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Vss Signal

9096 Views 13 Replies 6 Participants Last post by  z_power
K guys today i put the scope on the bmw's vehicle speed sensor output.

10mph = 30hz
20mph = 40hz
30mph = 55hz

My brain is mush right now so could someone figure if i'm right to say 1mph = 3hz?
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That is certainly not linear.

Where you experiencing wheel spin at the lower velocities?
That's odd. If it's a curve it could be:


Which you could test at 40mph. But that is still odd.
Yeh its weird. I'll try it at 40mph tomorrow. Could it be pwm and the scope just sees that as freq change?
yep agilent portable 2 chan scope. Not sure of the model number.
Try to capture the waveform or screen for future analysis :D
Ill likely need the same information soon for my conversion. :D
Which BMW?

The e30 uses a UAF2115 speedometer chip

Pin Function Connections
1 Flyback cathodes Connected pin 6, +V supply
2 Step M1 To odometer stepper motor
3 Pulse output To A pins on connectors (only)
4 Galv. curr set Resistor to ground, 75.0 ohm precision
5 Galvanometer out Stabilized w/ cap to +V
6 Positive supply +V supply, through diode+resistor

7 Current source diode clamped to ground, resistor
8 RC pulse width cap to ground, resistor voltage
9 Trigger input Resistor to ground, stabilized volt input
10 Stabilized +Vref To resistor
11 divider configure, connected to ground, div ratio 2^6
12 Step M2 To odometer stepper motor

This supplies the current, 1.15-1.5mA, to the magnetic reed switch at the differential. The switch is operated by the 9 vane magnetic chopper disk on the ring gear.

The speedometer constant is printed on the dial. It's something like "K=7784", which means 7784 pulses per mile (9 vanes * 864 revs per miles for the tires).
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Oh, and the "taximeter" output on the back of the speedometer has the same signal, just buffered.
Thanks. Its an e36.
New data:

10mph period = 31.6ms = 31.64hz. 17ms low , 16.8ms high

20mph period = 24.6ms =40.6hz. 13ms low , 11.6ms high

30mph period = 17.4ms = 57.4hz. 9.4ms low , 8ms high.

All speeds approx based on speedo reading. Pics of waveforms later. No numbers on speedo face.
The later cars have integrated body electronics, but still use the same switch mounted to the differential and slotted magnetic interrupter. I would be surprised if they changed the output signal.

Check the rotations per mile/kilometer for the specified tire size (look on, assume a 9 pulse per rotation interrupter disk, and see if the numbers work out. For the e30 with K=7784, 20MPH is about 43Hz -- pretty close to what you are seeing.

Remember that the speedometer is calibrated to always read high, both the offset (typically +3MPH) and linearity (+%5).

I would like to bring the fuel mileage to work and display the current on it.
The european version has as a display 0 to 30l/100km. So this could be useful to display up to 300Amps. This is enough for normal driving in my setup.
Does anybody know how it is controlled? Is it a PWM signal or directly calculated in the speedometer?
Elegancec, IIRC engine controller sends square wave signal to instrument cluster; frequency is ~16Hz I think, PWM% is proportional to opening time of fuel injectors. Then there's chip inside cluster which is doing calculation from rpm, %PWM and VSS signal so unless you develop algorithm for "fooling" this chip you will be better controlling needle's coils directly; it's just a (mili)ampmeter...
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