Yanmar 4JH57 will not power down

[Randy Nevada]

We are currently spending the summer cruising season in Grenada. Been here a week and so far so good. After flawless engine performance on the trip down here we were surprised to find the starboard engine powered up when we got back to the boat after supper. I tried to power it down with a long press of the power button but no joy. The engine was still powered up. A quick press of the start button and I was relived to hear the engine start and run normally. The stop switch also worked. I finally had to power down the engine with the main power switch, hoping that once I turned it back on all would be normal. Well turning the main big red engine power switch back on powered up the engine with out any input from the power button.

This morning thinking that the issue might be the Yanmar instrument panel I switched the starboard and port panels. This did not affect the problem. The starboard engine is still stuck in a power on state but it runs and stops normally.

There isn't much circuitry between the instrument panel and the ECU. Its hard to tell from the system diagrams but it looks like the accessory relay may play a role here. It seem odd that it would fail closed.

I would love any advice before I throw myself on the mercy of the local Yanmar dealer here in Grenada.

Cheers
Randy
High Sierra (Leopard L47)

[Randy Nevada]

After more investigation today it looks the ECU cant turn off the display but all else seems OK. If I turn on the master engine room switch the STB engine control panel beeps and lights up just like I pressed the power button. However the start button is not active. If I press the power on button for a second (as I normally do to turn the engine systems on) nothing seems to change but now the engine start and stop buttons work. If I stop the engine and press the power switch for 5 seconds to turn the engine off that seems to work but the displays dont turn off but the start button no longer works..

It seems like the ECU is working normally but the display instrument is always on. I would think the display is bad but the one from the port side worked the same when I switched them.

Thanks for any brainstorming ideas... this is a pretty strange one.

Randy
High Sierra

[tkeithlu]

I'm thinking stuck relay, particularly given some circuits are locked on and some are not. I suggest that you start at the engine and work backwards in the circuit that is on despite being turned off. You've got power there where you should not, so a voltmeter will show voltage relative to ground. Dollars to doughnuts you'll come to a relay that is letting power through despite having no voltage on its coil.

You are about to enter the world of electrical detectives....

[Randy Nevada]

I can only find two relays on the Yanmar schematic located on the engine, they were the same part so I swapped them. No change. There could be another relay somewhere on the boat that powers up the display and ECU. I will keep looking! What is weird was the engine was turned off for a few days after we arrived here in Grenada. I haven't messed with the engines at all and then for the circuit to power up on its own doesn't seem like a stuck relay. I cant imagine the system having a normally energized relay but maybe its sort of a fail safe so you can start the engines with a dead house battery.

Thanks for the idea.

Cheers
Randy

[Randy Nevada]

Well looking closely at the schematic there is a main power interrupt relay that is designed to protect the ECU from reverse polarity connections. It is simply a relay with the coil connected to a diode that energizes the relay when positive voltage is present else the relay stays open. This relay is always on except when the main engine battery switch is open which is rare on my boat. When I pulled and tested the relay the contacts were not consistently making zero ohms when energized and sometimes relay contacts measured open when energized. I had a spare main relay in my spares kit. I put it in and everything was normal. Its still a little puzzling why the ECU worked with a flakey main relay (IE start/stop/run normally), the only symptom was I couldn't power down the engine.

Thanks for the brainstorm idea....

Cheers

Randy