Hey guys I am finishing up my 4BT swap into my 1998 Ram 1500 ST. I wanted to keep the stock gauges so I decided to keep the PCM and just made a harness with what I only needed. That being said I am trying to figure out what signal is needed to get the Tach working. From what I can tell it comes from the CPS but not sure exactly what it needs to see.
Not only that but my battery is not charging and I believe this is because the PCM does not see the CPS signal and thinks the motor is not running and does not turn on the alternator to charge. If I am right this will help me kill to birds with 1 stone.
In the meantime I am installing an external regulator to at least get the battery charging. Any help would be greatly appreciated.

 

I'm not sure the specifics of your question, I would suspect you would need a tone wheel and sensor generating the same counts/pulses as the stock one. You could run into issues with lack of cam sensors and all that other stuff as well. I would be tempted to find a 98 Ram 12v Cummins PCM and change it out

 

I was actually just looking into that and the pin out for the 12v vs the v8. I will have to make my harness over again but it should not be that hard to do.

 

First off what motor is the pcm for?

 

5.9 v8 magnum

 

And you want that to run the tach? Not going to work. Pcm needs to be for the motor it's intended for. If it was for another 8 cylinder yes. Any other motor with different cylinder counts(4.6.10 etc) won't give you the correct rpm. If you ever installed a aftermarket tach, On the back there would be a switch for the motor it would be used for (4,6 or 8). This is assuming you have a crank and cam sensor hooked up.You MAY be able to fool the pcm in thinking it's a 8 cylinder. The crank sensor reads the open holes on the flex plate for the V8 motor. Not sure how it reads on the motor you are putting in. But it needs to see 8 not 4 inputs per one rotation to read correctly.

 

Thats what i was trying to do. Trick the PCM by using the harmonic balancer that the 12v uses. I just needed to get the correct signal. Maybe using a tone ring or machining the balancer to get the same count per revolution but also not sure about cam sensor if that needs to be there or not too. it looks like may just be easier to get PCM from 2500