First of all I want to thank everyone who posted their responses to my earlier request on how to solve my erratic speedometer issue.
Like most people with a speedometer problem, I have tried most of the suggestions but with no luck. I have worked with the wiring diagram from front to back, disconnecting and re-connecting as many joints that I could find - I have also used expensive silver grease to try and eliminate the fault at the connections.
I had limited success, but nothing permanant. Whenever the speedo stopped, I could get it going again by sitching the HRW on and off quickly - this always worked, even the blower motor would do it, but it wouldn't cure it.
Then I stumbled on tapping the binnacle cover above the multi plug locations on the circuit board on the speedo unit. I noticed that this would also start the speedo going, but not always.
I removed the binnacle cover and examined the plugs, there are two of them, a horizontal one with about 6 wires and a vertical one with about 4 wires( you know the ones!!) and decided to have a closer look at them. I had removed them previously as David G had suggested that this would be the problem, but again with only limited success. This time I removed each of the cables from the respective plugs, make sure you do it one at a time!! The ends are designed to tension and trip the respective pins, but the material is soft and can easilly become loose. By carefully readjusting each connection the grip of the plug is much more secure. It was obvious as soon as I refitted the plugs that there was a definite problem previously. I also used some foam to cushion the looms that attach the plugs to prevent them from moving about, with the weight of the spiral wrap on the wire loom, it is no surprise that the plugs move during normal driving over bumps etc.eventually reducing the tension on the connections.
The vertical plug ( the back one, which connects directly to the speedo) is more prone than the horizontal one.
Since I have made these adjustments, everything has been back to normal.( I have probably just put the kiss of death on it??)
Try it. It has removed countless frustrating hours trying to find the problem, which I was convinced that it was an electronic fault!!
Good Luck!!!!!