Tach reading too high, fixed it!

Dave M.

New Member
TECHNICAL Contributor
Oct 9, 2006
874
Hermiston, OR
Boat Info
270 DA
Engines
7.4L, Bravo II
Day before yesterday I took the boat out for a short cruise. My stated purpose was to make sure the fuel stabilizer I added got mixed in with the gas. :grin:

Anyway, I was on plane, set RPM at about 3500, and noticed I was doing less than 25 MPH. Chartplotter confirmed the slow speed. The tach was steady, like it really believed that RPM. Remembering from the old board the problems with the switch on the back of the tach, I stopped the boat and pulled the instrument panel. Then wiped the # cylinders selector switch back and forth a few times.

Buttoned it back up and tried again. Sure enough, just wiping that switch back and forth a few times added at least 5 MPH to my speed! :smt038

This thread belongs in the electrical section.
 
Dave M. said:
Sure enough, just wiping that switch back and forth a few times added at least 5 MPH to my speed! :smt038

Ok, I'll bite, How did it increase your speed :smt101
 
Chuck,

The older tachs like I have have a common failure mode where they read incorrectly, often too high. Sometimes this can be fixed by turning the selector switch on the back a few times, wiping the contacts internally.

In my case, the tach said I was doing 3500 RPM, which should give me at least 28 MPG, depending on weight and trim. But I could tell by the speed and the fact I was not dragging anything like my boat trailer behind me that I was not really getting 3500 RPM. I was probably closer to 3000 or 3200, and doing a bit over 20 MPH.

By stopping, turning off the engine, wiping the contact, restarting and getting back on plane, I really was doing 3500 RPM when the tach said so. This gave me the 28 MPH or a bit more that I should get at 3500.

So maybe I lied a tiny lie, in addition to turning the switch, I also would have had a more advanced throttle position on the second run to get a true 3500 RPM and 5 MPH more speed. But that would not make such a nice story! :grin:

What prompted me to write the original post was that someone had written to me saying they primarily use RPM to set speed (so do most of us, I think), as their speedometer is currently broken. They have GPS to confirm speed. But the RPM gauge is known to have this failure mode from time to time, so it is good to double check. If your speedometer is broken, you have to check the GPS.

There may be new people to the board that are not aware of the tach problem, I wanted to mention it.
 
Hey Dave thanks for the info. I thought maybe you had one of the special speedo that the other Dave had on his boat that keep giving him the extra fast reading :lol: :smt043 :lol: But seriously will keep it in mind if I ever run into the same problem. That is what this forum is all about helping each others
 
chuck1 said:
Hey Dave thanks for the info. I thought maybe you had one of the special speedo that the other Dave had on his boat that keep giving him the extra fast reading :lol: :smt043 :lol: But seriously will keep it in mind if I ever run into the same problem. That is what this forum is all about helping each others

Chuck,

My boat has done the same thing in the past. I've had to tap on the tachometer a couple of times lately to get the RPM down to the correct reading ('97 250 Da).
 
So THAT'S the solution. I've had this problem on my '94 250EC since I bought it several years ago and couldn't figure out the erratic RPM's. I used to tap (lightly) on the display glass and it would jump back down. Thanks for the info!
 

Forum statistics

Threads
112,943
Messages
1,422,703
Members
60,927
Latest member
Jaguar65
Back
Top