Sending units-tach

[Trojan]

I have 6-71N DD. I'm having intermittent tach readings on the fly bridge tachs. Both sides. Where are the tach sending units located? On the generators or the engine. They are 1973 vintage electrical driven. I cleaned the tach heads and fitting. Next work is down below. The gages in the lower station are working fine. I don't imagine there are any parts to repair anything.



BILL

 

[Nonchalant1]

DD's have 4 or 5 places they attach tach drives. One is up front on the right bank cam shaft. Most are in the rear coming off an adapter from the blower drive shaft, blower helix shaft or either cam shaft. It is usually a little right angle gear box and a very small generator, almost like a bicycle tire driven generator, with a wire coming off it to the tachs and a wire or cable going to your synchronizer.

If the lower helm tachs are working fine, the sender is probably OK too. It's a single sender for both the helm and the bridge tach for each engine.

My bet would be on resistance in the wires or connections somewhere between the sender and the tach on the bridge, particularly because the helm tachs work OK and both bad ones are on the bridge which is more exposed to the elements.

You can use a multimeter and a long wire to measure resistance between the sender and the tach head and then do the same for the lower helm tach and compare. If the bridge has much higher resistance, there's your problem. Resistance reduces the apparent voltage and tahcs really just measure voltage created by the little generator sender. To fix it, you would need to start cleaning the terminals very carefully and check again. If it's not the connections you may need to replace the wires if they are the problem. To find a bad wire, you can measure the resistance of each wire when all terminals are disconnected and clean. There should be almost zero resistance in the system, like you'll find at the working tach positions.

Good luck,

Doug Shuman

 

[REBrueckner]

Odd that BOTH bridge tachs would be intermittent...could there be a common ground wire with a loose/dirty connection..or maybe a partially broken ground wire?? If they both went intermittent together, sounds like a common cause...

Another trouble shooting method is have the engines running and while one person watches the tachs for erratic movement, another wiggles and jiggles electrical wires at connection points....

 

[Trojan]

They keep changing all the time. At idle they are correct. The higher the rpm the more they vary. Each one is different and each time you start the boat there different. I want to start in the bilge and work out. The wires must split somewhere. There are times when then they are correct. Also after a couple of hours of running they are correct most of the time. I think that its a dirty connection. But the engine area is an area that I have not ventured into yet. I cleaned the bridge connections. That helped some. I need to find that ( Y ) connection and start there.



BILL

 

[Maynard Rupp]

If the lower helm tachs work OK, the problem can't be in the engine room. The wires go from the sending units to the flybridge terminal block then there should be 2 wires on the opposite side of the terminal block. One set go to the bridge tachs and the other set to the lower helm tachs. I will bet that both upper tachs are internally corroded or the connections on the back are corroded. I have taken those AC tachs apart and tweaked them. They now work fine.

 

[TedZ]

My 43DC is wired with only two wires from the engine room terminal block to the lower station tachs and then from the lower station tachs to the flybridge. I believe that was the original wiring.

Just an FYI so you can look at all of the potential wiring issues.

Ted

 

[Trojan]

Well, I have a place to start. I cleaned the upper tach heads last year thinking that was the trouble . It helped but it did not cure it. I think I need to change the wires and clean the terminals. I don't think that there is enough extra wire just to cut them back to clean wire. Thanks for the ideas.


BILL

 

[REBrueckner]

Forgive me if this is too simplistic, but a simple way to quick check the existing wiring is to get a spool of cheap wire at home Depot, 500 ft maybe $20, # 18. Color code the tempy wires by wraping one of them in black tape at each end. Use this to run a pair of tempy wires from either the senders on an engine, or the connections on the lower tachs, (which ever is easier) and power the bridge tachs from these after removing the existing wires. (Label the old wire and terminals at the tach so you know which is which.) Just run the wires up the companionway steps, thru doors etc, any quick way to get to the bridge. If that solves the problem, then you can take the time and effort to run the wires permanently through bulkheads, wires runs, pipes or whatever.

I agree with the post here that suggests the likely problem is at the upper tachs..tha's more likely than separate wires failing...