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Old 12-11-2012, 05:44 PM   #1
K.Linchpin
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Default Speedometer and Tachometer both off

My 02 WRX wagon has a EJ207 (JDM ECU) and a 4.11 Final Drive (FD) ratio.

This may be a silly question, but does the Vehicle Speed Sensor (VSS) provide the Cluster and ECU with both RPM and Speed data?

With a 4.11 FD, my speedo reads ~65 mph when I'm moving at 70 mph. I have the 3.9:1 FD VSS in my transmission. Yes, I should get a 4.11:1 VSS.

What is weird to me is the fact that the Tachometer is reading ~250 RPM lower than what my ECU reads. I've been doing a lot of logging with Rom Raider lately and noticed this disparity recently.

Where does the ECU pick up RPMs from?

If I replace the VSS (Which I plan on doing), my measured speed will be reading higher than it is right now. And if the Tach uses the VSS as well, it too will read higher. For the Tach to read the same as the ECU, Either the tach needs to read lower or the ECU needs to read higher...


Thanks for your help.
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Old 12-11-2012, 10:04 PM   #2
lukeskywrx
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What MAF? -Speed Density!

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The physical Tach rarely reads dead on and it was never ment to be 100% accurate, although 250 rpm is more than I have ever seen on mine. My car reads a little over 100 RPM high.

RPM comes from the crank angle sensor right in the front of the motor, VSS comes from the tranny output. Are your wheels and tires significantly different than the factory ones that came on the car the 4.11 box is from? Different rolling diameter and slight speedo inaccuracy could easily make up the 7% difference you see, again your speedo is not calibrated, it is just an approximation.

I think you are getting worked up over nothing, Trust the direct ECU data not the display cluster.
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Old 12-11-2012, 10:28 PM   #3
K.Linchpin
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I'll check more carefully the next time I'm logging, but I'm pretty sure I saw the tack at 3250 when the laptop read 3000 rpm...

OD of tires is stock for a wrx.

Innards of trans are from a UK 95-96 Type R trans w/a 4.11 ring and pinion there to replace the 4.44. VSS is from 02 USDM WRX.

Quote:
RPM comes from the crank angle sensor right in the front of the motor
Maybe my USDM WRX cluster is calibrated differently than the JDM STi ECU?

Quote:
Trust the direct ECU data
I would certainly think the JDM ECU would play nicely with the JDM motor.

Anyways, thanks for the answer!

I'll verify my observation.
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Old 12-11-2012, 11:34 PM   #4
lukeskywrx
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What MAF? -Speed Density!

Default

The 02 wrx has a 3.9 final drive and a 3.54 rear diff so the 4.11 is not going to be too far off.

I dont think you are getting it. The speedo and tach are not officially calibrated in any way, they are just estimates for the driver. I am sure they test for some accuracy but they range and might be within 5 or 10% of the target value. your needles could be slightly off or the stepper motors are old who knows. Reality is everyones tach and speedo are slightly off the true values, most people just never know or care.

The point is that 250 RPM or 5 MPH dont really matter in the big picture. The car does not care what the cluster says, they just display the signal sent to them. All that matters are the values the ECU uses for its calculations, the ones you log in RR.

also UK WRX used 205/50s, the US is 205/55 so that is 864 vs 837 rev per mile so you start adding things up like that and your speedo being off not that hard to understand.

Last edited by lukeskywrx; 12-11-2012 at 11:42 PM.
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Old 12-15-2012, 02:50 PM   #5
Haelan
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The Crank Position Sensor provides your RPM readout. The Vehicle Speed Sensor provides speed feedback. If you changed the gearing not much you can do about the speed discrepancy unless changing the VSS as you mentioned.

The CPS should be pretty accurate as the ECU counts the pulses directly from the sensor. Try cleaning it--bits of metal stick to the magnet.
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Old 12-16-2012, 11:44 AM   #6
K.Linchpin
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RA gears 1-4 and 411 FD so quite different from stock WRX trim. But I'be been running ra gears forever with no issues tune wise. Changed to the 411 this past fall (a 5% difference).

Rpm's though different between rom raider and physical tach, produce a very clean signal.

Thank you for posting. Good to know.
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Old 12-16-2012, 11:51 AM   #7
SlowTrackImpreza
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mine used to do this, when the motor is idling, does the tach have a slight flutter to it?
I had to step up the voltage for the circuit in order to read right, always read low until i did this, I just ran a 10k 1watt resister tapped into the signal wire going into the cluster then hooked it to a 12v switched power source. now keep in mind this was a 2000 cluster. i dont know your voltage requirment.
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