Odd 1987 300SDL mono valve behavior - Page 2 - PeachParts Mercedes-Benz Forum

Diseasel300 · #16 12-17-2017, 09:53 AM

My car sat for 10 years in a field before I got it. I feel your pain.

My aux pump was seized and I still got heat most of the time. The culprit was the monovalve cartridge (about $30 for genuine OEM, don't waste money on the aftermarket). After sitting so many years, the cartridge would operate, but the shaft was gritty with rust and it would bind up. Do any system flushing you're planning on and refill the coolant before replacing the cartridge.

If the Aux pump is bad, bypass it with a 90˚ brass fitting. The coolant circulation in the 603 is strong enough that it'll heat just fine with the Aux pump gone. I deleted mine a couple months ago due to a leaking shaft seal and you can still get the cabin hot enough to cause a heat stroke when it's freezing outside.

Diseasel300 · #17 12-17-2017, 09:54 AM

It's also worth noting that the definitive test for checking Monovalve operation is unplugging it. The default state is wide open which should result in full heat all the time if it is working properly. The CCU energizes the valve to close it, not to open it.

oilslick · #18 12-17-2017, 10:12 AM

Quote:

Originally Posted by oilslick

I’ve unplugged the Auxiliary Water Pump and the Monovalve and still get very little heat.

Unplugging the Monovalve produces no change in the temperature of the air being discharged. Unplugging the Auxiliary Water Pump produces no change in the temperature of the air being discharged. Unplugging BOTH at the same time produces no change in the temperature of the air being discharged.

Diseasel300 · #19 12-17-2017, 10:48 AM

I'd start with the monovalve cartridge then. Make sure you buy OEM, the aftermarket are known to have problems at higher engine revs (weak springs).

vollpreuss · #20 12-17-2017, 01:50 PM

x2 on the MB cartridge recommendation.

From personal experience, the aftermarket units are rather suspect quality.

The first one I put in my SDL was no good out of the box. The heat was full cook mode after a few minutes of driving no matter the adjustment attempts.

The 2nd unit lasted about a year before the no heat at higher rpms issue.

Had some boneyard heater control valve guts that still had the original MB cartridge. Took a chance, put it in, and the no heat problem went away.

oilslick · #21 12-17-2017, 01:55 PM

I appreciate it. I know that the cartridge can be replaced. I’ve replaced a few over the years, but why am I not having full heat with the Monovalve and the Auxiliary Water Pump disconnected? Shouldn’t I be getting full heat at speed?

Diseasel300 · #22 12-17-2017, 02:14 PM

The monovalve cartridge is probably stuck closed or you have a clog somewhere in the heater core circuit. By unplugging the monovalve, you have forced the system to be "open". There is nothing to shut off coolant flow at that point, so either the heater core is clogged (unlikely unless you have a TON of silt in the cooling system), or the cartridge is stuck closed.

Pull the cartridge and see what kind of condition it's in. If you see rust on it, the screen is plugged, the rubber is torn, or the plunger doesn't move extremely easily, it's junk.

oilslick · #23 12-17-2017, 03:12 PM

Pulled the cartridge. The diaphragm was torn. I used an adhesive to mend the tear. I let it cure for about an hour and then reinstalled the Monovalve back into the car. My heater is working now! Hot heat!!! The Monovalve is responding to the temperature dial wheel also. I’m not sure this will last very long, but I know the core isn’t clogged now. Thanks for the help.

Diseasel300 · #24 12-17-2017, 03:29 PM

Good troubleshooting skills! Now you know for certain it's the cartridge. A new one should fix the problem for good

Maxbumpo · #25 12-18-2017, 10:12 AM

Interesting that the '87 SDL monovalve has a replaceable insert, but the '87 300D/TD do not. I would love to swap in a monovalve that has the replaceable insert, can someone take measurements and post a picture, so that I have an idea if this is workable or not? Ideally the dimensions would include intake pipe OD and outgo pipe OD, and some idea of the overall monovalve height / width / length.

Diseasel300 · #26 12-18-2017, 12:13 PM

Does the 124 not use the same monovalve as the 126? I was under the impression they all used the same part until the later redesign in the 90s. The forum sponsor has the OEM MB part for <$40

Maxbumpo · #27 12-18-2017, 01:50 PM

Quote:

Originally Posted by Diseasel300

Does the 124 not use the same monovalve as the 126? I was under the impression they all used the same part until the later redesign in the 90s. The forum sponsor has the OEM MB part for <$40

No, see attached. Looks like fitting the 126 monovalve to a 124 will be painful, but the 123 monovalve (which also has an insert available) might be possible to fit.

Maxbumpo · #28 12-18-2017, 02:01 PM

Looks like the monovalve inserts for the 123 and the 126 are the same part: 000-835-06-44-MBZ.