C3 Earthing problem runs with obd2 scanner attached

[Meannamate]

Hello or Bonjour to everyone! I’m creating a new post as I’ve searched my issue and can’t find anything specific. I have a 2007 C3 diesel that was kept garaged for years . It’s been running fine for two years. Had a recent full service, new battery etc. the other night I was on the way home and it died without any explanation. Dry conditions. Towed to a local mechanic. They looked into it and couldn’t find anything. So it’s back on my drive. Started googling and looks like a grounding issue. Upon starting it can choose various ways to be moody! Either starts first time. Ignition on but doesn’t start. Ignition on and cranks. Then throws up a myriad of messages. Anti pollution, abs, plip battery, air bag etc etc. it all points to an earth - probably around the ecu? I’ve visually inspected any earths around the ecu/fuse box. Engine earth strap. All clean and dry. I’ve looked for voltage drops and all fuses. Where to next?

[wheeler]

Id start by removing all the injection ECU & under bonnet fuesbox connectors & check for any signs of water ingress.
Are you saying it starts & runs ok with the OBD scanner plugged in?

[Meannamate]

Thanks for responding so quickly wheeler! Yes runs flawlessly with the scanner in the socket so I know it’s grounding through it. I had all the plugs off around the battery. Ecu, BSI and the small unit behind the battery. All clean and dry. Checked any earths I can see in that area. OBD threw up U1000, U1003, U0121, U1118, P0500 - all to do with comms, but these could be red herrings as everything system is working with the scanner attached…

[Meannamate]

Update- I’ve continued to work through any possible grounds I can find. I haven’t found anything that would point to a bad earth. Tested every system again- all working with the scanner in the socket. Starting to think it’s the ECU? Any way to test just the ECU alone?

[wheeler]

This is a weird one. You could try running a temporary earth from the ECU casing directly to the battery negative & see if that makes any difference.
Failing that just leave the scanner plugged in

[Meannamate]

Thanks again! That’s not a bad idea. I’ll try that. I thought about jumping the ground pins in the OBD connection. Worth a try! I’ll report back. I could also pick up the cheapest scanner for less than a tenner and leave it in! No harm leaving a scanner connected?

[wheeler]

No harm leaving a scanner connected?

Only thing i can thing of is drain on the battery as it will be powered all the time.

[Meannamate]

Thanks again wheeler! Ok I’ll keep searching and try earthing the ECU casing. It’s a brand new battery and the scanner has a power switch so at least I can turn it off (if I remember!!)…

[Meannamate]

Update - unplugged scanner. Grounded ECU case. Car started and runs fine. Then switched on all electrical systems. Engine stopped and threw ‘anti pollution’ and ‘brake system’ on dash. MIL came on then a few minutes later SRS airbag warning and light. Started again and switched a few things on abs off. Took key out of ignition, locked steering. Engine carried on running!! Took key in and out a couple of times. Still running. Plugged in scanner, engine stopped. Ran the scanner and got just U1118. Help!

[ozvtr]

No (or faulty) terminating resistor (load) on the CAN BUS? Which CAN BUS? Not familiar with the facelift C3, is it all one CAN BUS? Or are there multiple? Pre-facelift has multiple VAN BUS(es) and one CAN BUS.

Scanner acting as terminating resistor? Somehow correcting the line impedance?

I don't think the engine ECU is directly connected to the ignition switch on the facelift cars. I think it's power is switched by the BSM and the BSM is networked to the BSI.

Would also explain the coms errors.

I would have thought that the outright loss of the CAN BUS would have shut down the ECUs. So...it's sort-of working? I don't know enough about the CAN BUS structure to say.

If that's the case which end of the CAN BUS is the scanner connected to? That is: I would say the ABS ECU would be one end and the BSI at the other? I don't think there are separate terminating resistors on the BUS loom. I would have said they were in the ECUs. Dunno. Trying to think of a logical failure mode here. I cant think of how the BSI would fail but it could be corrosion in the connector of the ABS module, but that's on the other end of the BUS!?

Put a 120 Ohm resistor across pins 6 and 14 of the OBDII connector? See what happens? My research shows CAN high and low pins in the OBDII connector of the facelift model but not the pre-facelift model...Hmmm.

Just a thought.

[ozvtr]

Oh, if you have a multimeter you could check the impedance of the CAN BUS by measuring across pins 6 and 14 of the OBDII connector using resistance on the meter. Should be around 60 Ohms.

[wheeler]

No (or faulty) terminating resistor (load) on the CAN BUS? Which CAN BUS? Not familiar with the facelift C3, is it all one CAN BUS? Or are there multiple? Pre-facelift has multiple VAN BUS(es) and one CAN BUS.

The facelifted model from 2005 is full CAN

If that's the case which end of the CAN BUS is the scanner connected to? That is: I would say the ABS ECU would be one end and the BSI at the other? I don't think there are separate terminating resistors on the BUS loom. I would have said they were in the ECUs. Dunno. Trying to think of a logical failure mode here. I cant think of how the BSI would fail but it could be corrosion in the connector of the ABS module, but that's on the other end of the BUS!?

Im not sure if this applies to all manufacturers but on PSA cars the end of line terminating resistors are always the BSI & injection ECU.

Yes definitely worth checking the CAN line resistance, if you get anything but 60 ohms (give or take a couple of ohms) then you have a problem.

[Meannamate]

Thanks ozvtr! Ok, I connected a multimeter across pin 6 and 14 of the OBD port. Got nothing, either ignition on or off.

[wheeler]

Are you checking resistance across both those pins?
Can you also do a voltage check between each of those pins & earth? You should have around 2.5v on each.

[Meannamate]

Interesting. Just had battery out. Put it back in and get 65.3 ohms across pins 6 and 14 with ignition key out. 0volts. Ignition on I’m reading no ohms and 0.8 volts.
Pin 6 to ground is 1.2 volts 0 ohm. Pin 14 to ground is 0.5 v, 0 ohm