My truck will not start

Here's the Notes Page I made for use in the classroom. There has to be a bad connection at one of the seven places pointed out. Points 5 and 6 are hidden inside the solenoid where we can't get at them. A failure here is common on the little silver Nippendenso starters, but these contacts cause very little trouble on most other starters.

One of the problems with side-post batteries is we can't get at points 1 and 3 to take those two readings, and those are the two most common places for the problem to occur. Other readings will help us find those bad connections.

You have another great clue in the interior lights are working. Watch what happens to them when you try to crank the engine. If there is a bad connection at either battery terminal, those lights are going to go out during the attempted cranking.

This sounds like a neutral safety switch problem, but it looks like there isn't one on your truck. Look under the dash where the clutch pedal would be and see if you can find the jumper harness for the clutch switch. Use those terminals as a test point. You should see 12 volts there when the ignition switch is in the "crank" position. If it's missing, the ignition switch becomes the best suspect.

I can't find the neutral safety switch on either the diagram I just posted, or the original manufacturer's diagram I posted at first. My preference would be to go right to the ignition switch and find the purple wire for the starter solenoid voltage, but that can be hard to get to. That's why I sent you to the jumper wire. That connector is there for the clutch switch on vehicles with manual transmissions. Since you have an automatic, they didn't build a different wiring harness. There is no clutch switch, so they just plugged in a jumper wire. That makes for a convenient test point.

Your ignition switch is down on the side or top of the steering column, below the dash. It is mounted with two screws, and the mounting holes in the switch are elongated to allow for adjustment. It is not uncommon for that switch to work loose and shift out of adjustment. That would become evident by the 0 volts on the purple wire when the switch is in the "crank" position.

A second cause is burned or overheated terminals on the switch and its connector. There is no separate starter relay in this system. That also surprised me. That is fairly common on small import cars. It doesn't take as much current to run the starter solenoid, but on your truck, the solenoid could take as much as 20 amps. That's a lot to ask of the little switch contacts and of the connector terminals in that circuit. This would also show up as that missing 12 volts on the purple wire.

In other GM starter circuits, the neutral safety switch always came right after the ignition switch. From there current went to a starter relay. It was that relay that handled the 20 amps for the starter solenoid, so the two switches didn't have to handle that high current.

The issue remains, ... We have to see how far the 12 volts is making it from the ignition switch to the solenoid. The voltage readings will tell us where the break is in the circuit. Find that jumper wire or find the purple wire on the neutral safety switch. Probe that wire with the test light and see if you have 12 volts in the "crank" position. If you don't, work your way back to the ignition switch and check there on the purple wire.

I'm starting to feel pretty confident the ignition switch is out-of-adjustment, especially now that you've added these observations. It's on the side or top of the steering column, under the dash. The blue arrows are pointing to the slotted mounting holes. Loosen the two 5/16" bolts, then slide the switch up a little closer to the steering wheel.

You don't have to tear everything apart. You've verified the 12-volt crank voltage has made it to the splice at the green arrow. Next, check on the purple wire on the starter's "S" terminal on the solenoid. If you have it there, the starter is defective. If you don't have it there, what is not shown in this diagram is that purple wire has to go through the firewall. The pair of mating terminals in the bulkhead connector is a good place to find a corroded connection.

If you do find 12 volts at the solenoid, when the failure to crank occurs, be careful to not disturb anything, then connect a large battery charger or another vehicle with jumper cables, then try to crank the engine. If it cranks now, suspect the solenoid has one open coil of the two, or the brushes in the motor are worn. I'm working on a diagram to help explain this, but there's two electromagnetic coils inside the solenoid. The strength of both of them are needed to get the plunger to move and engage the ring gear, but once that happens, only one of them is needed to hold it there. The second one is switched off to reduce the amount of current being used. That second coil gets its ground through the windings inside the starter motor, provided the brushes are making good contact. When those brushes are worn, only the first solenoid coil is in the picture, and while it isn't strong enough on its own to pull the drive gear into engagement, it often becomes strong enough due to the higher voltage supplied by the battery charger.

This problem with worn brushes always starts out as an intermittent no-crank. Sometimes the problem occurs more often when the engine is cold. Sometimes it's when the engine is hot. Often it occurs after the engine hasn't been started for a few days.