I have inspected all of the wire harnesses around the manifolds and all look perfect.
Thanks for the ideas!

 

Page 22 of the schematic does not list circuit #775 in C3.

Circuit 1225, dark blue wire, should be in C3 pin #22 and it should be grounded.

Circuit 771, black with white stripe wire, should be in C3 pin #31 and it should be open.

 

Whoops...the third circuit is 776, C3 pin #26. This shows up as PRNDL P on the schematic. Should this be open or shorted to ground? If it is supposed to be open, it sounds like the Park Neutral Position Switch could be faulty or some of the wires coming from it are shorted. I did inspect these wires and they looked fine (no damage the wire loom) but I will lift the vehicle again today and inspect again.

 

It happens there's a lot of wires, connectors and circuits to sift through.


Circuit 1225, dark blue wire, should be in C3 pin #22 and it should be grounded.

Circuit 771, black with white stripe wire, should be in C3 pin #31 and it should be open.

Circuit 776, white wire, should be in C3 pin #26 and it should be open.

Disconnect the park/neutral switch at the switch, and see if ECM1 fuse blows. The engine will not start with the switch disconnected.

 

I have disconnected the park/neutral wire harness from the switch and I still have a short to ground from ECM 1 fuse contact closest to the firewall. While the park/neutral harness was unplugged I disconnect C3 from the ECM and pin #22 is still grounded as you noted above. However, Pin #31 & 26 are no longer grounded. That would suggest that the wire harness from the park/neutral switch is ok...right? I will attempt to probe the contacts in the park/neutral switch to see if they are shorted to ground. I think I will also try to disconnect C1 and C2 from the ECM to see if there is any change to the short condition.
Thanks for hanging in there with me.

OK so I started pulling the other connectors from the ECM one at a time and if no change to the short condition, I plugged them back in. So when I pulled C1 (BLUE) the short went away. To be clear, all of the other connections were plugged into the ECM. The only wire harness that remained unplugged was the park/neutral safety switch. So I probed the C1 connector looking for shots to ground and came up with the following:
pins 7, 25 & 26. All of the wires to go to heated oxygen sensors. I am assuming that these are supposed to be grounded until they reach a certain temp?

So I guess now I need to see if I can find any commonalities between C1 & C3. It just seems strange that the short condition will go away when either C1 or C3 are unplugged from the ECM.
Is there a way to test the ECM for internal shorts? Would it be safe to use an ohmmeter to test all of the contacts to determine if any are shorted to ground or not.

 

Well I finally got around to checking the crank position sensor and it's wiring. Sure enough 2 of the 3 conductors insulation is melted away and they are shorting together.
I took a couple of photos and if I can figure out how to upload them I will. Now it is off to the bone yard to see if I can cut a decent plug out of a wrecked vehicle and splice it in. Wish I would have done to "due diligence" thing and checked this 2 days ago:/

 

Here is the photo of the connector

 

Let me guess, one of the wires was pink, (power)

 

Yup! The Pink and the Purple wires both had their insulation melted back about 0.5". I was able to pop the contacts out of the plastic housing, wrapped some 3M Supper 88 vinyl tape around the exposed conductors, then slipped some adhesive lined heatshrink over the tape and the existing insulation and shrunk it down. Popped all 3 wires back into the housing (in their proper locations of coarse) and then applied more vinyl tape from the connector all the back to where these 3 wires split out from the main harness. The blazer is back on the road! I even washed it for her before I returned it.

As far as the A/C problem goes, I pulled the connector from the low pressure switch and jumpered it. The A/C came on. I told her to have the Tech that filled it up 4 weeks ago to check the pressure. If it is low then she needs to have him find the leak, but if it is not, then the low pressure switch should be replaced right?

Thanks again to all those that helped on this issue.

 

See post #2


Good job! Glad you found it!

EDIT: As for the A/C, best to start a new thread but, the cycling switch, (on the accumulator) should cycle the clutch. It keeps low side pressure cycling from 22psi to 42psi. If static pressure is below 42psi, the clutch will not engage.