I understand all that, and I'm not calling it into question.

What I'm asking is this: on the A/C compressor clutch switch--on the compressor itself--there are two wires coming off it. A green wire and a black wire. Where should that black wire go? I know it's not right at the moment--it currently runs from the starter relay to the AC clutch switch--but where should it go?

If I pull the black wire from where it is right now, that circuit to the AC clutch won't be complete, and my AC won't work at all. So where is it supposed to go?

With interior vehicle temperatures getting upwards of 150 degrees in New Mexico right now, and exterior temperatures nearing the century mark, having no AC at all is not an option.

Again, it's nearly 110 in my parking lot right now, so I won't be able to get to those pins for a while.

I really do appreciate your help. The weather just isn't cooperating!

 

The compressor clutch ground wire goes to the underhood fuse panel, C2-D12. Inside the fuse panel it splices onto the starter relay, DRL relay and the fuel pump relay, (might be individual wires or it might be a brass buss bar). Then it leaves the fuse panel and goes to G117 at the rear of the right cylinder head providing ground for the clutch and all 3 relays. We already know that the starter relay and the fuel pump grounds are missing in the relay sockets, (not sure about DRL's). Check resistance from C2-F7 of the harness connector, to battery negative, it needs to be 5 ohms or less. If resistance is more than 5 ohms: (don't cut the black wire), splice a piece of 14 gauge wire to the existing black wire, and run it to a good ground on the engine. I really think you'll find a problem here so take your time. NOTE: Both windshield washer pumps and the brake pressure warning lamp switch get their ground from this circuit too, and without ground, they won't work either.

There are 3 wires at the battery negative terminal: make sure all of the connections, (both ends), of each wire has a good, clean & tight connection. G119 grounds the engine, G109 grounds the body, and G118 grounds the frame.

There is a diode, close to the clutch connector, that connects from the dark green wire to the black wire. If you need to know how to test the diode, let me know.

 

So after a bit of thought, I've started replacing the entire computer wiring harness. Everything in the engine compartment, basically. We'll see if it fixes anything.

 

Pretty serious job, take your time and get it right. Keep us posted with progress.

 

So completely installed a new wiring harness into the truck engine compartment. This is the large harness that runs everything on the engine, and has 3 plugs into the computer.

I'm sure it fixed a bunch, but not everything. There was a whole set of grounds that were missing from before, and that's fixed now. Also, the P/N Safety Switch Plug on the transmission had paperclip wiring (see photo). That's fixed now. A few other wires that were abraded or missing were fixed as well. So hopefully a lot got fixed. Now for the next problem.

The truck now won't start. Gonna do a few tests tonight on the starter circuit if the weather will cooperate. I have a feeling it's the P/N safety switch on the transmission. I'll know more after I test the voltage.

We're in our "monsoon" season in NM right now, so who knows if it'll rain...

Good news--headlights are working right. We'll see if it fixed other things or not.

 

Cool, glad you're making progress. Post the results of the P/N tests, voltage, ground, etc at the starter relay.

 

Ok, so here's the results of my latest batch of tests.

IGN C Fuse - Battery Voltage (~12.5 volts) on both terminals
IGN A Fuse - Battery Voltage on both terminals

Starter Relay
Socket 30 Battery Voltage at all ignition switch settings
Socket 85 NO VOLTAGE at any ignition switch setting
Socket 86 0.0-0.2 Ohms to ground
Socket 87 When battery voltage applied from positive (+) battery terminal, engine turned over.

This is EXACTLY what I thought might happen, as I'm thinking at this point it's the P/N Switch that's bad--either out of alignment, worn out, or installed incorrectly to begin with. The p/n switch looks fairly clean compared to the rest of the underside of the truck, so I have a theory as to what has happened to this project.

Unless I'm really wrong, what I think happens is thus: The guy that owned it before me pulled the engine for some unknown reason. He didn't, however, know how to put it back together. When he installed the engine back in, he left off a bunch of parts, didn't know how to put the wiring back together, etc. I KNOW he left off two sets of grounds (because I replaced them when I replaced the wiring harness). He also had left off the knock sensor--also fixed. He installed a new p/n switch, but didn't do it correctly. From what I've read you have to have the truck in neutral to install it, and I bet he didn't do that.

So, in order to get the truck to run without a set of ground wires, a faulty p/n switch, and the incorrect assembly, he puts a bunch of jumper wires all over the truck.

Does this sound plausible? From what I've seen, the guy that owned this Blazer before me didn't have a clue what he was doing.

 

Page 52 of the schematic:

Pin #85 is what triggers the relay, the rest of the circuits are good. Next step is to see if the CRANK fuse (10 amp), in the underhood fuse panel is getting voltage from the ignition switch. Make sure trans is in park or neutral when testing.

Check both terminals of the fuse, they should both show battery voltage in the crank position only. Make sure trans is in park or neutral when testing.

If that checks good, check the purple wire at the P/N switch, it should show battery voltage in the CRANK position only. Make sure trans is in park or neutral when testing.

If the purple wire tests good, check voltage on the purple/white trace. It should show battery voltage in the CRANK position only. Make sure trans is in park or neutral when testing.

Post your results.

 

Alright. The results are in.

I replaced the Neutral Safety Switch on the transmission. SHE STARTS!

The AC works (kinda), the headlights work (kinda), it starts and runs, and NO jumper wires used anywhere.

So the AC kinda works -- the compressor will cycle on and off, but won't run all the time. I'm thinking either A) Bad low side pressure switch or B) Not enough Freon or C) Bad AC relay.

The headlights sometimes work perfectly--switch works, high and low work, etc. Sometimes the headlights are just on--the high and low work, but it's the mains that are on and not the DRLs. I'm thinking it's either A) a bad relay or B) a bad ground on the relay.

I have a few extra relays that I'll try first from when I replaced the fuse box. Then I'll start testing wires and see where the problem lies.

Also, the computer is throwing two codes that make me thing the MAF sensor is bad. Gonna clean that tonight with some stuff I got at Autozone, then try to clear the codes.

 

sorry