How do I test a set of wires in a loom?

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Tim4x4

Tim4x4

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OK pretty dim question but I think i have an issue with the engine loom on a Defender TD5.
Its the wires that connect to the injectors and its from the plug and socket at the head back to the ECU.
Some journeys I have poor running after a few miles If I stop and under the bonnet move the harness around a little everything is back to normal.
So I am thinking maybe the harness has some wires chafed and I am moving them to a better position to stop the poor running.
Not wanting to completely strip or replace the harness what electrical test can I do to
A) find out if I have damaged wire/s and
B) which wire/s are damaged
So that I can replace just the affected ones.
Be gentle I am no electrician
 
Known problem with td5 injection loom as they age. Cheap enough to just replace. It is only the section in the engine bay you renew.
 
I do a bit of wiring repairs on trucks and it can be a right bar steward to find the breaks/damage especially if the fault is intermittent.

Multimeter set to ohms with each pobe on each end of the wire and get wiggling, but unless you wiggle it in exactly the right way it may not show a fault.
 
tottot said:
Known problem with td5 injection loom as they age. Cheap enough to just replace. It is only the section in the engine bay you renew.
Click to expand...
Thanks tottot would like to make a test first £450:00 is a lot to shell out for the loom.
Where do they fail? Maybe I can take a look at the loom in that area.
 
lynall said:
I do a bit of wiring repairs on trucks and it can be a right bar steward to find the breaks/damage especially if the fault is intermittent.

Multimeter set to ohms with each pobe on each end of the wire and get wiggling, but unless you wiggle it in exactly the right way it may not show a fault.
Click to expand...
I see where you are coming from the wiggling test and it may not show the fault when there is a fault.
Is an insulation test with a megga of any use to me?
 
if the ECU is connected it a good way to destroy it pumping 500 volt to 1000 volt so no a insulation tester is not a good idea
the problem could be corrode or bad contacts on the connection have had problems on thin wall cables where the wire
turns to powder causing a high resistance ,the usual sign is a faint grey line on the insulation above a connection plug
 
The injector loom (harness) is only about £30, have you pulled plug at the ECU and checked for oil, oil can get down the loom to ecu, common problem fixed by fitting a new injector loom and cleaning plug, you need to clean oil out regularly until it clears up.
There are videos on youtube showing this problem and how to replace the harness.
 
Last edited:
Scooby22 said:
The injector loom (harness) is only about £30, have you pulled plug at the ECU and checked for oil, oil can get down the loom to ecu, common problem fixed by fitting a new injector loom and cleaning plug, you need to clean oil out regularly until it clears up.
There are videos on youtube showing this problem and how to replace the harness.
Click to expand...
Its not the injector loom and no oil at all its between the plug and socket at the head and the Ecu. Engine harness is costing £450:00.
I would like to trace the fault if possible but I have little electrical knowledge.
What would be the cost of replacing the engine harness by an auto electrician?
 
mystery said:
if the ECU is connected it a good way to destroy it pumping 500 volt to 1000 volt so no a insulation tester is not a good idea
the problem could be corrode or bad contacts on the connection have had problems on thin wall cables where the wire
turns to powder causing a high resistance ,the usual sign is a faint grey line on the insulation above a connection plug
Click to expand...

If I remove the Ecu and test the individual wires.
How do I make the test?
 
Last edited:
you could make your self a practical tester as shown this will high lite any bad connection or high resistance joints due to corrosion
I use large pins pushed in to the spade crimps as this give excess through the rear of the harness connections
but be careful not to apply any voltage to the loom connections whilst power is connected to the vehicle
PIC_0002.JPG
PIC_0008.JPG
as the ECU monitors some components when they are grounded to the return wire or chassis ground
and other by measuring the varying resistance and induction of the various components so if in doubt don't
 
What I have done today is drive the vehicle until the issue is present, stop in the nearest layby.
Disconnected both the plug at the head and the plug at the Ecu.
Completed a continuity test of each of the wires that fire the Injectors.
Each indicated continuity, I then moved the harness about whilst testing again each wire, I was unable to induce a break in the wire.
Now floundering I checked the continuity between different wires by picking one connection at the head plug and checking all the relevant connections at the E CU plug. Repstin this for the other connections. Now I think this has given me the problem because on one pair of wires there is partial connection between them.
I am thinking this is not correct and I should not have this reading.
Would anyone confirm this could be due to these wires chafing the insulation breaking down and causing the problem with the running.
 
Does seam like it. finding that point may well be a difficult job. Might it be possible to bypass those two cables with some temporary external ones to do a test ?
 
You may have to remove the entire loom and open it up, you will have to rewrap it again afterwards, self amagmating tape is the kiddy also easy to remove in the future with no sticky goo to contend with.
 
Firstly you have to blow down the ends to remove all the smoke then gently empty a large bucket of sparks in until full.
Lucas had a whole department for this. It was right next to the intermittent headlight research lab.
 
tottot said:
Does seam like it. finding that point may well be a difficult job. Might it be possible to bypass those two cables with some temporary external ones to do a test ?
Click to expand...
You would only need to bypass one of the suspect wires with a temporary patch as long as the one you are bypassing is disconnected.

Col
 
You say you stop and wiggle wires around and running gets better. Could it be the stop is what is making it run better and not the wiggle. Maybe fuel issues and the stop is allowing it to catch up or something is overheating. £30 wiring loom sounds like a good start for an intermittent fault that you just cannot find as loads of them have this fault.
 
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