no CEL / no start, cranks fine

[mr cribb]

Hmm. Diode at ignition. This might be the culprit.

Yes there's 2 sets of ground wires near pcm. And one is hidden behind the glove box. It is tight. I checked that last night.

 

[mr cribb]

Best part is that I have the other harness easily accessible to find this gremlin.

 

[MrShorty]

One or two of the PCM ground wires are routed up and splice into the pigtail off of the negative battery post cable. I had that splice corrode out on me at one point. If it looks like a problem with the PCM ground, there's another location to look at.

 

[mr cribb]

Mr Shorty are you leaning towards a ground problem versus pcm power problem?

 

[Roadrunner777]

Uploading PCM schematics here: http://www.explorerforum.com/forums/showpost.php?p=3320991&postcount=26

PCM and Power Distribution done. Grounds are next, maybe this afternoon.

 

[MrShorty]

Mr Shorty are you leaning towards a ground problem versus pcm power problem?

Sorry to be late back to the party.

I don't think I am leaning one way or another. You have indicated that the red wire (at least in the vicinity of the power distribution box??) has power, so that suggests that the EEC relay is closing. I would expand this search to include following this red wire to other components. At some point, obviously, this would lead me to testing this red wire at the PCM connector (I might initially disconnect the connector to be safe).

Once I determined that there is power at the PCM, then I would start to probe the ground circuit. With the PCM connector disconnected, measure the resistance between the three ground wires and ground (resistance should be near 0).

I power and ground both look good at the PCM, then I would start to suspect something has failed inside the PCM. Before making that conclusion, I would check the CEL light bulb (the other cheap explanation for "CEL does not come on").

 

[mr cribb]

Bulb works, as I started my truck with the old harness recently.

Pcm is from my 92 and has worked for as long as I have owned my truck.

There's something not right with this harness, because with the 91 computer in the original truck it did the very same thing.

 

[Roadrunner777]

I may not get to the ground pages tonight. Here is the quick version:

Pin 20 Black/light green, goes to the ground at the glovebox

Pins 40 and 60, both Black/white go to a ground under the starter relay by the battery.

Pin 16 is outgoing reference ground for the ignition system, that's not your problem, I don't think.

 

[mr cribb]

I'm probing the pins suggested.
Here's my findings:
Pin 40 reads 5.88 volts (power ground)
Pin 60 reads zero (power ground)
Pin 16 reads 5.88 volts (ignition ground)
Pin 20 reads zero (case ground)

 

[Roadrunner777]

Ok, well, it's not grounded if it's 5 volts. That's probably a floating voltage reading. I would tie 40 and 60 together, they both go to the same ground. Or just ground 40 any way you can. If you have a favorite beverage, now is the time to get it in the fridge. You are very close.

Don't sweat 16. It will come down when you ground 40. 40 is Black with white stripe... just making sure!

 

[larrydd999]

According to my Ford wiring diagram ('92 Ex.) #16 goes lots of places, so needs to be at zero any time the key is at run or start. Mr cribb, did you take the voltage readings with the ignition switch on or off?

 

[mr cribb]

So it seems like there's a break in the #40 ground wire.

So by grounding pin 40 everything "should" come alive?

Larry, I had the ignition switch in the on position.

 

[mr cribb]

Phil, would you tie 40 and 60 together at the computer connection, or somewhere else?

I always keep Jim Beam in the freezer

 

[Roadrunner777]

I have been thinking about this 2 wire to the same ground point deal. Why would they run 2 wires to the same place. I think it is because the PCM draws too much current for one wire but they can't put a bigger wire in the connector. So, they use 2 wires. With that in mind, you should cut 40, and insulate the end that we think is broken. Then, run a new wire from the pcm end to the ground. That might mean a trip to the store, but it's the safe way to go.

 

[mr cribb]

I have been thinking about this 2 wire to the same ground point deal. Why would they run 2 wires to the same place. I think it is because the PCM draws too much current for one wire but they can't put a bigger wire in the connector. So, they use 2 wires. With that in mind, you should cut 40, and insulate the end that we think is broken. Then, run a new wire from the pcm end to the ground. That might mean a trip to the store, but it's the safe way to go.

Ok, does length of ground wire matter or can it go to one of the 4 or 5 grounding points before the battery?

What I'm trying to get at is, does the new wire necessarily NEED to be 2-3 foot long?

 

[Roadrunner777]

Actually, before you do that, I would take that ground point apart. Maybe the wire is broken right at the lug. If so, all you would need to do is repair the lug.

As to which ground it has to go to... I would go to the same ground point it used to go to. I don't have a good reason for this other than that it's the way it was and they did it that way for SOME reason. Since I don't have a good understanding of the PCM guts, I'm going to go with that.

And, on second thought, I think my '2 wires to replace a bigger one' is false. I have not the slightest idea why they did it that way.

 

[mr cribb]

Ideally, you replace wire #40 to the same ground, which is the one near the battery by the starter solenoid. It's not about length, it's that 40 and 60 have to go to the same place.

Actually, before you do that, I would take that ground point apart. Maybe the wire is broken right at the lug. If so, all you would need to do is repair the lug.

Ok. I will look into that tonight after work. I wish I could afford to take a day off, but I can't.

Either way, I will still stop by the parts store to grab some wire and connectors. Living out of city limits you tend to minimize your trips into town if possible.

 

[mr cribb]

So I'm dissecting my old harness to see where this pin 40 goes. If I'm correct, it goes to a two pin plug.

Now looking at the one on the truck, the wire colors don't match.

I look in my Chilton manual, and I see 40 is tied into 60 and then grounded.

What is the best solution here? Take pin 40 and ground it separately?

When pulling apart the old harness, I found that at least 4 black and white stripe grounds are soldered together before parting their respective ways. It is really a difficult guess as to where pin 40 really goes.

 

[mr cribb]

Actually, before you do that, I would take that ground point apart. Maybe the wire is broken right at the lug. If so, all you would need to do is repair the lug.

As to which ground it has to go to... I would go to the same ground point it used to go to. I don't have a good reason for this other than that it's the way it was and they did it that way for SOME reason. Since I don't have a good understanding of the PCM guts, I'm going to go with that.

And, on second thought, I think my '2 wires to replace a bigger one' is false. I have not the slightest idea why they did it that way.

If I had the slightest idea where this dern ground went to I would.

 

[mr cribb]

I may not get to the ground pages tonight. Here is the quick version:



Pins 40 and 60, both Black/white go to a ground under the starter relay by the battery.

Wow I'm a genius, disregard my last two posts.