OK, so I'm going through this 2000 Blazer LS (2d, 4wd), trying to get everything working, and I've run across an electrical gremlin that I cannot figure out at all, so hopefully someone here has some insight.

Here's the deal:

If I shut the truck off with the headlights on, the HVAC blower will stay operating. It will shut off if you turn the headlights off, or if you cycle the HVAC control switch to off and then back to whatever you had it on. The 4wd selector button will also still work and will still control the transfer case as well (until you either shut the headlights off or cycle the HVAC switch). I should add that if you cycle the HVAC switch, you can hear relays cycling in the body relay block, which makes even less sense.

If you shut the truck off with the headlights off, the blower motor shuts right off like it's supposed to and everything is fine, it's bizarre.

So, I found the "anatomy of an ignition switch" post, and I noticed that the HVAC, 4wd selector, and cruise are all fed via the ignition switch from the IGN3 circuit (the big orange wire). So, I replicated the issue and checked voltage on that circuit. With the truck running, there's 13-ish volts there, which is good. But, if I shut the truck off with the headlights on, there's about 7 volts on that circuit, which makes no sense at all. If you shut the truck off with the headlights off, it goes right to zero.

My first thought was that something went bad in the ignition switch, so I replaced it with a brand new AC Delco one, no change.

I don't see how the IGN3 circuit can get power from anywhere other than the ignition switch (which is new), unless it's coming backwards through a ground or something, but I have no idea where to look. I have an Alldatadiy account, and have been going through wiring diagrams with no luck so far.

Anybody have an idea of where to look? This is seriously frustrating me, and I just want to get it corrected.

I have a meter with a clamp on ammeter and know how to check circuits, if I just knew which one to check.

Thanks!

 

I would start with your headlight wires and look for shorted or frayed wires from the headlights back.

 

I don't have an access to any Blazer schematics yet, but I have a tip for You. If You have 7V DC instead of a zero, then it means, the ignition line is powered through something from battery. It has some resistance, therefore from 12V battery You have 7V on a line and 5V on that *something*. If there is enough current to drive Your blower motor, then it passes quite a current. 5V times blower motor current consumption will give You an estimated power dissipation in that location. In my opinion it should be significantly hot.

Start with insulation measurements. Disconnect battery and measure the resistance in between the ground (chassis) and a power line that should be unpowered when ignition is off. In an ideal circumstances it should be invinitive, however above 1MegaOhm is still acceptable.

Next, measure the resistance in between a positive battery clamp and that line that should be off. You may expect a reading around 20Ohms (If You assume 5V drop and a current for blower el. motor as 0.3A-> 5V/0.3A=16Ohms. Well, You have a clamp meter - You may figure out an exact current easily). Try switching things off/on and observe when resistance goes significantly higher. Try disconnecting things while observing resistance.

If Your insulaiton readings are:

• not ok - search for a damaged insulation
• ok - try checking relays (sometimes, when they are worn off they have a tendency to end up with a welded together contacts), power receivers (by disconnecting and observing resistance changes)

In general, many strange electrical behaviours are related to a poor grounding.
Sorry for such a general answer, but I did not get the Blazer service manual yet Good luck!
Cheers,
Mike

 

Do you have the three button or four button 4wd switch? Do you have an auto or standard trans?

EDIT: Also, do you have automatic HVAC temp controls or manual?

 

This is a really strange one so I started looking for a place to start for some sort of direction/narrowing down. I took a guess on the wiring diagram since I didn't know the type of 4wd switch you have. The thing to remember is that on many sensors they have a 5 volt reference. That may be why you are showing the 7 volts left over hanging out. If you have auto temp controls, that also has a 5 volt reference circuit.

Started looking at the diagrams and noticed the A/C Compressor Clutch Diode. The diode is supposed to only allow current flow one way and stops current coming from the other direction. Also noticed that the DK/GRN traced that to the A/C Compressor clutch relay, which is fed from a hot at all time fuse. Anyway, what I noticed is that pin 1 on the clutch relay goes to the PCM relay control, coming out of the PCM (I’m leaving a lot out but you can see on your diagram). It goes to the A/C low pressure switch which grounds at G117 and also has a wire from the HVAC controls. It grounds in the same place where the circuit that has that diode grounds (G117). I had started wondering about that diode maybe being bad (didn’t investigate if it was possible for current to get through that diode if it was bad), instead went the direction of a bad ground. G117 is on the back of the driver side cylinder head (one of the real pain in the *** ground location).

I stopped looking around and I’m thinking you might have a block or body ground problem. One of the modules is seeking a ground to complete the circuit and its doing things like keeping relays closed it shouldn’t and probably a bunch of other whacky things trying to maintain that circuit by finding other ways to ground itself.

I’d take a hard look at your grounds. Check that strap on the passenger side head and see if it is broken. Check the ground at by the CKP real well. Check body grounds. Use your DMM connected to B(+) probe the ground locations (block and body), you should see battery voltage. If not, bad ground.

 

I have a slight update after some more troubleshooting:

For everyone's info, it's the 3 button case, and manual AC controls

Pulling the 10 amp 4wd fuse (fuse 15 in the IP fuse block) makes the issue go away. So, that kind of narrows it down?

Tracing that circuit out yields 2 options:

One is the 4wd switch itself, so I tried unplugging that, and that doesn't help.

The wiring diagram shows what looks like another branch of that wire running to splice SP200, and best I can tell from there, it goes to the big connector under the passenger dash, and then I don't know where it goes from there. I'd like to investigate that feed to see where it goes, if anyone is better at reading these diagrams than I am.....

It's wire K2 on connector 203, which simply says fuse output, and no other info as to where it goes...

Anybody got any idea?

 

[QUOTE=Mike.308;688457]If there is enough current to drive Your blower motor, then it passes quite a current.[QUOTE]

There is not. The blower on high runs from a dedicated power source through a relay. The 7 volts is enough to hold the relay closed, but that's it.

If the blower is in any position other than high, the 7 volts is still there as long as the headlights are on, but the blower doesn't work, as most likely the voltage is too low after passing the 7 volts through all the blower resistors.

 

If I'm looking at the correct one (Circuit 241)...it looks like it is spliced into three different wires (all BRN) in SP200.

In SP200 it enters (from fuse 15) into position "H" where it shares the spot with another BRN wire which goes to you TCCM position "C8" (12v IGNITION (3)).

It also is connected to the two BRN wires coming out of position "G" on SP200. One goes to the VCM Connector 4, Pin 13 (IGN 3). The other wire goes to position "A" on the 4WD INDICATOR SWITCH (on the front axle).

Hope that's what you're looking for.