[928uk] Speedo Failure

[Paul]

I've now taken out the dash and soldered the speedo connections as
mentioned by Dwayne's Odo documentation

http://dwaynesgarage.norcal928.org/1984%20Odometer%20Repair%20Procedure.htm

And finally confirmed continuity to the edge connector

https://thumbsnap.com/i/CvVdauy3.jpg

https://thumbsnap.com/i/8w8eDb6a.jpg

However, still no working speedo

What should I check or do next?

Paul
87 S4 Manual

On Sun, 5 Feb 2023 at 15:38, Paul via 928uk <928uk at lists.928.org.uk> wrote:

> Earlier I thought I'd retest the sensor, making sure both wheels were
> spinning (I had to adjust the handbrake)
>
> With the car in 4th and engine idling I did get highly variable impedance,
> which makes sense now
>
> There's no voltage across the car connector with the ignition on, which
> suggests that the dash connection is the culprit
>
> I'm relieved the sensor appears to be working, having looked up the cost of
> a new one
>
> Many thanks for the info and advice
>
> Paul
> 87 S4 Manual
>
> On Sun, 5 Feb 2023 at 13:53, Paul R Smith <smiffypr at gmail.com> wrote:
>
> > 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
> >> >
> >>
> >
>