Heat/Cold selector

Just when you think you know everything that your car does, you notice something you've missed.

This was the case when I was sitting idling this morning, warming up the car an waiting for my kids so I could take them to school. I started playing with controls to entertain myself.:iceslolan

Now I've always noticed there is a "notch" in the dial at the extreme cold and hot sides of it's travel, but never really thought anything about what it's purpose is. I just assumed it so if you set it to full heat or cold, it stayed there. Made sense to me.

Now here is the thing. My blower has always had a faint howl in it at the "2" position, one of those things that I can easily ignore but drives some folks nuts. And they, in turn, drive a service manager nuts by insisting that it be fixed under warrantee. (it's done it since it was new)

But I am very good at ignoring noises if they are harmless.:biggrin: That's why I get along with American cars so well, I think.:lol: So i'm sitting there, blower on "2" and howling softly, turning the temp selector back and forth, on and off that notch at the end of the heat range, when I notice that as I do, 2 things happen. The blower speed increases slightly as I put it on the notch, and decreases as I move off of it. At the same time, the engine idle drops very slightly, maybe 30-40 rpm, just enough to be noticeable like when you turn your headlights on, and then speeds up when the control is put back on the notch.

It's as if an electrical load on the alternator is relieved by setting it fully to the notch at the hot side. Thus the increase in idle, and the increase in blower speed. But I can't reconcile this with any part of the temp control system as I understand it, unless I've forgotten some detail that I used to know about.

I also don't think I've had the blend system apart on one this new ('97), since I stopped working in the shop in 2000, and most cars I saw for serious repairs were still early 90's and down.

So for all I know, the blend could have an electric actuator, and drops into a "rest" position and powers off when you go full one way or the other.

Anybody know?

I know...I have too much time on my hands:lol2:

wow, hmmm, now that you say that, I'm gonna go play. If you figure out anything else let me know. Sorry to get off thread here, but whats wrond when the air (wheather is be heat, ac, bi-level, or just vent) blows only out the defrost, and never moves. It does change to ac or heat or vent or whatever, but only blows out the defrost, and slightly out the floor, and even a little less out the dash vents. I thought it could maybe be a vacuum leak, since some cars use vacuum to controll this, right? But I know its controlled electronically. Whats is the first thing to check out, what could be the cause of such a situation. And by the way, sorry I know I know, lumina thread, but this vehicle is a 1995 GMC jimmy sle with the 4.3 v6 VinW . Thank you.

The OBD 2 system (which as you know all vehicles 96 up use) reads from the pcm. Almost all vehicles have a load % on the data list. Is it possible the pcm is detecting the extra load? That would explain the slight "drop" in rpms before actually picking back up to "target" idle rpm.
Even more probably is the fact that the pcm is set to idle at an rpm speed of such rpm and when it drops do to the extra load the IAC should open ever so little to get back to target rpm.

wow, hmmm, now that you say that, I'm gonna go play. If you figure out anything else let me know. Sorry to get off thread here, but whats wrond when the air (wheather is be heat, ac, bi-level, or just vent) blows only out the defrost, and never moves. It does change to ac or heat or vent or whatever, but only blows out the defrost, and slightly out the floor, and even a little less out the dash vents. I thought it could maybe be a vacuum leak, since some cars use vacuum to controll this, right? But I know its controlled electronically. Whats is the first thing to check out, what could be the cause of such a situation. And by the way, sorry I know I know, lumina thread, but this vehicle is a 1995 GMC jimmy sle with the 4.3 v6 VinW . Thank you.

Like I said, the ones that came out in the mid 90's and up weren't having problems like that yet when I stopped working in the shop, so they could have a nuclear reactor in the dash that operates the blend door for all I know.:lol:

But that sure sounds like a vacuum line leaking, you're right. Did you check the ball and see if it's cracked?

yes, thankyou, that was the first thing I checked was the ball. As a matter of fact, that vacuum line had fallen off of it for god knows how long, but replacing the bad vacuum lines had no evvect on how it operated. Thank you, I'll dink around some more when its warmer outside.

oh, yeah, and by the way jeff, "This Bud's for you". :smokin: I got this wiring diagram from mitchell ondemand. As you can see the temperature controll is nothing more than a variable resistor up in the left hand corner of the schematic. I sure hope your lumina is a 97 ls, thats what I thought it was, and if you would like the other half of the wiring diagram, let me know. It was just a pain to upload, so I thought, I'd only do one, unless you want both. I'll up the other half if you want it.

https://www.sharemation.com/tblake05/heater%20wiring%20diagram%2097%20lumina%20ls.JPG?u niq=nold42

oh, yeah, and by the way jeff, "This Bud's for you". :smokin: I got this wiring diagram from mitchell ondemand. As you can see the temperature controll is nothing more than a variable resistor up in the left hand corner of the schematic. I sure hope your lumina is a 97 ls, thats what I thought it was, and if you would like the other half of the wiring diagram, let me know. It was just a pain to upload, so I thought, I'd only do one, unless you want both. I'll up the other half if you want it.

https://www.sharemation.com/tblake05/heater%20wiring%20diagram%2097%20lumina%20ls.JPG?u niq=nold42

Cool man, thanks! That does answer part of the mystery, the other half of the diagram would probably nail it, if you want to upload it, I'd be greatful.

Dead on, '97 LS:iceslolan

hey jeff, sorry the website is down tonight, but I will try again tomorrow if I have a chance, I work tomorrow, so if I dont, I havn't forgotten about you. Any more wiring diagrams, or anything you need? I can put them in a zip and email them to you or something, it might be easier on me.

Just out of curiousity, anyone happen to have the vacuum diagram for the HVAC system, too?

You know, Jeff, I got to thinking.

The little flap that controls where the airflow goes to has a foamy rubber gasket around each end. When you hit the notch, the motor to the door seals the passage to prevent any air from coming from anywhere but the core or evaporator, as the case may be. This could cause increased pressure in the ducts, since the blower has to suck -only- from the passage. When you turn the control off the notch, the seal might be broken just enough to allow the blower to openly suck from the now broken seal, hence taking a strain off the blower and, in effect, drawing less power.

Same principle as a vacuum cleaner running, then putting your hand over the hose. Higher motor stress, more power needed, a slight demand on the alt. Just a theory.

As far as a diagram, not really. But ask what you need to know and I can probably photo/walk you through it. I am usually given cars with vacuum problems, for some reason, at the shop.

Jeff, some days, you make my head hurt! I almost cringe when I see a post you started, :lol:

You know, Jeff, I got to thinking.

The little flap that controls where the airflow goes to has a foamy rubber gasket around each end. When you hit the notch, the motor to the door seals the passage to prevent any air from coming from anywhere but the core or evaporator, as the case may be. This could cause increased pressure in the ducts, since the blower has to suck -only- from the passage. When you turn the control off the notch, the seal might be broken just enough to allow the blower to openly suck from the now broken seal, hence taking a strain off the blower and, in effect, drawing less power.

Same principle as a vacuum cleaner running, then putting your hand over the hose. Higher motor stress, more power needed, a slight demand on the alt. Just a theory.

As far as a diagram, not really. But ask what you need to know and I can probably photo/walk you through it. I am usually given cars with vacuum problems, for some reason, at the shop.

I think you got it. Makes perfect sense!

Jeff, I got to thinking again five years later...

I bet it's witchcraft and hobgoblins.