Digital speedo error - is calibration possible?
12-22-2007, 07:05 PM
#1
Rennlist Member
Thread Starter
Digital speedo error - is calibration possible?
Hi,
1990 S4 auto with 235/45/17 tyres on 17" cup 1's.
My analogue speedo is overreading 10% at legal limit. Digital is overreading 6% at the same. Given the larger tires & the fact the its digital I would not expect this to be the case. Did Porsche stuff up the maths when they programmed the digital sppedo to convert tire revolution rate into speed or is there a way to calibrate this to read correctly. I have found nothing in the WSM or through a search on rennlist - other calibration q's appeared to be related to the analogue speedo's.
Many thanks,
Myles Garmonsway
1990 S4 auto with 235/45/17 tyres on 17" cup 1's.
My analogue speedo is overreading 10% at legal limit. Digital is overreading 6% at the same. Given the larger tires & the fact the its digital I would not expect this to be the case. Did Porsche stuff up the maths when they programmed the digital sppedo to convert tire revolution rate into speed or is there a way to calibrate this to read correctly. I have found nothing in the WSM or through a search on rennlist - other calibration q's appeared to be related to the analogue speedo's.
Many thanks,
Myles Garmonsway
12-22-2007, 07:35 PM
#2
Race Director
235/45-17 is a 25.3" tall tire, so your speedo will be off by 4.1%...
http://www.miata.net/garage/tirecalc.html
Try this calculator to help figure out tire comparisons
On the calibration.....I dunno.....but I think the digital "might" be adjustable....
http://www.miata.net/garage/tirecalc.html
Try this calculator to help figure out tire comparisons
On the calibration.....I dunno.....but I think the digital "might" be adjustable....
12-22-2007, 07:40 PM
#3
Supercharged
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On the 1990 MY, each corner has a wheel speed sensor. It could be that the digital and analogue received their speed pulse from different corners.
12-22-2007, 09:36 PM
#4
I have checked the Digital Instrument Cluster "Diagnostic of the Instrument Cluster" technical manual, the electrical drawings and other technical manuals that I have and I could not find any reference for adjustments of the analogue nor the digital speed read outs.
The speed sensors at the four wheels are connected to the EKZ - CPU, LEH - CPU, the Cruise Control-CPU, PSD-CPU, The ABS as well as to the speed input at terminal 6 plug 4 of the digital dash. Unfortunatly I have no schematic drawing for the CPU or PCB installed in these units (I don't believe Porsche produce any drawing for external use), so unable to determine whether there are adjustment points available.
On my car analogue and digital speed are the same, however the actual speed is not correct, the speedo reading is approximalely 8 to 9 km/hr lower than the true over the ground speed, checked by radar and GPS with the correct recommended tyre sizes on 16 inch rims.
Usually speedometers are in accurate due to tire wear - reduction in the outside circumference of the thread and the mechanics involved, however, I have heared and read of inaccuracies of up 10% in some speedos. In my state the law only allows a speed differential of plus 3%, so speed cameras are a good revenue raiser for the local government.
Sorry I could not be of anymore help, however, Alan, our resident electrical and electronic boffin may have some better information that may assist.
Tails 1990 928S4 Auto
The speed sensors at the four wheels are connected to the EKZ - CPU, LEH - CPU, the Cruise Control-CPU, PSD-CPU, The ABS as well as to the speed input at terminal 6 plug 4 of the digital dash. Unfortunatly I have no schematic drawing for the CPU or PCB installed in these units (I don't believe Porsche produce any drawing for external use), so unable to determine whether there are adjustment points available.
On my car analogue and digital speed are the same, however the actual speed is not correct, the speedo reading is approximalely 8 to 9 km/hr lower than the true over the ground speed, checked by radar and GPS with the correct recommended tyre sizes on 16 inch rims.
Usually speedometers are in accurate due to tire wear - reduction in the outside circumference of the thread and the mechanics involved, however, I have heared and read of inaccuracies of up 10% in some speedos. In my state the law only allows a speed differential of plus 3%, so speed cameras are a good revenue raiser for the local government.
Sorry I could not be of anymore help, however, Alan, our resident electrical and electronic boffin may have some better information that may assist.
Tails 1990 928S4 Auto
12-23-2007, 01:08 PM
#5
Drifting
For the digital clusters on 89'-90' cars, the speed signal is from the differential speedo pickup sensor. Later cars with the digital cluster get the signal from the ABS/PSD controller via a speed converter relay.
For tire size changes on all 89'-95', you could install a electronic speedo recalibrater between the digital instrument cluster and the speed signal. This will compensate for changes in tire size. Similar to this product should work...
http://www.blackrobotics.com/yb_home.htm
This assumes the speedo calibration is correct throught the full range of the gage. If your digital cluster's speedo off by 10% over the entire range then you could either install one of these electronic recalibrators or reinstall the speedo pointer to the correct position. Otherwise you might have a problem that can't be fixed without replacing the cluster's circuit board, as there are no technicians that can repair the electronics of this instrument cluster.
For tire size changes on all 89'-95', you could install a electronic speedo recalibrater between the digital instrument cluster and the speed signal. This will compensate for changes in tire size. Similar to this product should work...
http://www.blackrobotics.com/yb_home.htm
This assumes the speedo calibration is correct throught the full range of the gage. If your digital cluster's speedo off by 10% over the entire range then you could either install one of these electronic recalibrators or reinstall the speedo pointer to the correct position. Otherwise you might have a problem that can't be fixed without replacing the cluster's circuit board, as there are no technicians that can repair the electronics of this instrument cluster.
