VIDEO How to test a Speedometer

[maxman]

Hi Everyone,

So after nearly a year of intermittent speedometer operation, I decided to get to the bottom of it. It's been very unclear whether the issue lies with the speedometer itself, or the sensor. On a hour long drive, the speedometer will cut in and out several times. So, I decided to pull it from the car and bench test it. Well, more like kitchen table test it. That way, I could run it for a couple of hours and see if it every cuts out.

Here is a video showing how I did the test, along with a schematic. If you end up doing this same test, PLEASE be careful and beware of the voltages at play, it is very possible to blow something up.











I've heard that the sensor in the transmission sends 8 pulses per wheel rotation. My 225/55/16 tires have an overall diameter of 25.7 inches, which means 1 rotation covers 80.68 inches of ground. At 60 MPH, I'm covering 63,360 inches (5,280 x 12) per minute, or 1,056 inches per second. 1,056 / 80.68 = about 13 wheel rotations per second, or 104 pulses per second

[Flieger]

Gee, I thought this was going to have a video of an Autobahn run.

[Flieger]

[tabasco]

I like it !

Which generator app did you use ?
How did you choose the transistor ? How did you set the volume on the iPad ?
Finally, I can't read the component names on the drawing.

-Guillaume

[rattlsnak]

you've just convinced us all to send you OUR speedos if we have a problem.

[jdbunda]

Very nice - so did you find the problem?

[maxman]

Well, I now know that the speedometer is not the problem. I've tested it for hours and it has not failed.

Time to look at the sensor.


tabasco,

The app is called Tone Gen Pro, it was 99 cents.
I just used an NPN Transistor that could handle 12 volts. It's a 2N3904.
The writing on the speedometer says "viewed from rear" for pin reference
The two wires on the bottom are labeled "Audio Hot (Tip or Ring)" and "Audio Ground (Sleeve)"

[village idiot]

interesting, this couldn't have come at a better time. I'm having problems with my speedo now and have been wracking my brain on how to test it. Thanks!

[jrumiano]

Good post! If you don't have an iPAD, a function generator is the way to go.

Something like this.


2 MHz Function Generator / Frequency Counter SFG-1002 | eBay


Good luck.

John

[ewave]

Originally Posted by maxman

Well, I now know that the speedometer is not the problem. I've tested it for hours and it has not failed.

That's not quite true. What you know is that your speedometer is reliable on a bench in an office with a stable voltage power supply.

1) You refer to the power supply as unregulated, but in your application where the current draw variation is almost zero, the output will be quite well regulated probably to within 50 mv. You want to at least be able to run the speedo from 10V to about 16V for testing purposes. (Hook an oscilloscope up to the supply voltage.)

2) Your speedo is subjected to considerable vibration in the car. If you are looking for an intermittent electrical problem, you need to simulate vibration of some kind. In situation like this I usually tap the device under test with the handle of a screwdriver. This works really well to find lose flaky connections.

3) In the car the speedo is also subjected to a considerable temperature variation. To simulate this a hair dryer works quite well. For cold testing the freezer is my choice.

[dshepp806]

Originally Posted by ewave

That's not quite true. What you know is that your speedometer is reliable on a bench in an office with a stable voltage power supply.

1) You refer to the power supply as unregulated, but in your application where the current draw variation is almost zero, the output will be quite well regulated probably to within 50 mv. You want to at least be able to run the speedo from 10V to about 16V for testing purposes. (Hook an oscilloscope up to the supply voltage.)

2) Your speedo is subjected to considerable vibration in the car. If you are looking for an intermittent electrical problem, you need to simulate vibration of some kind. In situation like this I usually tap the device under test with the handle of a screwdriver. This works really well to find lose flaky connections.

3) In the car the speedo is also subjected to a considerable temperature variation. To simulate this a hair dryer works quite well. For cold testing the freezer is my choice.

BINGO! There's still more work to do.....

You may want ID the adjustment for calibration, for your viewers,..as most people are in need of THAT!

Thanks for the time and effort.

Best!

Doyle