[pengineer]

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.

Thanks,

Fred Hudspeth
Tyler, TX

1990 SEC (45K mi.)
2104 E550 (17K mi.)
1982 300 SD (150K mi)
(all owned since new)

 

[MBL87560SEC]

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.

Thanks,

Fred Hudspeth
Tyler, TX

1990 SEC (45K mi.)
2104 E550 (17K mi.)
1982 300 SD (150K mi)
(all owned since new)

I'm with you Fred.. save ONE important detail: Usually, when one disconnects the power connector from the monovalve one should/will get unregulated FULL hot heat output..

With your description in mind here, seems to infer the valve works [opens] when external voltage is applied.

Sooo All that said.. If your initial complaint is no heat at road speed. .. OK at idle.... Rinse & Repeat....... That is classic failed monovalve insert diaphragm.

 

[pengineer]

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.)

 

[John350]

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.

 

[pengineer]

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)

 

[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 mboemparts.com,

1990-1991 Mercedes-Benz Dash Control Unit 126-830-09-85 | MB OEM Parts

1990-1991 Mercedes-Benz part # 126-830-09-85 - Dash Control Unit

tinyurl.com

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)

 

[John350]

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.

 

[pengineer]

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.

Fred

 

[John350]

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.

 

[pengineer]

Received, John -

That will help considerably!

Fred

 

[Gogi]

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.).
@pe
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 mboemparts.com,

1990-1991 Mercedes-Benz Dash Control Unit 126-830-09-85 | MB OEM Parts

1990-1991 Mercedes-Benz part # 126-830-09-85 - Dash Control Unit

tinyurl.com

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)

Fred,
Late last year I sold my 91 560sec. When I turned over the keys, manual, all maintenance dealer purchased parts receipts I also gave him a used ATC/PBU (push button unit) that I had repaired by flowing a deteriorated solder joint. The AC in that car was functioning normally. The car had absolutely zero problems but, being he was a young kid who flew in from Minneapolis I thought it would be a good gesture.

Instead of letting some parts changer replace the PBU just repair yourself. I feel you are confident and capable to carry out this repair. Engineers rock!!

P. S. I could text him and see if he will send that repaired unit to you.
i know he has it and it’s repaired by me with care.
Dave. (AKA) Gogi

 

[Gogi]

 

[pengineer]

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.

Fred

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)

 

[pengineer]

'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...

 

[John350]

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.

 

[pengineer]

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...

Fred

 

[John350]

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.

 

[pengineer]

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...

Fred

 

[John350]

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.

 

[pengineer]

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.

Fred