Mechanical tach question

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1 week 5 days ago #3329 by Robert Horn
Mechanical tach question was created by Robert Horn
Our FL has a mechanical tach. There is an odometer like readout which I assumed was engine hours. But bringing the boat from FL to TX, motoring or motorsailing all the way the display only showed a difference of about 80, obviously not engine hours. I pulled the instrument panel out and there is only one wire going to the tach and that is for the instrument light, so the display cannot be driven electrically, so not engine hours. So what does it register? Counts engine revolutions? I could not find any markings as to manufacturer or model number.

Bob

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1 week 5 days ago #3331 by DAVID JONES
Replied by DAVID JONES on topic Mechanical tach question
Can you post a picture?

dj

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1 week 5 days ago #3333 by Robert Horn
Replied by Robert Horn on topic Mechanical tach question
Next week when I'm back on the boat.

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4 days 13 hours ago #3336 by Robert Horn
Replied by Robert Horn on topic Mechanical tach question
Attached are two pictures of the tach. There are absolutely no markings of any kind on the case or dial to show manufacturer or model. Found a place called Dieselpro, they sell mechanical tachs, not sure if they only sell them or manufacture them. They did verify that they are totally mechanical, no electrical connection required. Ran the engine and timed the 1/100 dial, it clicked over at about 58 seconds, should have been 36 seconds, so it is slow.
Bought an electrical hour meter to install in the engine room. There is an oil pressure switch on the engine that is NO and closes on oil pressure. Plan to use that to power the electrical meter. It is currently not connected to anything, looks like it might have taken a bump. I'll have to rig some clip leads to test it while the engine is running. It's on the far side of the engine and I didn't want to lean accross the engine with an ohm meter with the engine running.
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4 days 12 hours ago #3337 by DAVID JONES
Replied by DAVID JONES on topic Mechanical tach question
Try running the engine at higher RPMs and see if the "calibration" changes. It may well be tied to RPM more so than time.

dj

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4 days 10 hours ago #3338 by Robert Horn
Replied by Robert Horn on topic Mechanical tach question
That was my original thinking. Unfortunately I was already on my way home when I thought about doing that. Next time.

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2 days 8 hours ago #3339 by Bill Chamberlain
Replied by Bill Chamberlain on topic Mechanical tach question
The engine hours on those old mechanical tachs is indeed a function of rpm. It's all about rotation. Essentially there x number of revolutions per engine hour, irregardless of time on the clock. As a result, for an off the cuff example: An hour of run time at idle might only run up a half hour on the engine hour meter, while that same length of time at max rpm runs up one and half hours on the engine hour meter. That company you found with the mechanical tachs probably can tell the number of revolutions per engine hour. With that you can figure out if the meter is working reliable.

If it's not, and the tach is giving you steady readings that make sense, you can rule out issues with the cable and the engine end. There are gears in the tach that move the hour meter that are likely worn and slipping.

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1 day 14 hours ago - 1 day 14 hours ago #3341 by Robert Horn
Replied by Robert Horn on topic Mechanical tach question
That was my initial thought, that the reading was related to RPM. I timed it at 1000RPM last time on the boat and the 1/100 digit clicked over at 58 seconds rather than 36 seconds if it was doing actual time. I'll take some readings at different RPMs this coming week. I will also verify the actual RPM reading with an optical tach.
I guess the issue is about service intervals which are speced in hours. If something is scheduled every 200 hours, for example, is that the hours displayed on the tach or should that be real hours?
Moderrn cars base oil change intervals on driving conditions monitored by the computer, rather than mileage.
I asked the company making the mechanical tachs about how they worked but never got a response.
Last edit: 1 day 14 hours ago by Robert Horn.

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18 hours 29 minutes ago #3342 by DAVID JONES
Replied by DAVID JONES on topic Mechanical tach question
I know this is not a simple answer, but if you find your hour meter is running correctly then you could begin a bit of a science project...Obviously if it's not, then it's not reliable and you should have a better method of measuring your engine hours. I actually like the idea of a meter that counts revolutions of the engine rather than simply time that it's turned on.

If you send your oil out for an oil analysis, you can gage how well your interval is working. Do this over multiple oil changes and you would have a very accurate understanding of what the timing on your meter is telling you. Modern lubricants are incredible. They do not break down anywhere nearly as fast as the old lubricants. The main concern is the oil filter. Oil filters break down now much faster than the oil used. If you have excellent oil filtration, you can run much longer between oil change intervals than typically recommended. Doing this, you could develop a schedule where after X number of hours, you change your oil filter (topping up oil as needed) and after Y number of these changes, you do the full oil and filter change. If you get what I mean.

There are diesel engines used on pipelines and such where they have set up special oil filtration systems on these engines. They run 24 hours a day for months on end without having any maintenance performed. The main factor in long term performance is the filtration system. You have to remove carbon, soot, from the oil over time. The modern fully synthetic oils last really long times without breaking down...

dj

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12 hours 24 minutes ago #3344 by Bill Chamberlain
Replied by Bill Chamberlain on topic Mechanical tach question
The scheduled maintenance hours in the manuals is based on the reading of the hour meter under whatever the manufacturer considered average conditions. They also expected the end user to figure out if usage was more severe and required that maintenance to be done sooner. Basically the computer for those old engines is our heads instead of the black box of a modern car.

The electrical hour meter you picked up will do time based hours based on how long it's energized. Most are just hooked into the ignition on of the key switch. Wiring into the oil pressure switch like you mention is a good idea. It will prevent running up hours just from the key switch being left on.

Assuming you have one of the old Lehman or Perkins engines, I don't think it will make much difference whether you use the tach based hours or time based hours for scheduling oil changes. I doubt there's enough difference between them under normal usage for the engine to really care. More important, take note of the oil's color when you check the dipstick. If it's starting to look dirty, change it. Even if it's under the scheduled hours. If still looks good when you hit those scheduled hours, change it anyway. An oil change is cheap insurance.

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11 hours 11 minutes ago #3345 by Robert Horn
Replied by Robert Horn on topic Mechanical tach question
Agreed, oil is cheap. Got a response from Dieselpro claiming their mechanical tachs, no electrical connection required, read actual hours. No explanation of how that works. I'll run some tests later this week at different RPMs to see if the hour readout changes with RPM. In the mean time, I already bought the electrical hour meter, so I will hook it up.

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