Hot air only sometimes, but monovalve seems fine

[Machiel]

Hi,

My name is Machiel and I am from the Netherlands. Since 2013 I have owned a 380 SEC from 1985. I hope my English is good enough to explain the issue I have since a couple of months. I have learned a lot already from this forum, yet I still have some questions left.

Let's start with the symptom. Sometimes, my heater only blows hot air, regardless of the temperature or push button setting. With the AC on, the hot air is cooled a little, which is the only way to prevent you from a heat stroke. I can unfortunately not force the heater to generate its problem. It appears (and dissappears) randomly.

I have removed and checked the monovalve and it seems really fine. You can manually move it up and down smoothly and the rubber has no cracks or something. Also the resistance level through the coils is ok (15 Ohm). My (preliminary) conclusion was that the monovalve is unlikely to be the culprit.

I also measured the voltage to the monovalve which was 12 volt. Unfortunately, when I did the measurements the heater was working fine. I could feel it vibrate / pulsate at intermediate temperature settings.

After studying the (attached) diagram, I concluded that it cannot be the temperature control unit that causes the problem. If the problem occurs and I set the temperature dial to MIN, the climate control unit (CCU) should bypass the control unit by making a direct ground connection. But the air remains hot in that case.

• 1. Is this correct? Can I ignore the temperature control unit?

If indeed the connection to ground in MIN is not well established, I reason that it is either the CCU that is corrupt or the temperature dial. I have read about bad soldering in the CCU. But before attempting to remove and opening up the CCU, I was wondering whether the temperature dial perhaps directly connects the monovalve to ground (G102 in the diagram).

• 2. Does anybody know if this is the case? Should the temperature dial in the MIN position establish a direct ground to G102, or is that signal processed through the CCU?

If so, I can perhaps first check the temperature wheel only. If this occasionally sends MAX regardless of the actual setting, this may also explain why I got hot air at all settings if the problem occurs.

All help is highly appreciated!

[Sugar Bear]

Welcome Machiel,

Your English is superb as are your diagnostic skills. Sorry that I cannot answer the specific circuitry questions; however, the symptoms point toward a defective monovalve. I've read that the BMW monovalve insert is available and works well. I've also read that aftermarket valves aren't reliable.

Good luck and please post your findings.

[Diseasel300]

If the push buttons are in any position other than "off", the climate control system is in control of the heater valve. The "Off" position you see on what appears to be a rheostat is for the pushbutton switches. Of course when the climate control is switches "off" the fan doesn't blow either, making testing further a bit more difficult.

Typically when the monovalve cartridges fail the diaphragm tears or the rubber gets very hard so that they are difficult to move. If the diaphragm is intact and feels very soft and supple (and you can move it freely) it likely isn't the problem here.

A more common problem is the silver "Tempregler" box that is the "brain" of the climate control system. There are a handful of electrolytic capacitors in it that leak and can corrode traces on the printed circuit board. When they dry out and damage the board, the climate control system can develop a mind of its own. The pushbutton unit is just that - the pushbutton unit. It contains the switches for the different modes, a couple of diodes, some relays, and that's about it. All of the controls are in the silver box mentioned above. Typically mounted behind the glovebox somewhere in combination with a black "Relay" box.

[Machiel]

Many thanks! So I misinterpreted the diagram, I guess. I noticed a direct arrow from the MIN of the temperature dial to G102 (in the center of the diagram), so I assumed that this would establish a direct connection to ground. Just out of curiosity: what does this arrow indicate?

[Diseasel300]

That arrow and that "dial" are the pushbuttons, NOT the temp wheel. The temp wheel is the "curly" looking arc that says "temperature dial" and is linked to the arcs on either side that say "min max" as indicated by that dashed line. The pushbutton switching is the arc/dial at the top left and linked to the one bottom right that say "Off EC NORM T/V DEF".

Sometimes these diagrams can be less than clear, drawing individual pushbuttons as a rotary switch tied to a second rotary switch (that actually is a rotary switch) doesn't help clarification.

[Machiel]

My apologies for the possible confusion, but I actually meant the curly looking arc. I drew a red 'circle' on the attached diagram for clarification. That one also has a direct arrow from the MIN position to G102.

But perhaps I am starting to understand now why it is connected to ground. I suppose the temperature wheel is a kind of variable resistance. And in order to establish a (closed) circuit through this resistance it must be connected to ground. The value of the resistance is then sent to the "tempregler" that in turn controls the monovalve.

And yes, then I also understand what you mean by the control unit being always in control of the monovalve. So I better replace the tempregler first then.

Thanks!

[Diseasel300]

The temp wheel only gives a setpoint reference to the silver box to control temperature, it has no connection to the monovalve. Unless the pushbutton is "Off", the silver box provides a transistorized ground to the monovalve to close it.

Your monovalve control circuit goes from the monovalve, into the pushbutton unit (PBU) through Pin 4 on PBU Connector 1, through the mode buttons, out of the pushbutton unit through Pin 4 on PBU Connector 2, into the silver box (SB) on SB Pin 8 (marked as heater water valve control), through the control transistor, and to ground via SB Pin 6 to G102.

Only in the "Off" position, is the monovalve directly grounded to G102 through Pin 3 on PBU Connector 1.

You can test the function of the monovalve by removing the control plug from it, and directly powering the coil with 12V. If the heat flow stops, you know immediately that the control unit is your fault. Be aware that the heater core can take a couple minutes to cool off when you energize the monovalve.

[Machiel]

Great information! Thank you so much!

I have found another (good working) temperature control unit (the silver box) that I can use for a while to see if this wil solve my problem. I will update this thread with my findings.

[Diseasel300]

If the new "good working" silver box fixes the problem, consider sending off the original one for repair/rebuild. Not sure if there's a competent rebuilder in Europe, but on this side of the pond it's hard to beat Kris Rose (Forum member James Dean) Overview — Cruise Control Repair

Age is not a friend of those boxes. Age/heat/use degrades the capacitors and they leak. It seems to be a poor choice of manufacturer of the component itself since most capacitors just get weak or dry out without leaking on the circuit board. A working box pulled from another car won't be a "working" box for very long as a result.

Chances are good that the system has never worked properly since you've owned the car (We've owned an '83 500SL since 1998 and it had never worked properly). You'd be surprised how well the system works when working correctly.

[Machiel]

The unit I will get is indeed rebuild. Part of the deal is that I will swap it for the one I have now, which will then be rebuild as well.

So far I have been impressed by the working of the climate control of my Benz. In winter you get warm air very quickly and in summer the AC has proven to be incredibly powerful. Coincidentally, like you I also own a 1994 BMW E34 525iA (my daily) and its climate control is no comparison to the one in the Benz.

The AC of the Benz was rebuild about five year ago. The original compressor was rebuild, and the condensor and dryer was renewed. Both compressor and condensor were leaking.

[Diseasel300]

I was referring to the automatic temperature regulation portion of the functionality, not raw performance. With everything working properly, the automatic climate control is just about clairvoyant. Starting to feel a bit chilly? Well it'll ramp the fan down for you just as you're about to reach for it. Feeling a bit warm? It'll ramp the fan up and cycle off the heater core just as you're about to reach to turn it up. I've found that I never touch the temp dial, it just does its thing without me asking it to. Then I get annoyed when I drive another car and it isn't automatic!

The first-gen W126 A/C is "ok" in our climate, the second-gen will blow you out of the car. You can just about hang meat in my 350SD on a super hot day, it handily has the coldest A/C of of any car I have access to.

The E34 is a close second. They have a pretty powerful A/C system, but like everything else BMW, they're picky about how well they're maintained. Replacing the capacitors on the IHKA down by the cabin filter makes a huge difference in how predictable the temperature regulation is. When they get flaky, they tend to randomly cycle hot water through the heater core. Also make sure the auxiliary fan is running on low speed when the A/C is supposed to be running, if it isn't, the performance of the A/C will suffer at idle or slow moving traffic. The resistor is a common failure part on them (I just replaced mine a week or two ago).

[Machiel]

You are right. It is indeed not only its raw performance that impresses me, but also the way it regulates the temperature inside. Great engineering.

My E34 does not have the automatic climate control, I think it has the IHKR type 3. One of the previous owners had the airconditioning built in afterwards, at least the AC is not mentioned on the original options list.

I usually have the fan running on low speed, but even on high speed I am not impressed by the amount of air coming out of the system. I have replaced the interior filter some time ago, but that did not make a noticeable difference. Neither does closing the lower and middle slide levers. So even if I direct all the air to the windshield, the flow of air is rather moderate when the fan is on max. It is not a big issue, but I suppose this is not how it should be.

[Machiel]

Yesterday I got the rebuild control unit. On my way back home the problem "fortunately" (it was about 90F outside) occurred again. I left the motor running and swapped the control unit. Result: no result. The heater kept on blowing hot air only, albeit slightly chilled by the AC.

Only after some random pushing on the buttons of the PBU the heating started working properly again. This makes me conclude that the problem is in the PBU or in its connections. My guess is that the PBU sometimes sends a wrong signal to the TCU that the driver demands full heat. Does that makes sense?

[Diseasel300]

The PBU is just several switches and a rotary potentiometer (temp wheel). It isn't like the 1986+ cars where all of the logic is in the PBU.

You need to definitively troubleshoot the monovalve. Get the car to act up, measure voltage on the plug to the monovalve *WITH IT STILL CONNECTED TO THE COIL*, you'll have to back-probe the terminals or use wire-piercing probes. It is extremely important that you measure voltage with the plug still attached to the coil. It is a negative switched circuit using a transistor, a complete circuit is required to have any meaningful value to the voltage you measure.

[Machiel]

I was not able to take the car out today, so I took the monovalve out in stead, took it home and connected it to a 12 volt power source. You were so right!

It seems as if the magnetic power is just not strong enough to overcome the force of the spring. The valve comes out a little, but that's all.

In this video below you can see what happens (i.e. after a little while, apologies also for the terrible focus). That cannot be right, can it?

https://youtu.be/FP1pMfgnBP0

If you move it by hand, it goes up and down smoothly. I measured the resistance in the coil, which read 14,8 Ohm.