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Tach question

ak diesel driver

ak diesel driver

6.5 driver
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Working on my son's 89 k2500. The PO had taken out the tbi engine and swapped in a (I think) 70s sbc. I got all the gauges to work and it charges now so so far so good. I read online that I can swap a 91 instrument cluster and everything should work, my question is about the tach. Is it as simple as hooking up the white wire that used to connect to the distributor and have the tach work in the new cluster?
 
I would think the tach on them would work off the negative terminal of the coil. I don't think 89 used a crank sensor, just the pickup under the cap of the distributor. does it still use the HEI dist or the old style that uses the coil on the fender?

now that I think about it, 89 used a smaller dist with only a pickup inside with the rotor, the module was mounted on top of the intake and coil next to it. it should use the coil pulse for the tach signal.
 
to my recolection the early 90's and late 80's ECM's read the RPM only for sensors and timing adjustment, it didn't re-output it to the cluster. the signal was shared to both the cluster and ECM. I could be wrong though, I would have to find a diagram to confirm.
 
I know it might be a crazy thought, but if you have the signal wire for the dash tach available to temporarily run a length out to the engine bay, you can use the alternator same as our diesels use and see how it reacts. on our diesels the signal from the alternator is nothing more that it reading the pulses from the field.

I would be inclined to test it with both the alternator and from the coil just to see if it reads accurately without any modifications. I would have to study how the aftermarket old school tach's work where there are terminals or a switch on them for the number if cylinders the engine has, but I think it's nothing more than a set value resistor inline on the signal wire for the tach to read correctly. a higher value resistor would slow or lower the tach reading where no resistor would be it's highest it would show. if my thinking is right, it would depend on running without a resistor if the tach is too low, then it would take something like the ECM to give the right signal.

I'm assuming the current engine is not using any of the 89's factory wiring other than power and grounds? if the ECM is still there with the harness, you might be able to connect the origional harness wires to the coil and other signal wires which might allow to serve only to give the tach the signal it needs. just a thought.
 
I'm using as much of the original wiring as possible. According to the diagram there's a white tach wire from the distributor to the ECM. It would be nice if it ran to the cluster but IDK.......
 
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