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For quite a while now, the speedo has been dead on my 1991 V12 XJS. I've searched the forums, which provided me with lots of helpful info.
I've been testing the output of the little box in the boot that turns the square wave signal from the diff sensor into a voltage which seems to give me about 6v constant, though this does increase by a tenth or two once into 5th gear and about 30 mph. I thought that it would be an intermittant voltage with gaps between signals that decreased according to speed, so not sure why I've got a constant(ish) 6v. I don't suppose anybody know what I should be seeing, as the only info I've found says to test the output of the box for a voltage output, but I can't find how many volts and whether it's a constant or an intermittant voltage I should be seeing.
I've changed the transducer on the diff for a known working one, tested that it's getting 12v and also tested that the speedo responds to a voltage input, so now I'm stumped!
Did the speedo stop suddenly or did it stutter all over the place before it failed?
Mine went down the stuttering route and eventually packed up altogether.
I replaced the black box thingy and it has been fine ever since.
Can't help you on the voltages I'm afraid, all I know is that it works like an ABS set up.
Did you take the two Multiplugs out of the back of the Speedo Binnacle then give them a clean and put them back again
That is all I had to do to get my Speedo working
If the Cruise is working, then its not the little black box in the boot
If the Milometer on the Trip Computer matches the same number of Miles, as the Milometer on the Speedo, as you are Travelling
Then again its not the little black box
And you may have changed the diff sensor for nothing, as they very seldom give a problem
If you clean those Multiplugs that go in the back of the Binnacle, then I wouldn't mind betting that your Speedo will work
Just be very careful, you don't damage the Printed Circuit that the Multi Plugs plug into
If none of that works, then you could try adding an extra Earth Wire
Carefully Clean the 2 Multiplugs and Sockets
Give the 2 Multiplugs and Sockets a good clean NOTE: The corrosion on the first 'Tang' of that Plug I used the Blade of a very small Screwdriver to carefully scrape off what looks like a build up of a Varnish like substance on the 'Plug Tangs'
Then finished cleaning them with electric contact cleaner and the end of a Pencil Eraser
One of the 2 Multiplugs that go in the back of the Speedo/Tacho Binnacle
Last edited by orangeblossom; 12-26-2017 at 04:42 PM.
Does the cruise control work? The CC and the speedo are connected to the same output of the speed interface. The speedo has electronics that turn the pulses into a voltage to drive the speedo. There is a Zener diode on each output of the interface unit, if your meter has a diode test function check the zeners they usually fail short circuit.
The diff sensor outputs a sinewave (sort of more a dirty sinewave) the speed interface unit has a counter which turns the input into 2 squarewave/pulse outputs. The TC uses one output and the speedo / CC use the other.
Do what OB said first as bad connections are 80% of the problems in the XJS.
The speedo (and trip computer) stopped suddenly, albeit after a period of about 9 months when the car wasn't used. I did remove the binnacle and have a look at the connectors, which were pretty tarnished, but thought they'd still be OK. I'l take the binnacle out again and give the connectors a really good clean, as suggested and see where that leaves me. There's definitely a signal of sorts going into the connector, but maybe too dirty to make it's way to the speedo!
The speedo (and trip computer) stopped suddenly, albeit after a period of about 9 months when the car wasn't used. I did remove the binnacle and have a look at the connectors, which were pretty tarnished, but thought they'd still be OK. I'l take the binnacle out again and give the connectors a really good clean, as suggested and see where that leaves me. There's definitely a signal of sorts going into the connector, but maybe too dirty to make it's way to the speedo!
If the TC and speedo stopped working then the problem is either the speed interface unit or speed sensor or connectors on speed interface. I'd be looking at the interface unit.
The speedo and TC use different outputs from the speed interface unit, although the chip that drives them is one unit inside the interface.
Hi Warrjon, sorry for got to mention that the trip computer stopped working about a year before the speedo packed-up. It doesn't even tell me the time! I'm going to try giving all the terminals a thorough clean and then, if no joy, look at trying another interface unit.
Alas, my speedo still dosn't work,despite cleaning the mutiplugs and the back of the instrument cluster. I forgt to add the additional ground wire, but might go back and do that later. I guess the next thing is to try a different interface unit and see if that helps.
Alas, my speedo still dosn't work,despite cleaning the mutiplugs and the back of the instrument cluster. I forgt to add the additional ground wire, but might go back and do that later. I guess the next thing is to try a different interface unit and see if that helps.
I have the same issue... no speedo or trip computer. I've had 2 different gauge clusters in mine with no difference(and ground). My thoughts always been towards the speedo sensor in the trans? Or it's interface unit more likely to go dead?
There is only 4 things, Speed transducer, Interface unit, speedo and wiring/connectors.
If you jack up the back of the car and put is on stands so you can put it in gear and run while you measure the voltage of the interface unit.
Use your Multimeter on AC volts (this WILL NOT work on DC volts). The Blue wire from the interface unit goes to the speedo so put your meter between the Blue wire and ground, you see about 5volts AC.
I think that was my first port of call and we got a constant-ish 6v out of the unit, despite putting the car in 5th gear and going up to about 1500 rpm. We followed this signal all the way to the speedo and it was still there at the input wire, but no mph read-out. I guess I was expecting to see an intermittent voltage where the time period between 'pulses' changed according to how fast the wheels were turning, but it just stayed at 6-ish volts, I think.
I'm not sure how a transducer failure manifests itself; is it no reading from the output wire or the wrong reading/type of reading? For some reason, I thought the quicker the voltage pulses from the transducer, the higher the speedo reads, but that is just my assumption, so probably wrong!
Last edited by HeWhoDaresRoyV2; 01-02-2018 at 03:34 AM.
Before my transducer died completely it was very erratic; sometimes reading, sometimes not and sometimes just bouncing up and down. and then stopping completely.
I think that was my first port of call and we got a constant-ish 6v out of the unit, despite putting the car in 5th gear and going up to about 1500 rpm. We followed this signal all the way to the speedo and it was still there at the input wire, but no mph read-out. I guess I was expecting to see an intermittent voltage where the time period between 'pulses' changed according to how fast the wheels were turning, but it just stayed at 6-ish volts, I think.
I'm not sure how a transducer failure manifests itself; is it no reading from the output wire or the wrong reading/type of reading? For some reason, I thought the quicker the voltage pulses from the transducer, the higher the speedo reads, but that is just my assumption, so probably wrong!
If this was with the meter on AC range then the speed interface and transducer are most likely working. Because the signal is a squarewave with 50% duty cycle (meaning the top and bottom of the squarewave are equal) the voltage will not change as the frequency increases.
Without a scope it's not 100% but my guess is to look at the speedo, there is an AC to DC converter in the speedo this is most likely the fault.
If the meter was on DC then the interface unit could be bad.
If this was with the meter on AC range then the speed interface and transducer are most likely working. Because the signal is a squarewave with 50% duty cycle (meaning the top and bottom of the squarewave are equal) the voltage will not change as the frequency increases.
Without a scope it's not 100% but my guess is to look at the speedo, there is an AC to DC converter in the speedo this is most likely the fault.
If the meter was on DC then the interface unit could be bad.
Have to say, I can't remember whether it was on AC or DC, so will do the test again!
One thing I did forget to mention, if your meter has a Hz function check the frequency as well as the voltage. The frequency will change as you speed up.
12-26-2017, 06:31 AM
Speedo Dead
