MD2030B electrical woes

[Grumpybear]

Dear All

I have one or more problems, and I would much appreciate your comments on my diagnosis. The story so far:


Two weeks ago my 1998 vintage 2030B (1750 hours) failed to start. The instrument panel would not energise. I traced the fault to a blown fuse in the 12v supply to the panel, transferred the outlet wire to another fuse and off we went, with only a slight residual unease about why the fuse had blown in the first place. Some days later I noticed the tachometer needle oscillating, and yesterday morning it dropped to zero for good shortly after leaving port. Yesterday evening I tried to start the engine to enter our home marina. Nothing, and changing the fuse again did not help. We somehow managed to sail into our berth, and this morning I nerved myself (as an unreconstructed believer that electricity is an invention of the devil) to investigate.

Fumbling with an avometer, I established that there is 12v at the panel button power switch (so there is o fuse problem) and that this is passed on when the switch is pressed to on. However, neither the alarm lights, the alarm test nor the glow plugs seem to energise. I also found that there is a voltage of about 10v present in the wire leading from the start button to the start relay, even when the 16 core cable is unplugged from the relay box at the engine end.

I conclude that there is a short within the 16 core cable between the 12v power feed and the wire back to the start relay, and possibly also the tach signal wire from the alternator to the meter. The solution would seem to be a new extension cable between the looms at the relay box and at the engine.

Before I invest the thick end of £200 on a new cable, does anyone have any comments, or an alternative diagnosis?

For those with access to a VP manual, the panel is the standard one without a key.

Any reactions gratefully received.

 

[earlybird]

Just one or two thoughts.
Digital voltmeters are not particularly good for voltage continuity testing as they are so sensitive that leakage currents can give misleading readings, eg your mystery 10 volts.
Can your 12v power feed to the button light a test lamp?
Tacho. failure might be failure of the alternator to exite, due to no voltage feed, hence no pulses, or just no power to the instrument.
First thing I'd check before spending money is the quality of all earth connections, esp. those onto the engine itself, they are often hard to see, let alone reach.

 

[Grumpybear]

Thanks for these thoughts. The alternator seems to be OK: it was charging the batteries when the engine was last running. I will check the earth returns though, and try the lamp test.

 

[Grumpybear]

Early bird, many thanks. After a lot of struggle I found a loose negative terminal on the alternator. Hey presto!

 

[john_morris_uk]

Early bird, many thanks. After a lot of struggle I found a loose negative terminal on the alternator. Hey presto!

That would explain the tachometer dancing all over the place as well. On the 2030 (like many other engines) the tacho relies on the alternator to work. In fact one of the first indications I have if ever our alternator stops working is that the tacho stops working..!