[willisodhiambo]

I am struggling with a very strange problem. The dme seemingly goes offline with dme power supply dtc. The dme has two supply lines. A direct line from passenger side junction box.

It has a second supply line actuated by dme itself through a relay in the junction box. A ten second battery reset brings it online and car can then be started. It falls offline everytime car is switched off.

Replaced the junction box thinking it was dme relay. Same issue. Then supplied that relay line directly bypassing the relay.

When i do this a new error appears, fuel pump activation! 

Mind you I already tested another dme.

I cant wrap my mind around this. Any similar experiences?

   

[willisodhiambo]

No where near resolving this issue!!

So I wired my own relay and connected S_HR dme ground to it and power supply through it. Then tested S_HR to ground with ignition on. About 3v which means there is voltage drop at the relay and car could start.


Ignition off, waited 10 minutes, tested S_HR, now 12volts!! DME stops giving the ground after short rest. Cranked car as I watched the voltage on this line, it drops to 6vs then back up. Almost as if the dme internal relay is attempting to gound the line but then releases without holding it to ground.


It is not dme fault because this happened with a replacement dme too. So my question is - How does DME know terminal 15 is on? Must be a PT-can message from CAS as the pinout for dme has no direct terminal 15 input. Without this message could it be that dme remains asleep even though it is receiving wakeup signal?


I am going to test with another CAS, if that does not resolve then I have no option but to give up on this one!!

[Node]

Where did you measure 3/6V? On the DME side or on the junction box side?

If DME change did not resolve the issue I would guess somewhere near junction box? Because I could not see how else it could behave this way. Then I bet there is a problem with a S_HR wire from DME to junction box. Corroded or something, anyway too high resistance so current is not enough to pull DME relay.

How this could work: while the wire is cold its resistance is lower so its able to pull relay. As its resistance is high, the corroded part of it heats up so resistance increases further. It is still capable of keeping relay engaged, but not to re-engage it.

According to schematics there is X6011 connector on that wire. I would first check if there is no corrosion inside of it or on S_HR wire near it. If that would not help measuring resistance of the wire from DME to that connector and from the connector to junction box would be useful.

P.S. I do not think CAS has anything to do with this because DME is clearly trying to pull down that wire (hence drop from 12V to 6V). It just fails to engage the relay.
P.P.S. I assume inside DME its transistor, not relay which pulls S_HR low, so it should be near 0V on DME output.
P.P.P.S. There may also be wire insulation damage and short-circuit to some other signal wire. But such cases are more rare and much more difficult to diagnose.

[willisodhiambo]

Where did you measure 3/6V? On the DME side or on the junction box side?

If DME change did not resolve the issue I would guess somewhere near junction box? Because I could not see how else it could behave this way. Then I bet there is a problem with a S_HR wire from DME to junction box. Corroded or something, anyway too high resistance so current is not enough to pull DME relay.

How this could work: while the wire is cold its resistance is lower so its able to pull relay. As its resistance is high, the corroded part of it heats up so resistance increases further. It is still capable of keeping relay engaged, but not to re-engage it.

According to schematics there is X6011 connector on that wire. I would first check if there is no corrosion inside of it or on S_HR wire near it. If that would not help measuring resistance of the wire from DME to that connector and from the connector to junction box would be useful.

P.S. I do not think CAS has anything to do with this because DME is clearly trying to pull down that wire (hence drop from 12V to 6V). It just fails to engage the relay.
P.P.S. I assume inside DME its transistor, not relay which pulls S_HR low, so it should be near 0V on DME output.
P.P.P.S. There may also be wire insulation damage and short-circuit to some other signal wire. But such cases are more rare and much more difficult to diagnose.

I measured the voltage on the dme side after relay load. When battery is reset, the voltage is 0-3v which is expected and car can start. anytime car is switched off and key removed, 10 seconds later problem recurs

1- Tested resistance of this wire, it is OK from 0ohms
2- Checked intermediate plug, OK
3. Checked use resistance - Fuse OK
4. Wired my own relay, dme still behaves the same

There is evidence that this problem has been probed before. I just cant figure it out. One strange thing. I supplied 12v directly to dme bypassing the relay and attempted a start. No start but with this bypass, fuel relay fails to energise!! How can that be? It is like when DME relay is bypassed, fuel relay fails. Maybe dme sense HSR line before engaging fuel line. 

One thing I notice, there is the brake warning almost all the time, as if brake switch is faulty or start attempt has been made without pressing the brakes. No DTC relating to brake switch though.