Volvo tacho problem

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snowbird30ds

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I posted this on the PBO forum but it's probably better here.
One of my tachos keeps dropping to zero, I have swapped sides and the problem is definately the tacho not the wiring as the problem switched sides with it, is this a common volvo issue and does anyone know of a fix without spending a fortune on a new tacho?
The numbers on it are:
430.240/004/002
858 654 4/88
12V/1225hz
I'm guessing the middle one is the part number, it's a vdo unit and the engine is TMD41A, the tacho looks like it can be taken apart.
The problem is intermittent at the moment but I don't suppose it will heal up on it's own.
Any help appreciated.
 
I have 42A's. It's not uncommon for me not to show rpm until the engine has run for a minute or two or rev's have climbed past 1.5k rpm before the tacho starts to work normally and rare but it does happen that the needle will suddenly bounce about when at cruise revs. The problem seems to solve itself mostly but when it hasn't it's always been an alternator issue (dirty terminals.)
 
Mine will drop completely while underway up at good rpm and stay at zero for a long time and then starts to read again, I've ruled out alternator and wiring by swapping to other engine, fault goes with tacho head for sure.
It's done it both at river speed and planing speed, it's also slow to pickup at startup.
 
I've had mine out to calibrate the revs with the dial at the back. They are if I remember correctly otherwise non serviceable.
 
Well I managed to get the thing apart for a peek inside after a bit of un-crimping and it seems the spade connectors on the outside are not directly soldered to the circuit board but sit in sockets and seemed to wobble about a bit so I have soldered in case it's just a case of loose connections between and will see what happens next time I am at the boat.
Conveniently the separate bezel covers the slight re-crimping marks.
 
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