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davchar 08-22-2008 09:11 AM

VSS Input Voltage Requirements
Hi All,
I have built an MPGuino using the iDuino. The car that I am going to install the unit in is a 1991 Nissan Patrol (Safari) 4WD with a naturally aspirated 6 cylinder mechanically injected diesel. I have bench tested the VSS input on the MPGuino and the counter ticks up each time the input is pulsed 5v.

The manual that I have for the car states that if a multimeter is connected across the speed sensor in AC mode, a reading of 0.5v should be detected. Additionally there is an output from the speedometer which when I tested it with a multimeter seemed to pulse 0.05v DC as the vehicle moved.

The MPGuino did not detect any VSS pulses when connected to either the speed sensor or the speedometer output. It seems that these input are either not generating sufficient voltage or they are generating a signal type not expected by the gauge.

Does anyone have any suggestions on what I might need to do to get the MPGuino to interface with my car's speed sensor or speedometer output?

Thanks in advance,

dcb 08-22-2008 09:21 AM

Just from informal tests, it seems the atmega pins will signal a 1 on a rising signal that reaches about 2.5 volts and return to a 0 on a falling signal of about 2.2 volts.

You realize that we don't actually have a working diesel version as of this writing? I have been casting about for an old school mechanical diesel victim though.

The information we need to get from the car:
throttle position

There may be another speed signal, or you can use a transistor to amplify what you have there to TTL levels.

Throttle position, might repurpose a TPS from the junkyard or parts store.

rpm, maybe an infrared transmitter/receiver positioned so they can "see" the holes in the pump pulley.

Plus lots of patience while we figure it all out :)

davchar 08-22-2008 08:09 PM

Hi dcb,
Thanks for the reply. Would it be possible to use the 0.05v signal to switch a 5v feed on the board? I'm not really knowledgeable in the area of electronics but from what I do know and what I've gathered from google I assume I need a transistor of some sort on the input.

I'm aware that the MPGuino is not yet working on mechanical diesels but someone has to go first :). I was planning to go down the twin flow transducer route (the units from and then modifying the code to suit. I thought this route would be good as it will also allow the unit to work on turbo diesels for which TPS and RPM does not provide enough info.

If you'd like to give the TPS/RPM route a try, my car is factory fitted with a Tachometer and I am sure I can locate a TPS. I also had a fish around for information about using piezoelectric film for injection detection, I came across one patent but I'm not sure whether it would provide injection duration information or how well it would work in a high-vibration environment.

I've always wanted a gauge like this, thank-you for making the circuit and code available to all!


dcb 08-22-2008 11:04 PM

1 Attachment(s)
You can try something like this if the signal really peaks at .5 volts, any general purpose npn transistor will do, but the 2n series is pretty ubiquitous:

davchar 08-23-2008 04:23 AM

Hi dcb,
Thanks, I'll give the circuit a try.


davchar 09-21-2008 09:16 AM

Unfortunately the voltage produced by the speedometer output was not sufficient to trigger the transistor. I did some probing and found out that the output was at ground between pulses. I then found a web page that detailed how to use a transistor as a ground triggered switch (Using Transistors As Switches). Using this circuit I have been able to get the MPGuino to read pulses from my speedometer output.

I have done some preliminary calibration and it seems about 2500 pulses/kilometre works well.

Now to get some fuel usage information!


dcb 09-21-2008 09:39 AM

Sounds good :) Glad you've got speed/distance :thumbup:

The vsspulsesPerMile is probably 4104, it's a Nippon-denso thing.

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