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Altering a Hight Level Tach to Accept Low Level Signal


Cameron Daline
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Hi all. I have an old Volvo 240 that I built a custom turbo motor for that I have running on LINK G3. It's all running beautifully and working great so far but I'm trying to tie up a couple more loose ends. I'm currently trying to figure out how to get the factory tacho to work with the LINK. The factory tacho originally ran off the negative side of the ignition coil. With the new motor I have it running on COP arrangement so no external coil to drive the factory tacho. Here's what I found, according to the LINK manual: Quote: "An auxiliary output configured at 'Tacho" produces a 0-12V pulse train to drive a low-level tach. There will be one output pulse for each time a cylinder reaches TDC. Connect an auxiliary output directly to a low-level tachometer. Link G3 ECU's will NOT drive a high level tach that is triggered by a coil's negative terminal. Using a high level tach on a multi-coil engine presents some problems, as each coils is not firing as often as a distributor engine's coil would. In this case the preferred solution is to modify the high level tach to accept a low level signal." I'd imagine I could wire the tach up by triggering off each coil and wiring in diodes to not send the signal back to the other coils, but single wire hookup off the ECU seems way easier since the ECU is already configured to send out the signal I need. I just don't get what is meant by the statement 'modify the high level tach to accept a low level signal.' I don't even know where to start with that? Can I just amplify the tach signal off the ECU somehow? Any thoughts?

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OK, another question related to this.... Since I'm not finding any info anywhere about how I might alter the tacho to accept the low level signal I'm also looking at installing an aftermarket tach. Most aftermarket tachos I find state that they will work on a "12V square wave applications." Would the tacho output from the LINK ECU be compatible?

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Hi,

'High level tacho' means that the tachometer requires a high voltage (aprox 400V) input from the coils negative terminal. Some companies offer the service of modifying the factory tachometer to a low level signal (12V).

Your ideal involving the diodes does work.

An after market tacho will work with the Link ECU if it states it is compatible with 12V tacho signals.

Regards,

Phil

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Hi Phil, Thanks for the response. Another question to follow up if I may, which will probably sound like a newbie question! As I'm running the COP arrangement in wasted spark I am only using the IGN1 and IGN2 outputs from the LINK G3. They are paired off to the coils of course. I am using COPs from a Toyota that have built in ignitors so each coil has 3 inputs -- 12v, ground, and the IGN input from the LINK to tell it when to fire. If I want to run the tacho with a couple diodes from the coils which of these 3 wires would I splice into? The IGN signal wires from the LINK? Thanks, Cameron

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  • 3 weeks later...

Hi all, So... still frustrated. Decided going aftermarket gauge would just be easier. Not proving to be thus far though. I got a VDO Vison tach to use with the car. It states it will work with '12v square wave' applications. In the install info that came with the gauge it shows it working either off a coil, or off an electronic ignition system (just like how the Autometer Phantom gauge is installed as mentioned above -- I compared install diagrams and instructions for both the Autometer and VDO). The VDO tacho I have just has dip switches that you change depending on config and # of cylinders. I've got Aux output 3 in LINK setup as 'tacho' output in the PCLink. According to the PCLink help, that signal produces a pulse each time an ignition event occurs. In the printed manual though, I find that it states that tacho ouput from LINK sends a pulse each time the motor is at TDC. Which is correct? No matter how I set it with the dip switches or what the info is set at in the PCLink, I get nothing on the gauge. I tried running the duty cycle low from the PCLink, I tried setting it all the way up, and in between. Nothing on the tach. I figured I'd at least get something and of course could fine tune it on the gauge and in the PCLink to calibrate it, but I just have nothing. I've verified the tach has power as the gauge 'zeroes' when power is supplied to it. It's just not reading RPMs. I tried every combination of settings on the 3 dip switches and no luck getting it to read anything on any configuration. What am I missing?

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Hi Cameron.

4K7 means 4700 ohms effectively, however you have shown a 1K (1000 ohm) resistor here.  In fact you would probably be better off with this one as it will provide a more positive 'pull up'.  I say this after discussing with my collegue and he agrees.  Yes, get the 1K 1/4W pictured in the link you provided from radio shack... Should do the job nicely.  Let us know how you get on.

Jurgen

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Thanks Jurgen! I've spent a lot of time on it and tidy was a top priority for me. And now, thanks to the help from you guys, I have a working tachometer finally! I wired in the 1000 ohm 1/4 watt resistor for the tacho output and that did the trick. It's a small victory, but very satisfying to have it working the way I want now :) Best, Cameron PS -- here's a link to more info and pics of the build up: http://www.ipdusa.com/blog-58.

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  • 4 weeks later...
  • 1 year later...

I cant comment of the Volvo.

 

But I recently got a 1990 Honda CRX up and running on a G3. As best I can tell, the tacho took it's signal from the coil ( built into the dizzy )

 

I fried the OEM ignition amp setting the damn thing up, and the coil no longer worked. I fitted a coilpack instead, but it left me without a tacho.

 

I just wired an Aux output to the wire from dizzy to tacho and set the output to Tacho ( annoying that the multiplier doesnt allow 2 decimal places ) and the tacho worked fine, apart from a slightly inaccurate reading

 

There are various signal modifier or adjuster boxes available. So maybe one of these might be of use to create a signal for you to use from a PWM output ?

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