Fixing Speedometer Problems

Joseph Iuzzolino writes:

I have an 84 XJ6 SIII with a speedometer that just stopper working the other day. It started out with not working till the car went about 5-10 miles or so and would just pop up into the speed that the car was doing ( I suppose it was correct speed). The odometer is also not working. Is there a cable that attaches to the transmission? I have never had this problem till just two days ago. The speedometer doesn't even "pop" up into a speed anymore, The needle just stays at 0 and the odometer still does not work. Any help with a way to repair this problem would be greatly appreciated.

From Brian Mee:

My speedometer dropped to zero the other day and remains there.

I checked the archives and got underneath last night to clean the connector. I took the two wire connector apart, used contact cleaner and brushed it and reassembled. The test drive showed no improvement. Did I get the wrong set of wires or should I look elsewhere?

From Greg Price:

You have a problem with the infamous speedo transducer. You don't have a speedo cable, you have an electronic sending unit screwed into the side of your transmission.

First step is to check the wires coming out of the transducer. Check and see if they are broken. Many times the wires will break from all of the transmission flexing. You can patch them if that's the case.

If that's not the case, then you will have to buy a new one. They're about $60.00 mail order and they're easy to replace.

From Michael Stanford:

The speedo on your car has an electronic pulse generator/speed transducer on the side of the transmission. It is not driven by a mechanical cable.

Most likely you have either a loose connection, a dirty connection or a bad transducer. On my car, the speedo was sometimes intermittent so I located the connection on the side of the output end of the trans, disconnect it and applied a generous dose of contact cleaner, to fix the problem.

You may also want to inspect the wires to make sure they haven't been burned by the exhaust or become damaged by rubbing the trans. I found that slightly crimping the circular contacts made the plug fit tighter.

If that is not it, I would check the wiring up to the speedo in the dash thoroughly before assuming that the unit itself is bad.

From Doug Dwyer

On a Series III XJ6, the speedomoeter transducer is on the right side of the transmission, towards the rear, with two wires (one green, one yellow). Speedometer problems are almost always due to transducer failure although sometimes the speedometer itself will be faulty.

This is a pretty easy circuit to test if you have a voltmeter. Green wire to the transducer should have 12V with key on (check fuse #4, too.) Also, the green wire at back of speedo should have 12V with key on. Black wire from speedo should be grounded (ground point G14 behind speedo). If all of these check good then you can test the transducer for output voltage.

General concensus is that the transducer produces a square wave form AC output. So, with your voltmeter set to AC and the igniton key "on", spinning the transducer cable (a drill motor would be nice for this) should produce some sort of a reading from the yellow wire. (Of course, checking the wave form would require an osciliscope so it's not easy to check the quality of the output.)

If no output at all you've got a dead transducer. If you do have output but still the speedo is inoperative then you've got a dead speedo or an open circuit between transducer and the speedo.

Jim Warren asks: Then again, my MPH is high by about 5% -- maybe just resetting the pointer is the thing to do. Since this is all electric, is the any calibration to the speedometer?

From Michael Stanford:

The meter is set up electronically and I did not find any potentiometer adjustments on the circuit board inside. The magnets have a zero, null and center field adjustment but unless you are VERY familiar with mechanical galvanometer movements, I would recommend against fiddling with them.

Mine has been quite accurate across the range and last night after the trip odometer repair, the speedo was dead accurate as always on my timed 1 mile run. If your speed is consistently high by 5% or so, you can move the needle a bit, but the mechanical movement will not follow the speed curve linearly. Setting it for accuracy at say 100 kmh, will not get you accuracy at 50 kmh in that situation. Best to have it re-curved by a professional instrument shop (if you can find a good one).

Good luck. This explanation assumes that you are using the correct wheel and tire size.