More weird electrical
 

I have a 2000 Blazer 4dr 4x4. Why would messing with the headlamp relay cause the AC clutch to work or not work?

A little more background to make it clearer. I just bought this a few weeks ago, and I've been trying to get the AC to work ever since. Inside the underhood fuse block are three hotwires to various parts. One from the HDLP PWR relay to the RT HDLP fuse, and from RT HDLP fuse to LT HDLP fuse. The other is from the F/Pump relay to ENG I fuse.

I was trying to get the headlights to work correctly, and by moving the hotwire from one pin to another on the headlamp power relay I was able to get the AC clutch to engage.

Only one problem with that--the headlights wouldn't turn off when I shut the truck off. So I moved the hotwire to the opposite pin--from 85 to 86 or vice versa...I can't remember which at the moment. Now the A/C will work intermittently when I have the headlights on, but not when they're off.

So...what does the headlamp relay have to do with the A/C clutch? The A/C is blowing really cold when the clutch is engaged. So I know it's not my A/C unit itself.

:icon_attention:

These wires in the fuse box..... are they above the fuses or on the bottom side of the fuse panel? Pictures would be very helpful.

On the top where the fuses/relays plug into the fuse box. I attached a couple of pics so you can see better.

As an addendum, without the hotwires NOTHING works--no headlights, no AC, no fuel pump. Everything else on the truck works though.

I see.... Well, if you want to trouble shoot this thing and fix it, I can help. First of all, can you read and understand wiring schematics? Do you understand DC circuits & terminology? Do you have a volt/ohm meter? Do you know how to use it?

Yes to all those.

Cool. Are there any jumpers on the instrument panel fuse panel? (inside driver door jamb.) If so, remove ALL jumpers from both fuse panels. With the ignition in the OFF position, remove the fuel pump relay and check voltage on all of the fuel pump terminals and post if there are any terminals with battery voltage. If so, which ones?

I'll do that as soon as I get home. I'm at work for another 15 minutes. There aren't any jumpers in the cab fuse box at all. Just inside the hood fuse box.

Ok so here goes. In perusing the forums here, I've noticed that a lot of people kind of stop mid-fix, and never post what works, what they did, etc. So I'm going to do something a little different, in case anyone here has similar problems. This might get a little long, but I don't really care. I want to get this fixed, and maybe you all will see something I missed.

CH was kind enough to email me a wiring diagram for my Blazer, which I'm sure will help. He also asked me offline to pull all the jumper wires and test the voltage on the fuel pump relay, and then test the voltage on all the fuses. So here are the results.

Since I don't know that it's wired correctly yet, I'm numbering the relay sockets, starting in the upper left and going clockwise. So a four prong relay would be 1,2,3,4:

1 - 2
| |
4 - 3

This numbering is with the big fuses at the top of your vision. See my pictures in a previous post to see how I'm looking at the fuse box.

So with the ignition switched to OFF, I get the following on the Fuel Pump Relay:

1-11.6mV
2-12.75V
3-0mV
4-5.8mV

Just for kicks, since I was there anyway, I did the same thing for the headlamp relay:

1-11.7mV
2-4.7mV
3-9.0mV
4-12.67V

Then, I switched the ignition to RUN, and tested the fuses. I'm not going to list the fuses that HAD voltage, just the ones that didn't/had less than 1V. Some of these you may read and go "Duh! That shouldn't have any voltage anyway!" To which I respond, yeah, but I didn't want to miss anything. Here goes:

RTHDLP
LTHDLP
TRCHMSL
VECHMSL
CRANK
HTDMIR
RRDFOG
HORN
LTTURN
RTTURN
VEH B/U
TRL B/U
TRLTRN
TRRTRN

Also, there are spots for the following on the diagram of the underhood fuse panel and empty spots on the fuse panel itself, but nothing to plug anything into--it's just an empty spot:

CHMSL (looks like a relay on the diagram)
W/W PUMP
HDLP W/W

In the driverside cab fuse panel, all fuses there had voltage.

READY....GO! :D

Before we get too far, does this vehicle, or did it at one time, have a remote starter or aftermarket keyless entry system installed?

Key ON: does the pink wire at the ignition coil show battery voltage?

In the fuel pump relay socket:
1)There should be one terminal that shows less than 5 ohms to ground at all times.
2)There should be one terminal that shows less than 5 ohms to ground for 2 seconds when the ignition is turned to RUN, then show an open circuit.

EDIT: Also, in one of those pictures, there is a blue wire in the right rear corner of the U/H fuse panel, where does/did it go?

To my knowledge, there never has been either a remote starter nor a keyless entry system.

Just to be clear, according to https://blazerforum.com/forum/articl...-switch-33465/ there are 5 positions for the ignition switch: ACC LOCK OFF RUN START. With the key in the RUN position, the pink wire shows battery voltage. In OFF it doesn't. Is that the ON you were referring to--RUN?

Terminal 4 (as numbered above) shows 0 ohms all the time. I haven't been able to check the RUN position as I don't have a helper. I should have one tomorrow night though.

The blue wire goes from the small yellow (CRANK) wire on the ignition switch wiring harness to (I THINK) the starter. I haven't had a chance to trace it down all the way yet, but it goes to something at the firewall side of the engine.