LB7 CRANKS BUT NO START

autospecialty

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Feb 22, 2021
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Has anyone ever replaced their BCM with a known good used one and were able to relearn the pass lock code. This where we have a no start and has no voltage to injectors. Has new ECM, FICM and all wiring check all powers and grounds good. Injectors (Bosch) 6 months old. It was driven home from work running great and would not start the next morning. Fuel rail pressure perfect. We do have access to a known good BCM from identical rig (except for VIN). I do have bi directional scan system.
 

Bradgrantham59

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Mar 6, 2021
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Prescott Arizona
Have you tried to do a pass key relearn?. If not I would try that I am doing a lb7 to LLY swap and had to relearn it. Have a full charged battery turn key try to start let go of the key it will return to the on position leave it for 10 minutes turn it off for 10 seconds do this 3x then it should marry to it and start hope this works for you
 

autospecialty

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Feb 22, 2021
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Have you tried to do a pass key relearn?. If not I would try that I am doing a lb7 to LLY swap and had to relearn it. Have a full charged battery turn key try to start let go of the key it will return to the on position leave it for 10 minutes turn it off for 10 seconds do this 3x then it should marry to it and start hope this works for you
yep, and it is learned
 

autospecialty

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Feb 22, 2021
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Is it working now?. Or does the security light still flash while cranking
Security (pass lock ) is working. The problem is no power to injectors (LB7 need 96DC volts). All circuits and harnesses are good. It was driving great, the customer drove home from work and would not start (it cranked great) the next morning. This all started mid Dec 2019. The ECM is commanding the FICM to supply the power to the injectors. We have perfect fuel lift pressure (has a Fass system), perfect fuel rail pressure, Rpm while cranking (all in GM CKP and CMP), and ECM commands the FICM the pulse with in which is it is doing. We have changed the ECM (programed and passlock relearned) and have changed the FICM from another running LB7. It has no codes and Bosch injectors and cups were changed 4k miles ago. We have a new (reman) BCM we are going to install, program and relearn sometime this week. Thank you for your help and with this info If you have any other ideas please let me know. I own my own shop and been doing this for over 45 years. We are having another GM diesel tech coming this week (this will be our third and the other 2 gave up). We also have good static compression and sprayed diesel start fluid in the intake and it runs good.
 

autospecialty

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Feb 22, 2021
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Yep, Yep and Yep and the FICM relay. You will not have any fuel rail pressure if they were not. We have 43mpl pressure which is 6235psi way more that enough to start.
 

dndj

Boost!
Apr 13, 2019
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Oregon
The FICM needs to see an engine speed signal from the ECM. The +12v reference on the CKP and CMP is shared, if it's shorted to ground neither will work and the engine won't start. Normally this is checked by observing the engine RPM while cranking. RPM will be zero if this is the case. Something to check?

You said the FICM has power so that means the 25A EDU fuse and EDU relay are good. Also check the 15A ECMRPV fuse which sends the FICM power back to the ECM. If the ECM can't see FICM power - probably won't start.
 
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autospecialty

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Feb 22, 2021
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You are correct. If you review my previous posts you will see all the details of what we have checked and done...
 

dndj

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Apr 13, 2019
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I did review, sorry I missed the part about the CKP and CMP. Hope you get it figured out, you've covered all the bases I can think of other than putting a scope directly on pin 94 of FICM C2 to take a look at the engine speed signal sent from the ECM.
 

autospecialty

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Feb 22, 2021
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Thanks, I need it. I have been doing this for 50 years and knock on wood it the first to stump this long. It happens to be a friend and neighbor to boot...We have a vendetta on this one....
 

autospecialty

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Feb 22, 2021
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It is nice to know (but not reassuring) that we are not alone in this boat. If you have read my previous posts you will see that we have resources and experience and most of all a not give up attitude. We have a J2534 system coming this week and I am leaning on the idea that the modules are not communicating properly even so that we have a no code situation. I do have all the bi-directional diagnostic equipment and a Tech II coming. I will keep you guys posted of the upcoming results. By the way all prayers are greatly excepted...
 

Bradgrantham59

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Mar 6, 2021
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Prescott Arizona
Yes please update as you figure it out maybe we can figure ours out also. The only thing I haven’t done is confirm signal to the ficm from ecm I don’t have a high dollar scan tool to check it unless you know another way to
 

autospecialty

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Feb 22, 2021
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You do need a good scanner. My newest one is a Snap on Modis Edge. I also have a Solus Pro (Snap on). Look for a used one online or find a Snap on dealer who may have a used one with current updates. I have seen the Solus between $500 to $1000. They were $5000 new. It is a must have if you are doing a lot of the work yourself. You have a minimum 9 modules on your truck and up to 17. If you get one let me know and I can bring you up to speed on using it. Also you can go to Alldata.DIY and for a small fee ($20) have at least a year of factory repair and diagnostics info...
 

dndj

Boost!
Apr 13, 2019
203
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Oregon
I had another thought about this last night, considering all the data you provided.

Given you've swapped all the main modules (ECM, BCM, FICM...) and checked everything over very well, I'm leaning toward some sort of a intermittent wire break in a wiring harness, especially since the no-start trouble started for no apparent reason out of the blue. Is it possible that all the electricals check out fine when the truck is static, but when cranking the engine the movement/vibration of the engine turning over causes a disconnect or intermittent connect/disconnect situation? You would have to back probe on the FICM inputs themselves and watch the signals while cranking. FICM inputs are just +12v (C1-5, C2-119), ground (C1-2, C2-114), engine speed signal and the 8 injector control lines from the ECM. Would be tough to catch an intermittent ground, so maybe just connect the ground pins elsewhere while cranking to rule those out.

Keep the data coming, I'll try to keep adding additional thoughts.
 

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