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Wiring a tacho? r33

dayne
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dayne Rising Star

dayne

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Hey guys, i've done a bit of a search but couldnt find much.

Anyway, just brought a tacho, and now im stumped as to how to wire it up. My original tacho only worked sometimes, very rarly. I've gone behind the cluster, and touched on the wires to the tacho to see if i could get signal, however couldnt find any on anywire.

So basically i want to know how to wire this tacho up? Can i grab it straight off the ecu? (im an Rb20E)

How has everyone else done it? is there any specific colours that the tacho wire usually is behind the cluster. Im lead to believe that a GTST cluster could go in, so it should be the same in theory...? kinda.

Thanks.

Dayne

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browny Proficient

browny

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You might have problems depending on the tacho type.

Basically you can try hooking up pin 7 as the signal wire and if that works you are laughing.

If that fails you can try a pull up resistor, then a tacho booster. Get back to us if you need more info on the last two.

igor Rising Star

igor

Posted
Posted

Actually, speaking of the tachos, what form is the ECU tacho output? Is it the same as a central distributor lead in other cars? The reason I m asking is I was trying to hook up a rev meter to my car recently and according to the manual I needed to use a wire that hangs out next to the O2 sensor connector (that was designed for that purpose). I did that but I was getting the signal that was about 6 times less than the actual rev figure. I guessed it was only a signal from one spark plug, not all of them.

Funny thing about doing that is that my tacho was going crazy while I had the meter hooked up. It was showing the right idle but going crazy while revving - it was dropping to zero. It actually looked as though it was working in "reverse"

So now I m wondering if I can hook up the device to the ECU... Not trying to confuse the OP

dayne Rising Star

dayne

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  • Author
Posted
You might have problems depending on the tacho type.

Basically you can try hooking up pin 7 as the signal wire and if that works you are laughing.

If that fails you can try a pull up resistor, then a tacho booster. Get back to us if you need more info on the last two.

Yeah i dont really know what that means, if you could fill me in a bit more it'd be great, im going to go try 'pin 7' now and i'll let yous know.

Dayne

browny Proficient

browny

Posted
Posted

Pin 7 is the signal wire that the OEM tacho uses (also need +12V and GND).

Some tacho's will work fine if their signal input is connected straight to Pin 7. Others don't work straight away and need a bit of fiddling, it just depends on the tacho. It seems less likely that the tacho will work if it was designed for a coil type ignition (eg some US tachos and EFI conversions into older cars) rather than an EFI application (eg Jap tachos).

dayne Rising Star

dayne

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Well thats what my tacho says, "Connect to coil ra ra ra" so i think i might need a tacho booster? is that correct? where can i get these from?? or what type of fiddling do you mean?

Also... how do i find wire 7 on the ecu? you guys said it was a green one, but the only green one i can seem to find is way up the list.

browny Proficient

browny

Posted
Posted
Actually, speaking of the tachos, what form is the ECU tacho output? Is it the same as a central distributor lead in other cars? The reason I m asking is I was trying to hook up a rev meter to my car recently and according to the manual I needed to use a wire that hangs out next to the O2 sensor connector (that was designed for that purpose). I did that but I was getting the signal that was about 6 times less than the actual rev figure. I guessed it was only a signal from one spark plug, not all of them.

Tacho output is a 0-5V square wave.

But...

All pin 7 does is switch between open and ground. So to get a signal you need to connect a voltage source to pin 7. AFAIK this is done internally in the tacho (+5V). If you don't connect pin 7 to a voltage source then the ecu is just switching pin 7 between open (0V) and ground (0V) and therefore you don't get any signal. The upshot is that if you have an OEM tacho, and it works, then the tacho connection should work.

But this is just background info and doesn't really answer your question.

From what I remember the tacho output is actually a cylindrical connector with a single pin, are you sure you are measuring from the right place and you're not measuring from the timing light pickup loop?

browny Proficient

browny

Posted
Posted
Well thats what my tacho says, "Connect to coil ra ra ra" so i think i might need a tacho booster? is that correct? where can i get these from?? or what type of fiddling do you mean?

Also... how do i find wire 7 on the ecu? you guys said it was a green one, but the only green one i can seem to find is way up the list.

Try connecting it straight to pin 7 first, if its a new tacho it might be designed for universal applications.

If it doesn't work you will still need to work out which wire is pin 7 anyway, plus you will need a +12V and GND sources, so you aren't really wasting any time if it doesn't work.

igor Rising Star

igor

Posted
Posted
Tacho output is a 0-5V square wave.

All pin 7 does is switch between open and ground. So to get a signal you need to connect a voltage source to pin 7. AFAIK this is done internally in the tacho (+5V). If you don't connect pin 7 to a voltage source then the ecu is just switching pin 7 between open (0V) and ground (0V) and therefore you don't get any signal. The upshot is that if you have an OEM tacho, and it works, then the tacho connection should work.

But this is just background info and doesn't really answer your question.

From what I remember the tacho output is actually a cylindrical connector with a single pin, are you sure you are measuring from the right place and you're not measuring from the timing light pickup loop?

Righto, so that's why you need a pull up resistor. Yeah, I was connecting to the single pin cylindrical connector lower down the left strut tower, not the pickup loop.

browny Proficient

browny

Posted
Posted

Because there are 6 individual coils in a Skyline (ie one per cylinder), which makes for some fun wiring it up and designing circuits so that all the signals don't interfere with each other. God knows the sort of trouble you could get into. It's not like a old single coil car where you just have to piggyback onto a single wire to get the tacho signal.

IMO its much better to mess with non critical instrumentation wiring rather than start compromising the integrity of the wiring thats essential for reliable operation of the engine (ie splicing into coil wiring etc).

If the tacho doesn't work straight out of the box, then a pull up resistor might work and that is going to cost about $1. If you have tried this and it doesn't work, a tacho booster will only take 5 min to install, but it'll cost $50-100.

Whatever option ends up working, you are still going to need to locate the tacho signal wire, +12V and GND, so you might as well start with the cheapest option first...

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