Electronic Speedos

On one of my infrequent sojourns onto the roads to keep everything moving in the Jaguar, I was dismayed to note that the speedo was not working. During earlier such drives I had noticed that the speedo needle would flicker, travelling slowly down as speed was increased. The service manual states that, due to the nature of the instrument a slight flickering of the speedometer pointer may be noticeable at below 15 kph (10 mph) However, this time no amount of tapping on the glass would induce the needle to move and it stubbornly remained on the zero against the rest. The odometer was also noted not to be recording the distance travelled.

On returning home and consulting the ‘Series 3 Service Manual’ the trouble pointed to a defective transducer or speedometer. The later Series 3 vehicles are not fitted with a speedo cable but have an electronic sending unit (an electronic pulse generator to the more technical minded) screwed into the rear of the transmission case. A zero reading can either be due to a fault with the transducer and its wires or with the speedo itself. The problem is to first establish which end is faulty…

A visual inspection from under and inside the car noted that the problem was not the result of either a broken transducer wire or a burnt out No. II fuse. It was possible that the wiring connections could be either loose or dirty. On jacking up the front of the car and. putting it on axle stands, I was able to get a closer look at the transducer end. The transducer and its wiring is along the right-hand side of the transmission case.

To confirm that power was getting to the transducer, all you need to do, is with the ignition turned on, your test lamp should show power (12 volts) at the green wire leading to the transducer. It should be noted that at the connection to the transducer, the green wire changes colour to yellow. The other wire is brown and is the earth wire and is grounded behind the speedo in the dash. There is no concern in getting the 2 wires mixed up as the plug is designed to connect only one way. This is the case with most of the Jaguar wire connections To remove the transducer it is possible to just fit a spanner between the side of the transmission case and the wall of the transmission tunnel to undo the transducer. I also removed the speedo driven gear assembly by undoing the 2 bolts holding the shaft securing plate. When refitting the securing plate, make sure that the lug in the bore of the plate relocates in the groove of the driven gear assembly.

At the other end you can remove the speedo from the dash panel by pressing its face into the dash panel and turning it clockwise until the rear of the speedo case releases from the locking tabs. All I was able to do at this end was check that the electrical wires were unbroken and that the earth connection was good.

With both ends of the green and black wires testing OK and with a new transducer estimated at about $170.00 it was thought prudent to test the old transducer to confirm it status. A quick visit down Keys Road, Moorabbin to Daley Electronics was called for. After testing, I was assured by Mike that the transducer was dead and a new one was required. When purchasing a new transducer ensure that it comes supplied with an oil seal to fit the end of the driven gear shaft and the transducer.

After refitting and a test run of the jaguar, the speedo was seen to back in working order again.

By Dawson Miller – JCCV November 2010