relay negative trigger from tacho.
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=128164
Printed Date: May 09, 2025 at 6:21 AM
Topic: relay negative trigger from tacho.
Posted By: rickyz_cube
Subject: relay negative trigger from tacho.
Date Posted: August 07, 2011 at 1:30 AM
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? ------------- Cube<3.com
Replies:
Posted By: howie ll
Date Posted: August 07, 2011 at 1:56 AM
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. ------------- Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: rickyz_cube
Date Posted: August 07, 2011 at 2:00 AM
so what goes to which pins? the light wires are 12v constant and 3v+ until shiftlight active then it becomes ground.
so
86 = 12v+
85 = switchable earth
30 = ??? ------------- Cube<3.com
Posted By: howie ll
Date Posted: August 07, 2011 at 2:17 AM
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.
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: howie ll
Date Posted: August 07, 2011 at 2:18 AM
Correction, 3 volt LED.
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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: rickyz_cube
Date Posted: August 07, 2011 at 2:20 AM
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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Cube<3.com
Posted By: rickyz_cube
Date Posted: August 07, 2011 at 2:22 AM
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 ------------- Cube<3.com
Posted By: howie ll
Date Posted: August 07, 2011 at 2:25 AM
Use the shift light to turn on the relay!
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: rickyz_cube
Date Posted: August 07, 2011 at 2:28 AM
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
Posted By: howie ll
Date Posted: August 07, 2011 at 2:48 AM
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.
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: rickyz_cube
Date Posted: August 07, 2011 at 2:51 AM
Is there Any Other Way to Sink More Current?? Sorry I Am not This Advanced With Relays
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Cube<3.com
Posted By: howie ll
Date Posted: August 07, 2011 at 3:04 AM
Now we ar3e going around in circles, PM Oldie or KP as I said before.
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
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