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Discussion Starter · #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.
 

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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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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.
 

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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.
 

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Discussion Starter · #6 ·
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.
Do you have any pictures or maybe some more in depth instructions on how you did that? For example, I’m not sure what you mean by the notches you made, any more details is appreciated for a novice like myself that’s learning as he goes.
 

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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.
By lead that "went to the coil" do you mean the tachometer connection from the coil? So 12v in to pickup then out to tach?
 

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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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Do you have any pictures or maybe some more in depth instructions on how you did that? For example, I’m not sure what you mean by the notches you made, any more details is appreciated for a novice like myself that’s learning as he goes.
The notches need to be deep enough and wide enough, 1/4 x1/4 or so to cause a dead spot creating a pulse. If the gasser motor taken out was a 4cyl then 2 notches and so on. The ABS (hall sensor) I used was only a 2 wire, it self grounded, one wire to 12v key on and the other to the tach lead that used to go to the ignition coil.
 

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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.
I have had success using the dakota digital tachometer converter with the stock CJ7 tach.

http://www.dakotadigital.com/index....t_id=128/category_id=694/mode=prod/prd128.htm

it essentially converts the W output on the alternator to a RPM signal that drives the stock tach
 
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