Electronic Speedometer / Odometer Calibration.. again

[thulsadoom]

Hi all...

I've been researching the calibration of electronic speedos on the boards, but still have a few questions. I'm going to get my hands on a pulse generator and plan on playing with the potentiometer, but figured I'd ask first.

Situation: I replaced my 85MPH speedo with a 150MPH one. It works, but reads high.

Issues:

1) It seems to read proportionately higher at lower speeds than higher speeds (reads about 30 when doing 20, but only 70 when doing 65). Does this graduated situation sound normal?

2) The odometer seems to be going faster than it should. I've heard that adjusting the speedo has no effect - How does one calibrate the odo??

3) Pulses for testing... someone recently posted that "the mechanic speedometer expects about 810 rotations per miile... produces 8 pulses per rotation..." So, if I send it 810 * 8 = 6480 pulses per minute it should be going 60MPH.
a) does this sound right?
b) what kind of voltage should the pulses be?
c) which leads get what signals (there are four)?

Thanks!
-John.

[jdowty3]

I stopped by a local Speedo repair show in Dallas to see about getting mine repaired last week. The guy tested it free and said it work fine. He also checked the speed calibration and found it was 8 MPH fast at 60 MPH. I think he would have adjusted it free if I had already drilled the hole to the adjustment screw. Turned out the reason it wasn't working in the car was I was only getting 2V at the power terminal instead of 12V.

[Early_S_Man]

John,

The 'formula' you cited/quoted is correct, but only for tires sized at 810 revs/mile or so ... 225/55ZR-16 or 245/50ZR-16 ... to use your own tire size, you would need to look up the specs at Tire Rack and substitute in the formula below for the 844 revs/mile I used.

For example, Yokohama A008P 245/45ZR-16 tire have 844 revs/mile per the Tire Rack spec page ...

The sensor for the electronic speedos is a simple pair of 'dry' [no Voltage present] magnetic reed contacts ... which can be simulated by a reed relay driven at 120 pulses per second by a wall transformer and full-wave bridge rectifier, which equals 7200 pulses per minute.

Using the A008P example ... 844 revs/mile x 8 = 6752 pulses/minute for 60 mph calibration. To use the 120 pps source, for the A008P's ... you would adjust the speedometer to: 60 mph x 7200/6752 = 64 mph.

The following diagram shows the speedo circuit and has the sensor connected to terminal 31b & Gnd.

[thulsadoom]

Early_S_Man, thanks for the diagram and explanation.

I didn't find the chart at tirerack, but google's first link was better - the quite obvious formula itself: distance divided by tire circumfrence.

Tire: 225/50-16 (rear), 205/55 (front)

Tire Width (mm) = 225mm * 50(%) = 112.5mm
Wheel width (mm) = 16in. * 25.4mm/in = 406.4mm
Diameter (mm) = 2 * Tire Width + Wheel Width = 631.4mm
Diameter (in.) = 631.4mm / 25.4mm/in = 24.86in.
Circumfrence (in.) = PI (3.1415) * Diameter = 78in.
Inches / mile = 5280ft * 12in/ft = 63360in.

Circumfrences / mile = 63360in. / 78in. = 812

This all seems to make sense except that it comes up with 817 (vs. 844) for your tire size (?!).

Anyhow, that would then be 812 * 8 = 6496 pulses per mile

Using the gadget you mentioned...
6496pulses/min / 60mph = 7200pulses/min / Xmph
Xmph = 60mph * 7200 / 6496 = 66.5mph

So, I hook up:
1) + terminal gets 12V
2) A terminal gets nothing
3) 31b terminal gets one end of pulse
4) - terminal gets both one end of pulse and ground

... And then I would twist the potentiometer until it read 66.5mph.

I'm not sure how you can have signal without voltage, but I'll ask my satellite-building housemate to explain how that gadget you propose works.

Does all this make sense??

Thanks again.