Bad ECM?

[N2BRK]

I’m trying to help my buddy with a new-to-him LBZ. All good until suddenly both P0090 and P0045 and limp mode. Truck commands and goes full rail pressure on startup. I checked wires from FPR back to ecm and no breaks or knicks. Key on I’m seeing 2.63vdc at FPR. I backprobed at the ECM and found the same 2.63vdc. I read that should be 12vdc. Does this sound right, or am I reading bad info? ECM no bueno?

Thanks in advance!

 

[Ron Nielson]

Are you doing your testing with the engine running?

Think about what the FRP actually does and what it would take to make it work that way. If it was supplied with the same voltage all the time, it would regulate to the same pressure all the time. So the voltage must be varied to regulate differing pressures. The FRP is a solenoid and depending on what pressure is desired, I believe you will have different voltages on that purple/white wire. I've never personally tested one, but that is what makes sense to me.

 

[2004LB7]

regulator is controlled by PWM so a regular meter will not give an accurate reading. need a scope to see the wave form. or a meter designed for showing duty cycle.

I would put something like efi live or similar scanner on it and read the pids to see what it is trying to do

 

[N2BRK]

The thing is that I scanned it and the truck is commanding full rail pressure. I used the DVT tool to try to back it down and it made no change. So I was trying to see if the ECM voltage to the FPR was wrong - I get that it's PWM but thought the RMS reading I got would at least make some sense... guess not. This is a log where I had the key off, then start up, and then toward the end I tried the DVT to no avail and then shut off. I was looking to see if any temp sensors were goofy perhaps but didn't see anything.

 

[2004LB7]

its possible that it is a bad ecm but more often it is a bad wire, rub through and shorting out or bad pressure sensor.

full voltage or 100% duty cycle will produce the least fuel pressure. less voltage or lower duty cycle is higher pressure.

the ecm is commanding full pressure but does the actual rail pressure pid show full pressure too? do the actual and demand match?

 

[N2BRK]

Commanded and actual match within reason at full pressure. I thought it would be a broken wire to the fpr but I checked it from ecm to fpr and it’s perfect. Maybe a different broken wire that’s effecting this problem.

 

[Ron Nielson]

See here: https://www.dtcdecode.com/Chevrolet/P0045

 

[2004LB7]

also, help info. the ecm controls the FPR on the grounded side and not the supply side. so it would seem to me that you either have a break in the wire, a short, bad connection at the ecm or a bad ecm

 

[N2BRK]

Thanks. I am positive there is no break from
ECM to FPR. I’m not sure if there isn’t some other broken wire causing the ecm to want to command full rp. I’m pretty convinced of a bad ecm, but it’s an expensive piece... I don’t want to be wrong when I tell him to get one! Lol. Thanks

 

[juddski88]

Thanks. I am positive there is no break from
ECM to FPR. I’m not sure if there isn’t some other broken wire causing the ecm to want to command full rp. I’m pretty convinced of a bad ecm, but it’s an expensive piece... I don’t want to be wrong when I tell him to get one! Lol. Thanks

When you tested circuit integrity, did you only test continuity or resistance? Or did you also load test it? Because I have had several of these circuits pass resistance but not pull even a 2amp load due to cold side pipes popping off and tugging on the harness. I've had wires pull off the pins, pins pull out of the connector, you name it... The ECM defaults to the last known command for some reason.

 

[N2BRK]

I did not place a load on the circuit. I tested unpluged and then tested again back-probed while connected to the FPR. I can re-try with a load. Thanks

 

[Ron Nielson]

Post #5 is likely your solution. Some part of the circuit is open or grounded.

 

[N2BRK]

I found a willing LBZ to let me swap in this truck's ECM just to know for sure, but given my efforts last night, I think there's a wiring issue. Since the wiring from the ECM to the FPR is showing good, it could be a break in the wire and it being touched by another broken wire - so both wires just have their insulation compromised but not their copper cut - I found this in another spot on the harness behind the ground "tang" on the driver side of the engine - fixing those didn't help this situation though

 

[Dallas S]

have you pulled the inner fender out on the left side? we see many LBZ with rubbed harnesses at the inner fender. cause all sorts of different issues. good continuity but cant carry the load. We have also see pins pushed out at the bale connectors. small needle nose and push/pull the male pin. I have had them look ok when un pluged, but the pin will push back when connecting the bales. push/pull the pin and it clicks into place again.

 

[MarkBroviak]

Also had am LMM locally with a similiar issue and it was the ecm pins themselves causing the issue.

 

[N2BRK]

Thanks - that was the first thing(s) I did. I just ran up to a friend with a test mule lbz - the ecm is good! This weekend I will tear through every wire if I have to!!

 

[N2BRK]

Thanks Mark - I’ll double check that too!!

 

[N2BRK]

Not much of an update, I’m afraid. I checked almost every wire at this point, as well as pins and connectors. When I key on, the truck throws the P0090 and will not clear. When I start it, the desired goes to 26k and actual to 29k. The only wiring I haven’t checked is under the coolant pipe that feeds the turbo.

I did notice that he has an aftermarket remote start. Wiring under the dash from it is all over the place. I can’t figure how that would matter though.

 

[dndj]

Something else to check on a P0090, from the .pdf in post #7:

" If the fuse that supplies voltage to terminal C1-76 of the ECM is open, this DTC will set"

 

[2004LB7]

I would try sticking a turn signal or brake lamp bulb in the pressure regulator socket. bend the pins of the bulb out and poke them into the socket. then supply 12 volts into the ecm pins. see if you get full brightness, consistent non flickering light when you move the harness