Technical Speedo and odometer not functioning

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Technical Speedo and odometer not functioning

AGoodwins

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Hi all,

had my B for about a year and just come back from a driving holiday in France (very nice:D). About half way through the journey my speedo and odometer stop working (while driving).

Mentioned this to a mechanic and his recommendation was to replace the sensor on the gearbox. So a couple of questions....

1. Does this sound correct? I have read threads that mention dry joints on the back of the instrument cluster. Is this more likely to be correct?

2. If he is correct where is the sensor located on the gearbox? Any pic's would be great (y)

Thanks all
 
Well if both the speedo and the odometer stop working then it is either a sender unit issue OR a cabling and connectivity issue.

I would first of all locate the sender unit which is/should be located on the top and rewards side of the gearbox towards the bell housing end. The sender unit is driven by the differential crown wheel (i.e. final drive).

STEP 1
Once found, unplug the cable and remove the sender unit. Check the cable and connector for corrosion, broken wires etc.

Look at the drive end of the sensor. You should see a small square drive shaft/pin. Check it is not rounded or damage. Also look at the gearbox end. You should see a female square drive.

STEP 2
Jack up RHS/Offside front wheel so it is just clear of the ground and securely support the car on an axle stand or blocks. Check handbrake is securely set. Start engine and put car into, say 3rd gear at let the engine idle. Do not rev. Now got and look at the female sender unit drive on the gearbox. You should see it rotating. If it is then all is well with the drive in the gearbox. (Note for safety I said RHS/Offside wheel because to look at the drive rotating you will be close to LHS/nearside/driver's side) and thus we do NOT want this wheel rotating as it could easly snag your cloths, leg etc.)

STEP 3
On the connector that goes to the sender unit are three wires.

1) Check that the blank wire connects to earth. Buzz it with a meter.
2) Check that the red wire goes positive (3V to 12V) when the ignition is turned on
3) Check that the middle wire goes postive (possibly less than one volt but you should see a reading movement if only very small.

If you get no voltage reading then you need to check the connections at the back of the instrument cluster. DISCONNECT THE BATTERY AND LEAVE FOR 15MINS before proceeding futher otherwise you will trigger and airbag error!!!! (they won't go off but the airbag warning light will come on). Look for loose connections. Then disconnect and reconnect in turn looking dirt, oxidation etc. Clean with alchohol as required. Often the simple process of disconnect/connect solves issues buy the wiping & self cleaning effect of the pins/skts moving appart and together again.

Reconnect battery and check fo the voltages at the connector Pins 2 & 3 again.

STEP 4
NOTE the next bit gets tricky and vague, but hopefully you won't need to go this far.

Next we need to check the electrical continuity of the sender unit. It has 3 pins. The pin to which the black wire goes is a ground connection. The pin to which red wire is connected is a +ve feed, and the middle pin is the pulsed wire. NOTE this sender is a hall effect probe so does not contain a coil as such. This means the when measuring with an ohm meter the leads will have to be swopped over on each pair of pins tested. In one direction you may get a reading and another none. It is a long time since I have tested a hall effect sender unit so I can not tell you what to expect.

Blank wine = Pin 1(outside), Red = Pin 3 (outside), other = Pin 2 (middle)

Measure restistances between, Pin 1 and Pin 2, Pin 1 and Pin 3, and Pin 2 and Pin 3 and for each measurement swop the meter leads over thus getting two readings per pain of pins. ALSO whilst taking these measurements spin/rotate the sender drive pin and see if you get and changes in the meter readings as you do.

Make a liitle table of results. You should see a pattern that indicates the presence of "something" behind the pins. I'm hoping also that when the sensor drive is spun that you also see some effect.

STEP 6
Connect a 10K (10000 ohm) resitor between Pin1 and Pin 2. Now rappidly connect and disconnect (with the ignition turned on and a rate of about 2 per second or faster) another 10K resistor bewteen Pins 2 and 3. Does the speedo needle move? If not try removing the resistor between pins 1 & 2 and repeat with just the resistor on Pins 2 and Pin3. Does the speedo move now? If not then try both tests again with a 5K (5000 ohm resistors) and lastly 1K (1000 ohm resistors).

If you can get the speedo needle to move at all then it is almost certainly the sender unit that is broken.
 
what I would do:

Connect pc and test with fiatecuscan if the speedo responds when you test the speedometer.
If nothing -) check connectors at the back of the speedo, but most likely you need to open it and resolder some points in the cluster.

If it works with fiatecuscan it is the sender. But you would be the first I know that has troubles with it.

You can find a speedo repair manual here: (see the points that needs resoldering)
http://www.fiatbarchetta.nl/smf/index.php?topic=4048.0
 
I thought it did. I need to recheck
but you could use the transistors like suggested too
 
You mean "resistors" not transistors :)

Anyway, MultiECUScan is a great bit if software (I'm a moderator on the site) but regardless of the diagnostic kit and software available their use is worth next to nothing unless you have basic understanding of the system operations they are trying to test.

And here in lies a problem that many garages and technicians fall into. "I'll just plug the box in, read the fault code and job done!" ...........
 
You mean "resistors" not transistors :)

Anyway, MultiECUScan is a great bit if software (I'm a moderator on the site) but regardless of the diagnostic kit and software available their use is worth next to nothing unless you have basic understanding of the system operations they are trying to test.

And here in lies a problem that many garages and technicians fall into. "I'll just plug the box in, read the fault code and job done!" ...........


for me that would be:
"I'll just plug the box in, read the fault code and I know where to start looking!"
 
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