Digital Speedo Sender help

[rknowles74]

Hi all,

I have a Smiths electronic speedo in my Nostalgia XK120 with a dedicated sender that senses the bolts on the prop shaft. I want to remove this sender as I think it could be causing some erratic readings on the speedo, and instead use the original sender from the donor XJS that is mounted into the differential.

In theory this should be quite simple, however the smiths gauge can cope with two type of sender, and I dont know what type the Jaguar sender is. It appears the two types are known as PNP, or NPN. One of them needs the gauge connected between the sender and ground, and the other needs the gauge connected between the sender and +ve 12v.

Does anyone know the right way of connecting up the original sender unit - especially since in the XJS the sender was connected to a speed interface unit that is not used in the XK120.

Thanks,
Richard

[poprivet]

Hi Richard,
First off I don't have a specific answer to the question but I have some thoughts on your existing layout which you may have already considered.

Is the sensor cable shielding electrically complete and earthed.
Is the gap between sensor and bolts correct and are all equal.
Is the sensor head clean.
Are the earth and other connections at the speedo known to be good.

I do have various wiring diagrams for the XJS, if you know what year the rear axle is from I will gladly scan and post them here.

Final thought, the last of the Series III saloons used a round electronic speedometer which looks suspiciously similar to the Smiths ones, I think I'm right in saying the sender was driven off the gearbox but was wired direct without an interface, I'm sure there is some basic information in the Haynes manual which may be another line of thought to explore.

Good luck with your quest,
Al

[rknowles74]

Hi Al,

Thanks for the response.
I can confirm that the current sensor head is clean, and it is accurately gapped from the bolt heads. I believe the sensor cable is earthed corectly, but the run is under the carpet and I have not yet stripped the interior and dash to check it.

The behaviour of the speedo is odd. Up to around 70 ish mph the speedo is accurate and consistent with its reading. After a while (10 minutes or so) going a little quicker than that (cough cough) the needle starts to indicate a faster and faster speed, until it hits the stop at around 140mph (no, im not going that fast). When then coming to a stop the needle will only drop down to 60 or 70 mph. Turning the ignition off and on again puts the speedo through its checks, and the needle returns to zero.

The manufacturer doesnt think its a sensor issue due the to behaviour at lower speeds, but Ive yet to hear back from them with any suggestions (they have had it back once to update the software, but that made no difference).

However - I would like to use the original Jaguar sensor as it is a much neater installation and I suspect it will be more reliable.

The axle (and whole drive train) is from a 1988 3.6L XJS. If you could dig out the wiring diagrams for it that would be great. I do have the workshop manual on CD, but it only has a diagram of the component locations, not the actual wiring for the speed sensor.

When you talk of the Series III saloons are you talking about the XJ6? If so then I will have a scout about for wiring diagrams for those as the lack of the speed interface may help me figure out the wiring.

Thanks
Richard

[poprivet]

Thats an odd fault, it sounds more like the speedo then the sensor almost asif it is switching to count double the pulses.

The first picture is of the XJ-S for your year: (Click to enlarge)

The second is from Haynes of the XJ6, late series III

If you go for the XJ-S setup, I'm not sure how many pulses the diff will produce per revolution or if you will be able to set your Smiths speedo accurately. I'm sure someone has done it but it's not something that I have experience of.

Hopefully this will be of some use,

Al
(Edit, photo placement is playing up on the forum!)

[rknowles74]

Thanks Al,

The second photo shows that the speedo goes between the sensor signal output and ground making it a PNP type (the simplest description I have found is here https://www.schneider-electric.co.uk/en/faqs/FA142566/ although this is pretty good too https://automation-insights.blog/2011/0 ... pn-vs-pnp/).

However the sensor is also a 2 wire type, whereas the XJS has a 3 wire type.

Ill need to digest the XJS diagram a bit more, but as a bit of a guess I think the R wire (red) may be +ve, the shield is connected to ground (via B) which means U (what colour was that?) is the signal, and it looks like it goees to the speedo and gets grounded there - so PNP too.

Many thanks
Richard