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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1691
Experience:  GM Grand Master Technician 2007. 14 year experience.
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1995 GMC Sonoma SLS (VIN# 1GTCS1447S8507279) with a dead

Customer Question

1995 GMC Sonoma SLS (VIN# *****) with a dead short between +12V and ground/engine block. The problem started as no crank no start at midday after a normal start in the morning. I removed the battery after the first failure to start and charged it overnight, although it seemed to draw minimal current. On attempting to start the next day, same problem. No cranking and no clicking, but I thought I might have heard the fuel pump running. The battery voltage monitor did seem to fluctuate when attempting to start, but on a later attempt while monitoring voltage at the battery there seemed to be minimal fluctuation (i.e. 12.5 to 12.3V) and the dome light didn’t seem to dim. I checked for voltage at what I presume is the starter relay on the firewall (The only one with a purple wire), and only found +12V on one pin (red wire). The next step was to try and verify continuity through the clutch safety switch, which checked good. When attempting to verify continuity between clutch switch and starter relay I kept coming up with about 2 kOhms to every pin. Shortly after that I realized that the positive and negative battery cables were reading shorted to one another. I have isolated the short to one of the fusible links connected to the +12V bus bar in the engine compartment. This link appears to be routed to the large connector at the firewall which I presume leads into the cabin. The thick part of the fusible links all seem to have a unique 3 digit number stamped on them, and I would like to know if there is a way to know which sub-system a particular link supplies. The link (splice?) in question is labeled 545. The wiring diagram in the Chilton’s guide is not very helpful. (I tried pulling the fuses in the dashboard fuse box one at a time without clearing the short, and all are intact. I also disconnected most of the electrical connectors in the engine compartment in the same fashion with the same result.)
Submitted: 1 year ago.
Category: GMC
Expert:  Tim Mohr replied 1 year ago.

Hi, my name is ***** ***** the battery take a charge when you charged it overnight?

Customer: replied 1 year ago.
The voltage didn't seem to change up or down after the overnight charge, so I assumed it hadn't actually been discharged. A week or so after the failure to start it is still reading 12.5V open circuit.
Expert:  Tim Mohr replied 1 year ago.

ok, if the wire that has the short in it is removed, does the truck start and run?

Customer: replied 1 year ago.
It didn't occur to me to try that, not knowing what this line supplies. Looks to about 12 gauge, so I assume its important. I did remove the lower half of the steering column cover and the backplate of the ignition switch, which revealed a terminal board with a number of 12 guage wires soldered to it. The shorted wire does not appear to be directly connected to any of these wires, read about 600Ohms to the red wire there.
Expert:  Tim Mohr replied 1 year ago.

yes, remove the fusible link at the junction block, and let me kn ow if it starts and if so, what does not work,Tim

Customer: replied 1 year ago.
That sounds risky to me. Do you have access to the wiring diagrams/schematics for 1995 Sonoma?
Expert:  Tim Mohr replied 1 year ago.

its not risky, just cut the fusible link, after the junction block, leave enough room to repair it with a butt connector, then see if its starts, I will post the wiring diagram below,Tim

Customer: replied 1 year ago.
after putting all but the suspected shorted link back together, the truck cranked but did not start. After realizing that the ‘short’ was due to my having left the dome light switch in the on position when checking the battery on a previous attempt to start, I reconnected the link and the truck started with no apparent difficulty. Drove normally for the rest of the afternoon, but the two times I started it later that evening (July 3rd) it made a grinding noise on the last few seconds of starting. There were no problems starting and it drove normally for ~three weeks, and then last night it failed to start on the second of three stops on the way home. Back to the not cranking state. So I suspect a ‘bad spot’ on the starter, but before I pull it out for test, can you provide a more detailed schematic of the starting system? The one which you provided earlier seems to show a direct link between the battery and the starter solenoid but I assume there has to be some sort of control by the ignition switch, presumably through a relay. Also whatever schematic follows the ‘Horn/DM’ fuse which I suspect connects to other circuits involved in starting.
Thanks.