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Old 05-30-1999, 08:20 PM
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I just bought a mint 1987 260E in South Carolina. It is freaking hot down here.

The air conditioner compressor and system works well (compressor fine - makes cold air and fan blows), except for this little really stinking annoying gremlin:

- Frequently, the thing simply decides to blow blazing hot air from the vents. It comes on fast, and only resets when it is ready.

In my limited time with the car, I have successfully reset the condition by monkeying with the controls (turning on/off) and/or restarting off the car.

The compressor still runs through all this and the lines indicate all is well with freon flow. (One hot, one cold).

Disassembly of control panel reveals nothing obvious. . . No sign of degraded vacuum lines under the hood or behind the control panel.

Please help!!! Take a guess... I'll try it.


Old 05-30-1999, 09:21 PM
MB Medic
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On the 124s the heater valve is electrically controled thru the pushbutton assembly. The 12v signal is constant but the ground is the controler. The valve needs both signals to close off the flow of hot water in the heater core. So you need to know if your power and ground signals are both present at the time of the problem. To check this unplug the heater valve element (located next to the battery, the brown valve with heater hoses going to it) and put the negative probe of your meter in one socket and the positive probe in the other and the result should be battery voltage. If battery voltage is not attained then you have a control problem or wiring problem. If battery voltage is present then the problem would likely be in the heater valve itself.

Hope this helps,

MB Medic
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop
Old 05-31-1999, 02:04 PM
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RESULT: I checked it as you noted. Thank you for the response.

There is a valve near battery, two prong connector. Unpowered it is open - water can pass. Powered with 12V directly, it closes properly. There is also a small pump further forward in engine compartment. It does pump water through the valve quite well when powered.

My problem seems to me electronic in nature, not with an electromechanical valve. The problem is intermittent.

When the AC decides to trow out heat, i can check the power at the valve connector and sure enough, it is zero. at the same time, the power at the little pump is 12V.

Is there a temperature probe in the car somewhere that may tell the controller that it suddenly got way too cold in the cabin? Where are connections that I might check for this. Is there a procedure that you know of to detect which electronics may be bad?

Old 05-31-1999, 09:16 PM
Posts: n/a
Now when the failure occurs, you have to determine if you are losing the ground or the power side of that 12v signal that you didn't have.
Split the circuit by using a known good ground for one of your voltmeter leads and probe the 12v signal with the other.
Check the ground by using a known good 12v source for one of your voltmeter leads and probe the ground signal with the other.
Old 06-01-1999, 11:59 AM
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I am losing the ground. This, I am told is the "switched" part of the circuit. I have temporarily jury-rigged this by jumpering the ground pin on the valve directly to the battery ground.

It works this way - prevents hot coolant flow - but whenever the system decides to go screwy again the aux pump runs. I gather the valve and pump work inversely to each other. One is on while the other is off.

Also, when the system goes screwy, the automatic cabin fan regulation is lost. Just stays on high and freezes you out of the car.

Now that I have lived with the problem for a while, it seems to me to be a factor of time... After about 15-20 minutes of operation, the screwiness happens. All is fine for the first 15-20 minutes of every day. After the system is allowed to rest (cool??) we are fine for another 15-20 minutes. Something electronic must be "overheating" and quitting, don't you think.

The good 12V on the heat valve with the rigged, good ground leads me to conclude that the valve is good.

Could this be a faulty temperature probe up by the sunroof switch? or the control unit itself? How can this theory be tested?

Thanks again.

Old 06-01-1999, 09:11 PM
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Have you had the pushbutton control unit replaced? I have seen the exact same symptoms cured by the replacement of said part. (After reading the good advise of RBM and evaluating the results)


Old 06-01-1999, 10:06 PM
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The in-car temp sensor is mounted next to the sunroof switch, it is responsible for maintaining the correct tempature inside the car. Whenever there is a short or open circuit in any of the system it defaults to the heating mode. ie; cold in Germany. The in-car sensor is a common cause of of this type of problem. Also the aux. water pump can cause this.
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