engine rpm

F

FordJeepMan

NAXJA Forum User

Location

Virginia

• Mar 25, 2010

• #1

This might be stupid but how does the pcm determine engine rpm. Its not from the distributor or cam position sensor because if you unplug it while the engine is running it does not change anything.

 

Talyn

NAXJA Forum User

Location

Radford, Communistwealth of Virginia

• Mar 25, 2010

• #2

What year would be nice.

 

joe_peters

NAXJA Forum User

Location

Grants Pass, OryGun

• Mar 25, 2010

• #3

Well, a dirty little secret is that the CPS/CKP crank sensor is actually an engine speed sensor.

This is not talked about in polite company.

 

Shorty

NAXJA Forum User

NAXJA Memorial Lifetime Member

Location

on the beach in Ocean View, VA

• Mar 25, 2010

• #4

quick hijack-- when the CPS determines RPM and sends it to the tach, will the signal work for a 6cyl and a 4cyl? I guess what I'm wondering is if a dash cluster from a 6cyl can be used on a 4cyl and vice versa??

 

joe_peters

NAXJA Forum User

Location

Grants Pass, OryGun

• Mar 25, 2010

• #5

Shorty said:

quick hijack-- when the CPS determines RPM and sends it to the tach, will the signal work for a 6cyl and a 4cyl? I guess what I'm wondering is if a dash cluster from a 6cyl can be used on a 4cyl and vice versa??

Click to expand...

Interesting. On the Renix the IP tach picks up the signal from the negative side of the coil for RPMs. I believe on the 91+ its fed from the PCM.

Did you have a particular year in mind?

EDIT: looked up the 87-90 parts, and the tach is engine specific for the Renix, just like any aftermarket tachs that aren't adjustable, it must be for the correct no. of cylinders.

 

Last edited: Mar 25, 2010

Shorty

NAXJA Forum User

NAXJA Memorial Lifetime Member

Location

on the beach in Ocean View, VA

• Mar 25, 2010

• #6

Boo Hoo-- that makes life a little tougher-- that's what I figured, but I was hoping against hope that the tach was using a CPS signal that was converted to a tach drive by the ECU-- no dice:rattle:

 

xjintx

NAXJA Forum User

Location

Eddy, TX

• Mar 25, 2010

• #7

I have an 87 2.5L and switched to a gauge cluster I think from like a 96 but not sure. The tach works but does seem off. So probly from a 4.0L ... is there a way to confirm and maybe fix?

 

joe_peters

NAXJA Forum User

Location

Grants Pass, OryGun

• Mar 25, 2010

• #8

TACHOMETER
8350 4229 1 4 Cyl. 1987
8350 4228 1 6 Cyl.

Pull the cluster and check the part numbers.