# Help with mimicking VSS output for running odometer mileage up

Discussion in 'General Electronics Chat' started by jonathaned3, Aug 23, 2015.

1. ### jonathaned3 Thread Starter New Member

Aug 23, 2015
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First of all, I'll be honest. Most everything here is WELL beyond my skill set. I'm a welder/fabricator. So I'm looking for some help from you guys.

I'm working on an old Honda that has a new(er) speedometer transplant. When we first did the speedometer swap, we weren't concerned with odometer mileage accuracy. Now we've decided we would like to make the odometer reading accurate to the original. This means we need to run up approximately 40k miles.

Dissecting the speedometer/odometer to roll the miles forward is NOT an option due to the way it's mounted and the irreplaceability of the gauge housing itself. We don't want to risk it whatsoever.

Me being mechanically inclined decided we'll do it with a drill or a small motor hooked to the speed sensor. This works, but isn't ideal, and we can't safely spin the VSS for extended periods of time without constant supervision.

I would like to run up 40k miles electronically by bypassing the VSS all together and tricking the speedometer into thinking it's getting a signal and it's running 100-130mph. I need a way to supply a signal to the speedo for a few weeks straight.

Some info on the VSS based on my testing and research:

-output signal from VSS to speedometer is square wave (no clue what this means! But I'm sure you do)
-output VOLTAGE from VSS to speedometer ranges from 0-5v. 5v when the speed is zero, and somewhere closer to 0volts as speed increases. (Yes, 5v at zero and decreases with speed)
-Honda uses a 4k pulse per mile

So knowing the above, I figured I need to supply somewhere between a 111hz and 144hz signal to the speedometer (1/4,000*111*3,600 seconds per hour)

111hz = 100 mph
133hz = 120mph
144hz = 130mph

I don't want to spin the speedo faster than 130mph due to mechanical limitation. 130mph will get me to my target mileage in approximately 12-13 days.

I will be supplying the speedometer unit its own 12v power source, so no problem there. I just have to figure out how to emulate the VSS signal.

I know there's a way to do this, but I'm not exactly sure how. I need your help.

Will this do what I need? Safely - for up to 2 weeks straight? Looks like it'll generate in the frequency range I need, but I'm not sure about the voltage. How does the Honda VSS's output voltage of 0-5v impact the situation? Or does it even matter as long as the frequency is correct?

Thanks for any insight you guys can provide. Keep in mind I can't make circuits and things like you guys can, that stuff is way way way over my head, so I would need to be able to buy something or two to accomplish this! And preferably cheap!

I really appreciate you taking the time, and I look forward to your advice.

2. ### tcmtech Well-Known Member

Nov 4, 2013
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The simplest way to get a 120 HZ pulse train would be to take most any common older type of power pack that has a transformer in it and simply feed its rectified signal to the speedo input via a potentiometer so that the output voltage can be varied.

Without any smoothing capacitor the output will be a steady 120 hz and the potentiometer will allow you to fine tune the signal voltage to where the VSS system is happy with it.

3. ### dl324 Distinguished Member

Mar 30, 2015
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Can you post the speedometer docs you have? Don't see why voltage is used to indicate speed. Frequency should be sufficient; 4k pulses in an hour = 1MPH.

If a higher frequency is tolerated, you could advance the miles faster.

I Google for info and it seems that the ECU also stores mileage and having different values could cause legal issues if speedo reads too low.

The pulse generator you referenced operates from 5-12V so it won't work if you really need pulses near zero volts.

4. ### Alec_t AAC Fanatic!

Sep 17, 2013
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1,134
I suspect the 'near zero volts' is an average resulting from the sluggish response of the meter used. Could the VSS be putting out fixed width negative pulses (e.g. from an open-collector output stage) at a speed-proportional frequency?

5. ### tcmtech Well-Known Member

Nov 4, 2013
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My experience with vehicle speed sensors is most are reluctance based which without the correct type of meter will show incorrect output voltages being as the frequency goes up most will start to show a bit of DC biasing which screws up most meters accuracy on doing AC readings.

The only way I consider to be a reasonable way to check there signalis with a O-scope and visually look at the signal.

Odds are with the system you have most any signal with a good deal of voltage ripple to it would work just fine.

6. ### cmartinez AAC Fanatic!

Jan 17, 2007
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I think the same way... a scope reading would be in order, perhaps you could ask an electronics-savvy friend to do it for you

7. ### jonathaned3 Thread Starter New Member

Aug 23, 2015
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Thanks for all the feedback guys.

Most of what you're saying is over my head. I have no electronic savvy friends! That's why I'm relying on you folks.

The factory service manual for the car the vehicle speed sensor came from states that to test the VSS, you check DC output voltage with a DMM and should see between 0-5volts while spinning the front wheels by hand. I assume this is so most average mechanics can easily check it with average tools.

At rest, the DC output of the VSS is around 5volts measuring with my Fluke DMM. At 80mph (the fastest I could get the VSS to spin with a drill, output was around 2.5volts).

I was thinking too stupidly and used these findings as an aha moment. I was thinking I could just supply somewhere between 0-5 volts to the speedo to make it run. Boy did I feel stupid when it didn't work!

You guys correct me if I'm wrong, but I suspect it's not the "voltage" that's important. The voltage could be anywhere in the 12v DC operating range. The voltage is just a byproduct of the pulse/frequency and is just an easy way for a novice mechanic to test that the VSS is doing something, anything.

In other words, I'm not sure the speedo needs to necessarily see any "voltage" persay. It just so happens that the VSS will output 0-5volts when rudimentarily testing it - but more precisely, the speedo is looking for a certain pulse/frequency. I suspect the factory service manual has owners test for voltage rather than pulse/frequency because the latter is much more difficult for the average mechanic. In my case, I would need to see somewhere between 100 and 150hz to get my desired speed.

Do any of you see any reason why the item in my link won't work, assuming the DC operating voltage range is incorrect? I could supply somehow in the 1-5volt range to the device, which I assume would keep the output voltage the same but have the "correct" frequency.

How does voltage relate to frequency? Or do they operate irrespective of eachother?

If I'm too ignorant to even be making sense I apologize! I'm really quite out of my league with this stuff.

8. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,698
2,752
Don't worry, we'll try to help you one step at a time, if wecan.
Can you get your hands on a Fluke multimeter with a digital display capable of showing a waveform? This could be used in place of an oscilloscope

9. ### tcmtech Well-Known Member

Nov 4, 2013
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So basically I am just talking to myself here.

10. ### jonathaned3 Thread Starter New Member

Aug 23, 2015
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I didn't understand the first part of your first response (over my head). I'm a dummy in a smart mans game!

I see at the end probably any signal will work - do you think the device in my link will provide adequate signal?

It's cheap enough I may try it.

I cannot get my hands on a Fluke with the capabilities the other poster inquired about . I actually have the fanciest DMM in our tech group, which is sad.

11. ### tcmtech Well-Known Member

Nov 4, 2013
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What I am talking about is simply taking common 60 cycle alternating current and running it through a full wave bridge rectifier to get a DC output that has a high level of 120 HZ ripple to it.

For the most part to the VSS systems input that will look close enough to a typical speed sensors output to make it work.

cmartinez likes this.
12. ### jonathaned3 Thread Starter New Member

Aug 23, 2015
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Thanks for the replies everyone. I would like to update this thread for future searchability. I hate when I stumble on a relevant older thread with no conclusion.

I ended up ordering two more of the 555 pulse generators. I soldered some wiring extensions to the leads to make it less cumbersome for hookup and lower the chance of my accidentally touching something wrong which is what blew up my first NE555 pulse generator. I have it running off a plain old 12v car battery but I have the voltage reduced to 3.5v as the speedometer per the FSM only should be seeing 0-5v so I would rather be safe than sorry. It probably would have been okay to not reduce the voltage but I already had the voltage reducer so I figured why not.

I have a car battery feeding an LM2596 voltage reducer (\$5-6 on Amazon) that I had leftover from another project to reduce voltage to 3.5 which then powers the NE555 pulse generator which then feeds to proper frequency to the speedometer signal input.

So far this seems to have done the trick.I thought it had a needle limitation on the speedometer but on closer review I was actually able to get it almost clock all
The way around before touching the 0mph resting needle (don't know the exact term). Doing some math tells me that I've got the speedometer turning at 192mph. It's been running unattended for a few days now and has already racked up over 10k miles.

I should be at my target mileage within a week. I'll update when it's done.