posted 01-08-2006 01:04 PM ET (US)
The tachometer signal is developed in the Voltage Regulator assembly. Unfortunately this assembly is a rather expensive replacement part, about $230 (U.S. funds). The assembly is mounted into an access hole in the engine block where it is designed to receive cooling water. The assembly consists of a large heat sink block with an encapsulated circuit board.You have to remove the Power Pack assembly (it looks like a cover plate) to access the Voltage Regulator assembly. You may be able to see the greenish encapsulating material. The circuit board and other components are embedded inside the assembly and are not serviceable. I tried to dig the circuit board out of my old regulator. It was quite difficult. I would consider it to be a non-repairable item. If the assembly has been overheated, the potting material tends to harden and crystalize, which makes it more difficult to remove if you were going to try to repair to the component level.
The tachometer signal is also a derivative signal from the main battery charging coils in the stator assembly. If the battery charging coils have been damaged this will also affect the tachometer.
It may very well be the case in your motor that there is also a problem with the battery charging circuit. The presence of an erratic tachometer signal may be an indicator of more problems with the stator coil and voltage regulator assembly.
The stator coil is also somewhat expensive. I think they are about $150 (U.S. funds). The problem with a stator coil replacement is its location under the flywheel. The flywheel must be removed to replace the stator coils. This can be difficult to accomplish because of the high torque needed to remove the flywheel retaining nut. You may also need a special wrench to retain the flywheel itself. Sometimes a strap wrench can be used.
Even with the flywheel in place, you can make a close visual inspection of the stator coil (as Seahorse suggests). You can probably see evidence of overheating if it was severe. You can also check the continuity of the stator coil.
Because the problem occurs when the engine has warmed up, it could very well be an intermittent component in the Voltage Regulator assembly or a problem in the stator coil which occurs when it heats up.
As seahorse mentioned, loose battery connections may be the underlying cause of these problems. Also, chronic low battery voltage and heavy charging currents can also cause a high load on the charging circuit in the motor. This in turn will cause it to be operating with a much higher temperature, leading to shorter life span.
Also, low water pressure and lack of cooling water to the Voltage Regulator assembly can cause higher temperature operation. Check the cooling system for good water pressure.