7000 RPM Bad Tach?

[KDAVID1]

1995 Merc 150 XR6--25P Pro V prop on a 1991 Ranger 354V. Dropped my wife off at the ramp and headed back out to run the motor wide open. Water had some chop--so I was running WOT trimming up to get the speed-looked at water pressure and it was perfect (18psi) glanced at the tach--and it was pegged at 7000rpm Scared the heck out of me. I immediately let up on the hot foot. Then started calculating in my head--150, 25P prop, GPS speed was 57mph, motor wasn't whinning--I ruled out spun hub as my speed was normal. I had noticed that tooling around at 35-40mph the RPM's were kinda high earlier in the day. When I got back to the ramp and was letting it idle out of gear it was sitting at 1500. Normal is 750rpm and the sound of the motor was proper. So my assumption would be a bad ground on the tach-would I be correct? Thanks in advance for your help

 

[Silvertip]

Re: 7000 RPM Bad Tach?

No -- but the calibration dial on the back of the tach may have jiggled to an incorrect setting or developed a bit of corrosion. Using a small screwdriver, note the current setting (should be 6P). Then carefully rotate that dial back and forth a few times (no force here -- I said gently). Then set it back on 6P. No try again. While you are under the console, check wire connections.

 

[KDAVID1]

Re: 7000 RPM Bad Tach?

No -- but the calibration dial on the back of the tach may have jiggled to an incorrect setting or developed a bit of corrosion. Using a small screwdriver, note the current setting (should be 6P). Then carefully rotate that dial back and forth a few times (no force here -- I said gently). Then set it back on 6P. No try again. While you are under the console, check wire connections.

Thanks. I looked at the connections while I was cleaning the boat out from this weekend and all was fine--the setting crossed my mind but couldn't figure out how it could have changed--but will take a look at it. Thanks again--will let you know.

 

[Silvertip]

Re: 7000 RPM Bad Tach?

The setting doesn't have to change! A slight bit of corrosion on the contacts for that calibration dial alters its resistance and hence the incorrect reading. Rotating the dial back and forth a few times burnishes the contacts and brings things back to normal.

 

[KDAVID1]

Re: 7000 RPM Bad Tach?

The setting doesn't have to change! A slight bit of corrosion on the contacts for that calibration dial alters its resistance and hence the incorrect reading. Rotating the dial back and forth a few times burnishes the contacts and brings things back to normal.

gotcha. I did it this evening and will be taking it out in the morning--fingers crossed.

 

[KDAVID1]

Re: 7000 RPM Bad Tach?

worked like a charm