stock '80 SC Speedo - square or sine wave?

[toddetch]

Hi all,

Since I bought my car last summer, the stock 85 mph speedometer has not worked properly, registering at about half the actual speed and mileage. The speedo has been checked and calibrated and it looks like only a few of the magnets in the transmission are registering. (That's a problem I'll address someday when I decide to get the transmission worked on...)

Between knowing that and just translating in my head to actual speed and GPS apps like Waze telling me the speed I've carried on. However, I know at least one other Pelican has successfully installed the Autometer GPS Speedometer so I decided to give that a try. I connected the power and ground to the power and ground for the clock and then the speed signal to the stock speedometer. I was easily calibrate per the instructions, but when I drive nothing happens.

I asked the manufacturer and they said:

We engineered this unit to work with AutoMeter speedometers. The output from it is a 12v square wave, typical to what you'd see on three wire hall effect style speed senders.

Your SC used a two wire, variable reluctance style speed sensor from the factory which outputs a sine wave signal which is A/C. The speedometer in your vehicle is probably not compatible with the D/C square wave signal coming from the unit.

Can anyone verify what they said, that my speedo is expecting a sine wave? If so, does anyone have a suggestion of how to convert the AutoMeter square wave to a sine wave? I'm a little surprised that it can move the speedo to calibrate but not while actually in motion if it is an issue with the type of signal.

Thanks for any help!

Todd

[brianlay]

The pickup I simply a magnetic reed switch which opens and closes, so I'd say the speedo is expecting a square wave. Also you said the calibration works which means you've got the right signal.
Sounds like the unit is not picking up a GPS signal

[toddetch]

That was my thought exactly Brian - it's been a long time since engineering school and I was never good at the analog stuff but to me the system of eight magnets inside the differential (I think) seems like it would produce a square wave.

If anyone knows for sure, it would be really helpful in pursuing solutions. The receiver is currently sitting on top of the dash so it should be receiving through the windshield no problem.

[fanaudical]

Autometer's response is not accurate. I believe the factory speed sensor is simply a reed switch which is sinking current to ground or otherwise triggering a counter circuit when a magnet passes. This would be "square wave-ish".

More than likely the sense of the GPS sensor (sourcing vs sinking) doesn't match the speedo input. This can possibly be solved by putting a solid state relay between the sensor and speedo. I will see if I can find more info for a suggestion.

[Dr J]

It is not a sine wave nor is the sensor VR. As others have said, it is a magnetic reed switch which closes and opens 8 times per wheel revolution. The output would be a square wave. In the tunnel in the back where you adjust the shifter coupler to the transmission, you will find a connector to the sensor in the transmission. One of those wires is grounded. If you cant figure out which just disconnect both and ground one of them. Take a 12v fused signal, put a resistor on it (like 1000 ohms) and connect the other end of the resistor to the second switch wire. Now take the signal between the resistor and the switch wire. Instant speedo 12v square wave.

My concern is that if some of the magnets in the transmission aren’t activating the switch you have a square wave that varies quite a bit in frequency within one revolution and this may confuse the autometer gage.

[toddetch]

Thanks @fanaudical, great idea. Any suggestions on the relay would be greatly appreciated!

@Dr J - the Autometer completely bypasses the speedo sensor from the tranny. It is a GPS speedometer sensor which is connected directly to the speedometer. So the issue I have is (apparently) how the Autometer GPS speed sensor sends its input to the speedometer. Admittedly I haven’t dug through the tunnel to check out the wiring, but when the mechanic looked at this last year, they told me they checked out the readings coming directly from the transmission and not all of the magnets were registering.

[brianlay]

Calibration pulse works...the problem is in the GPS unit.

[Dr J]

Originally Posted by toddetch

Thanks @fanaudical, great idea. Any suggestions on the relay would be greatly appreciated!

@Dr J - the Autometer completely bypasses the speedo sensor from the tranny. It is a GPS speedometer sensor which is connected directly to the speedometer. So the issue I have is (apparently) how the Autometer GPS speed sensor sends its input to the speedometer. Admittedly I haven’t dug through the tunnel to check out the wiring, but when the mechanic looked at this last year, they told me they checked out the readings coming directly from the transmission and not all of the magnets were registering.

Ahhh. I understand now what you mean. In my 79 the tachometer outputs a signal to terminal A at the speedometer.

[fanaudical]

I read through the instructions for the Autometer GPS speedo sensor and have a question.

You stated in your original post that you calibrated the speedo (presumably per Autometer's instructions). Did the Porsche speedo needle move during calibration?

[toddetch]

Originally Posted by fanaudical

I read through the instructions for the Autometer GPS speedo sensor and have a question.

You stated in your original post that you calibrated the speedo (presumably per Autometer's instructions). Did the Porsche speedo needle move during calibration?

Yes, that is what is confusing to me. I was able to calibrate it per the instructions, right up to 80 mph. But, when I back up and drive, nothing. As Brian suggests, I would think it is a problem with the GPS antenna, but their response to my question makes me wonder.

[fanaudical]

OK - Something else is going on here.

If you can get the speedo needle to move in calibration mode it should also move when driving (if the antenna is working correctly). That should be the same circuit and wiring. If you try calibration mode again, does it work? If not, then maybe something has failed in either the speedo or the antenna. Is the fuse to the antenna module blown?

[toddetch]

Thinking about it further, maybe I'll buy a little handheld oscilloscope like this or this and record the waveform from the GPS sensor and from the transmission. That would at least confirm whether or not the GPS sensor is receiving and sending a signal, and what each signal looks like.

[toddetch]

Originally Posted by fanaudical

OK - Something else is going on here.

If you can get the speedo needle to move in calibration mode it should also move when driving (if the antenna is working correctly). That should be the same circuit and wiring. If you try calibration mode again, does it work? If not, then maybe something has failed in either the speedo or the antenna. Is the fuse to the antenna module blown?

Good question on the fuse. I'll check (may be tomorrow).

[fanaudical]

A fast digital multimeter would probably suffice. I've got a 'scope but you'd need a long extension cord...

[toddetch]

Okay, I came down with something early this week so it took me a bit to get back to this. Attached is a video that I made using an oscilloscope (more on that later) to show the AutoMeter calibration and then driving with it, and driving with the stock speedo sensor. The oscilloscope probe was attached to the connector on the back of the speedometer.A few observations:

1. During calibration, you can see that the AutoMeter is sending a perfect square wave, and since the speedometer is moving to be calibrated, right up to 80 mph per the instructions, obviously the stock speedometer can handle a square wave as input. The AutoMeter support guy was wrong on that account.

2. Absolutely no signal coming through when I'm not calibrating, but just relying on the GPS sensor.

3. The stock speedometer signal looks pretty much like a square wave to me. Now, you can see it isn't a uniform square wave since some of the magnets are not registering, but the shape of the wave is square.

Now, going back to @fanaudical's original comment... You can tell from the waveform that the stock sensor definitely provides a sinking signal (getting pulled down to ground). He's probably right that the GPS sensor is sourcing - so driving the signal up.

However, since I am seeing no signal at all I'm thinking it is a problem with the Autometer unit in some way.

Any other thoughts are welcome!

Video: https://youtu.be/8IkX5XRyaiI

For those of you who might be interested in an oscilloscope for your car, this little thing is awesome:
https://smile.amazon.com/gp/product/B01MSJCUA7/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

Only $80 and really pretty easy to use...and I don't think I've actually used a scope since around the time Joseph Ketsler and I shut down the entire northern half of the Bell Northern Research building in Richardson, TX, circa 1995. Long story. Joseph was a little crazy.

[brianlay]

Exactly what I expected...good work!

There's something wrong with the unit, it should supply the same squarewave from the GPS receiver.

One other question, did you try it with the antenna on the roof instead of just the dash? It may not be seeing enough satellite signal. Otherwise get them to send another unit. If they want to argue, send them the YouTube link!

[toddetch]

Thanks - good suggestion. I’ll try that. I did already send them the link.

Originally Posted by brianlay

Exactly what I expected...good work!

There's something wrong with the unit, it should supply the same squarewave from the GPS receiver.

One other question, did you try it with the antenna on the roof instead of just the dash? It may not be seeing enough satellite signal. Otherwise get them to send another unit. If they want to argue, send them the YouTube link!