Under the hood: Truck won’t crank and tow will be costly

My 2008 Silverado diesel won’t crank. I’ve charged up the batteries and tightened the cables but still not even a click. Can you please provide some ideas? I live about 100 miles from the dealer, so a tow will be quite expensive.

Iwould start by turning on the headlights and having a helper observe brightness during a cranking attempt (key turned all the way). If the headlights dim only slightly, or not at all, this says the battery and battery cable connections are not at fault. Strong dimming indicates a discharged or faulty battery, cable connection fault, or — in rare cases — a starter or engine mechanical fault.

Since you’ve charged the batteries and checked connections, I’ll assume the lights might dim only slightly. Next, try holding the key to the crank position continuously as you wiggle the shift lever back and forth slightly, first in park, then neutral. If the starter momentarily engages, the P/N (park/neutral, also called transmission range) switch is faulty or misadjusted. Try also exploring various degrees of key rotation within the crank position, looking for a similar effect. If neither of these attempts is fruitful, the ignition and N/S switches are probably not at fault.

Next, locate the starter relay, located in the underhood fuse box. This device receives the starter request signal from the cab and in turn switches on an underhood power circuit to the starter solenoid (a giant switch/plunger mounted atop the starter). Think of the relay as a control hub, a great place to test! The fuse box lid (inner) provides a contents map. The relay

you’re looking for is a grey or black rectangular box, about the size of a large grape, slightly right of center in the fuse box.

Place a finger or thumb on the relay as a helper attempts to crank the engine several times. If you feel a click, the starter request from the truck cab was successful. In this case I’d swap the starter relay with the identical relay to its right. If the engine now cranks, the relay was faulty. If no click is felt, the ECMPCM (engine control computer) likely isn’t sending the request signal, due to a failed ECM fuse or internal issue, ECM-BCM (body control module) communication fault, faulty ignition or N/S switch, or other. This is likely to be tow truck stuff.

If a click was felt from the relay and yet swapping relays didn’t help, the fault may be fuse 57 (a large 40A fuse in the under-hood fuse box) is blown, or the starter is faulty. After removing rings/conductive jewelry, check all cable/wire connections at the starter for a secure fit, tighten as necessary. Smacking the starter is worth a try as well. Use a brick or similar, with a stiff item such as a hammer handle in between, to reach this difficult spot.

Going back to testing fuses, there are a half dozen possible offenders in the fuse panel, and they’re not labeled coherently. Using an inexpensive un-powered test light — alligator clip attached to grounded metal, key in run position — probe the tiny slit on each side of EVERY fuse. If both sides light the test light, the fuse is good. If one side only lights, the fuse is popped. If neither side lights, it’s likely not a circuit we’re concerned with. Fuses pop for a reason, replacing a blown fuse may provide only temporary function until the fault is corrected.