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No voltage output from automatic temp control to monovalve

652 Views 20 Replies 4 Participants Last post by  John350
I have a 1990 560 SEC (owned since new w/45K mi). I can no longer get cabin heat (AC works normally). Upon investigation, I find that there is no voltage output from the automatic temp control (ATC) to open the monovalve. When I disconnect the output from the ATC to the monovalve and apply battery voltage to the valve, coolant flows through it normally and the cabin heats up. There is no temperature control as the ATC modulates flow through the monovalve with on/off voltage to the 'valve (and that can't happen with the output from the ATC to the monovalve disconnected).

My inclination is to replace the automatic temp control. But before I do that, I would appreciate input from this forum on my diagnosis and ways to fix.


Fred Hudspeth
Tyler, TX

1990 SEC (45K mi.)
2104 E550 (17K mi.)
1982 300 SD (150K mi)
(all owned since new)
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I'm confused by your description of the behavior of the monovalve.

If you unplug the monovalve the valve should be open and you should get full heat. If you get heat at idle but not at highway speeds then the monovalve is bad.

If you get no heat with the monovalve unplugged at idle then either the monovalve is more bad than I've ever seen or heard, or you have a different problem.

For the controller to fail and cause no heat it would have to apply 12 volts to the monovalve. Normally it applies pulsed 12 volts to the valve; the length of the pulses controls the flow rate through the monovalve and thus the temperature. Depending on how your volt meter works, it might not read the voltage pulses correctly.

If you apply 12 volts directly to the monovalve it should shut off completely and you should get no heat.
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Except for the constant 12VDC output to the monovalve
OK, I think you may still be on the wrong track. The 12 volts to the monovalve is always on. It comes directly from the fuse box. The monovalve is controlled by switching the ground on and off. If you put a volt meter from the negative side of the valve (should be a yellow wire with a green marker) to a chassis ground, it will show 12 volts when the valve is commanded open and close to zero when the valve is commanded closed. Another way is to put the volt meter across the two terminals of the valve. In that case, if it shows close to zero the valve should be open, and if 12 volts it should be closed.

Keep in mind, the valve should be switching on and off over (IIRC) a 5 second cycle. If you see the voltage switching on and off, the controller is trying to manage it.

Keep in mind, to measure the voltage you either have to use both terminals on the valve, or the negative terminal on the valve and a chassis ground. Measuring from the positive side of the valve to ground should always show battery or system voltage.
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If there was (or is) an OEM manual on the HVAC system for this version of the W126, I don't have it.
Start here.


no resistance through solenoid
I don't understand what you're saying. I would interpret this to mean you unplugged the monovalve, then using an Ohm meter on the two terminals of the monovalve you got a reading of zero. This would indicate a dead short in the coil, not a good thing.
"If you see the voltage switching on and off "...It does this for about 30 seconds but then holds at 12.9VDC (battery voltage), thereby keeping the monovalve closed.
Again, how are you measuring this? If you say "12 volts on it" that means you put the red probe on the (+) of the monovalve (the side connected to the fuse, should be black/red/violet wire) and the black probe on a good ground. This is the expected reading any time the key is in the "Run" position. If you say "12 volts across it" that means one probe on each of the two terminals of the monovalve (red to (+)). This is the reading that should switch on to off and back to on over a 5 second period. Constant zero volts across it means the valve is not being grounded by the controller (or has no power, repeat test above for "voltage on") and should be open (full heat). Constant 12 volts means it is getting both power and ground, and should be closed all the time, and no heat.
ACC is not controlling the monovalve as there constant 12.9VDC to ground after about 30 seconds
There should always be 12 volts from the positive terminal of the monovalve to ground. Are you measuring on the correct terminal, or is there really no resistance in the monovalve coil? If you measure from the negative side of the valve to ground then that indicates the controller is not supplying ground. Unplug the wires from the monovalve and measure the resistance. I'm not sure what it should be, but I would expect it to be something on the order of 15 to 70 Ohms.

If the voltage across the monovalve switches on and off about 6 times over a 30 second period, this is normal operation. If it stops after 30 seconds then either there's a problem with the controller, or, maybe it's detecting a fault, like the monovalve or the aux pump drawing too much current, and the controller is shutting things down.
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there was continuity (i.e., 50+ Ω )
OK, so that's 50 Ohms of resistance, not "no resistance". Sounds about right.
I measured "switching" voltage at negative terminal of monovalve.
Where was the other probe of the volt meter? Voltage is the measurement of the potential between two points and without knowing what the other point is it makes no sense.
The voltage at the negative terminal of the monovalve was going to ground internal to the ACC and stayed there,
Then I would expect the reading to be zero. Answer the question above and we'll go from there.
Yes, if there is 12 volts on the hot side of the monovalve, and zero volts on the negative side then the valve is being grounded, most probably (since it works for 30 seconds) by the controller. One last thing to try is observe the behavior for the first 45 seconds when the temperature control is on max heat.
Does it still go through the 30 seconds of "normal" behavior?
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