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Old 12 September 2010, 15:41   #1
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tacho problem - wiring diagram

tachometer not doing anything apart from when i'm turning the engine over its working as soon as it starts nothing.

any ideas??

or does anyone know of a wiring diagram i download from somewhere.?
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Old 12 September 2010, 18:23   #2
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Originally Posted by sea_quest View Post
tachometer not doing anything apart from when i'm turning the engine over its working as soon as it starts nothing.

any ideas??

or does anyone know of a wiring diagram i download from somewhere.?
Are you saying that the tacho only displays a reading when you have the starter motor engaged? Has it always been like that?

You should have 3 wires going to the tacho:

  • + 12V (either live all the time the main battery switch is live, or more commonly when the ignition is in "run/on" (not "start/crank").
  • Ground
  • "Signal" - the feed from the engine which varies with rpm.
You may also have a extra wiring for:
  • Backlight (often wired into nav light circuit),
  • Any oil, temp or other warnings that are displayed on fancy multifunction units.
Assuming I've understood your problem right, and its been like this all along (or following some recent change - and not happened randomly) then it sounds like you have inadvertently connected the +12 V connection to the "start/crank" wire on the ignition rather than the "run" wire.
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Old 13 September 2010, 04:21   #3
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Assuming your using a Merc loom, you should have the Purple (switched live) to the + terminal, Black to the - terminal, and Grey (signal) to the signal terminal.
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Old 13 September 2010, 11:04   #4
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Quote:
Originally Posted by Polwart View Post
Are you saying that the tacho only displays a reading when you have the starter motor engaged? Has it always been like that?

You should have 3 wires going to the tacho:

  • + 12V (either live all the time the main battery switch is live, or more commonly when the ignition is in "run/on" (not "start/crank").
  • Ground
  • "Signal" - the feed from the engine which varies with rpm.
You may also have a extra wiring for:
  • Backlight (often wired into nav light circuit),
  • Any oil, temp or other warnings that are displayed on fancy multifunction units.
Assuming I've understood your problem right, and its been like this all along (or following some recent change - and not happened randomly) then it sounds like you have inadvertently connected the +12 V connection to the "start/crank" wire on the ignition rather than the "run" wire.
It happened randomly a few days ago which is why I'm a bit lost as to what it could be, All cables are connected as they were before on the gauge side of things (havent checked the motor side yet) and are all looking to be ok and it works fine as you said when the starter motor is engaged, but as soon as she fires nothing. Given what you've said about the ignition wire/run wire it sound like it could be that but it happened randomly and no changes have been done to the boat around the time its happened.


Dirk Diggler: yes its Connected exactly as you have described.
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Old 30 September 2010, 08:46   #5
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if i remember corectly you have two rectiyers/regulaters on your obm if so disconnect the grey wire from one rec/reg and put it in the other if that works you will need to find out why the rec/reg went wrong.
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Old 06 October 2010, 03:32   #6
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how about a loose connection somewhere, maybe near the engine. Maybe the vibrations from the starter motor shakes the wire so much that it works now and then.
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