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Discussion Starter · #1 ·
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's odd. If it's a curve it could be:

F=0.03*S^2+27

Which you could test at 40mph. But that is still odd.
 

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Discussion Starter · #4 ·
Yeh its weird. I'll try it at 40mph tomorrow. Could it be pwm and the scope just sees that as freq change?
 

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Discussion Starter · #6 ·
yep agilent portable 2 chan scope. Not sure of the model number.
 

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Which BMW?

The e30 uses a UAF2115 speedometer chip
http://users.tpg.com.au/cobra30/images/R31/uaf2115.pdf


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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Discussion Starter · #11 ·
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.
 

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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 tirerack.com), 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).
 

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Hello,

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?
 

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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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