Speed sensor connections
Discussion
Ok, I am trying to identify the wires that go into the speedo transducer box behind the dash.
The +Ve and earth were easy to find. Since my speedo box delivers a 60km/h signal when the car is at rest, this one was easy to find as well.
This leaves me with two unidentified wires: the one from the speed sensor and the one going to the speedo.
Unfortunately, when the rear wheels are moving, no tension can be measured at the connector of the speedo box ( and the speedo needle rests at 0). Does that mean my speed sensor is broken as well? Or does it mean the defective transducer box will affect the signal from the sensor?
The sensor has two wires, where do they come from and where do they go? Both through the transducer box or only the signal one goes there?
How to diagnose a faulty sensor without an oscilloscope?
Lots of questions I know....
Thanks for the hand.
The +Ve and earth were easy to find. Since my speedo box delivers a 60km/h signal when the car is at rest, this one was easy to find as well.
This leaves me with two unidentified wires: the one from the speed sensor and the one going to the speedo.
Unfortunately, when the rear wheels are moving, no tension can be measured at the connector of the speedo box ( and the speedo needle rests at 0). Does that mean my speed sensor is broken as well? Or does it mean the defective transducer box will affect the signal from the sensor?
The sensor has two wires, where do they come from and where do they go? Both through the transducer box or only the signal one goes there?
How to diagnose a faulty sensor without an oscilloscope?
Lots of questions I know....
Thanks for the hand.
The sensor is just a wire coil and magnet that generates a tiny voltage as the toothed wheel spins near it. So to test it you just need to do a resistance check across the connections to the sensor. The physical sensor wiring goes into the speedo calibration unit, but the speedo input is in parallel to the speedo calibration unit so it loops back out from the box, without any electronics being involved. Now from memory if you unplug the speedo calibration box, and locate the wires to the sensor, you can do a resistance check here to see if it loops back through the sensor as it should. I dont know the resistance off the top of my head, but at a guess it will be quite low as its just a copper coil- maybe a few k ohms at most. Do you get a speedo reading? If you do it cant be the sensor at fault.
The box doesn't do anything with the speedo signal - it reads what the sensor is outputting and if the wheels are moving then it outputs a constant 60kmh to the ECU to tell it you're moving, the speedo is connected to the same terminal as the box input so it's connected directly to the sensor. If the box is reading when the car is moving or not then the sensor is working but the speedo isn't.
Ok, so if the speedo signal goes to the speedo directly, either my speedo or the sensor is broken if the needle doesn't move from 0.
And the box is f
ked-up as well.... Nice.
So, I'll do the continuity check on the sensor, which will help me identifying the sensor wire that goes into the box and replace the broken component.
And the box is f
ked-up as well.... Nice.So, I'll do the continuity check on the sensor, which will help me identifying the sensor wire that goes into the box and replace the broken component.
Try opening up the speedo box, you'll note that two of the input pins are soldered together. One goes to the sensor, one goes to the speedo input. Try disconnecting the box, but then re-connecting those two wires in the plug that is now disconnected from the box with a piece of wire. If you do that then you've basically removed the box from the equation and it's just the speedo and sensor working independently. If the speedo still doesn't work then you only have the sensor, wires to/from and speedo to check...
I doubt both the sensor and the speedo box are knackered- its far more likely the sensor is open circuit so the wires are now acting as radio ariels and the speedo box is picking this up as a speed signal from the sensor and producing a false output. When I say loop back- if you do a basic resistance check on the sensor- the test voltage from the meter down one wire- through the sensor coil, and loops back to the test meter down the other wire. Water gets into the sensor coil and makes them go open circuit due to the corrosion.
Ok, did remove the center part in the cockpit and found the sensor wires.
The grey goes to the conditioner box and outputs as a blue that goes to the speedo.
But I am also open circuit on the sensor.... which means it is f*****!
Let's go and buy some new one.
The grey goes to the conditioner box and outputs as a blue that goes to the speedo.
But I am also open circuit on the sensor.... which means it is f*****!
Let's go and buy some new one.
Edited by e635815 on Thursday 20th July 08:19
The old sensor was open circuit and the seal at the wire end of the sensor was broken, thus allowing water to enter.
New sensor has a nice 457 Ohm resistance.
I re-assembled it and went for a drive.
It looks like this has also cured my idling speed holdup around 2000rpm.
The only thing left to sort is the needle reactivity: it only starts moving around 50/60 km/h.
Seems like the gap is too big.
New sensor has a nice 457 Ohm resistance.
I re-assembled it and went for a drive.
It looks like this has also cured my idling speed holdup around 2000rpm.
The only thing left to sort is the needle reactivity: it only starts moving around 50/60 km/h.
Seems like the gap is too big.
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