RPM Gage and Run Time Meter

[KZW]

The RPM gage on my 2005 C-310 stated acting up. The engine run time hour meter didn't increment after 40 minutes of engine on time today. On engine start, in idle, the RPM read 2,100. As soon as I pushed the throttle up on leaving the dock, the RPM pegged at 4,000. Given the sound and speed (3 knots) I believe the throttle was just under 2,000.

Folks in the dock indicated it may be the alternator? The voltage gage read 12.5 VDC when I turned the key to fire the glow plugs. On engine start it jumped to 13.5 VDC, so I presume the alternator is charging.

Anyone have any ideas on what the issue is, or what I should check?

 

[Stu Jackson]

The alternator. Really.

In addition to the AO (alternator output) there is the connection to the tach. In the back of the gauge in the cockpit are connections to that tach wire and the power to the gauge itself.

Whenever something electrical is intermittent, it is 99% of the time the connections.

Start at the alternator, since it is less usual to have connections at the back of the gauge go wonky.

You might also consider doing a search on "trailer connections" here, if you haven't done that yet.

 

[KZW]

Stu,

Thanks for responding.

Trailer connections???

 

[Stu Jackson]

Trailer connections???

You're welcome.

Try this, Engine Wiring Harness

http://c34.org/bbs/index.php/topic,5078.msg44056.html#msg44056

 

[KZW]

Stu,

I've been looking at the wiring diagram in the Universal Tech Manual. Looks like a brown wire from the Alternate goes to the tach to provide RPM.

However, the hour meter built into the tach didn't move at all in 40 minutes of engine run time. The hour meter should be on whenever the key is in the RUN position. All it should need is power and a ground. The diagram indicates power goes from the key switch to the Oil pressure gage, to the Water temp gage, to the Voltmeter gage, and then the tach. The ground for all the gauges runs from the G connector block (wherever that is???) to the tach and then to all the other gages.

All the other gages work, so the ground must be good and they are clearly getting power. The tach gage is getting an RPM reading because it is trying to track the movement of the throttle level as the engine runs up and down. Power is getting to all the gages, except the tach hour meter, which doesn't need the alternator to run (the hour gage will operate with the engine off and the key in the run position).

So, my best guess is power isn't getting to the tach. Alternator input is getting to the tach as the needle is trying to follow, but without a reference power reading, it won't read correctly.

Check the brown wire on the alternator anyway, because that is easiest thing to do. Taking the instrument panel apart is not easy and risks damaging something else. (A helicopter mechanic once worked with me on a motorboat and after a couple of hours indicated I should only be allowed to work with tools under close supervision).

Am I off base in my thinking?

 

[Tom J]

The tachometer on my 2003 C310 will often do what you describe. Following the advice of an electronics technician, I removed the tach and tried moving the calibration screw on the back of the tach to the left and right, after marking the original location of the screw. He explained that this helps wipe off oxidation on the contacts of the calibration screw. This worked fine initially, and it was a long time before the tach started acting up again. Recently, though, the tach gets intermittent very often, so I think it is time for a new tach, since the unit is sealed and cannot be taken apart. Unfortunately, this doesn't explain what could be wrong with your hour meter.

 

[Stu Jackson]

Am I off base in my thinking?

Good starting point(s). The more you learn as you go the better off you'll be. Step-by-step is usually the way it works, for me and most of the skippers here.

You didn't mention it, but one must assume here that you have a mechanical engine hour counter inside the tach gauge body. Sometimes they go south and get gunkied up insie and stop working even though the rest of the gauge (the tach) still works and the power issue is just fine. In that case, it's either a new gauge or a simpler separate engine hour meter installed somewhere convenient, since it doesn't have to be at the cockpit panel.

 

[KZW]

Well, it turns out I lied. The hour meter is working just fine. I took the console apart and checked the wiring vs. the diagram in the Universal manual. Aside from some differing wiring colors and a few minor differences, I was able to figure out where the ground, power, and signal from the Alternator were attached.

Turned the key to the run position and the mechanical hour meter ticks over just fine.

I then started the engine, with a voltmeter on the brown wire from the alternator. It read a constant 7.15 VDC, no matter where the throttle was located. I'm assuming the DC voltage from the alternator corresponds to RPM. However, now that I'm writing this, the signal might be AC. I simply don't know. I will need to find an alternator person and ask.

 

[KZW]

The tachometer on my 2003 C310 will often do what you describe. Following the advice of an electronics technician, I removed the tach and tried moving the calibration screw on the back of the tach to the left and right, after marking the original location of the screw. He explained that this helps wipe off oxidation on the contacts of the calibration screw. This worked fine initially, and it was a long time before the tach started acting up again. Recently, though, the tach gets intermittent very often, so I think it is time for a new tach, since the unit is sealed and cannot be taken apart. Unfortunately, this doesn't explain what could be wrong with your hour meter.

You wouldn't happen to have a picture or diagram showing the calibration screw would you? I just got back from putting the panel together and simply didn't recognize one for what it was.

Of course, I spend 5 minutes looking for my glasses (couldn't read the wiring diagram without them) and they were affixed firmly to my face!

 

[SimiSAilor]

You should be reading AC voltage for tachometer indication. Should be about 6VAC at idle and increase with RPM up to about 8 VAC at full throttle. Hope that will help.

 

[KZW]

You should be reading AC voltage for tachometer indication. Should be about 6VAC at idle and increase with RPM up to about 8 VAC at full throttle. Hope that will help.

Thanks very much!!!!! I will try that next to see what I get.

 

[KZW]

Measured the alternator output to the Tach this evening.

At idle it was 8.0 VAC
At about 1/2 throttle it went to 8.88 VAC.

I was tied up at the dock and didn't want to run the throttle any higher.


So, the issue appears to be the Alternator. Stu, take a bow, you were correct.

I presume I now need to find a shop that will rebuild an alternator, or buy a new one!

 

[rpwillia]

So, the issue appears to be the Alternator. Stu, take a bow, you were correct. I presume I now need to find a shop that will rebuild an alternator, or buy a new one!

When I was having problems it was $120 to rebuild and around $85 for a new one. The stock alternator is a Mando 55 amp alternator and if you check with an electrical supplier you should be able to find a compatible one. I recall the one I got was a Mercruiser branded one, but it was identical in everything but the label.

 

[Stu Jackson]

So, the issue appears to be the Alternator.

I presume I now need to find a shop that will rebuild an alternator, or buy a new one!

Congratulations. You've been pretty busy with this and the stuffing box, eh?

One of the best moves I ever made was to find a local alternator shop. Really worth the effort. Sometimes fixing the old one is worth it, but like everything else, you gotta do your own figuring it out. Comparative shopping is very helpful.

Here's one source I got from this board

http://www.ase-supply.com/Leece_Neville_110_603_12V_90_amp_ALTERNATOR_p/mo-110-603.htm

Good luck.

 

[SimiSAilor]

I was inspired by this thread to check my tach with an optical tach as I had a feeling it was reading low. Found it to be 200 rpm low, from idle to full throttle, that is for another topic. I did wipe the adjustment screw back and forth a few times before tweaking it, I also think that is a good idea. While I was there, checked voltage at idle and had 7.67 VAC @ 1100 RPM, increased to a little over 8.1 at 2000 rpm. I found troubleshooting info on page 65 of the service manual that came with the boat.

If you need the manual you can google "universal service manual 200554", I downloaded it from Westerbeke.

 

[KZW]

Congratulations. You've been pretty busy with this and the stuffing box, eh?

One of the best moves I ever made was to find a local alternator shop. Really worth the effort. Sometimes fixing the old one is worth it, but like everything else, you gotta do your own figuring it out. Comparative shopping is very helpful.

Here's one source I got from this board

http://www.ase-supply.com/Leece_Neville_110_603_12V_90_amp_ALTERNATOR_p/mo-110-603.htm

Good luck.

Yes, making a living keeps interfering with working on the boat!

 

[Tom J]

You wouldn't happen to have a picture or diagram showing the calibration screw would you? I just got back from putting the panel together and simply didn't recognize one for what it was.

Of course, I spend 5 minutes looking for my glasses (couldn't read the wiring diagram without them) and they were affixed firmly to my face!

The calibration screw is marked on the back of the tach. Reading the AC voltage to the tach from the alternator is just a quick check to be sure the circuit is functioning. The circuit is actually sensing frequency and converting that to RPMs.