relay negative trigger from tacho.

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rickyz_cube

Member - Posts: 6

Joined: August 07, 2011

Location: New Zealand

Posted: August 07, 2011 at 1:30 AM / IP Logged

hi all, i am new to this site,

i am trying to make my tacho turn a relay on when the shift light becomes active.

my tacho has a 12v positive and a switchable earth that triggers the shift light to come on.

when i hook this wire to the relay it has 12v earth, then when i connect the 12v+ to the other pins, the earth becomes 2.3v, why would this be?

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howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: August 07, 2011 at 1:56 AM / IP Logged

If you tried this configuration tach.bmp

and it still doesn't work, you might not be sinking enough current. PM Oldspark, or Kevin Pearson.

rickyz_cube

Member - Posts: 6

Joined: August 07, 2011

Location: New Zealand

Posted: August 07, 2011 at 2:00 AM / IP Logged

so what goes to which pins? the light wires are 12v constant and 3v+ until shiftlight active then it becomes ground.

86 = 12v+

85 = switchable earth

30 = ???

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howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: August 07, 2011 at 2:17 AM / IP Logged

I suppose since you can't read the diagram try the following:-

suspected tacho switching ground to 85.

Ignition live to 86 and 87.

Lamp output from 30.

Don't use 87a.

Don't forget the 1N4004 diode across 85 and 86 with the band towards 86 or you will destroy your tachometer.

Alternatively, if you use a 12volt LED, forget the relay and just run the positive side to 12v+ ignition and the neg side to the switching output.

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: August 07, 2011 at 2:18 AM / IP Logged

Correction, 3 volt LED.

rickyz_cube

Member - Posts: 6

Joined: August 07, 2011

Location: New Zealand

Posted: August 07, 2011 at 2:20 AM / IP Logged

im not using a shift light, just the output from tacho, the wires i have are:

12v+

12v+ switched

switching earth (shift light trigger)

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rickyz_cube

Member - Posts: 6

Joined: August 07, 2011

Location: New Zealand

Posted: August 07, 2011 at 2:22 AM / IP Logged

my relay works when i put the earth wire onto a actual earth, but it does not work from the output from the tacho, the voltage seems to drop when power hooked into relay

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howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: August 07, 2011 at 2:25 AM / IP Logged

Use the shift light to turn on the relay!

rickyz_cube

Member - Posts: 6

Joined: August 07, 2011

Location: New Zealand

Posted: August 07, 2011 at 2:28 AM / IP Logged

thats what i am trying to do, the two wires going to the shift light from the tacho are:

red = 12v+ constant

black = earth only when above desired rpm

the problem i am having is the earth is not triggering the relay,

it works the multimeter fine when not hooked into the relay (says 12v),

but when i put 12v into the relay, the multimeter says 2.3v,

what can cause this?.

Cube<3.com

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: August 07, 2011 at 2:48 AM / IP Logged

It's not sinking enough current to trigger the relay or you wired the relay wrong.

Ground (earth) to 85, 12V+ to 86 and 87, output (12Volt+)from 30.

Or you need a relay with 3 volt coils.

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