CAN Network wiring - 'twisted pairs' & adding a Wideband controller + OBD2 port

[UK Lee]

Hello , I'm going to be using a Link Monsoon G4x to support a Sierra Cosworth YB engine in my Mk1 Escort. 

I'm just finishing off the completion of an 'all new' - built from scratch, body harness for the car and am now at the point where I can begin building the engine harness. In addition to my Link CAN Lambda controller , I'd like to add an OBD2 port in the car . I've read through the following diagrams / instructions but am left with some questions -

 

I'm using the Link Type A loom , and my first question is regarding 'twisted pairs' . First , the basics -Why are they required to be twisted together ? does the wire gauge have any relevancy / effect on the performance ? What would happen if I didn't twist the wires together ?

On my Type A loom , pins 28 and 29 are the CAN High / Low terminals . The wires are Orange / Green + Orange / Yellow , they're the same diameter and are not twisted together ( like all the other wires coming from the plug_- is it necessary for me to twist them together myself when making up my harness ? 

I intend to lay the harness out according to my sketch below ;

1) Is it ok to 'spur' off the CAN high / low wires to feed both the Lambda controller and the OBD2 port ? 

2) Do I need to twist the CAN high / low wires together in my Type A loom - both before and after the spur for the OBD2 port ?

3) The Canbus wiring diagram states the need for a 120R resistor in some applications - do I need one ? where would it be placed in my sketch below ? 

4) In the sketch below - do my circuits look ok and wired correctly ? should I change anything ? 

Many thanks for you help - i really appreciate it 

Lee

[Adamw]

The first question is what lambda device do you have?  OBD2 needs to run at 250 or 500kbit so it wont be compatible with some Lambda devices.  

[UK Lee]

16 minutes ago, Adamw said:

The first question is what lambda device do you have?  OBD2 needs to run at 250 or 500kbit so it wont be compatible with some Lambda devices.  

It's the Link controller with a bosch sensor , I bought the kit the same time as the loom & ecu 

 

Sorry , also just noticed that I've posted this in the G4+ section , I'm using a Monsoon G4x if that makes any difference.  

[Adamw]

Ok, that is good then, the Link one can be reprogramed to operate at 250 or 500k.  

The CAN bus is generally constructed as a main "trunk" with short branches off it.  The trunk should be twisted, the short branches dont need to be.  The trunk needs a termination resistor at both ends - the ECU already has one built-in so you only need one at the far end of the bus.  

So to confirm, your orange/green and Orange/yellow wires will need to be twisted by hand from the ecu all the way to the far end. 

There are many ways you could layout the bus and still met the correct architecture.  I would typically do it "daisy chain" style like drawn below where I run the main trunk directly to each device and fit the resistor across the connection to the last device.  You should be able to crimp two wires into each pin on the OBD2 socket to save doing any splices etc.  

Heres how I normally do the termination resistor in a road car:

 

Seal removed from connector, fill back of connector with RTV instead of the seal:

 

[UK Lee]

Thanks very much adam , that's great. 

So I need to get a 120r 1/4watt resistor then ? I found these on Ebay- packs of 50 but they're only a couple of quid - would one of these do the trick ? 

[Adamw]

Yes 1/4W is fine. 

[UK Lee]

Nice one , thanks Adam .

Kinda disappointing that Link don't bother to  chuck in a couple of resistors worth just a few pence in a 'kit' costing hundreds of pounds when they're a requirement to get the kit to function. 

In any case , its easily sorted - thanks for the answers & advice . It's much appreciated 

[Adamw]

The Link CAN lambda works fine without the termination.  At least it does if you are using our CAN interconnection cables.  Your OBD tool likely wont though.