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post #11 of 37 Old 05-10-2019
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Re: Missing RPMs

Wiring. Same thing happened to me on my Yanmar. The old wires get corroded inside the plastic sheathing where you cant see it. Replaced the wiring to the unit and viola, fixed.
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post #12 of 37 Old 05-10-2019 Thread Starter
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Re: Missing RPMs

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Wiring. Same thing happened to me on my Yanmar. The old wires get corroded inside the plastic sheathing where you cant see it. Replaced the wiring to the unit and viola, fixed.
easy enough to run a test pair, check it and then use the old wires to pull the new ones through the conduit... it they are not zip tied. If not it's a bit of a PITA wire run.

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post #13 of 37 Old 08-11-2019 Thread Starter
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Re: Missing RPMs

I haven't tackled the last test... to replace the wiring...

But I noticed that the temp gauge creeps higher... not into the red zone... when I get the RPMs to produce hull speed. Bottom is clean. Back off the throttle a bit and the gauge shows "cooler". Engine is not overheating. exhausting water seems normal.

Is it normal for the engine temp tp creep up when the engine is driving the boat at hull speed and then a bit more RPMs... and then move down to a cooler reading when the RPMs are lowered?

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post #14 of 37 Old 08-12-2019
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Quote:
Originally Posted by SanderO View Post
I haven't tackled the last test... to replace the wiring...

But I noticed that the temp gauge creeps higher... not into the red zone... when I get the RPMs to produce hull speed. Bottom is clean. Back off the throttle a bit and the gauge shows "cooler". Engine is not overheating. exhausting water seems normal.

Is it normal for the engine temp tp creep up when the engine is driving the boat at hull speed and then a bit more RPMs... and then move down to a cooler reading when the RPMs are lowered?
Higher load means higher combustion chamber pressures and higher temps.
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post #15 of 37 Old 08-12-2019
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Re: Missing RPMs

Hello,

A few things:
The water is warmer now that it has been. That makes the heat exchange less efficient and can result in the engine running hotter
The higher the the engine RPM and the higher the load, the more heat the engine is producing. So if the heat exchanger is maxed out the engine will run hotter
The heat exchanger may be restricted by scale or other build up.

Barry
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Re: Missing RPMs

Good idea to de scale or flush raw water system... would running barnacle buster through and letting it sit for some time before flushing out remove build up?

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post #17 of 37 Old 08-12-2019
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Re: Missing RPMs

I prefer RydLyme. I set up a pump and a bucket and recirculate it through the raw water passages for at least an hour. If you let it stand, it will settle away from upper surfaces. Each setup is a bit different. I push the rydlyme in through the hose that ejects raw water into the exhaust elbow and then exit it below the raw water pump (opposite it's normal travel). The pump keeps picking it up out of the bucket. Works very well.

Of course, if the engine is raw water cooled, with no heat exchanger, you'd have to pull the thermostat and be certain you don't flood the cylinders.
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post #18 of 37 Old 10-21-2019 Thread Starter
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Re: Missing RPMs

back to the Missing RPMs

I located a used Tach... The part is no longer supported so that's the only replacement option.

By now I've changed / tested the two wires from the sender, replaced the sender so it seems to have to be the gauge itself.

My gauge works but reads slightly lower. I tested it with a portable laser tach. RPMS of under 800 shows as about 650... and it's proportional at higher RPMs.

So I wired in the used one and it shows almost 2x the RPMs./ Laser RPM of 800 shows as 1,500 or so.

This makes no sense. All I can think is that the ground is not working or the +12v is not 12v... but we know that that is variable based on the alternator and batteries. (batteries are topped up)

Why would the used and tested read high? When I called and asked him if he measured it with a laser... he said it ran at idle and SOUNDED to him like idle and read around 800 RPM YIKES. I don't trust that! I will send it back tomorrow. If he confirmed it matched a laser tach then I know something else is not right on the boat. He didn't.

Still looking for the missing RPMs.

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post #19 of 37 Old 10-21-2019
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Re: Missing RPMs

Is this a VDO tach? If so, it needs to be adjusted for the particular engine. Is there a small hole over a button on the back? If so, this is an adjustment button. Instructions for calibrating it can be found on the web.

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post #20 of 37 Old 10-22-2019 Thread Starter
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Re: Missing RPMs

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Originally Posted by colemj View Post
Is this a VDO tach? If so, it needs to be adjusted for the particular engine. Is there a small hole over a button on the back? If so, this is an adjustment button. Instructions for calibrating it can be found on the web.

Mark
It is a VDO and there is no screw or hole to insert a small screw driver to adjust.

A Volvo dealer who sells parts told me that the tach has to be paired to a specific rev counter sender. I replaced the OEM with a new volvo part and there was no change.

What is baffles me is:

The RPMs read from a laser tach does not agree with the original tach nor the used supposedly tested replacement tach.

So at idle..ie with the lowest fuel supply the throttle can deliver

the original tack now reads ~650
the laser reads ~800
the replacement reads ~ 1400

I formerly was able to get the revs to over 2600 (full throttle) with old gauge which now seems to be ~2100. So it seems to be reading ~15% low. Didn't test w/ laser. And didn't full throttle w/ replacement.

++++

I think I will keep the old and simply recompute in my mind. I am thinking it's a wire issue... but what?

there are only 4 connections and the OEM harness is labeled

-, +12, W, G (W & G are connected to the sender) I also ran new wires from send to see if results changed... they didn't so I conclude that the OEM wires are OK... connectors are new.

did not measure the voltage of the - and the +12 wires.

Could this just be a stiff sticky tach?

What's with the replacement?

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