Posted by: yachtanomaly | August 19, 2016

Engine Trouble, Part III

From the captain:

After motoring for 4 hours between Tofino and Ucluelet (gale warnings, NW 25-35 knots resulting in an actual 2-4 knots SE and dense fog) with no problems, I thought we were in the clear. From Ucluelet, we were going to the Broken Island chain, an archipelago in Barkley Sound. The distance was only about 12 miles, fog and very light wind. Just out of Ucluelet harbor I throttled down to clear the prop of some suspected seaweed, and the “low oil pressure” alarm sounded again. I immediately shut the engine down and investigated, again finding nothing observable amiss. I restarted, and no alarms, we proceeded slowly to our anchorage between Turtle and Dodd islands.

With the prospect of little wind forecast for the long day through the San Juan De Fuca strait, and no harbors of refuge there, I located a Volvo Penta mechanic in Ucluelet. There is cell phone coverage in Barkley Sound. Yes, he works on these engines, but no, he can’t possibly look at it until at least next week. All I needed to do (to begin with, anyway) was measure the actual oil pressure. This would tell me if the pressure indicating switch or associated electronics were faulty, or something more serious. Well, he could sell me a gage, but could not help otherwise. At this point I remembered that there was at one time, a pressure sending unit that was removed from the engine when it was converted from the A rev (all analog instrumentation and control) to the C rev (all Canbus controlled). And it might still be on board.

A search of likely storage spots uncovered it. While I had no gage to read the sensor, these are simple affairs, changing resistance with changing pressure. The Volvo service manual revealed a test procedure which illuminated its calibration. I had an ohmmeter. I began by trying to test the existing switch, supposed to close below 0.15 Bars pressure. It seemed to always be open, engine running or not, though occasionally I got some low resistance readings as the engine shut down. It appeared to be flakey. I will not question here the wisdom of specifying a switch which closes with low oil pressure, but someone should – a fault in the wiring will read always good. Replacing the switch with the pressure sender, I tried again, and got a different set of strange readings. With the engine stopped, I measured 10 ohms between the terminals as expected, but running it went open circuit rather than the maximum of 130 ohms.

The repair manual described a version of the pressure sensor that did not look exactly like the one I had. In another bit of documentation, there was an oblique reference to the ‘optional combined sensor and switch’, a little reverse engineering proved that this is what I had. One terminal to ground is the idiot light switch, the other terminal to ground is the pressure reading. With no pressure, the switch terminal is grounded and the reading between them looks like the other type of sensor as there is continuity through ground. Any pressure above 0.15 Bars opens the switch. Measuring the resistance of the sensor while running indicated 86 ohms to ground, this converted to 4 Bars by the sensor test, or 400 KPa, within the engine spec of 150 – 500 Kilo Pascals while running. So the oil pressure is good after all. Probably, the pressure switch is bad (jumping the terminals on the cable reliably produces a low pressure warning, so the electronics are probably good). The switch probably has been bad for some time. Due to the choice of a normally open switch, you would never know it.

We are swinging to anchor, there is one other boat in the bay some distance away. Two groups of kayakers have paddled past, the fog moves onto on off of the islands to the southwest but we are in the sun. Should I return to Ucluelet and try to get a replacement oil pressure switch? Not having it means if something really goes wrong I will not know. But it has probably not worked for some time anyway. I’m going to pour myself another iced tea and sit in the cockpit and think about that.

Sailing, engines, Jon

Captain Jon with his head in the engine compartment


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