S10 Heater vent control

I have not had any air coming from the heater of my 92 S10 Blazer for months now. You can hear a load hissing (vacuum leak) in the vent control area on
the dash.
I went to a junk yard and got a new unit, replaced mine and the leak is now gone. The little part that all the hoses connect to was leaking badly.
I still do not have any heat. If I reach down behind the vacuum assist on the heater area there is a spring loaded metal arm on the side of the duct area. If I grab it and pull on it I have heat. If I manually move it back I don't.
Is this part of the vacuum assist assembly or a separate part?
Can it be replaced?
Lannie
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months
now
back
There are two sliders on the HVAC controls. One operates the vacuum switch which you replaced, and the other operates a cable. Is that one intact? It's easy to disconnect it for service and have it slip back inside the dash.
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Yes, that cable is intact and I followed it all the way back to the heater core area and it was lose there. Tightened it and I have excellent heat, just no heat from the heater vents.
LS

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Sounds like the vac control pot is not hooked to the heat door. The cable controls a door between the heater core and the A/C. That door just puts air through the heater core or bypasses it. All the rest of the controls are vacuum powered. If you have heat coming out at the defroster vents but cannot get heat to the floor vents check the transfer door that is on the left side of the plenum above and to the right of the steering column, It moves the door that directs the airflow from the floor vents to the defrost vents. It is a PIA to get to. If it doesn't move when you move the controls from A/C, vent, defrost then either it is bad or it isn't getting vac. I would pull the heater control back out and use a small vacuum source to apply vacuum to the hoses.
Hose color (can be striped or solid color) - What they control Gray - Vac source from engine. Black - Heater/ Defroster control (one that is probably bad on yours) Orange - Outside air/ Recirculate valve. Tan - Panel door, This is the door on the passenger side behind the kick panel like the old manual vents and a control under the cowl panel below the wipers that allows outside air into the vehicle. Dark Blue - This one also goes to the Heater Defroster door, It only works when in Defrost and it allows the air to go through the A/C evap core to dry it out before putting it on the glass, helps prevent fogging over.
--
Steve

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I will check,. I do remember pulling that hose off from that vac and it made a suction sound as I pulled it off.
I will check all the hoses and etc let you know what I find.
I appreciate the detailed response!!!!!
Lannie

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I traced all the lines and they all look good and have a tight fit.
I took off the defrost line and put it to my mouth and blew on it. I could not force any air through it at all, which is what I expected. I pulled theblack line off from the heater vac and I was able to force air through it. Not much but I could. Is this the leak that causes my to lose vents and everything when giving the motor gas?
When I accelerate up a hill ev erything goes to defrost. Once I let up it goes back to vents.
The metal arm on the heater flap has two holes in it, Is there a spring or something that should attach to it or are the holes there but not used?
Ls

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Normal for my '89 S Blazer w/4.3 . With the throttle open far the engine vacuum drops to zero and the heater defaults to full defrost. When you lift the gas engine vacuum comes back up and pulls the vacuum motors back to where they are set. No matter what mode the heat / AC was in mine did that. Toss a trailer on it and even a small hill did it. Nature of the beast..........
--
John
"anything you say can & will be misquoted & used against you"
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my 94 blazer has started doing this as well and it wasn't always that way. just in the last year but the cpi replacement took precedence.
Semi looked yesterday but my ac/heat vent switch took a crap so if you put it to heat it starts to meltdown and little puffs of smoke eminate from the dash
mike
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I went to the junk yard and they sold me the whole vent control with all the knobs and vacuum lines for $10. There is a resistor pack on the firewall to the right of the blower fan that melts the wiring inside the dash. I replaced that and it hasn't done it since.
LS

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As a follow up to mine. I replaced the hose to the ball and all is good again. I had used a slightly bigger hose when i was looking for a stumble (which was a bad cpi) and never finished up checking everything
mike
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I haven't checked mine yet. I put a small bungee cord on the metal arm on the side of the heater box and pulled it to I have heat. Now I never lose vacuum and have heat. Of course it's been warm now and I don't have vent. :)
I will try and check it all out this weekend.
Lannie

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I took the cover off from behind the four wheel shifter so I could get a better view of the metal arm I was pulling on to get heat.
It has two holes in it and so did the heater casing. It looked like the arm was supposed to be screwed to the casing. I put in a screw to hold the arm in place and now everything works!
The vacuum is a little off on my selector but I think it might be a gear or two off when I put it back together.
Lannie

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NOT normal. If your vehicle is acting that way the vacuum check valve or the vacuum reservoir or lines are bad.
TO LS, if yours is doing the same thing then you have a bad check valve in the vac. reservoir OR it has a small leak. Neither are uncommon. What you are looking for is a plastic ball with a vacuum line on it. That line goes back to a check valve next to the intake manifold. On that check valve is a fitting that connects to the line that goes into the interior for the heater controls. The other end goes to a fitting on the intake manifold. If the line going to the vac. res. is bad or that check valve or the line going inside is bad you get the same problem you have. When you open the throttle the vacuum level drops and the doors go to the default position of defrost. On the 83-94s the reservoir is usually on the inner fender just behind the battery and under the 4X4 actuator. It is a large plastic ball about the size of a softball with a vac line going to it. Blowing through the line won't tell you if the valve is bad because the air normally goes that direction anyway. Use a vacuum pump (hand powered cheap unit from harbor freight or the like will work just fine) to apply a vacuum to the black line and see if it holds or drops VERY slowly. If it drops fast or won't even draw a vacuum then either the line, check valve or ball are bad. Check the ball next. Just apply a small vacuum to the port and see if it holds. If yes it is ok. Check the valve next it should pass air if you blow from the side with two ports, it should not pass air if you blow from the engine side. If it passes then one of the lines is bad. Hope this helps.
--
Steve Williams


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Thanks Steve! I will look at it tonight or if that doesn't pan out, it will be this weekend. I appreciate all the help!
LS

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Thansk for the heads up. I've replaced some vac line earlier this year when i thought my stumble problems were vacuum related. even bought a vacuum gauge. At some point i'll get to this .... it just seems some other little annoyance pops up and gets in the way.
two weeks ago came out to car to find drivers side mirror hangin. Luckily was able to pull it apart and fix it. woohoo. cheap fix
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