the tach hasn't worked on mine either,I too am thinking of replacing my tach.

 

I didn't do the swap, I'm just finishing it unfortunately. From the bulkhead connector forward all the wiring is 1992 except the lights. The coil is the regular old 2 wire 4.0 coil from 1992, positive wire is yellow with a black tracer, negative wire is green with a black tracer, the new wire is spliced into the negative wire right by the coil and goes straight to the tach which is receiving the signal but doing nothing with it. The Jeep runs fine so the coil is definitely working.

 

Well I just got in my new omix-ada tach, it doesn't seem to work either, and it may have a short inside too because when it's plugged up my dash lights go dim, when I hook the old tach back up the dash lights are normal brightness.

I just don't understand why I can't get either tach's to work. I'm getting a signal from the negative side of the coil all the way up to the green wire at the gauge, orange wire has power for dash lights, white wire (red on new tach) has power for gauge and the black wire grounds. On either tach when I first turn the key on I get a tiny twitch from the needle but that's it. Every single other 4.2 conversion thread people always just plug their tach up to coil negative and it works for them no problem.....what am I doing wrong?

 

Disconnect the battery.
Turn the key to ON.
Take a test light and a battery charger.
Ground the battery charger to a ground on the Jeep, and hook the positive up to the clamp on the test light.
Now you can test for grounds. The test light will light up whenever you probe a ground.

The idea is to probe wires that are normally positive, and if the test light lights up, you have a short. So probe wires around the dash and check for shorts. Just take the clusters out and probe the wires in the plugs. Sometimes you'll get a short on all of them if there's a short on one of them.

 

I'll give that a try, I was also digging through the tool bin and found one of my old dwell/tach meters, so I'm going to test with that off my coil signal wire too once I get my ecu back from the repair place and I can start it again.

 

Want the wiring diagrams for the newer dash style and the ignition system?

 

I've got most of the diagrams but thanks for the offer, the only 1992 wiring is under the hood, everything in the dash is still 1990. I'm still waiting on my ecu to get back from repair, once it does I'm going to try running my dwell/tach straight off the negative side of the coil. I noticed the PO messed with a few things under the dash because he's got the interior lights wires straight to the battery, it's possible he has tach wires switched somewhere.

If the dwell/tach works off my signal lead I'm going to try wiring the tach up under the hood and see if it (or the junky omix ada tach) works then, if so I'll have to figure out where things are crossed or just run new wires since I have the harness somewhat apart right now anyway.

 

Where did you hear about splicing the green wire? The new style tach goes through a few more steps, such as the PCM and the ASD relay. Compare the workings and frequency values of the old tach with the new tach.

"With the engine running, the tachometer receives an engine speed pulse signal from the PCM. An electronic integrated circuit contained within the tachometer reads and analyzes the pulse signal. It then adjusts the ground path resistance of one electromagnet in the gauge to control the needle movement."

Here are the frequency values:
33.3 Hz - 1000 RPM +/- 150
100 Hz - 3000 RPM +/- 250
200 Hz - 6000 RPM +/- 250

"PCM provides the tachometer signal to the electronic tachometer on circuit G21. Circuit G21 originates at cavity 43 of the PCM. Circuit Z1 provides ground for the tachometer's internal logic circuits."

You can see circuit G21 going from the PCM to the tach is gray/light blue. It has a black ground, circuit Z1.

The coil has a dark green/orange wire coming off of it. The wire coming into it is gray from the PCM (I think the switched ground).

The wire then goes into the ASD relay.

 

I don't have the 92-95 style computer controlled tach, I have the older 87-91 carbureted Wrangler style where it's an individual round tach and not part of a cluster like the 92-95 style.

The wiring on my yj from the bulkhead connector forward is from a 1992, since the signal coming from the ecm tach wire isn't the correct type of signal for the 87-91 tach you have to instead connect the tach to the coil like it would have been on the 4.2 that was originally in there. Which I've done but it's still not working, neither is the junky omix ada replacement tach, unless you count the slight jiggle of the needle when you first turn the key on. It's possible the po has crossed a wire or something somewhere.

The attack plan is once my ecu is back from repair I'm going to try to run my dwell/tach completely separate from the wiring and see if it works, if so I'll wire the real tach up externally and see if it works then, if so then I know the po has been screwing around under the dash and I can go about fixing that.