Odd 1987 300SDL mono valve behavior - PeachParts Mercedes-Benz Forum

Ether · #1 03-02-2014, 08:37 PM

My SDL developed an intermittent heating problem this Winter as in there would intermittently be no heat.... Turning the car off and then back on will sometimes result in coolant flow, sometimes. Simply turning the CCU off and then back on will never resolve the problem.

Voltage to the mono valve seemed to be correct but pulling the connector allowed coolant flow and heat galore. Did some credit card troubleshooting and installed a mono valve repair kit. No change. Hmmm, gotta be a flaky CCU, right? So I performed some more credit card troubleshooting and swapped out the CCU with a new rebuilt from Programa. Nuttin. Same intermittent issue.

Well, high time I did some real investigation and find out what's going on.

Thought I may have an aux water pump issue, but the fact that pulling the valve electrical connector results in getting coolant flow would seem to eliminate that I think. I noticed today that when in heat mode the voltage to the connector doesn't quite drop to 0 volts but about .6 volts. I surmise that this is probably correct as that would be about the voltage drop across a transistor to ground and certainly enough to release the valve. I have not been able to capture what that reading is during one of the times when the heat works normally to see if it is different.

So I have some data but I still can't put together an explanation for the intermittent nature of this problem. If anyone has ideas or experience with this type of issue please weigh in and tell me what I'm missing.

tangofox007 · #2 03-02-2014, 09:55 PM

Quote:

Originally Posted by Ether

Simply turning the CCU off and then back on will never resolve the problem.

The CCU is never completely "off" if the key is "on."

Any possiblilty of swapping in a known good temp regulator?

Ether · #3 03-24-2014, 10:53 AM

Still having this annoying intermittent heat issue. A longer time frame with the mono valve electrical connector unplugged has shown that the intermittent heat still occurs. So this is puzzling.

A disconnected (electrically) mono valve would nominally result in full coolant flow to the heater core all the time. However, the continued intermittent heat indicates something odd is still going on. I have a new aux water pump that can be installed but I'm not convinced that is the issue, as other than low flow at idle, I should still get coolant into the core regardless of aux pump status right?

Perhaps my recently installed mono valve repair kit is faulty and the culprit of my woes. I've come across several threads lamenting the quality of the MTC product. I reckon I will open it up and take a look as soon as I get a chance.

Zacharias · #4 03-24-2014, 11:51 AM

Have you found any correlation to outside (ambient) temperatures and this behaviour?

Ether · #5 03-24-2014, 12:44 PM

Quote:

Originally Posted by Zacharias

Have you found any correlation to outside (ambient) temperatures and this behaviour?

No, it seems to be completely random.

Although, as I am thinking back on the saga of this issue and gathering information from the driver of this car (wife) I'm beginning to develop a hypothesis.

Since I have owned this car (4 years) there has always been a small leak in the heater core that would require me to occasionally top off the coolant reservoir over the course of the Fall/Winter. Once warm temperatures return the level will remain constant all summer until the heat is required again. This has never been a problem since the level drops fairly slowly and I've been pretty diligent about keeping it topped off. That is until this Winter when my wife has been driving the car almost exclusively.

It seems that she ignored the low coolant light for a while and sometime in February made a stop at Autozone and had them top of the coolant with some 50/50. Interestingly, when I took a look at the coolant jug there was less than 1/2 gallon remaining so the level had obviously dropped rather low.
I'm now wondering if it dropped low enough to introduce some air pockets in the coolant system and just maybe that would explain the intermittent problem I'm having.

This is admittedly a bit of a swag but I've run across some threads where air in the system has been proposed as a cause of similar issues, so maybe???

michael william · #6 03-24-2014, 10:56 PM

Ether - I'm having the EXACT same problem with my 87 300sdl. Exact. I replaced the mono valve also (the old one did have a tiny tear in the the diaphragm). And the same thing happened.

I pulled the plug and drove it, expecting full heat all the time, but it went right back cold (at about 2200 rpm for me). I was thinking the CCU because when it is extremely cold outside - below zero- and I start the car, the CCU clacks and clicks and the Recurc button lights up and off and the heater fan starts and stops till the car warms up.

I'm willing to buy a new one, but I would really like to have a good idea that that is the problem. My antifreeze appears to be full - I've even put the car on an incline to see if I could get things to move, but still nothing.

Hopefully someone here can help us both. I will be watching this thread!

Good Luck

daw_two · #7 03-26-2014, 11:31 AM

There's a lot of this going around. Myself and another forum member in the Memphis area are both working on what sounds to be the same issue.

Vstech --- I sure could use a w126 monovalve assembly from your parts car.

funola · #8 03-26-2014, 11:50 AM

Quote:

Originally Posted by Ether

....................... I have not been able to capture what that reading is during one of the times when the heat works normally to see if it is different.

So I have some data but I still can't put together an explanation for the intermittent nature of this problem. If anyone has ideas or experience with this type of issue please weigh in and tell me what I'm missing.

Make a small 12V light bulb with light gauge stranded wires, unplug monovalve connector, wrap the wires around the monovalve terminals, add some tape so it won't unravel. Put connector back on, run the bulb out the hood so you can see what's going on with voltages to the monovalve.

Edit: Very important to make sure the wires don't unravel and short out, which will blow CCU trace.

eatont9999 · #9 03-26-2014, 12:22 PM

My grey car has a new Bosch monovalve and I have never had a problem with it. The silver car got a cheaper MTC monovalve last fall and it worked for a little while but now I have no heat in that car at all. Even if I unplug the monovalve, no heat. I did do some cooling system maintenance a few weeks ago. There may be air in the heater core but after driving on the highway for a few weeks, it should have been pushed out by now.

Ether · #10 03-26-2014, 12:34 PM

My replacement was the MTC brand which seems to be the only one available now. It's looking like I have a bum new mono valve

Going to take it apart this evening and just make sure I got all the parts assembled in the correct order.

eatont9999 · #11 03-26-2014, 12:53 PM

I have heard reports of the spring in the MTC unit not being strong enough to allow coolant into the heater core at higher speeds. I had such a symptom before my complete lack of heat recently. Some people also mention that you can pull the rubber bits off the MTC unit and install them on your old valve core. I have not tried that yet.

oilslick · #12 12-16-2017, 07:01 PM

I am having this same problem. Was a cure for this ever discovered?

vollpreuss · #13 12-16-2017, 10:40 PM

Getting the MB insert may a good a place to start. Here's a similar story.

http://www.peachparts.com/shopforum/tech-help/386013-300sdl-aftermarket-monovalve-again.html

Diseasel300 · #14 12-16-2017, 10:44 PM

Unplug the monovalve and go for a drive. If you have full heat all the time, it's a control issue. If you have intermittent or random heat, the monovalve cartridge is bad. The SDL will heat fine with the Aux pump dead, so a failed one will not cause a lack of heat.

oilslick · #15 12-17-2017, 07:39 AM

Quote:

Originally Posted by Diseasel300

Unplug the monovalve and go for a drive. If you have full heat all the time, it's a control issue. If you have intermittent or random heat, the monovalve cartridge is bad. The SDL will heat fine with the Aux pump dead, so a failed one will not cause a lack of heat.

I’ve unplugged the Auxiliary Water Pump and the Monovalve and still get very little heat. I will be flushing the system first and pouring in new coolant (Zerex G-05). The car sat for quite some time so I may be fighting a partially plugged heater core. I’ve also chased down and resolved several electrical gremlins in the car in the two weeks that I’ve owned it. I will not be surprised at all to find that both the Auxiliary Water Pump and Monovalve are faulty.