Are my tachs liars?

Mightychan

Member
Oct 8, 2016
157
Panama City FL
Boat Info
2005 DA 420, AB Dink
Engines
Cummins 6CTA
New to me 2005 420Da with 6CTAs. Port tach would wander around a bit, starboard was pretty stable. Wot registered 2700 on the starboard (stable) tach. Posted about this in the 420 Da thread and was told that there was a 4-6-8 switch on the back of the tach that would corrode and cause this behavior, just move it around and all will be fine. Pulled the panel and did not see what I would consider a 4-6-8 switch. What I found was a rotary control with 3 positions marked 103T, 126T, and 159T. Having no idea what those meant, I rotated the control between settings a few times (no detents) and returned them to the middle position, where they had been to begin with. The good news: port tach is now stable. The bad news: they both now read 2500 at WOT. So, have I messed up the calibration of the tachs? I don't understand why they would not both read 200 RPM less. Suggetions? Many thanks....
 

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The numbers refer to the number of teeth on the flywheel. There is a sensor picking up the teeth and counting them as the flywheel turns and converting that into RPM.

On Caterpillar engines, the flywheel specified has a tooth count listed and that’s what you would set on the back of the gauge. I would expect Cummins to be the same. Boat diesel.com shows a SAE #3 flywheel. I think 126 teeth is correct.

As to why they both now read 2500? They could both be correct, or both be very wrong. Only way to accurately know is to use a strobe tach to check actual rpms and compare to the tach.

I don’t think these tachs are know for high precision. Some people have gone to digital techs.
 
Dtfeld is correct. The Searay tachs are known for these issues. My advice is to check the Rpms with a photo tach. To be honest you will probably need to replace the tachs. Aetna is a good choice or you can order exact replacements from Searay-parts.com.

As far as the actual rpms... most of these boats are overpropped and need adjustments to the props. Your wot rpm should be 2650 fully loaded, I set mine at 2750 to be safe. Tony Athens is the pro on the 6CTA. Check out his tips at a sbmar.com and boatdiesel.com.
 
Thanks guys. Tell me a little more about the strobe tach. Never used one. I see some pretty inexpensive ones on Amazon, I assume that I just ride in the engine room with it pointed as something spinning while the person driving reads the analog tachs and compare the readings?
 
You got it. You can just measure the shaft rotation rpms if you want and muiltply by the transmission reduction and get the rpms that way. The photo tachs come with a little piece of tape you put on and it reflects the infrared light then calculates the rpm.

I actually have an old fashions stroboscope I used but many of the guys have photo tachs.
 
Bill

That is correct. I keep a pair of ear muffs on my ladder to the engine room for this task.
 
So like, you gotta stand in the engine room holding the strobe while the engine is roaring away right next to you at 2700rpm?

Bill, you can make it a little easier if you put the reflective tape spots on your prop shafts or coupling flange, or I put mine on the spare seal carrier, then take your reading and multiply it by the gear ratio. In our case, 1.483. Saves you from crawling up to the front of the engines.
You can see the tape in this pic.

DSC_1039.JPG
 
My reflective tape is on the outside of the harmonic balancers. I can open the hatch and point/shoot both engines from the cockpit. That might not be possible in some boats......but, gotta love the 450DA engine room design and access.
 
I have never done this at high rpm.... so you guys are saying to run a Cummins 6CTA engine up to max rpm's in neutral with no load?
 
I have never done this at high rpm.... so you guys are saying to run a Cummins 6CTA engine up to max rpm's in neutral with no load?

I believe it would be best under load. Someone piloting at the helm and confirming RPMs on Tach as you yell them out from your strobe reading.

I need to do a similar procedure on my outboard to correct my tach. However, I'll be using an inductive timer that clamps onto the plug wire and has a very long lead to the display unit. thankfully, I won't be sitting next to a 135HP with the hood removed.
 
I have never done this at high rpm.... so you guys are saying to run a Cummins 6CTA engine up to max rpm's in neutral with no load?

That is what is referred to as high idle speed. Gear in neutral, push the throttles to the pins and a Cummins 6CTA M-3 450 Diamond should turn between 2920 and 3020 rpm. Obviously for that, you need to get to the harmonic balancer, which is almost totally covered by the belt guard so remove it first or you can just sneak the laser in under the guard very low on either side of the motor just above the motor mount. (your v-drives may vary) This test is rarely needed.
That test is usually performed to verify throttle cable adjustment is good or that full fuel is being delivered and burned. It's generally done when the underway wot described below comes up substantially short as a check to be sure it will reach "high idle" when not loaded.
When we are talking about wot rpm, that is the rpm that the motors turn up when in gear, underway, with the boat loaded down pretty good and a slightly less than clean bottom, AKA everyday boating with full fuel and 2 or 3 other couples out for a sun set cruise.
Cummins says that when you are running your boat at wot, if you are turning less than 2600 rpm, you are overloading the motors. Basically, the boat is too heavy to allow the motors to reach their rated wot rpm. The solution is to reduce pitch of the prop to allow the rated rpm to be reached, however, there has been much written and proved that it is better to prop the boat to achieve 2700 rpm or even a little more under these conditions. I have mine propped to turn between 2725 and 2750 loaded down.
 
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