308 Early Speedo Drive Sensor Repair

I had a dead speedo drive sensor as used in 308 GT4s and early 308 GTB/GTS, and since they are a bit costly to replace (Superformance offers them for £145/$220) i thought i have a look how complex a repair is. Good news, it is quite simple and all the components are still available.

On my sensor, a bit of oil found it's way through the seal and dissolved capacitor C2, the other components were still good. To be on the safe side i replaced all transistors/caps and the zener diode. If you don't have tools to test the sensor, simply replace all parts except the coil pickup and the resistors (these usually don't break), part cost is about $2-$3:

C1: 0.47uF 100V film capacitor, 7.5mm raster
C2: 1.5nF 160V film capacitor, 9.5-10mm raster
T1 & T2: BC327-16 or BC327-25
D1: 1N4743A Zener Diode, 13V

The wires and sleeve were quite bit brittle due to the heat in the engine compartment, drilled them out and replaced them against new wires/sleeve/AMP connectors then sealed the opening with black epoxy.

See below for component and circuit diagram, as well as a scope shot of the repaired sensors output signal. About two hours of work and well worth it.

Good luck,
Adrian
Image Unavailable, Please Login
Image Unavailable, Please Login
Image Unavailable, Please Login

 

Photos of the repaired circuit board, the PCB mounting plate with the two new wires in place (before adding the epoxy) and the finished speedometer sensor.
Image Unavailable, Please Login
Image Unavailable, Please Login
Image Unavailable, Please Login

 

106324: First sensor version, conical body. Green (VCC) and red/black (GND/Signal) wires.

124146: Second version, replacement of 106324. Brown (VCC) and yellow (GND/Signal) wires. (Note: The wiring in the connector of 124146 seems to be often swapped, while installing check that the sensors brown wire goes to the cars green wire, and the yellow wire goes to the cars red/black wire).

From what i know 106324 and the successor 124146 have been deployed starting from the early 308 GT4 up to 308 GTB/GTS as well as early (81-82? US only?) 308 GTBi and GTSi's. The 308 QV uses a different sensor type (part number 116987), which is not interchangeable.

Will do!

The transistors and the zener diode were ok but capacitor C2 was gone, it literally fell apart so was easy to spot.

 

Paul, i searched for 'RJay' and found this thread, is that the one you meant?
http://www.ferrarichat.com/forum/technical-q/101159-308-speedo-odo-sender-system-fix-3.html

The circuit board shown there looks identical to the one in i found in my sensor. I just found another PCB photo posted by Brian here, also looks identical: http://www.ferrarichat.com/forum/137886082-post17.html

RJays circuit diagram lists a slightly different value for R2 (120Ω compared to 110&#937 but that is nearly within the tolerance of the resistor values so can be ignored. It looks like RJay mixed up the emitter and collector terminals in his circuit diagram though.

Interesting, that is probably the different PCB layout used in the later type 124146. The circuit is similar but the components had a different orientation on the PCB due to the now horizontally mounted sensor.

 

Jon,

i am using a Wiha Picofinish 268 10P where the tip has been filed down to a sharp end. It's similar to a screwdriver with a bent and filed down tip, this allows you to get below the crimped edge for carefully levering it upwards. Take your time and go around the edge several times, each time lifting the edge just a bit so it won't break off.

good luck,
Adrian
Image Unavailable, Please Login

 

That looks similar to mine, oil had entered the inside of the sensor and dissolved the capacitor. Looks like an easy repair, good luck!

 

Good question, i did not try to disassemble the rotor/body assembly as it looked like the rotor/axle was riveted into place. My guess is that the seal in question is at the gearbox side.

Rob, the oil enters from the gearbox side, i think the o-ring seal at the connector side is fine.

 

BTW, you could protect the repaired board by applying silicone based conformal coating to the component (top) side. This will create a thin transparent layer which protects the components against humidity and oil, if one of the seals should fail. Available as spray or paint (spray is a bit easier to use, apply one thin layer, let it dry and apply a second one).

Buy Conformal Coatings RS transparent 200mL Aerosol Conformal Coating For PCBs, Silicone Resin, -70 to +200°C RS ERDCA200D online from RS for next day delivery.
Amazon.com: MG Chemicals Silicone Conformal Coating : 422: Everything Else

 

Yes you can, but you need a resistor (any value between 470 and 680 ohm should do) for this test. Wire the Green wire to +12V (a 9V Battery should also work nicely), and the red wire via the 470 ohm resistor to ground (Red--[Resistor]--Ground).

Measure the voltage between the Green and Red terminal of the sensor. When you turn the axle, the voltage should alternate between less than one volt and several volts, if the sensor is ok.

 

Correct. As long as you see a change in voltage while turning the axle your sensor should be good. With a 12V source and a 560 ohm resistor, my Agilent RMS multimeter shows 4.5V without turning the axle, and between 4.5-10V when turning the sensor axle by hand.

Values are a bit different if you go through the filter circuit in the Speedo (as shown in the scope shot), but that extra work is not required for just checking if the sensor is still working.