I have searched everywhere and cannot find an answer. When I am using a GM Tech 1 scanner (or, currently, EBL P4 Flash) the speedometer (in the ecm; not on the instrument panel) reads about 8 mph slower than the actual speed of the car. Why would this happen and is there any way I can correct this?

 

Both the stock $8D code and the P4 code have a VSS PPM parameter to dial it in. For the EBL P4 (affects both magnetic & optical inputs):

VSS - Pulse Per Mile

And in $8D (magnetic input only):

L8015: FCB 230 ; PULSE/MILE ROAD SPEED CONST (MAG SENSOR)
; ARG = 921600/VAL

I trust that you verified the dash speedometer in some manner.

RBob.

 

yes, I did check the speedo with a gps. I was sure the ecm reading was off since both ebl and tech 1 scan tool were reading the same. funny thing is it was off in the factory ecm and off in the replacement ecm I just got from you. might be something up in the tranny?
anyway; I see the above places where I fix it, but can you give me some guidance on what this is actually doing and where I might start? thanks.


ps: so far, the EBL is AWESOME. Thanks for the support you have given me so far.

 

The listed parameter sets the pulses per mile to expect from the VSS. The ECM uses that to calculate the vehicles MPH, lowering it will cause the ECM to calculate a higher MPH.

RBob.
.

 

ok. I will check that today. Just one more question...would it be typical to have to adjust this? I just wonder why both ecms would be reading incorrectly?

 

My guess is that the speedometer if actually off and the gears in the trans are not the correct ones. But in such a way that the speedometer reads correctly.

This would leave the ECM MPH incorrect.

RBob.

 

crap. Now that I think about it; when I first ought the car the speedo was off and I had it calibrated. Now, this confirms there are probably the wrong gears in the trans. Thanks for the info

 

I wouldn't worry about it, just tweak the VSS PPM in the calibration so that the ECM also has the correct speed. The only other area it affects is the cruise control. But that doesn't matter as it goes by a relative speed not absolute.

Note that the ECM's VSS PPM parameter doesn't affect the speedometer, just the ECM.

RBob.