Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
ECM is offline and I've got no 5v ref at The throttle body. After looking at a diagram, it doesn't look like any sensors share the 5v ref external to the ECM, I don't see any splices. I assume they would still use the same voltage regulator in the ECM, but since they all have different wires going to the ECM, could one shorted sensor bring down the 5v ref or is this not one of those situations?
A short anywhere in the reference circuit can take out the PCM 5V supply. I would check the engine harness thoroughly. Probably a pinched or rubbed wire somewhere.
By the looks of P0643 there are several sensors which can pull down the 5V. Interesting that it can set a code so maybe a shorted 5V doesn't cause a no comm?? Have you checked ECM powers and grounds yet?
I haven't had much time with it, but will tomorrow. All I have done so far is confirm the ECM no comm, confirmed no 5v ref. After I found this out I looked in the manual for all of the sensors with a 5v reference value and unplugged them. I also confirmed power to the ECM. That's where I will be picking up tomorrow. My plan for tomorrow is to check grounds at the ECM and shorts to ground in the harness on those sensor circuits. If all of that checks out I guess I'll be recommending a new or used module.
When I first got in the car waiting for my push help to come out I noticed that the fuse cover was off and there was a roll of new wire on the passenger seat. Fortunately it was still in it's packaging. Then I found the covers off in the engine bay and some evidence of tampering. There was a 30a fuse in the fuel pump slot instead of a 15 (see my carnage post in the off topic thread on some pictures of a customers harness who did that a couple of weeks ago on a shorted circuit). For fun, I popped the seat out and the fuel pump cover to find two of the wires on the pump had about a 1/4" of insulation stripped away to touch anything they want to. The customer shared with the writer today that they had someone put a fuel pump in it when it wouldn't start.
I am looking at that P0643 now and I'm wondering the same thing. It says that it goes into limp mode and throws the code. I've never seen this code and this is the first time I've had a Nissan with the 5v down like this, so I'm not sure.
Thanks for the replies so far. I'll let you know what I find tomorrow but will check back here a few times to see if any more info has been added!
Ha, you're right, and I don't remember what he found! One of the problems is that service info for these older models is ridiculous. Nothing is in a logical place. I finally stumbled across a section that had an image of powers to the ECM, and I missed one the other day. A black/red wire that should have b+ in start or run does not. It comes from the ignition switch and has one splice. Two techs just left so we are getting slaughtered, so I didn't get to this until the very end of the day. I did check the ignition switch and have power in and out. I suspect that splice is where I'll find the issue. That's where I left off today. I'm working tomorrow but probably won't have much time with it. If I get a chance I'll see where that spice goes on a diagram and see if that component has power, then I'll at least know there is power to that spot.
The old manuals are so bad that I usually just use alldata or mitchell. Especially for diagrams.
I saw that somewhere else recently but for this 2011 the description was different, basically just saying that it could be a shorted APP sensor, TPS, or CPS, or a short to ground on the 5v to those sensors. It directs you to clear the dtcs and go through all of the nonsense, but I don't have a live ECM. It doesn't mention the two systems or all of the other sensors. This is a 2.0 too, if anyone was wondering.
SI is BAD for this one. The power distribution, from PG-14 on, doesn't even show the ECM feed. The splice does feed a bunch of stuff, including several fuses, so it shouldn't be difficult to check some power is passing thru' it.
In Engine Controls>Power Supply and Grounds the feed to the ECM is drawn as a straight shot from connector M6/E19. Might be worth checking how many wires come out of that connector.
I'm in my garage finishing up a heater core on my uncle's car and am in part 2a of Paul's Sentra No Comm premium video that Tyler mentioned and I forgot how similar this is! Right down to customer fuel pump replacement! I got excited when he had no power on that wire until I remembered that he had the connector orientation off and there was no pin in the cavity. I've never seen Paul so frustrated in a video and man, these connectors are stupid. I had my Phil's tool out today with it too, ha.
I wish I could just get some time to focus on this. I'm spending more time digging through the ridiculous SI than anything else.
I believe I found E19 where the yellow ignition wire turns into black /red and goes through the firewall loom. After the firewall I believe I have found that wire wrapped up with another B/R wire. They join together and go to the IPDM. I have power on all of them. I just thought I should have done a continuity test to make sure it was the right one, but they are the same larger gauge and I think they are.
Si info says that section of wire is part of a sub harness that I have not find a diagram for yet.
I'm totally shot and had to walk away but wanted to throw everything out there that I found. I'm not thinking clearly, but this could be it. Ignition in on position and harness connected to ECM, I jumped b+ to that terminal and my light lit up. That makes me think there is a short to ground in the ECM. I also unplugged the connector and did a continuity check on that wire for a short to ground and found none.
Even if the ECM is toast, I still have to find why I'm missing power to that pin!
Tutti57 wrote: Ignition in on position and harness connected to ECM, I jumped b+ to that terminal and my light lit up. That makes me think there is a short to ground in the ECM.
ECM is probably trying to draw more current than the test light can flow. More likely to be a lower resistance than a test light path to ground than a short IMO.
I'd be tempted to supply voltage to that terminal with a big test light, or a fused jumper, and see what happens. If the ECM is shorted it ain't gonna damage anything that isn't already broke.
I agree with Matt. You don't have much to lose by trying the fused jumper. :silly:
For whatever it's (not) worth, I did try shorting both 5V reference circuits on my Sentra with the engine running. Both stalled the engine, both set a bunch of codes. Always restarted afterwards, and always had communication.
Not saying you were wrong to check! I just wanted to see how the PCM handled it. :evil:
I actually tried providing 12v with my PP and it didn't help. It didn't trip the breaker either, so what you guys are saying makes sense.
I just spent some time trying to go through the diagrams again and I keep going in circles in whatever that "sub-harness" is. If I trace out of the ECM, it's a R/B wire that hits the sub-harness showed as a black dotted line (like it's in another diagram) and also says it's R/B. Since I know it comes from the ignition switch, I follow that one down until I hit a B/R wire that goes to the IPDM. I am not finding a link between the ECM and IPDM for this wire. It's gotta be somewhere in that sub-harness bermuda triangle, but I am not seeing it.
Well thanks to Nissan SI, I was chasing my tail. The colors were not accurate on the connectors, so I was on the wrong wire. I did have good powers and grounds, so I replaced the ECM yesterday and am back up and communicating. Instantly got two codes, one for the fuel sending unit and one for fuel tank temp circuit. Now it's a cranking no start with no fuel. It could be out of gas or something is up with the "new pump" with the hacked up wires. We will see.
Knowing that both codes and no fuel pump activity were all common to the connector on the pump, I started there. Scan data showed fuel tank temp at like -48* indicating an open and fuel gauge fixed at something like 3.04v. Wiggle test revealed a bad connection there since scan data changed and I was able to get the pump to turn on and off.
I suspect that it never actually needed the fuel pump that the other shop put in it. I've been trying to speculate on whether or not leaving those wires bare at the pump and having the 30a fuse in the pump slot instead of the 15a could have caused the ECM damage. I believe the three wires that were stripped were fuel pump power and ground and the FTT signal. With the latter being a thermistor circuit, grounding that shouldn't be a problem but wouldn't a short to ground on the pump power wire just blow the 30a too, or could it take out the driver first? I still wouldn't think it wouldn't cause the no comm issue and would just prevent the fuel pump relay to turn on if the driver was shot.
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