Labda sensor earth problem

[Minio]

I have problem to get my Chimaera 4.3 through the Swedish MOT because of way to high emissions. After some diagnostics I found out that the ECU don't regulate on the lamda sensors, and it turned out that neither the lamda sensors nor the ECU pin 4 are connected to ground.
The schematics show a common ground point for ECU pin 4 and lambda sensors and now I try to find out where that point can be located in the car.
Does anybody out there know where I shall start looking?

[blitzracing]

Its earthed through the cable that feeds it- there is a join in the cable inside the loom, where the lambda ground wire connects to a braided screen (like and audio cable) that connects to the ECU directly. It does not use the car chassis at all for ground. I would unwrap the tape around the cable that leads to the Lambdas 3 wire connector and locate the earth screen connection inside it.

[Minio]

OK thanks for a quick reply.
Maybe I misunderstood your answer, but as I understand it the lambda sensor must have a heater ground. From the ACT fault finding guide:
'With the ignition switched off, test for continuity between the White ground wire and the engine block. There should be no resistance (i.e. a short circuit).'

I have no connection from the white wire on the lambda, or the black wire in the wire loom, to ground.
I have no connection from ECU pin 4 to the lambda white wire or ground.

I have connection from lambda sensor signal wire (black/blue) to ECU and get 12 V , relative engine block, on lambda heater when fuel relay is on. I have replaced both the lambda sensors and the extension cables.

[blitzracing]

OK

Black wire is lambda output- this is not connected to ground at all
White- this is the ground wire and connects to the cable screen that runs inside the loom, and then is grounded at the ECU pin 4 and to chassis- all the earths for the ECU signals have a single point of connection at the ECU to ground- so you should get a near zero ohms reading. The point I was making its it does not use a a local engine or chassis ground to reach the ECU, it has to use the ground screened wire to get from the probe to the ECU.

[Minio]

Well it's a bit confusing with the colors changing from the probe into the loom but I guess we are talking about the same ground wire i.e the white wire from the probe. As you say there shall be a common ground for the white wires at ECU pin 4, but are pin 4 grounded internally inside the ECU, or dose it have an external ground point at the chassis? If so where?

I have disconnected the ECU plug and do the following measures:
Pin 4 to chassis ground - no connection
Pin 4 to white wire on probe - no connection
White wire on probe to chassis ground - no connection

I suspect that there are a wire break or a lose connection somewhere and hoped for a lose ground nut or something but it's a 'bit' hard to follow the wire loom. It's also strange that both the left and right lambda probe are not connected to ground, that made me thinking that there shall be common ground point that have come lose.

Any suggestions for further diagnostics or measurements?

I really, really want to get the car back on street because we have only spent one summer together so far.

[blitzracing]

There will be a ground plane as part of the PCB tracks in the ECU. Id suspect that the lambda probes earth pins would connect to this, but Ill have to open up a spare ECU to check as there are several ground pins on the ECU plug, pins 14,27,and 40

[Minio]

Hi again.
Now I have had another session in the garage trying to find out where the lambda probe earth point is. I unwrapped the wire loom from the ECU and followed the black wire from pin 4 that are supposed to be connected to earth together with both earth wires from the lambda probes. The wire disappear together with the wire loom into the center console under the dashboard and I suppose that it shall be connected at one of the earth points under the dashboard. According to the images in Steve Heats Chimaera book the earth points right under the dashboard panel.
Does anybody now how to reach those earth points?
Do I have to remove the dashboard and panels completely?
Has anybody seen the wire loom in the 'free' or know where the lambda probes are supposed to be connected to ground?

[blitzracing]

You should not need to look for a chassis earthing point, as the ground connection is fed through the screened cable directly to the ECU. So it goes like this: White wire earth wire from probe goes to black wire from loom. Inside the loom tape this black wire connects to the screen around the lambda signal wire. At the ECU end a black wire is connected to the screen at the other end of the lambda signal wire, and this then goes to Pin 4, so you have a direct earth connection. The chassis earth is not used for the lambda signal as its prone to interference. Ive an old loom laying around ill try and take some photos.

Edited by blitzracing on Friday 23 September 12:35

[Steve_D]

blitzracing said:

.......there are several ground pins on the ECU plug, pins 14,27,and 40

Also worth noting that these pins earth at the front of the engine to a bolt beside the timing cover (close to the oil warning switch).

Steve

[Minio]

Thanks a lot for your patience. This give me some hope.
I'm not sure I got it right but when I traced the black wire from pin 4 it disappeared, together with the wire loom, into the center console under the dashboard. I guess, according to your answer, that the black wire from the lambda connector are connected to the screen in the loom inside the engine bay or right after the loom pass through the firewall where its hard to get to.
I have to go to the garage again in the weekend and check how the screens are connected in the ECU end.

And does't the lambda probe white wire have to be connected to ground somewhere for the probe heating return? At the ATC 14CUX page Lambda (Oxygen) Sensors it says:
With the ignition switched off, test for continuity between the White ground wire and the engine block. There should be no resistance (i.e. a short circuit).

If I understand your reply it locks something like this:

pin 4 ---------------|-------- screen ----- black wire in loom ------ white wire on probe a
|
|
|-------- screen ----- black wire in loom ------ white wire on probe b


pin x ----------------------- blue wire in loom ------ black wire on probe a
pin y ----------------------- blue wire in loom ------ black wire on probe b

+12V ----------------------------------------------- Orange/White ----red wire on probe a and b

[blitzracing]

Minio said:

pin 4 ---------------|-------- screen ----- black wire in loom ------ white wire on probe a
|
|
|-------- screen ----- black wire in loom ------ white wire on probe b

Thats correct


The 0 ohm ground point is correct as the lambda earths are connected to the ground pin 4 on the ECU, that will be connected to chassis ground via one of the ECU grounding tabs. Where you are getting confused is you should be looking for continuity between pin 4 and the probe white wires as your starting point, not to chassis ground. I think the heater is also grounded via the screened wire and ECU ground, not directly chassis ground. If the ground wire is open circuit to the ECU the heater supply voltage will end up on the white ground wire as it leave the probe, so you would measure 12 volts on the red and white wires, not just the red. Ill double check when I get home tonight on the old loom.

Edited by blitzracing on Friday 23 September 13:57

Edited by blitzracing on Friday 23 September 14:06

[Minio]

Well confused I am!

It all started with the sentence from ACT and so far I have the following facts.

1. No connection from any of the white probe wires to ground.
2. No connection from any of the white probe wires to pin 4.
3. No connection between the white wires from left and right probe.
4. No connection between pin 4 and ground when connector plugged into ECU
5. Black wire from pin 4 disappear into the center console (as far as I know)

I guess there are a simple explanation but right now it looks rather complex....

[blitzracing]

Looks like my memory has failed me on this one! Only the lambda wire is connected to the inner core of the screened wire. The earth wire runs separately alongside the screened wire and eventually ends up in this plug. This plug appears to have multiple earth points going into it, but I dont know what it plugs into. I cant find it on my Ginetta- Id suspect its in the back of the engine bay, as its a major branch of the main loom. This would seem a very likely point of failure if it had come unplugged.

[ClassiChimi]

This is a Tvr wiring loom from a 2000 year 450 Chimaera. The bit that comes through the fire wall and into the Footwell.

And here's the other end of it that sits in engine bay.






Viewed from the left hand side of the car if it was fitted.
I hope this may be of some help.

[blitzracing]

Well thats it with a loopback plug plugged into it near the main ECU plug in the footwell loom. Mine must be taped up somewhere as Ive never seen it in the footwell on mine.

Edited by blitzracing on Friday 23 September 21:23

[ClassiChimi]

Could it be as simple as this connector being dislodged !

[Minio]

Thanks a lot for the pictures and information!
It looks really promising and if the plug was missing it explains why all ground connections was missing.
It made me run all the way to the garage in the weekend, but no, my wire loom from 1993 dosen't look like that. There are no extension going to a loop back plug like that. I can identify the communication plug and the tune resistor but that's all, except for some relay sockets, that's hanging out of the loom.
The screens from the lambda probe cables are solder joined with some thick black cable inside the wire loom, just before the ECU plug.
My hope right now is that the loop back plug is in the engine bay where I can find it. Another theory is that something went wrong when the AC compressor and the compressor clutch relay was removed by the previous owner.
If I don't find the plug or any other fault I have to route new cables to the lambda sensors and connect them together with the screens and pin 4 to a common ground point close to the ECU as it was supposed to be.

[Minio]

The mystery is solved, at last!

I had another nice session in the garage yesterday where I started to search for the connector, mentioned above, somewhere on my wire loom.
It was not inside the car so I start looking in the engine bay. There was 3 black suspicious wires coming out of the loom right before it passed through the fire wall.
After removing the inlet manifold I was able to spot the failure, they where hit by some blunt object right before the join. Or maybe attacked by a rat.
Anyway it all explains why nothing was connected to ground.
Thanks for all help and encouragement.

[Steve_D]

Well done...pleased for you.

Steve

[Sardonicus]

Give that man a C.I.G.A.R