07 LT 5.3L Sudden no start/crank
- Thread starter J-Herkel
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J-Herkel
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I'll check it when I get home. I know what you're talking about. The firewall side is good. Haven't checked the engine side.
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J-Herkel
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Alright so after some more diagnosis this evening, I had my buddy hook up his $7000 scanner and scanned the entire vehicle. The Theft Module is good. The BCM was good. The ignition switch is reading key postion correctly, the signal is being sent when the key is turned to the crank position. The ECM was good but detected a fault in the Start Relay Circuit (P0615). So we used a schematic and traced the wires to the connectors below the Fuse box that the Fuse box itself plugs into. We tested the relay terminals. 30 is good (12v). 87 is good (12v). 86 is good. 85 (ground) is not good. No reading. We took the fuse box off and out and tested the terminals on the plugs below. The power (+) side to the relay is all reading. The ground (-) is also reading fine. Next I pulled apart the fuse box and we tested the contacts on each side of the board. The power side has continuity. The ground terminals do not. We touched the relay ground terminal to every terminal in that bank that it leads to and no continuity. As weird as it sounds, it seems like the fuse box relay circuit went bad on the fuse box circuit board. I'm assuming I can put it all back together, use a thin wire and stick it in the ground side of the relay, ground it to the frame, insert the relay and see if it cranks. Thoughts? Ideas? I'm not an auto mechanic but I'm doing my best here.. I did see a little bubble on the board. Maybe it has nothing to do with it. Idk. I'll attach a picture.
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Nice sleuthing! We've seen corrosion take out the circuits on the bottom of the fuse block in the engine compartment and cause weird issues. So yes, you're likely on the right track.
rockola1971
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That sure is odd. That circuit board doesnt appear to be a multilayer circuit board. So just the traces on the top there in the pic and likely ones on the underside as well. Not alot to go wrong there and if it does it usually shows a physical defect. Id have to so better pic and the ground terminals marked to see if your "bubble" is the culprit and it very well could be. The bubble can be bridged with jumpers on the trace of the board. Ive had to repair many obsolete boards over the years when I worked in the private industrial field. Running your own ground at the relay socket (top or bottom) is fine as long the correct wire gauge is ran AND the proper automotive rated wire with the correct temp rating is used. Remember it it always hot as hell under the hood of a vehicle.
you might not think you're a mechanic but this is better trouble shooting than most mechanics can do these days.
definitely add a ground to the relay and see if it all works, if so you might consider picking up a new fuse box at a junk yard. Just incase it starts having issues to other relays long term.
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J-Herkel
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Once I get it all back together I'll try a jumper ground wire. Appreciate the compliment.
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J-Herkel
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So I went back again this morning and did some continuity tests from the top of the board, starter relay terminals, to the bottom of the board terminals that plug into the banks of wires that run to the other components. Relay terminal 87 (F1)(to 40amp starter fuse) has continuity and is good. Relay terminal 86 (C5)(to ECM starter relay) has continuity and is good. Relay terminal 30 (A)(hot at all times/main power) has continuity and 12v and is good). Relay terminal 85 (N5)(ground to engine block) has no continuity to the N5 terminal on the back of the board. I ohm tested the wire it plugs into under the fuse box on the vehicle, and it tested good for a ground. Since the computer is sending the crank signal, it's reading the key in the crank position, no theft control module is active, the starter jumped off the relay and worked, my only thought that it can be is the ground (N5) on the board. Without the ground it won't activate the relay. And I wasn't getting any clicking in the relay when we were trying to crank it yesterday. I'm attaching more pics of the "bubble". By the way, I also checked the N5 (ground) circuit which also leads to the ground terminal on the "run/crank" relay and I had continuity there so it does seem like there is a broken connection between the N5 plug on the bottom and the ground (85) terminal for the starter relay.
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J-Herkel
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Since the starter relay and the run/crank relay share the same N5 ground on the bottom of the board and the run/crank N5 seems to have continuity and the starter doesn't, I added a jumper wire. I'm going to see if it works. If so, this will be my permanent fix. This jumper is just a ground wire.
