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Jeep CJ7 tachometer

5K views 9 replies 6 participants last post by  smoothoperator 
#1 ·
Need some help. I’ve got a 1987 4BT in my 84 CJ7 which came factory with a tach, that I’d like to keep. Will I need a new tach? If not how do I wire this one up? My mechanic mentioned something about Autometer selling something to attach to the alternator but he wasn’t very sure. Any help is appreciated.
 
#2 ·
Yep, my standard tachometer wouldn't work with my 4bt as well. I bought a Autometer diesel tach and installed it. It has a pickup that attaches to the alternator, as viewed in the pic below. They come in various sizes and work good. No complaints.
 

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#4 ·
Mine was 2-5/8 inch. I think they also sold a 5 inch diesel tach too. But, it's been a few years ago. They may have different sizes now.
 
#5 ·
I have had success making the stock tac work with diesels, I put notches in the crank pulley and mounted an ABS sensor to read pulses. 2 notches for a 4cyl, 3 for a 6 and 4 for a v8. Two wire hook-up one to 12v and the other to the lead that went to the coil.
 
#8 ·
In my CJ-7 I made a bracket to use a hall effect sensor to read the 4 bolts on my crankshaft that holds the pulley on. The idea is that as the bolts of the crankshaft pass near the sensor, a pulse is created for the tachometer to use. As an 8 cylinder engine fires 4 spark plugs every revolution of the engine, setting the tachometer for 8 cylinder operation is a must.

I ordered the sensor from Digikey, part number CH416-ND. It comes with 3 wires to wire up- 12V, Ground and Output. The output goes to the tachometer pickup, which is set for 8 cylinders for an accurate reading. You'll need to solder a 2.4K ohm resistor between the 12V and the output wire you'll be all set. There's a link to the Datasheet on the product page, which shows the setup of the sensor. I don't have 10 posts yet to attach links, but hopefully this gives you another option, similar to user fte.
 
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