Quick Question About Cranking RPM

Going to do a compression test tomorrow, just wondering if my torque app will still show engine rpm with ESS disconnected? I want to log my rpm with torque to find out my true compression numbers

 

I believe but am not certain that the ESS is how the ECU picks up the RPM

So IMO no it won't, sorry

edit: actually I might be wrong mmmm I should go out and try tonight.

I was going to delete this post but ... I'll let it go as my...
sub'd

 

I'm pretty sure it uses the ESS. Mazda typically grabs it from there. It would be by far the most accurate method, I think everything else would have to be derived from other factors?

 

Yeah I read that you disconnect the ESS when doing the compression test to stop spark and fuel but it made me wonder if I would still get the rpm signal, ill find out for sure tomorrow

If it turns out that there is no rpm signal, would I just be able to pull my fuel pump fuse and crank the engine with ESS plugged in to see my cranking rpm?

 

Another problem might be that I think power to the OBD2 port is interrupted during cranking...

 

disconnecting the ESS will disable the Rpm signal.

however you can get cranking RPM before and after the test and just average it.

 

All you have to do is hold your gas pedal to the floor while you crank. There is no need to disconnect the ESS to test with Torque.

 

I've tried to log my cranking rpm with torque last week and it did work, but it only cranks for a second or less so it doesn't give me a steady cranking rpm.


How do I make it so I can just crank the engine without starting while the ESS is still plugged in? Just disconnect the fuel pump fuse?

 

I believe disconnecting the ESS kills the ignition.
Holding the pedal down or disconnecting the Fuel Pump fuse, kills the fuel.

 

So Holding the Gas Pedal down Fully while cranking kills Fuel? So I guess even when im doing the compression test I wont need to disconnect the ESS since I will be holding the gas pedal down fully

Good to know thanks

 

Pulling the fuel pump fuse is one way, although I'd honestly also disconnect the coils from the harness. During the test you will have 1 or more plugs that are no longer grounded, and a coil firing without a place for the current to go will fry fast. How fast? Unsure, but from other forums and other cars it can be as fast as ~10-15 minutes of firing time. Even if you are firing for less than that, no reason to put your coils through that.

Disconnecting the ESS kills both fuel and ignition WCS. The ECU needs it to know when to fire the injectors, and won't fire them without that information.

 

Check

I thought this was the case, but for whatever reason I changed my mind.
My mistake.

 

+1 well put! and/or i wish i wrote that

 

Good to know, Ok I will disconnect ESS when doing compression test and will do a quick cranking rpm check by holding the gas pedal all the way down before and after the compression test and average them like j9fd3s suggested

I would disconnect the coils but I would have to remove the intake and tube to get at them which is more work then I want to do, 5-10 seconds worth of cranking when checking for rpm hopefully shouldn't hurt my BHR coils ;/