Speedo connection to 2 wire sensor

[Mick]

Has anybody connected a 2 wire sensor Audi 01E to VDO electronic speedo. Recently install a new transaxle actually late model 016 has the same 2 wire sensor as the 01E. I have the wiring diagram for the VDO, it gives the option for both the 3 wire & 2 wire sensors. I have tried the suggested connection option with out a result can anyone tell me if the is a trick to getting it to work.

Mick

 

[Ian Clark]

Can you post a pic of your instruction sheet? If the Audi sensor is an un-amplified magnetic reluctance unit that may be covered in the two wire setup, the VDO gauges in CAVs used a powered proximity sensor (three wire).

Is the gauge capable of self calibrating? Do you have the dip switch setting formula or charts? Without knowing the pulses per mile/klm you could be so far out on the calibration the gauge just wont register even with everything else correct.

Hope this helps

 

[Tom Democko]

While upgrading my 016 transaxle I got rid of the magnetic pickup style signal to my CAV's speedo. VDO makes an inexpensive GPS speed sensor. Very easy to install and calibrate. The speedo is no longer driven by half shaft rotation. All the wiring changes occur under the dash and was easy to incorporate with the ani/digital VDO speedo.

 

Hi Mick,
I hooked up the sensor on my 01E gearbox to a Classic Instruments (USA) speedo using their SN74Z 'box'. It's a little tricky to calibrate but a helper, straight private road and a GPS Sat Nav got it done. The Audi gearbox sensor is a reed type so I checked it by putting a meter across the terminals and passing a magnet over the sensor to ensure it opened and closed. Cheers Roger Allen

 

[Mick]

Have attached the speedo wiring instructions showing the 3 options.
Ian the speedo does have auto calibrating functionality.
Roger I assume the Reed sensor is the same as the inductor sensor? This may be where I'm going wrong
Tom will look into the GPS option if I can't resolve the problem.

 

[Ian Clark]

Hi Mick,

I think Tom has the best solution if your wiring is ok and the problem persists. Wiring diagram #2 corresponds to standard CAV practice however speedometer issues are not unusual in the pre s/n 100 cars so I'm going to fit the VDO GPS to an early car and see how it goes. Here's the link to VDO GPS Sensor Instructions: https://www.vdo-gauges.com/media/instructions/A2C59519787 GPS Speed Sender .pdf

Cheers

 

Hi Mick,
I'm not an electronics expert but I think it goes like this. The reed sensor is basically two 'reeds' which come together to form a contact and complete the circuit when a magnet passes over them. The inductive sensor generates a signal by passing over a small gap and 'inducing' the voltage change - like a mini dynamo. I believe the Classic Instruments SN74Z can work with either - maybe have a chat with them to nail it.
Cheers
Roger

 

[Paul Hendrickx]

Hello, to check a inductif sensor(suposed 2wires) you have a magnet with a coilaround just like an relay switch, so you should be able to mesure the resistance over the coil.... can be from 50 ohms to roughly 500/750 ohms. if you move a little metal in front of it the ohmmetre should vary in reading, equal to the mouvement.

then you have inductive active sensors 3wires. this means you have still your coil around a non magnetic ferous metal, so you could again mesure the ohm's, and the third wire, is an 12 volts one who wile energised will make the metal magnetic(same as the function of a relay, 3wires 1)12v 2) earth 3) signal(same line as the 12volts entree.

the other category are hall effect sensors who have again 3 wires +12v earth signal, and here you only can detect signal by means of open up (in serie) the signal line and to mesure in millamps the strengt and in parrallel the outcomming voltage . the latest ones - hall sensors are two wire ones actif where only the drop in voltage is mesured

 

[Tom Democko]

Mick the GPS option is really accurate and offers a second benefit. As we all know the rearview visibility is really poor. The connector in the rear used for the inductive pickup speedo system provide an ignition sourced power supply for the rearview wireless camera I just recently installed. I know, heated seats and multiple cup holders cant be too far away!

I bring this up because the use of this available power source allowed me to install the camera system without drilling any holes or running wiring from the back into the cockpit area.

 

[Mick]

Thanks for the advice guys,

Roger are you sure the O1E is a reed sensor? All indications are that the sensor is a magnetic (inductive) looking at the manuals (which by the way is piss poor on detail).

 

[Mick]

Ian I love the idea of the wireless camera.

 

[Mick]

Hey Roger,
did a bit more research, discovered the difference between the reed & inductive sensor. Inductive unit induces a current the Reed doesn't it simply opens & closes. As you have suggested a converter is required to change the signal from the Reed switch to something that the speedo can see so will try Classic Instruments SN74Z.

Mick

 

Hi Mick,
My understanding is that the reed opens and close as a magnet passes close by. So there's a mechanical element with this.
Cheers
Roger

 

[Mick]

Hi Mick,
My understanding is that the reed opens and close as a magnet passes close by. So there's a mechanical element with this.
Cheers
Roger

Yep that's how I understand it Roger now the problem is how to get a signal to the speedo. As there is no current/pulse coming from the reed switch when it is opening and closing I think all I need to do is apply a low voltage current say 5v to one end of the Reed sensor from the speedo using a capacitor and the other back to the ground connector on the speedo. I'm concerned about applying voltage to the reed sensor and if it will allow a signal to be registered by the speedo. The reed sensor will create a square wave the same as the Hall effect sensor I think which the speedo should see as a pulse. That being the case I should be able to connect to the same pins on the speedo that the hall sensor would. Any electronic wizzes able to concur.

 

Hi Mick, I just put a fused 12v supply to the sensor and fed the output to the Classic Instrument box. I guess the output is a square wave. It works fine. Cheers Roger

 

[Mick]

Your a gentleman Roger Thanks should do away with the crap add on's every body say you need. Well Worth persevering.