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How to get the Tacho working...


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44 replies to this topic

#1 gadget1382

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Posted 29 September 2006 - 12:41 AM

As stated above... My comments in Italics.

Using the method of running the four diodes off the igniter.


Posted Image

Here's a pic of the ca18det ignitor, the 4 wires that go to the coilpacks from this are the ones you splice 4 diodes onto then join the "cathode" (striped) ends together and join them to one wire which you run to your tacho. There is a light green wire at the back of the tacho, which you cut and join to this wire. You have to remove the instrument panel to get at the connector that has the light green wire in it.

I am going to do a wire trace from there an hopefully find an access point alot more accessable as i'm connecting other wires in as well at the junction box

For those not in the know...

Posted Image

The tacho works by counting the rate at which ignition pulses are being fired. With the CA20 (and distributor-based ignition systems in general), all four pulses are on a single wire, so all four pulses are counted by the tacho. With the CA18/SR20 (and distributor-less ignition systems in general), each pulse is on its own seperate wire so any given wire carries only 1/4 the pulses of a distributor system, like so:

CA20 _|-|___|-|___|-|___|-|___
CA18#1 _|-|_____________________
CA18#2 _______|-|_______________
CA18#3 _____________|-|_________
CA18#4 ___________________|-|___

So, you should get 1/4 the tacho reading from a CA18 that you do from a CA20
hence you need 4 wires instead of the CA20's 1 wire

What the diodes that Gerry has suggested do is to allow you to mix the signals from the CA18 wires onto a single wire, without affecting the original signals. The output from the junction of the [edit]cathodes[/edit] of the diodes then looks like the CA20 ignition signal:

_|-|___|-|___|-|___|-|___

so the tacho is happy and displays what you want

Q) Are they just called "diodes"? Do they need to be a specific wattage or voltage?

A) iirc the 1N4004 is a 1 amp 400 volt rectifier diode. They're as common as dog turds. If you're looking for an alternative, I wouldn't use a signal diode, but any reasonably rated rectifier should be fine. Bear in mind that you're working around the ignition which can have sizeable voltage transients (caused by backlash from the coils) so a 400V rating is not unreasonable.

From the later part of the thread, i learnt that a number of people experianced failures in this method by chosing low voltage diodes and also "fatigue" due to not being mounted properly after 3 odd months. Suggestions were made to use hot glue and mount them to a board. If propperly connected within conduit and tightly wrapped i fail to see why they would break. only time will tell.



For full info refer to... http://www.club-s12....s...=14975&st=0

PS. Thanks for the info Gerry.


#2 S12-FVR

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Posted 29 September 2006 - 09:51 AM

ive installed 1 and it works fine, but the standard tacho goes really crazy....it says its idoling at about 3500 when its actually around 1000 on the aftermarket one... so in otherwords, does anyone know how to fix my stock tacho while keeping the aftermarket one?
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#3 gadget1382

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Posted 29 September 2006 - 02:08 PM

Did you use the diode method? and due to my memory not being great do you have a CA18 or a SR?

#4 S12-FVR

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Posted 29 September 2006 - 03:38 PM

It's a CA20 and i'm not sure,my dad was doing it and i got sidetracked somehow so i didnt pay much attention, from what i know he just hooked up the tacho underneath the dash with a simple wire then pulled it through the firewall and hooked it up to some thing near the starter lead? (sorry im no car smart i am not) hahha.
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#5 gadget1382

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Posted 29 September 2006 - 03:51 PM

Well looks like ur getting 8 spark signals now... rather than 4 or one...
If your timing is out... you could get all 8 spark plugs firing sending a signal, which would then get closer together as the rpms increase.

CA20 _|-|____|-|____|-|____|-|___

Bad tune/spark signal
_|-|-|__|-|-|___|-|-|__|-|-|__

CA18#1 _|-|_____________________
CA18#2 _______|-|_______________
CA18#3 _____________|-|_________
CA18#4 ___________________|-|___

Thats my guess anyway.

#6 S12-FVR

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Posted 29 September 2006 - 03:54 PM

Wow, i have no idea what you just wrote :D :)

I'll show my dad when he gets home lol.:)
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#7 Fester

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Posted 29 September 2006 - 06:56 PM

ooooh I've never been plagiarised before! Feels good! :D

Yeah, that sounds like a reasonable diagnosis gadget. I think the tacho measures the time between pulses and scales that to rpm, so mixing two pulse trains with a modest phase offset (say, 50 degrees instead of 180 degrees) could look to the tacho like a higher than double rpm. I'd expect it to be quite jumpy though.
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#8 Fury-S12

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Posted 30 September 2006 - 02:12 AM

thats seems like a slightly hard and time consuming way to do a tacho isnt there a easier way or am i dreaming again

#9 TOK-079

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Posted 28 December 2006 - 09:53 PM

Right. I have a problem.

WTF is the pinout on the back of the tacho? I need to bypass the harness, I broke something in the bay. x_X

EDIT; I did so installing a new engine. Please be nice!

Edited by TOK-079, 28 December 2006 - 09:57 PM.

Location; Eaglehawk, outside BENDIGO, VIC (3556).Call me; 0411 604 294-- Keep in mind I don't answer "private" calls generally.

#10 Umai Naa!!

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Posted 28 December 2006 - 10:43 PM

Blue wire with green trace on the plug at the rear of the cluster.

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#11 TOK-079

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Posted 29 December 2006 - 07:36 PM

I figured out which one it was, but which wire does which, if you don't mind?

EDIT; Wait wait, that's the tacho -signal-... So black is ground and .. what do the rest do?

Edited by TOK-079, 29 December 2006 - 07:40 PM.

Location; Eaglehawk, outside BENDIGO, VIC (3556).Call me; 0411 604 294-- Keep in mind I don't answer "private" calls generally.

#12 Umai Naa!!

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Posted 29 December 2006 - 09:48 PM

The wires at the rear of the cluster?

Or the ones at the ignition coil that generate the tach signal?

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#13 TOK-079

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Posted 29 December 2006 - 10:03 PM

Ooh. Okay, I've busted -more- stuff then o_O

Both ends please? :wub:
Location; Eaglehawk, outside BENDIGO, VIC (3556).Call me; 0411 604 294-- Keep in mind I don't answer "private" calls generally.

#14 Umai Naa!!

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Posted 29 December 2006 - 10:41 PM

Can I ask which engine you're running?

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#15 gerryb

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Posted 30 December 2006 - 08:33 AM

s12-FVR Dude this method of wiring a tacho is ONLY FOR CA18 or SR20 engines, NOT CA20 engines.
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#16 Kingos

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Posted 30 December 2006 - 09:03 AM

Hey guys ive finally got my s12 on the road. Fester i met you in wollongong at jade's house getting spare parts,
I had the white commodore ute, and i've already met Ash (FLY PSI) who i got a spare bonnet off.
I will post some photos today after i wash it.
I am having problems with both my tacho and speedo.
My speedo is out by about 10 - 20 km/h, and my tacho is out by something ridiculous.
at idle its showing about 250rpm.
I am running an SR20DET and gearbox with a microtech.
Does anybody know how to fix my speedo problem.
Is it to do with the SR gearbox ?

#17 Umai Naa!!

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Posted 30 December 2006 - 09:50 AM

Wheel and tyre size will obviously affect the speedo.

Or, find the correct speedo drive to suit your diff ratio.

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#18 Fester

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Posted 30 December 2006 - 09:52 AM

Hello :wub: Glad to hear you've got her up and about.

What signal are you feeding the tacho with? The Microtech tacho output, or off a coil input? It sounds like its out by 4:1, which suggests you're running it off a coil input?

When you say the speedo is 10-20kmh out, does the amount vary depending on the speed, or is it out a constant amount regardless of speed? If the amount varies with speed then its almost certainly your gearbox change. In this case I believe you can get a little mechanical dingus that goes in line with your speedo cable and alters its ratio - basically an extra little gearbox to speed up or slow down the speedo cable rotation to match the wheels.
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#19 Umai Naa!!

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Posted 30 December 2006 - 10:06 AM

The speedo drive out of the previous gearbox would fix the problem :wub:

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#20 Kingos

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Posted 30 December 2006 - 10:17 AM

Fester, the speedo varies with speed, faster i go the more it goes out.
I currently have a 9 inch diff with bad 2.9 ratios, i will be changing to 4.11 in the new
year. I am hoping this may fix the problem.

I am unsure as to how the tacho has been wired, wether to the coil or the microtech.
i can ask the guy who did it for me and find out.
if its off the coil then i can use the method above obviously. If its off the microtech where
does that leave me.

Cheers mate




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