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Discussion Starter · #1 ·
So, I'm just about finished pouting about my XJ, and mostly convinced NOT to do another OM617a. I am also convinced that decent, somewhat inexpensive, used B3.3Ts do not exist in my section of reality. That leaves me with the 4BT as the next best option to replace my OM617. Having gone through what I've already gone through, I know most of this drill. One thing I never figured out is my tach. I no longer have my PCM, nor do I really want one. My question to the brilliant minds on this boards is, how do I drive my stock tach? I can get a tachometer signal modulator device, but how do I future out how many pulses I need for my stock tach to match the reading the engine should give? The Jeep trigger wheel has 12 teeth on it. This doesn't necessarily mean the tach takes 12 pulses per rev as input from the PCM, if it even uses it. I have a 1991 XJ with the full gauged cluster (6 gauges). I would prefer to not use the diesel tach thing. Getting 1 or 2 or 4 pulses per rev off a 4BT should be rather straightforward, I just don't know how many to send in. My gut instinct is 3 per rev since it is a 4 cycle 6 cylinder.

Thoughts guys?
 

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If you didn't change your injection pump settings then the factory recommended idle would be 700-800RPM or so i found online. Now once you got the signal modulator hooked up you can figure out how many impulses would give you a reading within those numbers. It makes sense in my head but i'd ask for a second opinion:)
 

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Discussion Starter · #3 ·
Not a bad idea. If a friend would let me borrow one of those diesel mini tach things, I could play with my output signals. Hmm..

Anyone actually done anything with the XJ tach? Is the 6 cylinder tach different than the 4 cylinder tach? That would be telling.
 

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You need to find out how the factory gets signal if it is for example 20 teeth on a plate on the flywheel of the crank shaft, then you need make a setup on your crankshaft pulley or better your dampner. If it works off spark fire and is set for 6 or 4 or whatever then make it read with magnets and magnet reed switches on your dampner.
 

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Discussion Starter · #5 ·
So just to follow up for future swappers..

Dakota Digital makes an array of signal pulse modulator boxes. They can be used to basically do just about anything with a pulse based signal. The idea would be to run a w-term alternator rig and get a Tiny Diesel Tach (fits over a diesel injector hose). Use the tiny tach to ensure we know the current engine rpm. Modulate the w-term signal until it matches the engine, and bam, you have stock tach running a 4BT. Rev it up and down the band to ensure the OEM tach is accurate throughout.
 

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Discussion Starter · #7 ·
It's an older XJ, and a non-bus type cluster. So yes, the plan would be what I outlined above. Thank you for your reply. It will highlight another variable for future swappers. This site is by far the best for swap info. I got plenty of help, and like to give back.
 
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