Tachometer Accuracy

Tin Diesel

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There's a lot of threads with "Tachometer" in the title. This one is about tachometers that are inaccurate and have always been inaccurate, and about one possible cause. Its not about tachs that were OK but then broke. By the way, you probably don't know whether your tach is accurate (and there's a good chance its not) unless you check it with a second gadget (there are options...).

When I bought my new Mercury Vesselview Mobile module, it has a tach monitor. It showed a very different RPM than the analog gauge on my dash.

When the Vesselview showed 5000 rpm at WOT (and it sure sounded like 5000 RPM!), the analog gauge showed about 3500 rpm. At idle, the difference is about 200-300 rpm. So, I started researching how to calibrate tachometers (I think this is for older boats, because newer ones seem to have the ability to calibrate with buttons on the front of the tach).

My boat's tach brand is Teleflex. I found a lengthy troubleshooting guide by Teleflex for all of their gauges. For tachometers, their first trouble-shooting recommendation is to check your tach and make sure the engine-type selector is set to the right type of engine. They said it is very common for boat builders to install these tachometers and not set them correctly. It will still give readings, but it will be the wrong reading. (In their defense, boat builders may not always know what type of engine will end up on the boat)

See the catalog photo below for the selector switch for what kind of engine you have. Mine should be set at 8C for eight cylinders.

However, my tach is actually set to the factory-original of 6P (which is for outboards).
I won't get a chance to adjust it for a week or so, but when I do I'll report back here.

Plan B is to adjust using the CAL. screw on the left side of the image.

I know that "Take It Back To The Dealer" is a common response, but in most cases, this is an easy check and fix... easier than getting it to the dealer and waiting a couple of weeks!

1595960266721.png
 

Tin Diesel

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Well, I finally got a chance to adjust the tachometer and test it.

As I noted above, I set the tachometer to 8C. I started the engine in the driveway - - idle speed on the dash tachometer was now much more accurate.

It was a little higher than the Mercury app showed by about 50 rpm, but I thought that was pretty good.

Yesterday I tested it on the lake and the higher the RPM, the more accurate it seemed to be - - usually within about 25 RPM of what the Mercury app showed. I'll resist the urge to get anal and try to tweak it to get closer! This is good enough.
 

Tin Diesel

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Oh, and WOT for my Mercruiser 5.0L is 4600 to 5000 rpm.

At 5050 RPM, I hit 38.5 mph (3/4 tank - 45 gallons, just two of us, bimini up, mostly smooth water)
Good enough for me.
Toooooo fast for her!
 
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