[928uk] Speedo Failure

[Paul R Smith]

To test the sensor, put a magnet next to it and it should go to 0 ohms.

To test the rest, connect and disconnect the pins on the car connector. 
To register 10 mph, you would need to do it at 18 times per second, so 
you would need a mechanism to do that. Probably not worth the bother. If 
the sensor doesn't respond to a magnet, you know it's the sensor (I 
might have a spare).

Simple test for continuity to the speedo is to check for voltage across 
the car connector when the ignition is on.

Yes, that other sensor will be a magnet with a coil around it, just like 
our crankshaft position sensors. The 928 speedo sensor is just a reed 
switch that is closed by magnets in a disc on the side of the 
differential. (For MY '91 they changed to using the ABS signal to 
generate the speed signal).

Smiffy

On 05/02/2023 13:11, Paul wrote:
> I found the 4 pin connector in the wheel well, pins 3 & 4 for the 
> reverse switch and pins 1& 2 for the sensor, but there was no 
> continuity across the sensor
>
> https://thumbsnap.com/i/nKaT8Q7R.jpg
>
> With the car on axle stands and the car running in first at idle I 
> checked again, no continuity, no voltage, no current with the meter 
> across the sensor
>
> https://thumbsnap.com/i/BFnFVL8W.jpg
>
> My car does not have an LSD, so only the left wheel was spinning, I 
> presume that does not invalidate the test
>
> Is there a way to test the speedo by connecting some sort of signal 
> generator to the connector, if so what should I use?
>
> The speedo sensor in my other car has a 300ohm impedance and produces 
> a 5V output, I guess that's a completely different type of sensor?
>
> https://thumbsnap.com/i/RT52J3is.jpg
>
> Any advice welcome
>
> Paul
> 87 S4 Manual
>
> On Sat, 3 Dec 2022 at 20:38, Paul via 928uk <928uk at lists.928.org.uk> 
> wrote:
>
>     Thanks that's exactly what I needed
>
>     Will check shortly
>
>     Paul
>     87 S4 Manual
>
>     On Sat, 3 Dec 2022 at 18:11, Paul R Smith via 928uk
>     <928uk at lists.928.org.uk>
>     wrote:
>
>     > It could be that you haven't got the connector on the dash all
>     the way on.
>     >
>     > If you find the connector in the spare wheel well with two brown/red
>     > wires and separate the connector and put your meter across the
>     two wires
>     > that go out to the diff, the resistance should go from 0 to infinite
>     > about eight or ten times per turn of wheel, so you can just roll
>     the car
>     > back or forward a foot.
>     >
>     > The other side of the connector, the two wires, one is ground,
>     the other
>     > goes to the dash, there should be some voltage across these two
>     when the
>     > ignition is on.
>     >
>     > When the connector is plugged together that voltage will come
>     and go as
>     > the diff turns.
>     >
>     > Smiffy
>     >
>     > On 03/12/2022 10:40, Paul via 928uk wrote:
>     > > The speedo/odo has stopped working
>     > >
>     > > The last work I did on the car was to fix issues with the dash
>     lamps,
>     > which
>     > > are now sorted
>     > >
>     > > So it's tempting to assume that I broke something while fixing
>     the lamps,
>     > > however I'm not convinced
>     > >
>     > > What else could have failed, and is there an easy way to test it?
>     > >
>     > > I do have a USB oscilloscope, so I could check there's a
>     signal from the
>     > > transducer which the rear on axle stands
>     > >
>     > > Any suggestions?
>     > >
>     > > Paul
>     > > 87 S4 Manual
>     >
>