How to test the circuitry?
Hello all.
I have resumed work on my 1988 XJS and I am trying to diagnose why the speedometer is not working.
Background:
After I got the car running and driving, the speedometer and all of the gauges were not working. I removed the binnacle and did the recommended cleanup, re-installed same and still they were not working.
I replaced the instrument cluster with one from another car and all the gauges and some of the lights started working.
The speedometer and the green turn signal indicator lights still do not work.
The speed transducer is mounted on the differential and the wires from the transducer goes to the speed interface on the left hand side of the trunk area.
Before I remove the instrument cluster again, I need to test the system from the transducer into the speed interface for the signal.
How do I go about doing that?
Do I test for signal before the interface? If so, how?
Do I test after the interface?
What kind and what value of the signal I should see?

pic of connectors from transducer to speed interface and from interface to instrument cluster

I will appreciate any and all assistance with this. If additional info is needed, please let me know.
Thanks

 

The one thing I forgot to mention was the 1988 non-working instrument cluster was replaced with a cluster from a 1985 XJS V12.
I do not know if that makes a difference.
Also, which terminal at the instrument cluster is the one for the speedometer?

 

No difference. The easiest way to check which tracks go to the speedo is to follow the coper track from the connectors on the speedo to the multiplug.

 

NO difference in the clusters.
Teis may help ID the speedo signal feed. (the speedo also has an earth feed).
Just check it it is accurate for your cluster by following the copper trace on the blue circuit flexible board. Somebody kindly posted some corrections to it, but I cannot find them. This is allaegedly a 1989 digram pinched from Jaglovers.

 

Thanks Greg. When I remove the cluster I will verify if the 88 XJS has the same circuitry. I would assume that the 88 is pre-facelift.

 

sanchez,

The facelift XJS was introduced in 1991, so yes, 88 is pre-facelift.

Do you have a trip computer? If so, us ut registering distance etc? This might help understand if you're getting signsl from speed interface and diff sensor.

Paul

 

I removed the Trip Computer and found that the PO had cut the connector off the TC side of the harness. Where it was supposed to be connected to the vehicle harness, I saw two connectors. One is black and the other is red. I will have to look at the electrical schematic to see where the TC is supposed to be connected and see if I can use spade connectors to complete the TC circuitry. Will post pics of the cut harness tomorrow.

 

Here are some pics of the cut harness and connectors. I believe the TC is connected to both plugs. I will have to check and see.



connectors for TC

 

Using a schematic I found on the other Jag forum, I was able to make up the connections for the TC.
RESULTS:
Car up on jackstands, Car in Drive: With the voltmeter set on the 20v AC scale I had a reading of 0.59 volts ac on the blue wire coming out of the speed interface. I read somewhere on this forum that the reading should be 5 volts AC.
The TC seems to be working now. That means it is getting a signal from the speed interface.
Clock works.
Ave Speed varies when I accelerate from idle to about 2000 RPM

Idle speed. rear wheels turning 19,7

change on acceleration from 19.7 to 21.5

Distance 3.3 miles

distance changed to 3.8 miles

 

sanchez,

I understand that the signals run separately from the interface unit to the Trip Computer and Speedo rather than in series.

Notwithstanding that, your test shows that the likelihood is that the interface unit is working correctly, which would imply that the problem is more likely to be at the instrument cluster interface or the speedo itself.

Good luck

Paul

 

Sanchez, assuming your 88 has the vehicle speed transducer in the diff rather than the transmission, the signals for the speedo and trip computer are different. There is an interface box in the trunk on the left hand side that receives the signal from the transducer in the diff, and converts the signals in to something the speedo, trip computer and cruise control can use. The guide at the link below has both the 87 and 88 XJS wiring diagrams, including the speedo and trip computer. The 88 diagrams are at the back of thecdocument.
https://jaguarrules.ru/wp-content/up...ical-guide.pdf​​​​​​​

 

@jal1234: My post clearly states that my car is a 1988 Jaguar XJS. Thanks for the electrical diagrams, I already have those.
I understand that the speed transducer( diff mounted) signal is split 3 ways from the speed interface. one for the speedometer, one for the TC and one for the Cruise control.
As stated above, there is some kind of signal from the speed transducer (0.59 v ac),whether that's the proper signal, I do not know.
I will be checking the schematic to see which of the wires from the output connector take the signal to the speedometer and post results.
Thanks again.

 

Fig 18-1 of the S57 schematic shows that the green wire from the speed interface goes to the TC. This green wire has a splice that goes to the speedometer.
I will have to check the voltage on this green wire but I do not know whether the voltage leaving the Speed Interface is AC or DC voltage nor do I know what value I should be looking for.
I read a post from Warrjon and he stated that I should see 5 volts AC on the blue wire but he does not state whether that voltage reading should be the input to the Speed Interface or the output.
Given that I saw 0.59vac on the output of the Speed Interface, I don't know.
Is it AC voltage as the input and DC voltage as the output at the speed interface? Anyone?

 

The green wire is a 12v switched power feed from fuse #4 at the top of the diagram.
Does your cruise control work? If it does, then you know it's a speedo problem and not a feed problem since the speedo and cruise control get the same signal.
You really need to look at the blue (U) and red (R) wires coming out of the interface with an oscilloscope. They are pulsed signals. A voltmeter isn't going to tell you much.

 

I have a mini oscilloscope but I am trying to figure out how to properly use it. The instruction guide is not very user friendly. I did use it once to check for signal on the dreaded white wire on my 85 XJS coupe at the ECU. My main query is whether to set the scope up to read AC volts or DC volts. I am still in the trunk area. After I am sure that all is well in the trunk area, I will move to the instrument cluster. I believe that the pulse signal to the Speedo is on the white/yellow wire at the cluster. If the speedo is bad, I have the original cluster just in case.

 

Should be set to DC volts.

 

Oscilloscope Test 2 DC
Oscilloscope Test 1 AC
Here are the reading from pin #7 of the connector to the Instrument cluster.
At the end when I increase the wheel speed the spikes get closer together.
However when I reconnect the speedo (2 of them) no joy.
I guess I will be looking for a used speedo.

 

Since you have the sensor in the diff, this should be the correct speedo. I bought one of these when I converted my 87 to a diff mounted sensor. They say new, but are actually factory rebuilds with a zero mile odometer. Even included a sticker to say that the speedo had been changed. Works perfectly and only $64!
https://www.ebay.com/itm/15531550550...mis&media=COPY

 

I replaced the speedometer with a used one that I had.
The speedometer works but the Odometer and the mile-ometer? do not register the change in mileage.
I read somewhere on the forum that I should take the speedo apart and lubricate the gears that make them to turn, but no details as how to do that.
Anyone knows?