Checking subloom integrity

[Doc007]

I've a Duratec coil subloom that I think is causing an intermittent misfire. Is it possible to check where the problem lies (and fixing it by unpicking and remaking a connection) with the subloom off the car? Perhaps by checking the continuity of each of the circuits? I have a multimeter but would need to know which holes to stick it in. So to speak...

[KnifeySpoony]

What are the characteristics of the misfire? I'm curious, as I'm diagnosing one as well. 

[John Vine]

Your loom looks like the standard one rather than the uprated flexi-loom.

The standard wiring is notoriously fragile and subject to wiring fractures.  The permanent cure is to fit this uprated plug 'n' play flexi-loom.

The most likely culprit is connector #1.  Have a look right inside the connector and check for fractures. If that's ok, check for discoloration of the pins (due to arcing), or general looseness of the contacts (caused by engine vibration).  If there's a problem, as a temporary fix you can cut off the old terminals and crimp on new ones (always assuming you have spares).

You can check continuity on each of the four coil circuits.  The wiring diagram (from my 2008 Assembly Guide) is:

 

WO (white/orange) are the ground wires.  The other colours (power) should match your existing loom.

I'm away from my car at the moment, but IIRC the 6-way connector pins are:

1 - power cyl #3
2 - ground
3 - power cyl #4
4 - power cyl #1
5 - ground
6 - power cyl #2

By the way, what year is your car?

JV

[Doc007]

Thanks John. It's a 2015 car. 
The symptoms are a stutter n the acceleration when you floor it usually in 2nd or 3rd from lowish revs. It then clears and accelerates hard. 
My electrical knowledge is "limited" but I'd like to try to test it. What setting should the multimeter be on? And which pins do I compare with which probe?

Thanks.

[John Vine]

Set up your multimeter like this, selecting the "sound/continuity" symbol:

 

Check by touching the probes together -- you should hear a beep.

To check continuity for a given wire, touch a probe (doesn't matter which) at each end. 

To get good access to the connector terminals, you can either prise out the yellow/red locking plates or (better) make your contacts by "back-probing".  To do this, starting with wire NP (cyl #1) in the 6-way connector, push a pin or needle into the back of the connector, parallel to the wire, such that it makes contact with the wire/terminal inside.  Do the same for wire NP at the coil connector for cyl #1, and touch the probes against the two pins/needles.  Repeat for wires NG, NU, NW, and then the WO wires.

G = green
N = brown
O = orange
P = purple
U = blue
W = white

I'd be very surprised if you find any continuity problems, but you might well find loose connectors or fractured wires as noted above.  Let us know how you get on.

JV

 

[ScottR400D]

Wouldn't it be easier just to change the sub loom? Is it that costly? 

[John Vine]

#6  Maybe the sub-loom isn't faulty?  That's what the OP wants to check.

But if it were my car, I'd opt for the flexi-loom anyway.  But it seems CC is OOS.

JV

[ScottR400D]

I get that John but there's a chance the fault might not be found with a meter and for £50 or so I'd have thought it would be a much quicker way to eliminate the loom or fix the issue. Wouldn't a 2015 car have the updated loom anyway?

As you say though, OOS!

[John Vine]

#8  Wouldn't a 2015 car have the updated loom anyway?

Well, you'd certainly hope so given the well-known problems.  But the OP's sub-loom is definitely not the flexi one.

JV