07 4.0L NO CRANK

PCM is not tied to the starting circuit in any way on 2nd gens other than it recieves a STA signal to tell it to turn the fuel pump on while cranking, you can completely unplug the PCM and the truck would still crank over it just wouldn't start.

 

This just gets better.
I determined that there is a fault in the NSS. Several afternoons of soaking and prying and the NSS slipped off. Comparing the two units showed that two pins were missing from the original.

I found them.
My thinking is that the adjacent holes are for releasing the connector from the plug body.
How fun is this?

 

You may have to de-pin those 2 pins to do so but if you can get the broken off pins out you can probably get by with cleaning the connector good and reinstalling it.

 

The saga continues...
Pictured below is the W/B wire which should be the ground for the STA Relay inside the Engine Room Relay Box. In the upper left corner you will see a burn mark where it attached itself to another wire.
OK. I found a problem, but what caused it?

 

Looks like that wire got hot.
Which is a sign of high resistance.
I’d inspect the connections where the wire plugs in. It not unusual to see corrosion in the connectors. Some times the connector melts.

 

Looks like too much current went through it more than high resistance, being that is a ground wire that to me would point to something having a bad ground and finding a path to ground through that wire or something bing shorted to that wire somewhere.

 

You’d think the fuse would blow?
Unless something else rubbed through the sheathing of the wire?

 

Grounds are almost never fused and individual wires going to a particular ground are typically sized for the circuit they are meant to ground so if anything extra tries to ground through them that can damage them.

 

Something, or things, are definitely pulling more current than it’s designed for.
The STA relay ground is grounded in 2 different places. I don’t think both grounds would be “bad”.
But you never know. Maybe in the splice or 2A connection?

Or another wire is rubbed through into that ground wire?

 

IMO that appears to be an over current condition as the insulation is bubbled or appears to be along a stretch of wire vice just a localized charring.

That being said the circuit should be like this:

Key switch >> ST1 >> ST Fuse >> Park neutral Safety Switch or Clutch Switch >> STA Relay Coil >> Chassis Ground.

I Don't see anything in that circuit that would cause a Ground wire to melt. A loose connection anywhere would have higher resistance and cause a reduction in current. An open would stop all current. A loose or open ground would also reduce current or stop it.

The only thing I can see that would cause that would be shorting a wire across the relay coil terminals. Or the coil is shorted inside the relay. But that should have blown the 7.5 amp STA fuse for the relay.

If the wire isn't damaged too bad you could cut out the melted section and splice in a new wire of the proper size.

 

Another day tinkering on the taco. Went over everything once again. Starting at the ignition switch ST1 circuit, STA fuse, (2A, 2A, PB, and PA plugs), NSS, etc... Something is drawing down voltage to 8.5v.

I must have missed this. I don't know if this is new today or was there before. There is something running with the ignition ON. Sounds like a fan motor coming from the rear of the plenum. (not the heater fan) Decide to run through the circuit again but unplugging the various plugs and testing voltages at each unplugged. I notice when I disconnected the plugs the noise stopped and voltages rose to 12v+. Reconnecting the plugs, the noise returned, and voltage dropped.

Here's where it gets good. I decided to say "F" it and have it towed out. Buttoning it back up I put the relay back in. The motor noise subsided. Hmmmm... R&R the relay several times proved this repeatable. Whatever... Needing to unlock the steering column to access the screws for the cover I turned the key to the start position and the truck came to life.

I'm mentally exhausted. It runs, but I can't say it's something I did to fix it. Maybe there was a poor connection at one of the plugs. I don't know. I'm afraid it will malfunction again while I'm out on the road.

Anyway. Thanks for listening to this Olde Geezer and for all the expertise you shared.

 

Hey Guido:

I'm having a similar problem to your posts back in April. I turn the key and......nothing. This happened out of the blue with no prior warnings. I too heard something running whilst working on the starter relay(I believe that it was the fuel rail being charged by the pump????). Anyway, did you ever figure out what the issue was? I'm at wit's end here.

Bob

 

What is your issue?
Will the truck not crank?

 

Hey Deogee:

My troubles are outlined in the post "Could It Be The Key". The discussion involves TNShooter , me, and yourself, and quite possibly Oldeguido, I hope.
It seems that only one-half of my starter relay is powered. The wire from the Ignition Switch (green w/black) is not sending voltage to post 5 on the relay. All components of the starting system are new. As soon as the rain stops I will start searching for any unknown security features that the previous owner might have installed, but I'm fairly sure there aren't any. Any suggestions as to where to start looking?

Bob

 

Bob,
I can't say what the true problem was. I can only guess that there was some loose or lightly corroded connection. After pulling all the plugs related to the neutral safety switch (anything with a purple wire IIRC) the truck decided to work again.
Be careful with the neutral safety switch plug. The wires are very fragile. I ended up replacing the plug and the switch which was "not" the problem.
There are a bunch of plugs in the junction block behind the glove box. I had pulled and replaced all of them that had the purple wire. Sometime after doing that the truck decided to behave itself.

I wish you luck. If you do identify the issue, please post it back to this thread for others to find.