2015 GM Truck 2500 HD 6.6 LML No Crank No Communication With ECM Diagnosis pt. 1

Well good evening today we have a

2015

chevy silverado

2500

hd with 66 dirty max just towed here because apparently it stopped while the customer was driving, he said the stabilization light came on, the engine stalled and wouldn't start, I went back to it. a day later and the battery was dead so I put a battery charger in it. Now service to generate a follow-up message. I see a little fuel light and it's not trying to start so let's hook up a scanner to this thing and see if we can talk to any drivers so let's try to hook up to this thing to see if we can talk to anything here and it's a

2015

.

Let's see if it will identify itself automatically and the ecm is not communicating so I will have to identify it manually so I will get the VIN number so I manually entered the container and it still didn't decode it so let's do a code scan and see if anything is will communicate on the CAN bus. We have

communication

with the transmission

communication

with the abs module and it complains of a loss of communication with the engine control, so the transmission lost communication with the steering angle sensor body control module lost communication with the electronic brake control module low voltage while the battery was dead lost communication with the engine control low speed can be disconnected, it's interesting, so let's go back. 16 controllers talk but the ecm doesn't so I don't think we have a network problem per se unless it's limited to the ecm there is no communication so the first step is to check the powers and grounds. to the ecm check the fuses that supply power to the ecm so on another note the check engine light is not on which would explain or be another indication that the ecm is not activated.

Problem with the fuel gauge on the dash because the information is coming from the ecm probably that's why it shows low fuel so we have a 30 amp ecm fuse. We have several fuses here so let's check all the fuses in the fuse center under the hood, according to this cover fuse 34 is an etm. fuse and 54 I think it was 51 this 30 amp fuse here both are good 134 is this one here well I will check and test all the fuses here to see what is needed and if they all test good They are three leg fuses in this panel of fuses.

Any fuse that has no power on both sides is probably not on so I don't worry too much about that, where is the ecm down here? I think I printed the OE diagram. Fuse 34 is Good fuse 57 is fine, the engine control relay feeds power to fuse 30 and that is fine, so the ECM is activating this relay, but I don't see the EEC. I'm turning on the check engine light so this is fuse 57 it's one of these triple fuses okay well we're going to have to go to the ecm to connector x1 x1 this is the ecm here and hopefully it's marked, it's usually marked, you would think it's this connector on the right side here or the one closest to the rad.

I mean one x1, but we'll double check, so I'm 99 sure that this middle connector is actually x1, while the logic module is labeled j1 in the real ecm and this one is labeled j2, so that this is pin one and this one down here is Pin 73 has the same design as this connector here and I have to answer the phone, so let's check pin 48, which is supposed to work with the ignition voltage of

crank

1, so it should be 52 51 50 49 48, that should be the correct pin. there's one, two, three, four in that row, so I'm going to use just a conventional test light, but then we're going to upgrade to a better test light, so we have power on that pin that's key on.

I'm going to use a, I think it comes from a 15 amp fuse, yes it comes from a 15 amp 48 fuse, so it should turn on a headlight without problems so that the circuit supplies enough power to run a headlight without problems so one is okay, so next I'm going to rate 67, I think it's on the next page, here the pages are pasted here. 67 positive battery voltage so that should be the count from the end so that's pin 67 you can see that one turns on a headlight okay so next we get to rating 73 but that one needs power from the relay k75.

Now there was power on this 30 amp fuse so if I bypass this relay and send it because I disconnected the ecm it will not be able to ground the relay so Here that relay is called kr75 engine control ignition relay and here is relay number 70 ecm, which is this one here, sorry, this one here, which is actually this one here, so let's jump from 30 to 87 with a relay jumper which is these two diagonally or the which are facing each other and should send power to pin 73 and turn on my headlight. I put a relay switch there when I turn that switch on it supplies power to pin 73 of the ecm x1 so that's the power, that power and that power which are just the three feeds to the ecm rated reasonably reliably so next We must qualify these five lands, four of them are in connector two, which is the connector that is closest to this end face. to the rad so let's check those grounds so there is pin 73 on x2 which is a ground that uses battery power and lights a headlight so the ground is reasonably good there are just a couple more grounds here 17, 18 and 21, which are 17 18 and 21 okay, so we'll check them.

Now they're known as terrain, but they all go to this g122 here, so we'll rate those 17, 18, and 21. So there's 21 and it provides enough terrain to light a lighthouse and 17 and 18 or this one and the next one, so those three Grounds are good, so there is only one more ground on the three pin 73 connector, which is obviously the one I haven't unplugged, so there is one pin 73 on the x3 connector and as you can see that it provides a sufficient ground. good for running a three to four amp headlamp so we have rated all the powers and all the grounds reasonably reliably in my opinion why is this ecm not activated?

This relay turns on these fuses and I had power on these fuses that meant the ecm was supplying ground to turn on that relay hmm very interesting so I have all three connectors plugged back into the ecm I have the key on and if I remove this relay and I reinstall it. You can see that it is turning on and it is supplying power to this 30 amp fuse and this 15 amp fuse, so the ecm is partially active. I'm wondering if it's a network issue, but why wouldn't the check engine light be on? I think the check engine light is networked, I think it's hardwired but I haven't looked at that part of the schematic so I was right the check engine light is wired even though it's an LED ignition that comes from of the group through the light and is controlled by the ground side.

Now I just noticed another power on this pin 26. I didn't check it so let's check that pin 26 says it is accessory activation serial data which is not power cable accessory activation serial data but here it says on, but they are data communications. so let's take a look at the can bus cables. These are probably 46, 66, 65 and 45 are can bus cables, but I think I'm going to call it overnight pickup in the morning, some brain food, so we get back to work. on this 2015 chevy silverado

2500

hd and I'm looking at the can bus activity the yellow trace is can h and the green trace is can l using a breakout box on the dlc and yes I'm using pin number five here as a ground It should probably go to the negative battery post, but it seems like a pretty good and perfect tin sign.

I'm going to measure the terminal resistance, so I'll see if it goes to sleep if I turn the key off. because I really don't want to disconnect both batteries, that clicking sound you hear is the relay, the pcm relay works and it shouldn't do that, we still have a bias voltage on the two circuits, but if it drops to zero I'll put an ohmmeter between six and 14 and see if it reads between 60 and 62 ohms okay we're going to give it a few minutes here so I ended up having to disconnect both batteries because the can bus bias voltage wouldn't go down so I didn't want to. to affect the full rating of the meter, installed the junction box on the dlc, disconnected the scan tool, disconnected the batteries, measured the resistance at 6 and 14 and we have 61 ohms so the can bus is not shorted, It's not open, I'm sure there is a terminal resistor in the ACM so I don't think it's an ECM problem.

I guess we could check the can activity in the ecm, but I'm going to find out what that serial data trigger signal is, so I hooked up the labs. to the serial data on the pin, I assume it's pin one, I can't see this stupid number, yes, pin one of the DLC, there are a couple more patterns here and we have a nice serial data waveform approach , it looks reasonably good, what is this here that must be? inside can or something so yeah it's what's the pin yeah 11 12 13 12 and 13. Okay like I said I'm going to look up what's the serial data trigger signal for the pcm so This is what I have confirmed so far. is on this is on this is on if the relay is on these four grounds on connector two are good this ground on connector three is good I found a case on identifix that suggests you are supposed to have 12 volts on that pin 28 i I think it's either 26 pin 26, so let's check that and see if we have that.

Call that circuit a serial data wakeup 26, maybe it's 28. No, that's the x2 connector, this connector, this schematic here it comes in the pages are stuck. together 26 accessories activate purple and yellow serial data, but identifying the case suggests that it's supposed to be 12 volts with the key on, so let's take a look with the lap visor, so that's pin 26 from connector 2 and it shows 12 volts on that pin with the key on, well, no activity on that pin. I'm going to look at that schematic a little later in a little more detail and see exactly where it's coming from because all of these tests indicate that the ecm has failed and that's not common for ecms to fail out of nowhere, in my opinion it's usually a wiring problem.

Well, according to my research, this serial data trigger turn-on voltage circuit comes from the bcm, it originates from the bcm and gm says. test it with a conventional test light so with the key on a conventional test light comes on so we have confirmed all the circuits on the bc in this ecm and in theory that means the ecm has failed. It's very unusual but certainly possible, I know this is tuned so that has nothing to do with it, but I see this disconnected connector here that looks like egr. I guess I'm not sure so I reconfirmed all the power cables that go to connector one with the load pro and I get a maximum of 0.1 volt Drop when I put the load on it which is about half an amp 500 milliamps and now I'm on the relay control wire here and if I load the test you can hear the power relay turn on if I put a power on here and turn it on then I have power to pin 73 on this x1 and it also passes the load test so I go to recheck the reasons once again just before condemning this ecm, so I reconfirmed all the reasons.

Connector 2 and ground on pin 73 on connector 3 connected the load pro and you can see the reading on the load pro is negative because the polarity is reversed, it goes from 12.74 and then when I charge it it drops to 12.68 so the ground test good that the serial data trigger voltage is there so let's order an ecm for this