Cat C15 starter question.

As far is the not cranking problem, your starter lockout is controlled by the Namux ECM, which powers a relay based on the programmed inputs, like a clutch switch, neutral safety (automatic), whether or not it thinks it already running, etc. You usually have to turn the key on, wait for the gauges to sweep before the Namux will allow cranking. If you try too soon, nothing happens. That's normal.

If that's not the issue, you can figure out if the issue is on the Namux side or the key switch/starter side by jumping the starter lockout relay. If you don't have the issue with the relay jumped, there is something going on the Namux side preventing cranking.

The most likely cause for a starter staying engaged is a stuck solenoid. If it has the original starter it's probably a 39MT (or the Paccar copy of one) and has 2 solenoids. Either one sticking will cause the starter to stay engaged. It could also be something else powering the start wire or a mechanical issue not allowing the gear to retract, but solenoids are the more common cause.

 

Just to save you the trouble if you cant get it to turn off again, look at the battery connections and you should have 2 or 3 small wires with inline fuses going to a positive post. Just pull the fuses till the motor turns off.

 

Most likely its starter solenoid stigking. When truck is cold you have no problem, but when hot metal parts in the solenoid sticking. If its 39mt starter they have them a lot with defective solenoids. I already replaced two starters on my truck: one after a year, another after a month.

 

It's going to be a solenoid. The previous posters filled you in with great detail more than I can say.

I remember a mack that failed to crank, used a bic pen shoved into the solenoid on the right hand side to get it kicked over once or twice. I realize this is not a Cat engine, but those kinds of lessons tend to be useful.

Give that engine a chance to go through it's boot up seeing the gauges sweep and back. Otherwise check your batteries for voltage and load amps.

Another lesson with Cat I learned. You can smell overheating coolant and wonder now what? While driving all day. But the moment you shut it off as I did for sleep the water pump cools off and contracts allowing all the fluid to pool under you and your neighbors to both sides. Those Cats will run their hearts out for you even if they are hurting with a leak.

 

I overlooked the 387 part and was thinking 388/389 (newer stuff with namux). I'm not sure how your starter is wired, I think you only have 1 main starter relay, not the extra safety interlock relay. Let me look up a schematic and I'll let you know. What year 387 do you have?

 

OK, you only have 1 starter relay that should be in your fuse panel behind the glove box. DO NOT jump it, if you jump it, it will start cranking. They're not expensive, you can replace it to try, if that's not it you have a spare to keep in the truck just in case.

I'm thinking that you have more of a stater solenoid issue now. Or possibly a problem with the thermal protection switch on the starter is it has one. It's probably a 42 MT starter in a 2006, which will be the big heavy iron one. If it's never been changed, 10 years is a pretty long time for those. I'd recommend replacing it with a 39 MT, which is lighter, higher torque and have less failures. Better to do it now than loose a load and pay double for a road failure when it fails.

Here is a schematic for your whole truck. The ignition switch side of the starter is on page 5.
387Schem.pdf