Speedometer Off.
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I think this is fairly normal. On most any vehicle. One of the benefits on the Raider of swapping the rear tire to the 240 was that it helped correct the speedo at highway speeds.
I went thtough this.....it's not easy to do.
Requires a scope and a signal generator
If you want to know how it's done, PM me, I'll walk you through the process.
Requires a scope and a signal generator
If you want to know how it's done, PM me, I'll walk you through the process.
They are allowed to read 10% high but 0 low by law. My Road Star reads about 4 mph high.
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Itd be awesome if you’d just explain it here so you don’t have to do it multiple times.
How fast of a scope, and how good of a signal generator?
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Basically the scope is used to obtain the frequency and form of the drive signal for the instrument (at the instrument)
The signal generator is used to apply an input (VSS) for the instrument on the bench.
I have a 99 Roadie that needed a better speedo, all I was able to find was a unit that was left over from a warranty claim that worked but was off by quite a bit.
There is a trim capacitor on the early Roadie's board for calibration. Most instruments (automotive and motorcycle) from the 90's have these, on later style clusters the calibration is carried out at the software level.
Correction factor is obtained by comparing GPS indicated speed to instrument indicated speed.
Requires knowledge of how to run a component on a bench.
I have a Snap-On Vantage Ultra scope and a Pico 4 channel setup and an AES signal generator.
Any scope suitable for automotive is plenty fast enough.
Don't remember exactly what waveform the roadie was as I did that repair over a year ago, most speedos I see run off of 5V 50% duty square wave.
Don't use raw signal (sine), must be the same as the buffered output.
Like I said this isn't easy,
The only reason I did the Roadie's speedo is because I have the equipment and work on 4 or 5 speedos a year.
A Speedo Healer is plug and play and much easier........
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Who wants their speedometer accurate anyway.
If you get pulled over by the cops then you don't have any excuse.
If you get pulled over by the cops then you don't have any excuse.
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212 Posts
Super consistent issue, especially with motorcycles. The reported speed is affected by the circumference of the tire which changes more on motorcycles than on cars so the reported speed has to be higher to accommodate the various stages of wear on your tires while staying in the 0-10 over reporting value. You'll be closer to accurate with new tires but as they wear, you'll be further and further off.
There are multiple ways correct this (Speedo Healer, signal generator, tire size change, final drive gear adjustments, just to name a few) but there will almost always be some variation unless you are always driven off GPS.
There are multiple ways correct this (Speedo Healer, signal generator, tire size change, final drive gear adjustments, just to name a few) but there will almost always be some variation unless you are always driven off GPS.
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8 Posts
ok. So a square wave generator with a variable frequency and a 20 mhz scope… but how do you figure out corrected output when it varies depending on speed?
I try to get them as close as possible around highway speeds. My Roadie was like that.
Most of the cars I work on, the owners are happy if the thing will just go down the road without having to be towed home.......a speedo that tells a smaller lie is just icing on the cake. Some work out well, some not so much.
The Snap on scope runs at about 6mhz, it does the job fine, sig gen will go up to 20khz.
Sometimes run into triangle or sine and odd voltages on 80' and early 90's cars.
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8 Posts
Exactly that.
Say your instrument reads 60mph, GPS indicates 55mph actual= 5mph overeport. I use the sig generator appy sig to the instruments input to bench run it to indicate 60mph, then using the trim adjustment bring the needle to 55mph.
I only use the scope to find what waveform the instrument needs to see so I can run it on the bench, also allows me to see if sig is clean.
I'm not altering vehicle's signal to the instrument, just the instrument's response.
Good to compare speedo/ GPS discrepancy at different speeds to see if the error percentage is consistent. If it isn't I concentrate on accuracy at highway speeds.
I only use the scope to find what waveform the instrument needs to see so I can run it on the bench, also allows me to see if sig is clean.
I'm not altering vehicle's signal to the instrument, just the instrument's response.
Good to compare speedo/ GPS discrepancy at different speeds to see if the error percentage is consistent. If it isn't I concentrate on accuracy at highway speeds.
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That makes a lot more sense. Thank you. I can build a signal generator with an Arduino and I have a, frankly, really nice Rigol scope. Using the test equipment wasn't the issue, it was using that equipment to fake a signal to the speedo that didnt quite make sense.
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