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  1. #1
    Todd Frye
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    Testing heater control valve

    An earlier post in the archives suggested disconnecting the electrical lead from the HCV, to see if that changed anything. I tried that, and I still get hot air at idle, cold air at highway speed. I'm thinking, why not pull the innards out completely, which, as far as I can tell, should let coolant pass that juncture unobstructed. That should tell me if the valve is possibly being forced shut by higher system pressures from higher RPM's. There were a few suggestions about replacing the HCV with a copper elbow. Seems like removing the HCV innards would give the same results of unostructed coolant flow. Your thoughts??
    Thank you,

    Todd Frye
    88 535is
    162K


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    Re: Testing heater control valve

    Pull the valve and check the rubber diaphram. Mine was ripped. $40 and 15 mins later, I had perfect heat again.



    85 535i
    90 325is



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    Removing 12v from the valve will result in

    Hot air. The way it works is at a full CCW you have 12v at the valve and it will be closed, you will get unheated air. Turn the dial for warmer air, CW, the valve starts to pulsate allowing more and more hot water through it the more CW you turn the dial until you have no voltage and the valve is open all the way.
    -Rick ©
    BMWCCA 162285
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    00 328i Sport Steptronic Steel Grey/Grey (problem child)
    87 325, The Hummer of BMW's, ???
    83 633 (gone to BMW heaven) fire is an ugly thing.
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  4. #4
    TJ
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    Re: Removing 12v from the valve SHOULD result in?

    If the valve is defective such that there is only heat when the car isn't moving then does the electrical connector matter? It sounds like Todd tried removing the connector and the heat remained off when cruising.

    I've had this problem since I got my 528e (not very high up on my list of things to fix). I have often read on this board that the heater valve is the likely cause but never picked up on the details. On my winter BMW (e30) I bypassed that fartknocker and I still have the option of either heat or no heat (I'm assuming partial heat is what I lose?) but what happens on the e28 if the valve is bypassed? heat all the time?


  5. #5
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    If the rubber diafram is defective it wouls allow

    water to pass all the time and he would get heat wouldn't he? It could also be the heat control unit. Since the valve is so easy to take apart, I would check to see its condition before going any further with the diagnosis of the problem. Then after it's condition is confirmed I would measure the voltage at the connector full cold, 1/2 way to hot and then at full hot. You can expect to see a voltage difference at each step of the way. Not seeing it could lead me to think control unit os maybe inside temp sensor. While at it he might just see if the vacuum line in connected to that sensor. These are just WAG's. I'm no service tech my any means.
    -Rick ©
    BMWCCA 162285
    87 535is 5 speed Diamond Black / Black
    00 328i Sport Steptronic Steel Grey/Grey (problem child)
    87 325, The Hummer of BMW's, ???
    83 633 (gone to BMW heaven) fire is an ugly thing.
    95 540 sport (gone with DotCom)
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    That isn't haw the heater valve works...

    Here's my Heater Valve 101 lecture, revised and expanded...

    Welcome to BMW e28 Heater Valve 101

    In this lecture, we'll discuss how the heater valve in the BMW e28 works. First we'll talk electrical, then mechanical. Then we'll talk about how the valve commonly fails, and what you can do to get heat until you get around to replacing the valve core.

    This discussion probably won't make sense unless you have a dismantled heater valve in front of you.


    ELECTRICAL

    The valve core is a solenoid with a plunger rod. It is either open or closed, not in between. When power is applied, the plunger goes down and the valve is closed (no heat). That means if you unplug it, you'll get maximum heat. When you have the temperature knob set in the middle, the temperature control unit opens and closes the valve every few seconds. If you listen, and your engine doesn't make too much noise, you can hear the valve clunk.


    MECHANICAL

    The valve body has an upper inlet on the side, and a lower outlet on the bottom. Hot coolant enters from the upper inlet on the side, and exits from the lower outlet on the bottom, on its way to the heater core.

    The hydraulic part of the valve core consists of a stopper and a diaphragm, connected by a hollow tube. This assembly fits loosely over the solenoid's plunger rod. The stopper is on the end of the plunger rod, below the side inlet, and the diaphragm is above the side inlet. All you engineers out there: note that the diaphragm has a larger surface area than the stopper. This is important when the valve opens, as you will see.

    There is a small weep hole near the diaphragm, which allows coolant under pressure to SLOWLY move to the space above the diaphragm.

    Finally, there is a small plug on the plunger rod. It's just above the diaphragm, and more importantly, it's just above the end of the hollow tube. The plug is how the plunger rod pushes the hollow tube to close the valve, but, like the diaphragm, it's also important when the valve opens.

    So, we have a stopper, a diaphragm, a weep hole, a hollow tube, and a small plug.


    How the Valve Closes (No Heat)

    When power is applied, the valve closes. The plunger rod goes down, which causes two things to happen:
    1. The small plug plugs the end of the hollow tube.
    2. The small plug pushes the stopper into the lower outlet at the bottom of the valve housing.

    The valve is now closed. No coolant can get through. So far so good.

    Important: Note that the coolant has nowhere to go, and is applying pressure throughout the chamber at the inlet, including on the diaphragm. Coolant seeps through the weep hole so the pressure above the diaphragm is the same. Coolant does NOT go down through the hollow tube, because it's closed by the small plug on the plunger rod.


    How the Valve Opens (Heat)

    When power is removed, the solenoid is no longer pushing the seal down, and the return spring in the valve core is trying to pull the stopper up. BUT when the engine is at speed, there's all that pressure from the coolant above the seal, which is sufficient to keep the valve closed. So what opens up the valve?

    Well, all that pressure from the coolant...pushing on that BIG diaphragm forces the LITTLE stopper open.

    But isn't the pressure above the diaphram the same as it is below, keeping the diaphragm from moving? Nope. Remember the small plug. The solenoid return spring can't lift up the stopper, but it can lift up the small plug. Why? Because it's so small (low surface area), so very little force is required. Now all that coolant above the diaphragm rushes through the hollow tube to the other side of the stopper. The pressure above the diaphragm drops, and the pressure below it pushes it up, which lifts the hollow tube along with it, which pulls up the stopper.

    Notice that there's a lot more going on to open the valve than to close it.


    VALVE FAILURE

    So what about when you're driving at 60mph and suddenly you have no heat? What usually happens is that the BIG diaphragm is ripped. So now, when power is removed from the valve and it's supposed to open, the coolant goes right through the rip up into the valve core and down the hollow tube, without pressing on the diaphragm.

    So now you have a static situation, i.e. the coolant is at the same pressure on both sides of the diaphragm. This means the coolant is NOT trying to open the valve. In fact, all that pressure is trying to keep the valve CLOSED, keeping maximum volume in that little chamber above the stopper.

    The higher the engine speed, the higher the pressure from the coolant, and the more tension is required from the return spring to open the valve. Enough pressure, and the valve stays closed.


    SO HOW DO I GET HEAT ***NOW*** WHILE I'M DRIVING?

    Do the following:
    1. Turn your heat up all the way. That way, the valve will never close, unless the maelstrom of coolant forces it shut.
    2. Momentarily put the engine out of gear and let it idle (low rpm). This will reduce coolant pressure, allowing the return spring to open the valve.
    3. Don't drive too fast, or the coolant pressure will shut the valve.

    Class dismissed.

  7. #7
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    Umm... "how" the heater valve works...


  8. #8
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    I like heater valve 101. Thanks for the info.


    -Rick ©
    BMWCCA 162285
    87 535is 5 speed Diamond Black / Black
    00 328i Sport Steptronic Steel Grey/Grey (problem child)
    87 325, The Hummer of BMW's, ???
    83 633 (gone to BMW heaven) fire is an ugly thing.
    95 540 sport (gone with DotCom)
    <P><IMG SRC="http://webpages.charter.net/rirrgang/535is/both700b.jpg" WIDTH=700 HEIGHT=205><IMG SRC="http://webpages.charter.net/rirrgang/325pics/bmwlogo.gif" WIDTH=100 HEIGHT=100></p></body>

  9. #9
    Todd Frye
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    Re: Testing heater control valve

    Well I spent last night pulling the HCV. First, I tested my voltage, as per Rick’s suggestion. Yup...12V in the off position, .003V full on. So my temp switch is good. Removed the 4 screws that secure the solenoid/etc. Took it inside to have a good look and the rubber stopper (similar to a rubber washer on a faucet valve) was in real bad shape. Kind of crumbled when I touched it. the rubber on the diaphragm appeared to be in decent shape. Now that my curiosity was satisfied and I knew the rubber parts needed replacement, I ordered on line and reassembled the valve with the bad parts. HOWEVER...even after taking heater valve 101, I still don't understand. Here's why. After reassembly, I crank it up to check for leaks. All is well. I turn the temp switch to cold and I get cold air. I turn the switch to hot and I get hot air. (This is at idle). Take it for a test drive...as soon as I reach speed, cold air. Initially, I assumed higher RPM caused higher pressures in the cooling system, which possibly overpowered the internal operations of the "valve action" in the HCV. Then the thought of testing for heat changes while static (car stopped in the driveway) came to mind. At idle with hot air blowing, I brought the RPM's up through 2, 3 and 4K. Still blowing hot when not moving at 4K RPM. Took it out on the road, and the temp dropped to cold. The only difference was that the car was moving. Does air flow factor into any of Heater Valve 101? I'm pretty sure the new part is going to fix things; I just wish I knew why. Thanks all for your help. Each suggestion shed a little more light on the subject. Todd Frye

  10. #10
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    WAG: Car moving vs. high rpms at rest...

    The only things I can think of that differentiate moving vs. at rest at the same rpm as far as the heater valve is concerned are vibration, and airflow around the engine. Airflow around the engine would reduce the coolant temp a little, causing the thermostat to not open as much.

    In other words, when you're not moving, the water pump is circulating coolant through the engine and the radiator. When you're moving, it's still doing that, but more through the engine and less through the radiator. This may result in more pressure at the heater valve when you're moving. This additional pressure could force the valve shut if the diaphragm is torn.

    That's my WAG, anyway.

  11. #11
    Todd Frye
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    Re: WAG: Car moving vs. high rpms at rest...

    To bring closure to this subject, I just put in the new (Bosch) mono valve kit and we're back in business. Mornings have been 7 degrees (for 3 days so far) so I am so thankful for this problem being fixed. Those of you with E-28s with 120 K or better on the clock may want to replace this when it's not so dang cold out, as a” preventive maintenance” project. Thanks everyone for your ideas. Todd Frye

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    Awesome Ron, I have this exact symptom on

    the 5er I just purchased. The heater worked perfect on what is now my parts car and since I don't think it was damaged when my wife installed the Suburban hood ornament should be good to swap it over to the "new" replacement 5er...

  13. #13
    FL Bimmer
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    Heat/ defrost or feet ok dash vents it's cold, why

    My heat works fine in defrost or at the floor vent, but when I put the dash vent to open, cold air comes out why? History" I just replaced the heater blower, it was stuck.

    Also have several wires under dash that do not connect to any thing. Also I found this black box thing ,with two silver plugs-1\2 inch size on one side, under the dash/driver kick pannel
    anybody know what it is.

    Thank You, Gecko1

    P.S.1988 528e super eta 5 speed not sport. no history. solid car. \Bought 3weeks ago, Help.


  14. #14
    TJ
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    that's a "feature" on E28s

    in other words, that's normal. You don't get heat out the front of the dash, only on top or down below.


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