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After using various GPS navigation & speedo apps on my phone, I've found that my FD's factory speedo has a significant error, and the odometer does as well - the speedo reads significantly faster than actual road speed (as compared to GPS), and the odo & trip meter will click off more miles than actually traveled, roughly 5% more. For example, if the OEM speedometer is reading 70mph, my actual speed would be about 65mph. Which is the exact opposite of what should be happening given my tire sizes - the 275/40R17 rear tires are 3.2% larger in diameter/circumference than the OEM 225/50R16 tires, so my actual speed at a OEM speedo indicated 70mph should be right at 72.2mph with those larger tires.
Not really sure if this matters or not, but the output of the Vehicle Speed Sensor (VSS) on the transmission is shared with the OEM dash speedo and my Link G4+ ECU. I did this by basically wiring the VSS in parallel with the 2 loads (OEM dash and a Link G4+ digital input), as that was the only way I could think of to share the sensor at the time I built my wiring harness. The VSS provides the Link G4+ with driven wheel speed, which is used for a number of features in the ECU. I was able to calibrate the Link to display the correct speed as compared to GPS speed and with mile markers on the highway (i.e., cruising at exactly 60mph it should take exactly 1 minute to cover 1 mile on a marked highway)
Perhaps the parallel wiring of the shared VSS sensor causes an impedance mismatch that is effecting the OEM speedometer & odometer accuracy? Is this an error that a guy like Micheal Gagne could calibrate out during a speedometer/odometer restoration job? I was thinking of sending him my cluster next winter anyway as preventive maintenance to replace all the old capacitors and other inevitable failure components.
The other option might be to revise my ECU harness so that instead of sharing the VSS sensor directly like I did, instead tap the OEM speed signal output that goes from the OEM cluster to OEM ECU, and then re-calibrate the Link to read that signal instead, which is derived from the VSS.
The other option might be to revise my ECU harness so that instead of sharing the VSS sensor directly like I did, instead tap the OEM speed signal output that goes from the OEM cluster to OEM ECU, and then re-calibrate the Link to read that signal instead, which is derived from the VSS.
i would be inclined to do this, although i also am skeptical that it would do anything.
the VSS is an (AC?) voltage, and it does go up with speed
i would be inclined to do this, although i also am skeptical that it would do anything.
the VSS is an (AC?) voltage, and it does go up with speed
Yup, the VSS puts out an AC voltage, it's basically a variable reluctance (VR) type sensor. IIRC, the FD Service Highlights says it puts out 8 AC pulses per revolution of the sensor, and it shows as a sine wave in the block diagram. Same diagram shows the "speed output" to the OEM ECU as a square wave, which gave me the idea of using that instead of going direct to the VSS as an input to the Link.
If the speed sensor was a scaled analog voltage you might do it the other way but for that sensor type, if both a reading consistently you won't gain anything by changing it, just the the ecu input vulnerable to a dash failure.
If you search speedometer adjustment on this forum (the thread linked above is good anyway) you will find the details required to trim the speedometer. Dash out, drill a hole in the side of the cluster so you don't have to strip the entire thing while tweaking it, adjust trim pot between runs with a gps. Be mindful a lot of speedometers seem to have about 1mph offset as well as scale offset across theur range so confirm scaling at both suburban and highway + speed is where you want it.
I see the thread I did on it is linked, that's the way to go. Hardest part is just taking the car apart to get to the cluster, actually doing the calibration is pretty easy.
After consulting my own wiring schematic for my Link ECU install, and the FSM wiring diagrams & Service Highlights, I realize that because I connected the (-) terminal of the VSS sensor to my Link ECU's sensor ground, pins 1A & 3E on the cluster are now connected together, effectively shunting part of the R-C circuit that feeds the comparator to ground (see Service Highlights diagram below). This won't matter for DC signals, but for AC it certainly does, especially as the frequency output of the VSS sensor goes up with speed.
So the first thing I'm going to try is to sever that connection between the (-) VSS terminal and my Link ECU sensor ground in my harness. That will be an easiest wiring mod, and if it works, the OEM speedo will be accurate again, and I won't need to adjust the calibration of the Link ECU speed input signal.
If that doesn't work, then the next approach is to do a simple wiring revision to my harness to tap the OEM speedo cluster output (pin 3E) that normally goes to the OEM ECU and use that as the speed sensor input to my Link ECU. Basically I'll be tapping pin 3E from a G/R wire that terminates in the OEM connector B1-01, (F) harness and feeding that to one of my Link ECU's digital inputs. From the Service highlights, that cluster output is basically a square wave, with the frequency of the VSS sensor divided by 2, which should actually work better for my ECU as a speed input. Fortunately I built my ECU harness with a combination of Deutsch DTM connectors and the OEM style connectors to do plug & play integration with the factory wiring via the OEM "X" connectors, so this will be a pretty simple wiring revision.
07-05-23, 08:56 AM
