Won't Start.

[Ara T.]

Last week I did a diesel purge on my mom's 1986 300SDL (around 176k miles). It was running very nicely for about a week, until today. It won't crank. The instrument panel/coolant temperature will come up but when i turn the key one more notch to crank, it will just cut off everything, and no cranking will occur. I tested the battery's voltage, and it comes out around 12.3V. When I try to crank it, it will hold that same voltage. I checked the fuel lines and everything that I messed with during the diesel purge and they were fine. I guess it's an eletrical problem. Could this be a relay somewhere that went out? Or perhaps a fuse? I can't find the fusebox on the 603, i've only really poked around in my 617. Any ideas? As always, I appreciate any answers or advice. Thanks.