Of course, now when the weather has decided to head towards 0 F, my '81 300D ('84 eng/tran) has decided to take a dump heat-wise...sort of.

I haven't had a problem with heat for the one+ years of driving (this the second winter currently), but today, what was happening was that, at slower speeds, but definitely at idle, the air was...lukewarm to cool at best coming out of the side "bullet" (window?) vents. However, if I stomped on the gas or was going at a good rate of speed (above 50 mph), the car's heat acted normally. Note this was in the "defrost" setting, which is supposed to give you all heat, all of the time, as I understand it.

Changing to other settings and playing with the temp wheel did nothing; the wheel behaved normally depending on its setting, but was still doing what I mentioned above.

So...where to start? I've looked at some posts on this forum and elsewhere (using my Google Fu), but am at a loss where to begin. My thoughts as to causes (from what I've read):

1. Probably not the monovalve. If monovalve was wonky, it would behave in reverse of what's described (hot at idle, cold at road speed). I bought an MTC valve and replaced last year---which did the hot idle/cold road speed thing; put original back and normal "defrost setting" behavior returned. Only reason I replaced was because I thought monovalve was defective; instead...

2. Found last year that the stupid foam tube above the glove box...had become a crumbly mess. Replaced with a piece of PEX and...heat, monovalve and temp wheel worked like a charm afterwards (using original---how original I have no idea---monovalve as well).

3. With that in mind, got me to thinking---could it be the temperature sensor in the dash? Of note, today when I was driving, the sun was facing right at the dash, and the coat I was wearing was very warm to the touch because of the sun. I really don't think it's the sensor, because IF you are using the "defrost" setting, the heat is supposed to be MAX regardless of the temp wheel setting.

4. The aux water pump? Since I've driven this car, I've always noticed the heat was not as good at idle; from forum posts, I've read that likely meant the pump was bad. However, the heat was never quite as bad at idle as it was today. I'm real disappointed that, upon quick searching, I only found a $500+ aux pump on eBay, and that the Pelican only has an MTC pump for $89 besides the super-expensive original. Thought about going the route of a universal Bosch (see below), but it is a 90 degree flow angle, not a straight pass-through. Plus, the wiring harness would have to be adapted from the car to it, I believe. I'm sure I'd get that part (from Estonia) about the time I'm drinking in a pub to celebrate...St. Patrick's Day.

BOSCH universal auxiliary additional electric water pump for car/vehicle/heating | eBay

5. Heater core full o' garbage? Backflushing the core: of course, this is JUUUST the type of messy repair I'd like to tangle at 10 degrees F outside this time of year---not!

6. The frickin' CCU head unit/control itself? Just had the cruise control box electronically refreshed by JamesDean here on the forums (thank you, sir!). Perhaps it's time for it to be as well.

With my head full of too many potential solutions; what I thought I would try first troubleshooting are these things:

---Completely unplug electrical connector from monovalve. If I understand right, that will "default" the valve to a "wide open", yes? If the heat works perfectly then, well...time to use the MTC monovalve solution below:

MTC Monovalve No Heat Issue and Solution

---If that makes no difference, completely unplug the power from the aux water pump. Perhaps there's something with that goofy thing causing the issue.

Folks, I appreciate your responses. This car, while a neat old car, is starting to frustrate the living he77 out of me. Seemingly every month or two, something breaks on this damned thing! I realize it's 35+ years old, but I'm starting to lose my patience with it being such a damned diva.

Since it wasn't mentioned in your posting, what is the engine temperature? Normal?

Monovalve could have a plugged screen. If so, that'll really hurt your flow.

Not sure about the 123, but my 126 has an unplugged Aux pump and the heat works just fine without it. I'm in no rush to replace it. Granted I'm not at 0˚, but it works fine in the 20s like it was today. I keep the dial set at 20˚ since I don't like the car being rocket hot, but it had no problem keeping the cabin comfy.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Engine temp very normal; once car is warm (let's say, 5-10 min. of in-town driving 30-40 mph), temp sits right around 90 (visually, picture it about 2-3mm above the 80 mark). Always has sat right around there; perhaps a shade warmer than that in hot summer temps, but not much.

Really sounds like the problem is with the auxiliary water pump. I recently rebuilt my AC system and found my auxiliary heat pump was bad. I did not replace it at that time because it was warm out and I didn't need the heater. Well, a week ago it started getting colder outside and I started noticing the air from the vents would drop to ambient very quickly. I had actually bought a replacement about a year ago so I installed it a few days ago. Just went out for a little jaunt this morning while temps were in the low 20's. With a good auxiliary heat pump the vent air remained hot all the time. Speed of the car did not influence vent temps at all.

YMMV!!

Good luck.

Oh BTW, on my car I can not hear the little pump running but can feel it when I place my hand on the pump.

__________________
Baton Rouge, LA
1977 300D Non-Turbo

The auxhilary pump is the problem . Remove the plug and put 12 volts to it see if its running or not. Chack your manual as to what wires are possitive or nutral. If its running then you mono valve wants a rebuild . But have read in some place about the part inside the car that is faulty .

Yes, optimus, I believe you're referring to the Temperature Regulator Relay, that sits above (?) the glovebox, and can be seen upon glovebox removal (see link below).

Mercedes Temperature Regulator & Blower Control Relay R107 W123 W126

Haven't had time today to unplug the monovalve or aux pump for a test drive, but hope to tomorrow.

If you have heat while riving I'm going to echo the above and say aux water pump is probably your guy. My 1979 300D had terrible\no heat while sitting still but was fine while moving. Replaced the bad aux water pump and boom, I could practically cook a roast on the dash....

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

In fact the aux pump was added by Mercedes specifically because of your complaint, little/no heat at idle/low speed.

While you're in there, add a fuse to protect the controller from the high current that a failed aux pump can draw. It can fry the electronic pc board in the push-button climate control unit. About 1-1/2 Amps should do it.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

So, with a mid-afternoon temp of 19F with a feels-like of 6, I test-drove the vehicle today to operating temperature. What I noticed was that the air was fairly warm out of the ducts above 40 mph; at idle, had some heat, but significantly reduced. Then, I disconnected the aux water pump's electric plug, along with the monovalve's electrical connector. The monovalve already looked like it was working ok, since the metal "rod" was sticking up about 1cm or more above the unit. Test driving, the car felt like it was overall a tad bit warmer, with the car putting out some warm air at idle, but not much. I realize leaving things in this position basically means "all heat/all the time", but I can cycle the switches if necessary. Regarding the aux pump, can it become blocked (or partially so)? Also, what do you folks think of the MTC replacement pump; anyone have any experience using it?

One thing I did notice is that the force of air was markedly stronger (and therefore the cabin had more heat) the faster the car went. Not sure what that tells me, but here's my thoughts. Tuesday it's supposed to be near 50, so I'll open up the monovalve casing, and clean out the valve and that area if needed. May try to put the MTC valve in with a rubber o-ring spacer as mentioned previously in this thread. Otherwise, I may simply attempt a hose bypass of the aux water pump temporarily, until I can get a new one. If I'm doing that, I may also go ahead and flush the system (especially the heater core) to see if there's blockage there; I hope not!

Will keep ya posted.

I agree with Jeremy5848, put a fuse in line for the Aux water Pump.
If it shorts out it can cause the PBU (push button unit) to get hot enough to cause a fire.
There are some pictures of burned circuit boards.


This guy lost his whole car,


I lost it all....electrical fire high up in the dash
.
.
.
.

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

So, after being sick for a good portion of the month, just got around to fixing this issue. Here's how I (actually my nephew did most of the work) fixed the problem.

1. First, drained all of the old coolant, both from the radiator (screwdriver-style petcock on driver's side bottom) and from the engine block. I couldn't find the engine block drain when underneath the car, but my nephew did. A bit of a pain, because on the OM617 turbo engine, the turbo is "in the way" of getting to the bolt from above, so it has to be gotten at from underneath the car. OR...as luck would have it, you can do this by hand, feeling for it from above...IF (believe it or not) you have a hand-screw petcock that someone at some point installed in the drain block instead! Made things a bit easier. Of note, in order to reach it from above, we had to remove the air cleaner assembly as well.

2. From that point, we drained the engine block coolant. Even though the radiator coolant looked terrific (nuclear green---almost like new), the stuff from the block looked green but had a lot of oil mixed in. That said, believe the block drainage was picking up oil deposits from the bottom of the engine as it was draining.

3. Next, according to the radiator flush I used (details on it later), we were to rinse the entire system with water. Once filled with bottled water, I started it and let it idle for about 15 minutes with the defroster on. Then, that was drained from both radiator and engine block.

4. My nephew's idea next was to backflush the heater core using a garden hose. Here's how he did it, and I think it's a good idea to do so in this manner. He simply disconnected hose #4 from the engine block and #7 from the monovalve (see link below for pic to see which hoses I'm referring to). Using a garden hose connector tip that narrows at the end and is controllable in force by twisting it open or closed, he was able to both front and backflush the core. When first inserted, there we a few burps, then some clear fluid momentarily came out, then some lumpy looking goo came out, followed by the nuclear green coolant and then clear water. After the backflush, flushing back forward yielded clear water, so the heater core flush was now complete.

(Scroll down the webpage below to see the pic of the hoses I'm referring to.)

Coolant Hoses - Mercedes-Benz Forum

4. Now it was time to use our radiator flush. My flush of choice was Blue Devil Radiator Flush, since it claimed to get out "grease, rust, grime or any other build up" and was readily available at a local auto parts store. I would have used Irontite's flush (as recommended here on the forums), but seems I can't find it locally, so...went with the Blue Devil. After emptying it into the reservoir, once again, topped off with bottled water and ran for about 15 minutes. This was then drained from the radiator and engine block.

5. At this point, everything was looking very clear, but...Blue Devil said to run another water flush to get rid of any of its remaining residue. That was done for 15 minutes, then the drain procedure.

6. Finally, was ready to dump in new 50/50 mix. Yeah, I know, should've used the Zerex G-05, but I used what was on hand (the nuclear green stuff, don'tcha know!). We filled it with around 2.5 gallons, then let her idle away for about 15. Problem: not getting any heat! Solution: rev the engine a bit, off and on, wait for the temp. gauge to come up, and viola! HEAT! I should note that somewhere in here, we electrically reconnected the monovalve and aux water pump, as I had disconnected them previously as a test to see if they were the issue.

FINAL THOUGHTS: Would it have been better to drive the car around, especially on the final fill, to get the heat working in a faster manner? Sure. I'd also say driving around is the way to go in order to "burp" the system---it's possible the "no heat" issue after final fill was caused by air in the system. That said, I don't see the need to do anything special to "burp" the system; your mileage may vary on that. I also believe that, should your system be really dirty, driving around is much better to help the fluid circulate more to clean it out. Also, I have not totally ruled out the aux water pump as faulty; the car does turn out the heat at idle now, but with less force than when the car is moving. It'll get replaced at some point.

Refill process: Look up capacity. Install 50% Zerex. Remove upper radiator hose. Fill the block through the hose. Reconnect. Start engine and top up as necessary.

You have to get the engine full on the other side of the thermostat.

I have a friend who is in his 80s. He had a local shop replace the water pump on his Jeep. It over heated by the time he drove the few miles home. Rather than drive back and let the monkeys mess with it and over heat en route, we filled through the hose and everything was fine. Fortunately, getting hot didn't mess up his engine.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.