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No voltage output from automatic temp control to monovalve

647 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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Thanks - that is an excellent point. Coolant flow, however, was likely just the reverse of what I described in applying 12V to the monovalve. I thought I might have had that backwards when I wrote it (I did the diagnosis about 6 months ago) . I had a known-good monovalve and installed it. Testing it with alternate applications of 12VDC had the effect of restoring cabin heat as with the monovalve in place when first diagnosing the problem.

If there is, in fact, no monovalve problem, that takes me back to suspecting that the automatic temp control module has failed, at least in the heat mode. 'Would appreciate any experience by owners in diagnosing the ATC control before replacing it (i.e., disassembly, visual inspection of electronics, etc.)
Thanks, John -

Based on your comments and those of other owners on this forum, I need to
1. recheck the voltage output from the ATC module when "calling" for more heat. I had the mistaken opinion that 12VDC output from the ATC opened the monovalve.
2. re-check the operation of the monovalve (a) with the connector "unplugged" and (b) with local 12VDC applied.

I considered it improbable that the installed monovalve and a "known good" one are both defective. That may ultimately be an error in my logic.

Thanks to owners who commented on this. It has had the effect of putting on hold my replacement of the ATC without further diagnosis. I will report back to the forum after another run at solving the problem.

Fred (aka pengineer)
Now for the "rest of the story"...

With the harness disconnected (i.e., 0 VDC) at the monovalve, the cabin floods with heat. In checking the output voltage from the automatic temp control module, it is a constant 12VDC, regardless of the setting at the temperature "wheel". As noted by John350, a constant 12VDC keeps the monovalve closed.

Except for the constant 12VDC output to the monovalve, all other aspects of the ATC module appear to work as designed (AC is modulated properly, diverter doors' operation is correct, etc.).

This, to me, suggests that the fix is repair/replacement of the ATC module. If that is a valid conclusion, 'would welcome experience of the owner community, if any, with ATC disassembly, visual inspection of electronics, etc. before ordering a replacement part. The OEM part appears to be available at, but at a cost of about $600. There are many sources for rebuilt ATCs starting at around $150 but absent favorable experience in the owner community with rebuilts, I will seek the lowest cost of an OEM replacement.

Fred (AKA pengineer)
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Thanks much, John -

Re "Measuring from the positive side of the valve to ground should always show battery or system voltage" - and it does.

Re "The monovalve is controlled by switching the ground on and off" - did not know that!

Re "...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." - I will check for that. 'Hope it does as that would seem to eliminate the controller as the source of the problem. 'Will advise findings...

'Appreciate your staying with me on this... If there was (or is) an OEM manual on the HVAC system for this version of the W126, I don't have it.

Received, John -

That will help considerably!

Thanks, Dave - as a petroleum engineer, I am not highly competent in diagnosing/repairing electronic hardware. I, however, find the electronics fascinating and always make an attempt at repairing failed hardware before replacing. Often, the cause of failure is highly visible (or detectable with a multimeter) and repairable.

With the W126 HVAC manual furnished me last night by John350 on this forum, I hope to be able to prove (or disprove) the failure of the ATC module. If indications are highly probable that the module has failed, I will remove, disassemble, inspect and attempt to repair any identifiable causes of failure. If I am unable to salvage it, I will contact you about your buyer possibly providing me the one you repaired.


PS - 'beautiful SEC. Front-on, looks identical to mine, except for the 'Jersey plate (I have the Texas historic plate on mine but required only for the rear)
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'Have done further diagnosis of the monovalve. It "passes" all of the electrical tests (i.e., no resistance through solenoid, audible evidence of valve switching on/off with 12VDC applied and touching negative terminal of valve to ground alternately, etc). And - when 12VDC is removed (and valve is open), cabin floods with heat.

Re "...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." It does this for about 30 seconds but then holds at 12.9VDC (battery voltage), thereby keeping the monovalve closed. See notes on circuit diagram attached.

Conclusion: the ACC is not controlling the monovalve as there constant 12.9VDC to ground after about 30 seconds (has effect of keeping valve closed/no cabin heat).

I welcome critiques of my diagnosis and conclusion...


Thanks for your feedback, John -

Re "no resistance through solenoid" - there was continuity (i.e., 50+ Ω ).

I measured "switching" voltage at negative terminal of monovalve. 'Switched 5-6 times over approx. 30 second and then held steady at 12VDC. The voltage at the negative terminal of the monovalve was going to ground internal to the ACC and stayed there, keeping the monovalve closed.

As a further check, I removed the connector from the monovalve. With ACC on/engine running, temp wheel set at 80F, I measured voltage from + side of connector to - side of connector. Result was same as with connector attached to monovalve.

I discounted the auxiliary water pump as a contributor to the problem due to so little use, its undisturbed state and difficulty of accessing.

I replaced the valve assembly in the monovalve about two years/1000 mi. ago. George Murphy/Performance Analysis Co. furnished the replacement assembly - Bosch 000 835 06 44, about $80. 'Found that I didn't really need it as the OEM part was still OK.

Except for the monovalve service/diagnosis, nothing under the hood of the SEC has been disturbed since new/45K mi. (other than that associated with scheduled maintenance).

'May not have yet "touched all of the bases" but signs point to a malfunctioning ACC. I am prepared to remove, inspect electronics/mechanicals, repair obvious causes and, if I cannot repair, replace the ACC. 'Not really a high priority repair as need for cabin heat is minimal in this part of Texas. But as a "purist", 'want it to perform as designed...

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Re "Then I would expect the reading to be zero" - upon closer examination, 'agreed with your logic. So, I rechecked my connection of ta lead wire from the negative terminal of MV (very tight quarters!) to measure voltage. Because of the interference of the lead wire with connector firmly seating at the negative terminal of the MV, I got an erroneous reading of the voltmeter-to-battery ground yesterday. After reworking my leadwire connection at the MV negative terminal, the voltage was/is 0 VDC (except for the first 30 seconds of the test when the voltage was switching between 12.9 VDC and 0-4 VDC). After switching to constant 0 VDC, current was then flowing through the MV coil to ground, internal to the ACC and keeping the MV closed to coolant flow.

My apology for the mis-diagnosis as a result of faulty VOM readings...

Re "observe the behavior for the first 45 seconds when the temperature control is on max heat. " - 'have done that - several times and at longer time intervals. There is never any cabin heat, suggesting that with 0 VDC at negative terminal of MV, the MV current is going to ground through the ACC and keeping the MV closed.

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