Speed sensor for aftermarket ECU's

[nreed]

I have noticed that there is some confusion out there when it comes to easily interfacing a speed sensor to a newer technology ECU like Megasquirt, AEM, ECUMaster etc. Here is an explanation of the problem and a very simple, bullet proof solution that I use.

Some people have little to no success trying to use the Porsche OEM Reed switch sensor that is used by the 911 dash electronic speedometer/odometer. One of the reasons for this is "switch bounce". Any mechanical switch will bounce when activated. Normally in microcomputer systems, hardware and software debounce techniques are used.

The Reed switch that is activated by the 8 magnets rotating around in the transmission can be "debounced" with some success using a "schmitt trigger" circuit. This works "sort of OK" if you know what you are doing. Basically the schmitt trigger has some hysteresis which mitigates the oscillations when the Reed switch bounces. I don't like this solution.

The new ECU's all have 12V inputs, usually with pull-up resistors for interfacing many different types of sensors. The Hall effect switch that I chose has an "open drain" (mosfet transistor). This allows a very simple circuit because the ECU already has a pullup resistor into 12V.

The Hall switch has three pins, 12V, Ground, and the open drain output. Hook up a clean 12V and ground to the power supply pins of the sensor. The output from the sensor then goes directly to the ECU input. No mess, no fuss, no bounce issues, because the Hall switch is electronic. You could add a decoupling capacitor across the 12V and ground pins but I don' think it is required.

I could not find any information on the strength or polarization of the transmission magnets. Some experimetnation was required to find a Hall switch with the correct sensitivity and type. There are many different types of Hall switches for all sorts of different jobs. I chose the AH3572 device from DIODES. This switch is a "Omnipolar" type which means it will operate with either a North or South pole and simply releases when that pole is removed. It operates at a nominal 20 Gauss and releases at a nominal 10 Gauss. Here is the link for the datasheet:

https://www.diodes.com/assets/Datasheets/AH3572.pdf

The only trick is to mount the Hall switch. I slightly modified the OEM sensor with a Dremel tool to enable the Hall sensor wiring to fit in the space between the OEM sensor and the transmission sensor pocket hole. You can use small gauge wire like 28-26 AWG. Below is a picture of my modified OEM sensor. Also the device schematic from the datasheet showing the power supply pins and open drain output. I should try to post a picture of the installed sensor on the transmission but I think you get the idea. I put more epoxy over the sensor pins after I took the photo.

I really hope this helps someone. This forum has helped me so much over the years. If there are any questions or things you would like me to make more clear, please ask.

[911obgyn]

Brilliant solution!

[Dr J]

I was able to get vehicle speed into the Infinity by connecting the speedometer terminal A to the infinity. No electronic mod required. This terminal is used for cruise control. I do not have cruise control on my car.

[nreed]

Thanks DR J

Here is the speedometer IC schematic. Pin 3 is the pin connected to the "A" speedometer connection.

I hope this helps someone.

['78 SC]

Norm and Dr J,

Just to confirm...

The UAF2115 circuitry is embedded in the stock speedometer and generates a clean pulse output (terminal 3) that mimics the stock reed switch mounted on the differential?

This is available on 1979-1986 speedometers (I'm going by your signatures) with or without cruise control?

Thanks,
Steve

[nreed]

Yes, sorry I should have said that the UAF2115 (Google for the datasheet) chip is inside the speedometer on a circuit board. The signal on the speedometer terminal A comes from a monostable inside the chip. I have not personally verified what the signal looks like. Dr J knows more about that than me and I am not sure what years have this setup. MIne has this setup and it did not come stock with cruise control.

Originally Posted by '78 SC

Norm and Dr J,

Just to confirm...

The UAF2115 circuitry is embedded in the stock speedometer and generates a clean pulse output (terminal 3) that mimics the stock reed switch mounted on the differential?

This is available on 1979-1986 speedometers (I'm going by your signatures) with or without cruise control?

Thanks,
Steve

['78 SC]

Excellent. I'll try to get a photo of the back of my speedometer to show where the "A" connection is. I have a 1983 that came with cruise (which has since been stripped off).

Steve

['78 SC]

According to the wiring diagram, the speedometer output "A" goes to the mileage counter that triggers the oxygen sensor light (every 50k miles, I think). So it is likely later catalyst equipped cars (US 1980ish onward) would have this output. My 1978 does not have have an oxygen sensor so no mileage counter.

Speedometer terminal "A" also connects to the Tempostat Control Unit (cruise control) terminal 11.



Thanks to Dr J and Norm for picking up on this useful signal. Who would have thought these antiques would have ICs lurking about.

['78 SC]

As promised, a photo showing connections on rear of speedometer...



Terminal A (middle connector) has two wires piggy-backed onto the same terminal:
white/red (oxygen sensor mileage counter - terminal G, see above diagram)
black/brown (cruise control - Tempostat terminal 11)

[zaphodtheprez]

Originally Posted by Dr J

I was able to get vehicle speed into the Infinity by connecting the speedometer terminal A to the infinity. No electronic mod required. This terminal is used for cruise control. I do not have cruise control on my car.

Dr J - in your infinity sensor settings, what type of sensor did you use.
I don't have an Infinity, but on EMU Black the settings are VR or Hall.

Hopefully your answer will save some time experimenting.