Subpage of MembersPage/MarcellGal/PowerAndTraction
Overview
The dash wheelspeed has been junk since I have the car, mostly ~0 (showing some activity but showing usually much lower than real speed), later speedo's been staying at 0 permanently.
Now I have the VEMS v3 ECU logging the wheelspeed to SD-card, and [Display-only Round] showing the ECU wheelspeed in digital format
- among other runtime data, the wheelspeed is sent over the RS232 in [AIM format], besides wheelspeed: lambda, MAP and EGT are also displayed in my setup (using 2 display-only rounds).
Wheelspeed Sensors overview - some more info below
Audi-200 (1989) has at least 5 wheelspeed sensors:
- transmission front-left shaft (3-pin AMP connector, but only 2 pins used with a reed-relay between pin2,3).
- 4 high-pulsecount ABS sensors (VR ?)
I plugged in [PS2 wheelspeed cable] to the PS2 connector of my old 3B ECU with (using 2 wires: pin2,3 of 3pin injector connector for the audi transmission side)
- it worked the first time
- calibration: 112 Hz (at 100 kp/h) with only rising edge: 27.7 m/s / (112 1/s) = 0.248 m (sounds like 8 pulses: 0.63m*pi / 8). Verification needed.
- After the winter the ECU-wheelspeed showed mostly 0 or 255.
- We suspected firmware problem first (fw was upgraded a few times), but on bench all was perfect.
- We installed a 1uF SMD (0805 size) capacitor in the DSUB9 of the wheelspeed-cable between the Base and Emitter of the BC817 (sot323 SMD) NPN transistor.
- Soldering was easy: there was provision for a 0805 component. The 1uF * 2k7 input pullup = 2.7msec which limits the max freq to appr 350 Hz ~315km/h (the top-speed is below 290km/h anyway). 220 nF was almost enough, but there was still some noise. The 1uF seemed to fix it perfectly.
Calibration
Anyone knows the number of pulses per wheelrot ? 8 or 9 ?
- since 100 km/h = 27.8 m/s
- If wheel circumference is 2m, and 8 pulses per wheel rotation, than 1 pulse = 2m/8 = 0.25m
- so configuration in vemstune 27.8/ 0.25 = 111 (Hz / 100)
With GPS hooked up to notebook, (and configured in VemsTune) the GPS is sometimes higher, sometimes a bit lower than the wheelspeed (GPS lags a bit behind). Will need to get this (and gear calibration) straight on clear roads.
Brainstorming
I'm also thinking of lighting up a "stayaway" light (different color than the red shiftlight) in the speed ranges:
- 72..75 km/h (with limit=50km/h they pull your licence above 75km/h)
- 86..90 km/h (with limit=60km/h they pull your licence above 90km/h)
- 100..105 km/h (with limit=70km/h they pull your licence above 105km/h)
- 114..120 km/h (with limit=80km/h they pull your licence above 120km/h)
- 128..133 km/h (with limit=100km/h they pull your licence above 133km/h)
- 161..172 km/h (with limit=130km/h they pull your licence above 172km/h)
This would help prevent serious speeding while allowing some slight speeding.
Gearbox wheelspeed sender ("geber" in German :-)
I found the connector that should go to the transmission speedo output (we already have the backup lights hooked up properly and working ;-)
The 3pin female connector has
- pin1 black-blue +12V (when ignition is on, fuse-relay panel pin B15A fuse#12) supply for HALL sensor
- pin2 brown-red pulled up by the car with 100 Ohm (strong pullup!)
- pin3 brown: GND
My 6 gear audi transmission sensor seems to have a simple reed-switch between pin2-3. It's a passive device, does not need supply on pin1 (with the [PS2 wheelspeed cable] I did not connect any supply).
This matches the [fig09.pdf instrument panel schematic] perfectly.
- the outer housing (shell) of the 3pin female connector is broken off, the inner part keeps the 3 receptacles together but does not guarantee perfect mating (also, one has to be carful with the direction!)
- when I pushed the connector on, and drove around with the car, there was some speedometer needle activity. It was fluctuating, not stable at all, which I blamed on the half-broken connector... but something else might be broken here, read below...
The speedo sender in the transmission
- only has 2 pins in the 3 pin connector. Pin1 is missing.
- it seems to be a reed-relay (same type as in your bicycle speedometer), that does not need +12V supply. Obviously the same harness is used for older (or other) sensors that have HALL sensor inside, these do need the +12V supply
- when pushing the car around (in neutral gear), the Ohm-measurement between the 2 pins flip from near 0 Ohm (1..2 Ohm) to near infinity (>2Mohm)
- that's what you expect from a reed-relay
- it seems to give more than one pulse per wheel rotation. One is a bit reluctant to do this measurement properly when pushing 1500kg back and forth. From some internet page (todo: find again) I remember 1670 pulses per mile (isn't that the ABS sensors ? No, the ABS must be much more, sg. like 20..100 pulses per wheel-rotation). 225/60 R16 tyre circumference is (16*25.4 + 225 * 0.6 * 2) * pi = 2125 mm (is this correct?). 2 pulses per wheel rotation would be 1600 / 2.125 *2 = 1506 pulses per mile ... more measurments needed to find out the truth
- duty is obviously much "higher" than 50% (the resistance is 0 most of the time, rarely infinite - obviously infinite when the magnet is close)
Maybe I have to hire a car-electrician to disassemble the instrument cluster, and do what [SJM] suggests. I've seen Scrambled Auto Check "OK" display recently, which is another symptom of instrument cluster solder-over-wear (Flash Gordon was there in silver underware).
Frequency signal to the brown-red pin2
- generated wav file with ElectronicDesign/TriggerSignalGenerator, using c004 100-100-100-100-200-200-200-200-300-300-300-300-.... 7800
- using a recent signalgen, the secondary trigger pulse was 50% duty. Earlier, it was narrow. I look at the wavfile (and play them) in audacity (a nice program, works in linux and windows)
- the secondary trigger output (notebook soundcard 3.5mm jack speaker output, right channel) was connected through 1kOhm to the base of a small NPN transistor (BC817 sot23, but almost any type should work), the base was also pulled up via 10k to +5V (this should not be necessary as the soundcard output is several Volts, opens the transistor easily)
- the NPN output was directly connected to the brown-red pin2
- without wav-file played, I measured appr 7.8V DC voltage
- with wav-file played, I measured appr 3.9V DC voltage (makes sense, if the NPN pulls down the signal to 0V 50% of the time)
- To my surprise, the speedometer needle didn't move away from the resting 20km/h
- no, I didn't forget to have ignition on
- maybe it's sensitive to the duty of the input signal ?