Binary interior temperature (heat) control - suggestions?

[MarkD in NJ]

I'm driving a 1984 533i with binary heat temperature control. With the temp knob full left (lowes temp), I get cold air through the heat vents. Any other setting provides full on heat.

I've searched and found and will also go through the troubshooting perscribed in the Bentley manual.

Anyone ever have and solve this problem? If so, what was your solution? Most of the posts I found were from folks who had the problem, but had not solved it.

Is there any way that a rebuild of the heater control valve will solve the problem?

I have a running parts car and I can swap some of the easier components over to the 533: Temp control unit in the dash, interior temperature sensor, heater control valve. How hard is it to reach the temp sensor in the heater core? Obviously, I'm going to try to diagnose the problem prior to swapping parts.

Thanks in advance,
Mark

[stuartinmn]

Heater valve.

I rebuilt my heater valve several times to cure the typical no-heat-except-when-stopped problem, but after the last time I did that the heater went into the same binary mode you're experiencing. I bought a whole new valve, not just the rebuild kit, and it solved the problem.

[a]

Uhhmn, there are differences in the HVAC between a 533 and a 88 528e. The later model HVAc is motor operated not cable operated like the earlier models. FWIW, thats mostly how I choose to operate the heat. Full on, vents shut, no blower= low heat, Full on , vents open, no fan = medium. Full on, open vents, fan on = high. Usually thats for defrost.

[RonW]

Two possibilities come to mind, and a bad heater valve isn't one of them.

1. The temperature sensor is unplugged, which would make the car think the interior temperature is very cold. The temperature sensor is mounted on the driver's kick panel, near the hood release lever. The sensor has a 3-pin electrical connector.

2. The temperature control unit has a bad transistor. The heater valve is a solenoid that is energized to turn OFF the heat. When you turn the knob all the way to the left, an electrical switch in the knob grounds the solenoid, which energizes it and turns off the heat.

When the knob is in a middle position, a transistor in the unit does the job of powering the heater valve, and the control unit turns the transistor on and off to open and close the valve and maintain temperature. If that transistor is fried (which happened to me once when the solenoid coil went bad) it won't close the valve. I got a replacement transistor at Radio Shack for a few dollars, but I had to bend the legs around to match the old transistor.

[Mike W.]

Ron, I've got the same heat situation, all or nothing, but A/C works just fine, modulates well and all that. Would it still be the module or would you suspect something else. The temp sensor in the lower dash cover is there and hooked up, but I haven't tried to verify it's operation. Again, the puzzling part to me is the A/C works like it should.

[MarkD in NJ]

[patrick]

[MarkD in NJ]

For the record, I tested the interior temperature sensor and heater core temperature sensor and they both appeared to be working properly. I rebuilt the heater control valve and verified that it was working properly.

Last night I replaced the temperature controller in the HVAC control panel with a known good one from my '88 528e and that solved the binary heat problem - now the heat works as advertised. The job took about an hour.

I replaced the controller through the radio opening - removed the soon to be replaced factory radio, disconnected the wiring from the A/C switch and fader and removed both of them from the panel. Then I cut the zip tie that holds the HVAC wiring harness to the back of the temp controller and pushed all of the wiring up to the right inside the space behind the panel. I used a small 2" long phillips screw driver to remove the three screws that hold the controller to the panel and removed it through the radio opening. I installed everything in the reverse order that it was removed. You cannot see the screws or where they go when replacing the controller, so it helped to have a spare HVAC panel and the old controller to use as a guide. I 'glued' the screws to the end of a jewelers' screwdriver with a small dab of 3M body seam sealer to help get them back into the panel. Having small hands was a definite advantage.

Mark