TC35A Tachometer is whacky

boatyard · Feb 6, 2020

The tach on my 2005 TC35A has never liked moisture. If you were caught out in the rain it would stay at zero until completely dry, a couple of days. A couple of months ago it was out overnight and we had a very heavy dew. The needle sits at 2500 rpm with the engine shut off and pegs itself at 3000 rpm when you start the engine, clearly not an accurate number, as the engine is still at an idle. It remains at 3000 rpm at all engine speeds.
What is wrong and can it be fixed?

seville009 · Feb 7, 2020

Maybe pull it out and check the seals and wiring. If it looks like moisture is getting in from the top, let it dry out and put a thin bead of silicone wherever the moisture is entering.

dlctcg · Feb 7, 2020

Yeah... the NH TC's had an issue with the gauge cluster developing condensation inside & sometimes causing havoc with the gauges, seems to be a pretty common problem. I would definitely pull the cluster then unplug the wire harness plug & check for corrosion. If there is some clean it up & Test it, if it resolves the issue I would pull it again & put a light coat of dielectric grease on it. (this fixed on of my gauge problems)

Also the tach gets it reading via a a single wire from the alternator (usually either a green or reddish/pink wire); I would check that connection to make sure it is clean & making good contact. You can check continuity on that wire from the back of the alternator to the cluster harness, but is is probably fine.... It should be pin #20 on the connector plug (bottom row second pin from the left).

I'm pretty sure your gauge can be removed from the instrument cluster so you could clean up those connections & test the gauge itself while you have it out.... Procedure below.

Hope this helps...

boatyard · Feb 7, 2020

dlctcg said:
Yeah... the NH TC's had an issue with the gauge cluster developing condensation inside & sometimes causing havoc with the gauges, seems to be a pretty common problem. I would definitely pull the cluster then unplug the wire harness plug & check for corrosion. If there is some clean it up & Test it, if it resolves the issue I would pull it again & put a light coat of dielectric grease on it. (this fixed on of my gauge problems)

Also the tach gets it reading via a a single wire from the alternator (usually either a green or reddish/pink wire); I would check that connection to make sure it is clean & making good contact. You can check continuity on that wire from the back of the alternator to the cluster harness, but is is probably fine.... It should be pin #20 on the connector plug (bottom row second pin from the left).

I'm pretty sure your gauge can be removed from the instrument cluster so you could clean up those connections & test the gauge itself while you have it out.... Procedure below.

Hope this helps...

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