Tachometer Accuracy

[Chrises Craft]

I have a 97 Mainship 350 with twin Volvo Penta TAMD-41p-a Diesels. Both tachometers on the starboard side read about 400 rpm too low. Any ideas why this could be and how to correct? Thanks

 

[Brooksie]

Your tachs run off your alternators. Make sure your belt isn't slipping of worn out. Then adjust using the screw on the back of the tach. Get a digital tach at Harbor Freight and check them both.

 

[LarryM]

I guess the obvious first question is whether or not the starboard engine is actually running 400 rpm too low. You should check it with a handheld digital tachometer to know if there is actually a difference between the indicated and actual engine speed.

I'm pretty sure the Volvo TAMD41 diesels all use a tachometer sensor p/n 862251 which is located on the timing gear casing. Here is a parts breakdown for your reference. If so equipped, make sure the sensor wiring is clean and tight. You might also make sure the sensor is clean and well seated. As the gear teeth pass the coil in the sensor, a pulse is created that triggers a counting circuit in the tachometers.

If there is no sensor, but only a blank plug, then your tachometers are sensing the alternator speed, so wiring, loose belts etc. could be the cause. Like Brooksie said, these tachometers generally have an adjustment.

In either case, you should use a digital tachometer to determine your actual engine speeds.

Good luck

Larry
M/V Boomarang

 

[boatpoker]

or switch the tachs and see what happens.

 

[bayview]

yes use a photo tach to verify what is happening.

Do they read the same at idle??


you could briefly run both engines at wot in idle. If they match in rpms and the tachs reads the same then it is not the tach

 

[ancora]

While we are on the subject, a friend has a Camano with a TMD41, and installed an AETNA digital tachometer. He had to glue magnets on the harmonic balancer to get it to work(?). I have TMD40s and the AETNA tachs plugged right into the original wiring. Why the difference?

 

[Capt.Bill11]

Cheap laser photo tach

Digital Photo Laser Tachometer Non Contact High Accuracy MPU LCD Display Handheld - Rakuten.com Shopping

 

[READY2GO]

I put the Tiny Tach on our boat and am very happy with in. It connects to one of your fuel injector lines.

Design Technology, Inc., Home of the Tiny-Tach - Diesel Tiny-Tach Tachometer

 

[LarryM]

I put the Tiny Tach on our boat and am very happy with in. It connects to one of your fuel injector lines.

Design Technology, Inc., Home of the Tiny-Tach - Diesel Tiny-Tach Tachometer

Now that is cool!

 

[Astral Blue]

Just curious... How were you able to confirm they are running 400 RPM's off? Were you able to use a laser tachometer or similar device to test it. Also, if the laser tachometer's strip is attached to a pulley, you will get the RPM readings of the pulley, not the engine.

 

[psneeld]

Just curious... How were you able to confirm they are running 400 RPM's off? Were you able to use a laser tachometer or similar device to test it. Also, if the laser tachometer's strip is attached to a pulley, you will get the RPM readings of the pulley, not the engine.

most will put it on the pulley on the crank end so it is the engine rpm.