steeler Posted December 10, 2020 Share Posted December 10, 2020 I must be cursed with mystery problems! I'm working on a GU with an MP7 that will occasionally either die and not restart, or just won't start at all.......UNTIL you unplug the engine ECU. Then it will fire right up and might run trouble free for 3 days until it happens again. When the problem occurs, I have no communication with the engine e cu. No coolant temp, oil pressure etc. No matter what I do, it won't start until I remove power from the ecu. I have replaced the ecu, but that didn't fix it. Sometimes when you turn the key on, the ecu will communicate fine, but as soon as you hit the starter, it will clunk and then the engine ecu goes dead. Other times, it's just dead from key on. I've tightened and cleaned every cable on this thing. Frustrating! Any ideas? Quote Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/ Share on other sites More sharing options...
41chevy Posted December 10, 2020 Share Posted December 10, 2020 (edited) I would look very close at the ECU and VECU connectors and pins for corrosion, loose in the connector housing where the pin pushes back a bit when plugged in or bent / miss aligned pins. Check the harness it self for worn or rubbed places Edited December 10, 2020 by 41chevy Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/#findComment-465179 Share on other sites More sharing options...
Mackpro Posted December 10, 2020 Share Posted December 10, 2020 Check the fuse panel by the clutch pedal for water damage. Pull one of the bottom relays out and look for while crusty corrosion. There was a recall for this on some units . Follow the battery cables all the way to the starter . Look for them rubbing the frame rail or transmission. Check the ground lug welded to the frame behind the battery box. Is there any GPS/data logger/Qualcomm hooked into the diagnostic connector? We had a faulty Qualcomm cause lots of issues till we unhooked it . Check the terminal strip by the fuse box on top of the dash , there are a few ignition, battery and ground connections there , also 2 data link terminals, make sure nothing is touching them . We had a guy hook a CB up and his power wire terminal twisted over and touched the data link terminal causing a problem that took a day to figure out . Quote Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/#findComment-465185 Share on other sites More sharing options...
steeler Posted December 10, 2020 Author Share Posted December 10, 2020 Thanks guys I'll check those out. My fear is that I'm going to kill power to it which will bring it to life again for another 3 days and not be sure I've fixed the problem. These are the jobs that I wished I still worked at a dealership. If it wasn't fixed after tossing a couple thousand bucks at it, oh well! Can't do that at this shop though. Quote Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/#findComment-465191 Share on other sites More sharing options...
90millas Posted May 5 Share Posted May 5 Hello mr.Steeler, I have the save mistic electrical problem whit my GU713 Mack 2015, CA you please give me any lid how you fix the problem. Thanks in advance Quote Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/#findComment-551135 Share on other sites More sharing options...
JoeH Posted May 5 Share Posted May 5 On 12/10/2020 at 1:33 AM, steeler said: Thanks guys I'll check those out. My fear is that I'm going to kill power to it which will bring it to life again for another 3 days and not be sure I've fixed the problem. These are the jobs that I wished I still worked at a dealership. If it wasn't fixed after tossing a couple thousand bucks at it, oh well! Can't do that at this shop though. Bump Quote Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/#findComment-551141 Share on other sites More sharing options...
steeler Posted May 14 Author Share Posted May 14 I'm still here, but damn that was 4 years ago. I'm trying to jar my memory as to what I did to fix it. I've had so many cluster f jobs since then they all run together. I wasn't keeping very good notes back then. I'll keep thinking!! Quote Link to comment https://www.bigmacktrucks.com/topic/65213-mystery-2015-gu-electrical-problem/#findComment-551716 Share on other sites More sharing options...
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