Xterra ECM relay control
I've got this xterra here... it's a long story involving an accident and multiple PCM's. It came to me with a crank/no start. The ECM was not communicating and dragging down the network. The 5v power supplies out of the ECM KOEO were just over 2 volts. I sourced a used ECM to continue my search for the possibility of an ECM killer.
After replacing the ECM the only anomaly I have found involves the ECM power relay and (I presume) subsequently the PCM output power supplies. The issue is that when the key is shut off the ECM relay remains powered up sending power to the ECM pins 119 and 120. At that point all the power supplies out of the ECM remain hot (12v and 5v)(IAT, MAF, Evap sensor, Battery current sensor, AC sensor, ECT sensor, Power steering sensor, throttle, pedal, etc) There is no ECM communication during this time.
When this occurs the ECM power relay remains held to ground by the ECM (self shut-off) pin 111. The shut-off circuit should release the relay a few seconds after the key is turned off which would then kill the power going back to the ECM from the relay. I can not find any information about internal logic regarding the ECM power relay control. I would assume it would rely on input from the ignition switch to the ECM pin 109. I would think that when the ignition switch input goes to 0v then the ECM would command the release of the ECM power relay which would then power down the ECM except for it's KAM. Then when the ignition switch was turned on and the ign input was sent to the ECM it would power back up.
Another thing to note is that this fault occurs when the engine is running and then turned off. If the ECM is powered on and off without starting the engine then it will release the power relay. Start the engine, shut it off, it stays powered up. Then go KOEO and then turn the key off and it will shut off the relay.
Any ideas how this control is supposed to occur? What inputs does the ECM need to command ECM power relay on/off function? I don't know if I've got a bad used ECM, or something else confusing the ECM logic and it's relay control. I don't like used but I went used as a cheaper test step to get to the next stage. It already had a new ECM installed and bricked after it was operational prior to me seeing it so I wanted to find a killer without taking out another one. Is something causing this to do this and somehow this is causing the ECM to die? Is this just a bad ECM? Do I still have an ECM killer after this is resolved? So many unanswered questions!
So, any thoughts on the ECM internal logic regarding the ECM power relay shut-off circuit?
... How LONG does it stay latched?...try disconnecting the rad. electric cooling fan (and maybe the HVAC fan)and see if that changes the situation on a start /stop /staying latched thing...That's about all I can think of.Dan H. ...South Texas...
Rio: Try disconnecting the alternator to see if the latching problem goes away.
Good morning Rio, is there any possibility of a remote starter or aftermarket alarm in this vehicle? Does the voltage on ECM pin go to zero when the key is turned off? Will it release the relay if the brake pedal is depressed? I see you lose communication with the ECM through the scan tool is it possible the can bus is still active and another module is keeping the ECM awake? It looks like the…
As Rob suggests, it appears the main difference is the higher voltage of the alternator when engine is running that seems to trigger the circuit that keeps the ecm relay on. Just for kicks, with the key on engine off, I would connect a charger with the max output for a minute or two to confirm if higher voltage is the culprit and this may narrow down the possibility of a bad ECM. Good luck.
So heres a couple thoughts after looking at some wiring diagrams. Like you said only pin 109 is IGN power. So if your sure thats at 0 volts key off then that eliminates that. Another way that modules can be woke up is over the network . This truck has CAN and a K-line to the ECM. Are both networks at 0 volts on a scope when the problem is occuring? Have you cut the wire at pin 111 when this…
Thanks for the thoughts and ideas so far. Some good thoughts in the mix. I'll just talk about them all here. I did not replace the relay as it is integral to the IDPM and can not be serviced. As far as I can tell it is functioning as intended, as commanded by the ECM. Unplugging the fans has no effect. They are only connected to the ECM by data through CAN. The relay stays connected…
The relay can be serviced. For the life of me I cannot find the number but Keith at new level auto on youtube did a video and tells you the number. But rereading your story I see that you said that the relay is being turned on from the ECM so sorry for not understanding that part before.
No worries Philip. From my understanding the serviceable relays were in the earlier generation. But, I’ve been wrong before
Hello Rio I am wondering if this is a retained accessory power issue it would make some sense as to why the key cycle unlatches the ECM relay perhaps the BCM is keeping the ECM awake. Just another thought.
Just a thought but, look in the theory of operation section for the EVAP and see if it mentions staying awake for like 45 minutes to check fuel volatility or/and "EONV leak detection".
This one does not, but I wonder if the vehicles that do have key off evap checks have communication capability with the ecm during this period?
My educated guess would be "yes" since, from a design standpoint, it's simpler to have it just be fully awake to run it's timer and monitor pressure/vacuum, than to engineer an additional "partially awake state" for the ECM to do certain things but not others. I was watching a GM video the day before you posted this, originally, that mentioned an ECM taking 45 min to go to sleep, due to it's…
Updating this post with recent info. It kind of took the back burner for a bit, but today we got the replacement (new) ECM and installed it. The ECM power relay now functions as SI describes. When the key is turned off the relay is unlatched after a couple of seconds. So the used ECM we ordered was causing an issue. When the ECM powered down after engine running it would hold the ECM relay…