2005 Buick Lacrosse
Hi guys this is my first posting here so go easy on me. I have here this buick that came to the shop as a no crank/no start. I verified complaint sure enough no power to the starter from starter relay. I ran an auto scan of all modules. They all communicated except the ecm. The only code set was U1000 in many modules. I scoped the computer data line at dlc pin 2 and I had a nice waveform 0-7 volts performed same test at the ecm and I got same results. I also scoped pins 6 and 14 witch is gm LAN bus and I got nice waveform 2.5 to 3.5 volts on high speed and 2.5 to 1.5 volts on low speed and had the same results at the ecm as well so I checked powers on pins 1,3,17,36,58 and I got good powers and a good ground. Next I checked the 5 volt reference and I am missing that. I disconnected all sensors and still no 5 volt reference. I have checked wiring for shorts and everything checked ok. I decided to crank the engine from the starter relay with key in the on position and to my surprise I had the check engine light illuminated on instrument cluster than I cracked the engine with the key and the car started and of course I had communicatio with ecm. I discovered if I disconnect the battery I get a no crank no communication with ecm but if I crank the engin from the starter relay I get everything back to normal. I think the ecm is not getting a wake up signal from the bcm . I have checked all the signals from the ignition switch to the bcm and they are all there what am I missing. ANY help would be greatly appreciated.
Try jumping wake up signal to ecm to constant 12v if you get comm and starts bcm would be your issue
The BSM wakes up the PCM? I never realizes that. Never ran into a situation to need to know this. interesting.
I had an 06 lacrosse a while back with an intermittent no crank that I could barely duplicate, the bolts that hold the connectors into the fuse block by the battery were loose. Long shot, but it only takes 10 seconds to check.
I’ve seen this exact situation twice. It needs a new PCM
I did not mention it but it already has a new ecm
There isn't any ‘wake up’ signal that I can see in the wiring diagram anywhere, neither in the BCM, nor the ECM diagrams. There are indeed, circuits used for waking up the BCM, but on this vehicle there isn't a dedicated wake-up signal. That's not your issue here. You need to recheck your basics. Powers and grounds should be checked with a significant load on the wiring, (I use a headlight to…
The PCM won’t communicate for diagnostics till you by pass the starter relay and it comes alive. Once it’s alive, the symptoms are gone for a half hour or so.
Yes, but jumping the relay, does not provide power to anything at the ECM, just to the starter…
Incidentally, you'll have a comm signal on your scope when you turn the key on from almost any point in the system, as they're all spliced together on the comm circuits. The only way to ascertain whether you've got a good comm signal to the ECM, is to depin the splice and attempt comm directly from the DLC to the PCM. That will tell you whether the PCM is actually ‘talking’.
Hi Timothy I have disconnected all modules from the splice pack and connected one at time and I can communicate with all modules except ecm
Then you've got a dud ECM. These are a high fail rate item. I've yet to see why/how a starter relay jump (if jumping 30/87) would have anything to do with the PCM coming alive, as that circuit is not connected to the ECM directly…
Is it possible to get a hold of a Tech2? They have a “ping all modules” test, which should substitute the scanner to wake up the PCM If i am thinking correctly. You may see some BCM data in there which could be helpful as well. Please confirm all the pins fit tightly, and load test the powers and grounds ( with a headlight bulb).Consider Stabilant22 on the male pins for extra assurance. The…
I dont remember any type of wake up from a bcm on this ecm. I've bench flashed many of them and from what I remember only use KAM,RUN & Grounds to power then can lines directly to scanner. This ecu is a very common failure. I would not be surprised if your replacement is a dud same as original.
That is why I am confused why just cranking the engine from the starter relay the ecm comes on line, yesterday I installed another pcm from dealer with same results
Im not sure I follow. If you jump the starter relay the car will crank and runs or just cranks over but you can communicate with ecu?
I’ve seen this exact situation twice With the same powertrain. The ECM is “asleep” and won’t communicate. Car won’t crank. If you bypass the starter relay, the car will start normally with no DTCs and will restart normally for about ½ hour. After it rests for a ½ hour shut off, the no-start reoccurs. A replacement ECM fixed both of the vehicles that I was brought in on.
You can’t have power on pin 1 and 17until pcm is wake up. Pcm control power relay. Most likely have relay stuck. Pin 3 and 58 is wake up from ignition switch. Check your power accordingly to the state ofthe ignition switch.
I want to thank everyone who was able to help. Unbelievable 2 computers from worldpac having the same problem. I told my boss to to get one from dealer and everything is good working well
Looks like some one already had those pcm order and returned.
Good job sticking with it Selvin! What a lot of stress and testing time for some bad parts… Very unusual to get 2 bad ones with same weird issue in a row I think most of us would have been second guessing ourselves.