While crossing the bar on the Clarence River in New South Wales, Australia, in 2007, the reading on our oil pressure gauge dropped to zero. It turns out that the engine was fine, and the culprit was the oil pressure sending unit which I later replaced for about $100.
Last year the oil pressure reading again became erratic, but this time I had frequent Internet access. A bit of Web surfing revealed that this is a fairly common problem with Yanmar oil pressure sending units, so I thought that I could solve the problem by switching to another brand of sending unit. Yanmar, however, uses a proprietary sending unit, which meant that I had to replace the sending unit and the oil pressure gauge as a matched pair.
I ordered a Teleflex gauge model 80180P at 0 to 80 psi and “C” sender from Defender Industries for a total cost of $63 plus shipping, still cheaper than a new Yanmar sending unit alone. The Teleflex oil pressure gauge has a black background and white pointer so it matches the Yanmar gauges surprisingly well. The diameter of the Teleflex gauge is the same as the Yanmar gauge so it will fit the panel cutout with no modifications. The Teleflex sending unit has a 1/8-inch NPT male thread which is the standard tapered pipe thread available in the USA. A quick check in the Yanmar engine service manual confirmed that the pipe thread was 1/8 PT, which I thought meant it was the same as the Teleflex unit (but more on this later.) On both sending units there is only one wire terminal to connect. My onboard stock of electrical spares provided a blue ring terminal that fit the ground terminal on the Teleflex gauge.
Installing the oil pressure gauge
When you have the new gauge and sending unit, begin the project by turning off the battery switch to the engine to remove power to the instrument panel. Gain access to the rear of the instrument panel and locate the rear of the oil pressure gauge. Disconnect the red wire that supplies 12 VDC power from the starter key switch to the gauge. Disconnect the yellow wire that goes to the sending unit on the engine. The ring terminals on both wires will fit the new gauge, so be sure not to damage them. Remove the small Phillips screw that attaches the ground wire to the gauge. Or, just cut it off as the ring terminal is too small to fit on the new Teleflex gauge. Now is a good time to install the blue ring terminal on the ground wire.
Remove the Yanmar gauge by unscrewing the two Phillips screws and lift out the gauge and clamp plate, being careful not to drop the two screws as they will be reused. Now unplug the lamp from the back of the gauge. It will take a bit of a struggle as it is a tight fit. Don’t damage it as it will also be reused. There is a small gasket resembling a flat O-ring between the gauge and the clamp plate. Remove it now and place it on the new gauge. (It’s easily overlooked. Don’t ask me how I know.)
There is also a gasket that fits between the gauge and the instrument panel. This gasket will be reused also. Now, carefully place the clamp plate over the new gauge while aligning the mark on the clamp plate with the vertical center line of the gauge. Note that the clamp plate will be about 45° from horizontal. Insert the two hold-down screws loosely, and then check the face of the gauge to ensure that the gasket between the panel and the gauge is properly aligned. Tighten the two hold-down screws. Have a look at the face of the gauge to make certain that the gauge is indeed horizontal. Connect the ground wire on the gauge. Connect the red wire to the terminal marked “I” and connect the yellow wire to the terminal marked “S.” Note that the Yanmar and the Teleflex gauges have these two terminals reversed, so don’t be misled by wire positions. Recheck the tightness of the nuts on the three terminals.
Install the gauge lamp by building up the diameter with a wrap or two of rubber electrical tape (self-amalgamating), as the Teleflex lamp hole is a bit larger than the Yanmar lamp holder.
Beware of this problem
I only discovered the problem when I installed the Teleflex sending unit. While the threads made up easily enough, they seemed to firm up with only very few threads engaged. Although I felt a bit uneasy about the sending unit, I started the engine anyway and confirmed that the gauge and sender worked correctly. I shut down the engine, and unhappily found a trace of oil on the threads of the sending unit.
A search on the Internet found the information on the various pipe thread standards, and that led me back to the Teleflex website, where at the bottom of the pages on sending units was a statement that its sending units were U.S. standard threads and therefore could not be used on engines with metric threads such as Yanmar, Volvo, Lehman, etc.
Yanmar, a Japanese company, uses all metric fasteners and fittings in its engines. Now I know that its pipe threads are to Japanese Industrial Standard JIS B 0203, which is identical to British Standard and DIN which have a thread angle of 55° and 28 threads per inch in the 1/8-inch pipe size. The U.S. Standard NPT has a thread angle of 60° and only 27 threads per inch in the 1/8-inch pipe size. The two thread sizes are so close that visual comparison is inconclusive. The threads will make up, but they will not seal properly.
Not to be defeated, I sketched up an adapter, with U.S. NPT female threads on one end and metric male threads on the other, and set out to find a machine shop to make one for me. As luck would have it, I showed my sketch to the friendly folks at the local chandlery, and five minutes later, walked out with an off-the-shelf adapter for less than 10 dollars. Now, why could Teleflex not offer the same?
Installing the oil pressure sending unit
Make sure that the battery is off as you surely don’t want the engine to start while the oil pressure sending unit is removed. Not only will there be a terrible mess to clean up, but the resulting loss of engine pressure will probably cause engine damage.
Remove the wire from the sending unit. Using two wrenches, one wrench on the sending unit and one wrench on the pulsation damper, carefully unscrew the sending unit while not disturbing the pulsation damper. It will probably take considerable force to start the threads, as no thread sealant was used in assembly at the factory. There’s a good reason for this: the threaded connection on the sending unit is the return path for the measurement signal, and a clean metal-to-metal contact is required for a reliable measurement.
Screw the thread adapter onto the Teleflex sending unit. Clean up any spilled oil and wipe off the oil in the pulsation damper threads as best you can. Carefully insert the new sending unit and the thread adapter in the pulsation damper and make up the threads until resistance is felt. (You didn’t put any pipe thread sealant or Teflon tape on the threads, did you?) Again using two wrenches tighten up the sending unit until reasonably snug, but take care not to overdo it. Connect the wire to the sending unit terminal.
Start the engine and inspect for oil leaks and proper operation of the new gauge and sending unit.
Total cost of the project is less than $80 and it should require less than a half day to complete depending on access to your engine and panel. This idea can be extended to the replacement of the temperature gauge and its sender, and of course, the instruments on other makes of engines can be replaced similarly. Just be mindful of pipe thread standards!
Harry Hungate and his wife, Jane Lothrop, have lived aboard their Corbin 39 cutter, Cormorant, since 1997 and entered the Mediterranean Sea in 2009. They spent last winter in Gaeta, Italy.
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