Sigma misfire - Dead ECU (was wiring diagram help)

[Bellissimmo]

So, I'm just home from working 'abroad' (Glasgow*smile*) and the 7 is misbehaving.  Co-pilot son took it out for a run last night and it was fine (Honest Dad!).  I took it out today and it is misfiring badly.  Looks like its not firing at all on 1 & 4.  Son (BMW trained and RAC patrol) is baffled - mostly by the wiring diagram.  

Is there a version of the wiring diagram that actually tells you where the wires go - or maybe an explorers guide to understanding the one that comes with the build manual?  We suspect a dodgy earth or a short to earth somewhere but struggling to trace the wires and find the various earth points We have three wires into the coil - assuming centre is for earth and the other two for 1&4 and 2&3.  Any assistance gratefully recieved.

 

Steve

 

[Jonathan Kay]

I've never seen a diagram than does that, and they're now even more schematic than they used to be.

Have you got the table of "standard" wiring colours as well as the Assembly Guide?

Looks like its not firing at all on 1 & 4.

• Is that from the appearance of the plugs?
• Have the tops of the plugs got wet? Any chance the leads have been swapped?
• There's a known problem with interference on crankshaft sensors, but I can't remember which engines it affects... should be in the archives.

HT testing kit is available, and one Member has some.

Jonathan

[Bellissimmo]

No 'table of standard colours - but would be nice to know.

Not firing on 1 & 4 deduced by pulling off the leads one by one.  Pulling off 1 & 4 made no difference, pulling off 2 or 3 stopped the engine running.  Also had the leads off one by one and can see a spark from 2 & 3 and not 1 & 4.

No chance of the leads being swapped.  Worked fine last night, didn't work this morning.  Bonnet not been off between nor has there been rain or other water. 

Would be nice to confirm the function of the three wires into the coil.

[Jonathan Kay]

"COMMONLY USED COLOUR CODES FOR BRITISH CAR WIRING"

Jonathan

[Tom_Arundel]

It`s not unknown for the coil packs to fail on Sigma engines, they are wasted spark engines.i.e 1- 4 spark together  and 2-3 spark together. Failure usually due to big plug gaps and high revs.

[Bellissimmo]

That was our first stop.  Euro car parts had them at 22.50 so we took a punt.  swapped them, problem still there.  One of our outstanding 'things to check' is whether the the centre wire is live or earth, so if you could shed any light there, we'd be another small step forward. 

 

[Jonathan Kay]

Could you add a photo of the cap and those three wires?

Jonathan

[martyn]

I imagine a faulty earth connection would effect all cylinders and not just a couple of them.  So instead I'd be focusing on the plugs, HT leads, and coil pack.

Replace the plugs to rule them out.  Its a good thing to do regularly anyway.

Then with the fresh plugs, try swapping the HT leads around.  Obviously dont run the engine like that but just see if you get a spark.  If the failing spark follows the HT lead you know its your culprit.  If the failing spark stays where it is then you know the problem is upstream from there.. coil pack, or wiring to the ECU.

[Jonathan Kay]

Agreed

Jonathan

[Bellissimmo]

So I'm just back from my Tartan-to-Garden commute and co-pilot son has done some more diagnostics and it looks like BAD NEWS.

He's got a NOID light (never heard of it?  me neither).  He can see power on the low tension pair that is supplying cylinders 2&3 nothing on the low tension pair that is supplying cylinders 1&4.  He's had the 1&4 low tension cable off and has continuity from ECU to coil so it looks like no 'signal' from the ECU for 1&4.

 

So the question now becomes repair or replace?  Are they repairable?  Anyone know a specialist that could take a look?  Anyone got a spare ECU they want to sell?

Just looking for options now.  :-(  

[Jonathan Kay]

He can see power on the low tension pair that is supplying cylinders 2&3 nothing on the low tension pair that is supplying cylinders 1&4.  He's had the 1&4 low tension cable off and has continuity from ECU to coil so it looks like no 'signal' from the ECU for 1&4.

Does that exclude bad connection from ECU to lead?

What sort of ECU and have you run any built-in diagnostics yet?

You might find someone who's prepared to swap to test... 

Jonathan

[Bellissimmo]

I ass/u/me he's tested for continuity from the pin on the ecu end to the cable end, so "probably", but I guess there's scope for corrosion inside the ECU connection.  He's very thorough but I'll check when he gets home from fixing other people's cars.

I've got the 'OEM' ECU as supplied from Caterham - don't know what manufacturer.

I'm hoping someone's upgraded and got one laying about but a swap-to-test maybe an option.

Steve

 

[Paul Deslandes]

Is it an MBE ECU?

If so and assuming you're correct re the ECU 1-4 ignition drive being faulty, don't rush to buy a new ECU.  Contact MBE directly rather than going via SBD who are their agents, and see if they'll accept it for repair.  I know of one member who had his repaired at very reasonable cost and with a pretty quick turn around.

The only other possible cause I can think of is a short in the LT ECU to coil cable although it's pretty unlikely.

You could try swapping over the LT feeds and the 1-4 and 2-3 HT leads on the coil but I think your analysis has already taken you further than this.

Paul

[Jonathan Kay]

If I'm reading what you know so far correctly I think it could still be connector to connector... I'd get him to try that probe directly on the ECU output as well.

Good luck

Jonathan (Crossed with Paul's)

[Bellissimmo]

Thanks Paul, as you've probably read, I'm working in Scotland so all communications between me and Andy (co-pilot son) have been me knackered from a long flight or him knackered from working late into the evening. So I haven't even seen the ECU yet let alone reviewing or re-doing any of the work he's already done.   

Hopefully this weekend, we can get our heads together and finally confirm where the fault is and get it sent off to MBE or whoever the maker is.

We have swapped the LT feeds and the fault stays with the 1-4 side of the ECU so I'm pretty convinced it's at the ECU end.

Assuming a short in the LT to coil would stay constant, I think we've already eliminated it but I'll double check this w/e.

Steve

[Bellissimmo]

It's an MBE, so I'll give them a try.

Steve

[Dorset7]

I've just been down this road.......

My car was cutting out and when I checked I had a poor spark on cylinder 4.

I replaced the coil pack, plugs, leads, crank sensor, cam sensor and air temp inlet sensor - none had any effect on the issue.

I had my ECU checked and it was fine (I'm informed from a VERY reliable source that they very rarely fail).

The culprit - a failing ignition circuit relay in the immobiliser. Now the immobiliser has been  replaced it runs just fine.

Having subsequently spoken to a Caterham specialist (and racing team owner) he tells me they fail on a regular basis and they often remove or bypass the immobiliser completely.

[Bellissimmo]

Oooh, that sounds interesting.  ...but how do I isolate the problem?  don't really want to end up replacing both.

Steve

[Paul Deslandes]

I agree that the MBE ECUs very rarely fail and I can see that causing the engine to cut out intermittently, but not just loss of spark on one half of a wasted spark coil.  Worth a look though.

Where's Andrew (revilla) when you need him as he knows his way around the immobiliser wiring and relays.

Paul

[Dorset7]

There are six wires coming from the immobiliser, two are thin and operate the LED and the induction loop.

The other four work in pairs - two for the starter motor circuit and two for the ignition circuit. They are colour coded but only inside the immobiliser box, white and blue pairs as I recall (colour coding on the wires is removed on installation).

If you can determine the correct pairs (white and blue) you can bridge them with a scotch lock and this will remove the immobiliser relays from the equation. If the car starts and runs correctly there's your issue.

The alternative is to get your ECU checked to rule it out then, by a process of elimination you're left with either the immobiliser or a wiring fault.

I had my immobiliser replaced (insurance requirement) but I know of plenty of cars that have had them removed completely.

As for cost I stumped up £130 for a new unit and complete installation worth a punt when you consider a new ECU from CC is £630!!

[revilla]

@Paul - I am here!

Unfortunately I only really know the K Series setup. The K Series immobiliser arrangement doesn't use any kind of external relay, the actual immobiliser function is really in the ECU software, all the external immobiliser box does is exchange an encoded key message once it has verified the key fob. If the ECU doesn't get the coded go ahead it cuts the fuel pump (I guess this bit is via a relay) and just stops sending out pulses to the injectors and ignition coils, so no way to bypass it without either providing a dummy external box that acts like a permanently disarmed 5AS immobiliser or by changing the ECU software. None of the ignition or injection signals actually pass through the immobiliser.

None of that is particularly relevant here, and I'm afraid I have very little knowledge of the Sigma immobiliser arrangement. 

Sorry to disappoint! Wish I could be of more help.