Could It Be The Tach Sensor??

[Joe Mullee]

My tach has never worked correctly in the eleven years I've owned my boat. Steadily it's been getting worse. The engine is a 1983 Yanmar 3GMF. When I first bought the boat the tach reading on the engine panel maxed out at 2200 RPM's. Today it only registers about 1600-1700 RPM's when the engine is wide open. When it's at idle it registers about 700-800 RPM which I believe is correct. A few years back I had a local yard use a timing device and it read 3400 which is the rated capacity. With a clean bottom I can cruise at 7 knots. I believe my engine is in good shape.

I've checked and cleaned the wires and removed the sensor on the engine and cleaned that too. Still can't get it to work properly. I've managed to do OK without it by listening to the engine and using the throttle to estimate the RPM's but I would like to fix it.

My thoughts are to first purchase a new tach sensor for the engine. At a cost of $139 from Torrensen this was less expensive than a new tach on the engine panel but I've never read here where anyone else found that this was the culprit of the problem.

Has anyone had any success with fixing this thing?

Thanks,
Joe Mullee

 

[kloudie1]

Joe, A quick check on the sender is to read the resistance across the connections. It should read 1600 ohms (+- 0.1K @ 20C) .. The installed clearance between the tip of the sender and the flywheel teeth is also critical and should be no more than 1.3MM .. The tachometer instrument is most likely the bad guy ..too many hours of sun and humidity. You can either get a new Yanmar one (the 97 gear tooth reader) or get a different kind that reads the alternator frequency. Most of the after-market ones that read the alternator can be adjusted (calibrated) for your particular alternator pully ratio.
My tach reads low as well and I plan to take the instrument apart and see if I can clean the bearings and make sure that the needle is free and the internal connections are good. Haven't gotten to that yet.

 

[alexco38]

I had the same problem on a Volvo 2002. After replacing the tech, as it turned out, it was the "Alt" light bulb on the instrument panel that was having intermittent connection.

The Tech on diesel engine depends on the un-rectified AC from the alternator to count cycles. There is a wire terminal (W terminal) from the alternator that carries the AC to the tech.

However, there is no magnet in the alternator. The voltage regulator can not provide voltage to the rotor to "excite" the stationary coils. Before the alternator can generate voltage, there must be some initiation "exciter" voltage for the rotor. The engineer designed the alternator using a small light bulb (ALT) to provide a little current to "initiate" the voltage generation. When that light bulb is blown or loose connection, there will be no AC generation.

You should also check the DC voltage output. In your case, it may be much less than 14.4 volt. i.e. not charging.

 

[kloudie1]

Tachometer 3gmf Yanmar

Here is the shop manual section for troubleshooting, Rich

 

[seadaddler]

As mentioned

First check the sender first and if OK than a new VDO from Defender $130.00 is what I just got and installed rather than $400.00 Yanmar part and working good and some change of wires on the plug and recalibrate,easy to do but first check all wires going to plug.
Nick

 

[Bill Roosa]

The sender is just a magnetic induction coil. if the continuity is good and there is no short to ground on either terminal you should be able to read an AC voltage across the terminals with the engine running. I don't know the required voltage the sender has to produce to get the tach to read but if you get anything at all there is almost no possibility the sender is the problem. Also measure the voltage at the tach to confirm there is no massive voltage loss in the wires.
The tach sender sends AC voltage that varies in frequency (just like the alternator stator winding types) and the tach head is just a frequency meter with a face that reads RPM.

 

[Benny17441]

Every day I learn something new. Always thought that the abreviation for tachometer was "tach" but have recently learned it can also be "tack" or "tech". I guess there are regional variations.

 

[RichH]

Agree with kloudie .... BUT dont remove the sensor to do the checking ... and check the wiring to the sensor AND the sensor at the same time - first. Yanmar wiring is notorious for corrosion, etc.
On the Yanmar QM series engines (tach sensor measures the proximity 'pulses' of the flywheel (starter) teeth) its ALWAYS corrosion / high electrical resistance in the tach sensor wiring, hardly ever the sensor.

 

[Ralph Johnstone]

Suffered With the Same Problem ..........................

..................... almost from day one when the boat was new.

Initially it happened very rarely ................. the tach would read either high or low by a few hundred RPM. Then it became more frequent as the years wore on. Checked the sender and it always came out with flying colours. So, it was time to invest in a new readout.

I figure it had to be a VDO rather than what Yanmar was peddling for the 2GM20 F. And from what I've seen on other posts here, much cheaper as well.

My favorite pastime is getting the lowest possible price I can get for a name brand item on the web. And I'm talking nickels and dimes here. Never been burned yet by a bad vendor. This time the winner turned out to be Don's Sport Vehicle Sales at:

http://www.egauges.com/vdo_ind.asp?Type=Tachometer&Series=Vision&PN=333-163

Egauges is their service department for guages. What I purchased was the VDO Vision series 333-163 for $99.95 which saves a few dollars over Defender, who is actually one of my favorite suppliers.

I would strongly recommend this gauge as it is adaptable to ANY OUTPUT from the engine. I think this thing can also read a heart monitor if needed. The final bonus is that it fit the old Yanmar gauge cutout perfectly.

 

[Marc14857]

If the TACH is getting the signal off the alternator you need to make sure that the TACH is properly calibrated for the number of poles in the alternator. Most TACHs are initially set for 12 poles but my high output alternator had 14 poles causing the unit to read falsely high. I had to recalibrate the VDO unit for 14 poles and it now reads the same as the hand held TACH.

 

[kloudie1]

Ralph !! Thanks for that post.. That VDO looks like the ticket since it can accept the output from the original Yanmar inductive sensor.. It looks like the solution to my problem as well.. at a fine price..

 

[juslookin]

When checking the system

Do not forget to check the plug in the harness, between the panel ant the engine. I have had hell with these, and actually tape the plug, to keep it from seperating

 

[Joe Mullee]

Thanks For The Info

I wish I understood more about engines, altenators, and electrical circuits to understand what everyone is talking about. I pulled up the site with the VDO gauge and reviewed the installation instructions. Maybe with the help of my dockmates I'll be able to install it. Thanks again. I'm one step closer to having a working tach.

Joe Mullee

 

[Ralph Johnstone]

I wish I understood more about engines, altenators, and electrical circuits to understand what everyone is talking about. I pulled up the site with the VDO gauge and reviewed the installation instructions. Maybe with the help of my dockmates I'll be able to install it. Thanks again. I'm one step closer to having a working tach.

Joe Mullee

Nothing could be simpler. All you need is a grade four reform school education and you'll have no problem installing it :dance:.

Seriously, just follow the instructions and you'll find it quite easy once you get into it. The instructions are excellent. Go slowly and don't be intimidated. Once you do this one, the next electrical project will be easier.