Erratic Tacho

[seventeen2]

Please could some offer any advice to a problem i have with my volvo 3.0ltr petrol engines?
The tacho gauges for both seem to give very erratic readings, i.e the needles to the gauges seem to jump about a fair bit?
The engines are only two years old, but this problem has been there since installation?
Also...while im on, I have had a problem with corrosion to the outdrives, of which i have since fitted a galvanic isolator to te shore power circuit, but have since been told i only need this if the shore power is connected all the time?
I would be extremely grateful, if someone could offer any advice as im getting very confused and dont want to put the boat back into the water if this doesnt cure the problem.
The outdrives are fitted with aluminium anodes and are sx drives in salt water.
Many thanks

 

[sailorman]

it the tacho alternator driven. if so is the alternator belt slipping

 

[A_8]

If you are in saltwater you need zink anodes, not Alu. That would explain the corrosion.
Yes, the isolator only help when shorepower is connected.

I have a similar problem with one of my tacho gauges and its not the belt slipping.
I have not got around to fixing it yet but when I do I will put in direct wires from the alternator to the tacho

 

[leomagill]

If you have a magnetic tacho pickup they tend to jump about if set too close to the flywheel or trigger point, I found this on my old bmc diesels.

 

[carlton]

Check the contacts at the back of the gauges.

 

[seventeen2]

thanks for the replies, most grateful!
I will be checking out the tacho circuit as informed.
Could anyone suggest anywhere i can get zinc anodes for the volvo sx drives as ive searched the internet but only seem to be able to get hold of aluminium?

 

[sailorman]

asap supplies ??????