Certainly on my 1990 project 7 3CX the rev counter takes a feed along a co-axial cable from the alternator - the common problem is that the electronic unit in the display head fails - JCB regard it as a sealed unit and they cost over £300. There is a chap flogging them on ebay at the moment for £99. Be carefull regarding the connectors - modern ones use round plugs with square pins - whereas the older ones use squarer plugs with round pins ! There is a converter assembly available from JCB for about £45
Andrew Mawson, April 2009
