Faulty Tachometer - Volvo Penta MD2030

[onlinelee]

Hi friends,
further to my other post about my engine idle speed being very high at 1200rpm.
It turns out the analogue tacho is giving a false reading and the rpm is in fact around 850.

Before I call an engineer to investigate the instrument is there anything members can suggest that I could check myself?
Thanks in advance, Lee

 

[Capt jgw]

Is it wired to the alternator or cable driven, and did it ever read correctly? If wired to the alt, check all the wire connections for power, ground, and the sense wire. Loose or corroded connections on any of these can make it act up. If cable driven, the cable may be going bad or need lubrication. Either type may have a calibration screw on the back that needs adjustment. If it never read right that may be all it needs. I don't think that adjustment would change though, so if it used to read correctly and now doesn't there's something else going on. Have you changed alternator or the alternator pulley? That would mess with an alt driven tach, that's why they have a calibration screw.

 

[onlinelee]

Update... I am informed by the last owner that the tacho had read correctly till the engine was rebuilt 6 months before I bought it. Im sure its a wired connection so will check those and lift the binnacle to see if the tacho does in fact have an adjustment facility.