Strange tach behavior

Mightychan

Mightychan

Member
Oct 8, 2016
157
Panama City FL
Boat Info
2005 DA 420, AB Dink
Engines
Cummins 6CTA
Hi all, 2005 420 DA, fully mechanical 6CTA engines, original gauges and VHF. I just noticed that, at idle, if I key the VHF, the starboard tach rises to about 1000 RPM (from about 550 RPM idle). Once I am over 1000 RPM, keying the VHF causes no change, but at any speed less than 1000 RPM, keying the VHF will cause the tach to rise. Obviously, the engine is not accelerating. This doesn't seem to be causing any problems, I am just wondering if anyone else has experienced this, and if it is something that I should address. Thanks!
 
Two options:

Pull the tachs and look for a switch on the back rotate a couple times...wash rinse repeat. The teleflex tachs are notoriously inaccurate.

Or buy a set of Aetna digital tachs and get read outs +/- 1 rpm.
 
dtfeld said:
Two options:

Pull the tachs and look for a switch on the back rotate a couple times...wash rinse repeat. The teleflex tachs are notoriously inaccurate.

Or buy a set of Aetna digital tachs and get read outs +/- 1 rpm.
Click to expand...
I agree with dtfeld this happened to me in the past and this process resolved my problem.
 
Check the ground bus for good clean connections and that the bus bar connections to ground are clean and tight.
 
I second flipping the switches on the back of the tach. They get oxidized and switching them to other settings and back usually works. It's easy, so try it first!
 
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