Can someone tell me what inputs the ECM is using to create the signal sent to the tach? I have just swapped in a cummins 12V and the tach is not reading correctly. If I check RPM with a SCT X4 in the obd port, it is reading the correct value at idle, around 800. But the tach in the cluster is showing about 1400. The ECM must be using some other info besides crank sensor. That is what I have wired up and it appears to be sending the correct information.

 

Wait, what? You put a Cummins in it, and are still using the original V10 PCM?

 

Yes. I plan on using it to control the trans also, with a custom tune. Of course plans have been known to fail, but no one has been able to tell me why it will not work.

We have the correct RPM in the ecm to use for shifting, trying to figure out why the tach on the guage is not reading the same as the engine rpms in the ecm.

 

Was the tach accurate to begin with?

The tach, in this generation truck, should be driven directly by the PCM.

Just checked the EVTM and the tach goes directly to the PCM - so if the PCM thinks the RPM is correct, I'm not sure what else there could be.

Do you have both the camshaft and crankshaft position sensors emulated somehow?

 

My thoughts exactly. You put in a Cummins and you are using the computer from the V10 gas? How does that work?

 

The ecm is not controlling the engine. It is a 1stgen 12valve. All it needs is a fuel line and 12V to the fuel pump.
ECM will control all the other stuff and trans.

I believed that the PCM/ECM controlled the tach. A single wire going to it. But in most conversions, they put a 4 tooth ring on and bypass the ECM, going directly to tach and that works. So the ECM is taking the normal input (crank has 40 - 1 ring) and using that to create the 4 pulse signal used by the tach. Obviously it is using something besides just the crank signal. Since the X4 is displaying the correct value for engine RPM, that signal must be correct. ECM uses crank to get the RPM. But it is doing something else also in deciding what to send to the tach.

I don't have the cam sensor wired up. Could be the problem. Have seen conflicting info on what signal it generates. Some say it is a single pulse at TDC, others say different. Compounded by the fact that different places seem to confuse cam and crank sensors.

Just like references to ECM and PCM. Later trucks have separate ones but this has just one.