Replaced the monovalve which at least got some heat but now the only time it blows hot is when you are sitting. Quits blowing hot air after you get up to speed. Engine coolant level looks good. Don't have any history on maintenance but it is running green coolant. May try to flush heater core out. Any thoughts/ideas are appreciated.

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Jim

That is classic monovalve failure symptoms. What did you replace the monovalve with? I replaced with one from the junkyard and it failed. The diaphragm tore shortly after I installed it. It didn't "look" bad until I looked closely. I had to move the diaphragm to notice the tear.

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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.

Thanks, that is why I swapped it out. I can disconnect it and it will run you out of the car it gets so hot. Swapped complete solenoid and internals with one from a good working one (got to verify) and I get a little bit of heat but not much. Got some more testing to do. I'm wondering if a failed push button unit would cause this? I think I have a spare around here somewhere I can try.

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Jim

I would like to know too.

I have the same issue, but only after a restart (warm or hot engine). Sometimes loses heat at low speeds as well.

But after a cold start the system works fine all the time.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

The climate control is not reducing the signal to the monovalve to put heat to the cabin. Got some more troubleshooting to do to see what is up.

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Jim

I've heard the aux water pump might also be a factor? Please feel free to refute this argument, it's only something I heard. Not parading it as fact.

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Never be ashamed to ask for help.

1985 300D
1987 300SDL
1970 Jaguar E-Type OTS
2002 Dodge Intrepid (Traded In)
2011 Ford Crown Victoria

The auxiliary pump keeps the coolant moving in traffic or at idle. The failure symptom should be the opposite: no heat at idle.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Different car (92 300D), but I did have a situation in which a failed AWP was adversely affecting heat at times other than idle. Just for grins, you might want to disconnect lead to AWP and see what happens.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

A friend had this same issue in his '84 300D. Replacing his temperature regulator (silver box behind the glove box) with a spare solved it. Another member over on BenzWorld found that the control devices (push button, temp regulator, and temp sensors) had a few faults that when fixed brought his heat back at highway speeds. So it's quite possible it's control related, not any physical issue with the monovalve.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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I'm going to start digging into the control side of things some time this week.

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Jim

It very well could be a problem in your temperature regulator or push button unit. Cold solder joints, failed capacitors and (rarely) bad IC chips are what gets these issues.

I think these problems are more so common with the 81-85 style three piece system than the later types but it can still happen to them.

I'd be happy to re-work the unit(s) for you if you are interested. Just let me know!

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

I had very similar problems with the heat in my '84 300CD. There was little to no heat because the plunger in the mono valve was being pulled down and therefore closed by coolant flowing past it. I had installed two MTC rebuild kits, both of which have too weak a opening spring for the plunger it turns out! I have recently install a brass solenoid valve, 1/2", 12VDC, normally open. Now the heat works perfectly! The best way to check if your mono valve is closing when it shouldn't is to bring the engine to operating temperature and shut it off by using the manual fuel shutoff "stop Button". Once off you can hear the Aux water pump and the air blower running. I then unplugged the AWP and then plugged it in again. I could hear the coolant start to flow and then a thud as the plunger in the mono valve was pulled down and then there was no water flowing sound. The sound the AWP makes also changes when there is flow vs. no flow. I had previously checked the operation of the AWP in a bucket of water and I also checked for free flow through the heater core by disconnecting it at both ends and pouring water down a temporary hose on one side. The core should flow water freely. The original mono valve has a check valve built in for one way flow and my new valve does not. Does anyone know if I will have trouble because of the missing check valve?

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1984 300CD (sold)
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
1988 300CE (sold)
1997 E420 (Totaled)
1998 E320 Wagon

Thanks for the info and advice.

I also replied to your thread with the Q on the check valve (no answer, just a request for more info on the retrofit).

Even a pic of the new valve in situ would assist my understanding (also make/model info on the valve, maybe?).

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Same thing 86 sdl. A bad insert but no heat above 50 mph. Slow down it works fine. Unplug mono valve and to hot. I put a ball valve from the plumbing store to regulate or shut off, but I have to open the hood to make the adjustment.

Here are some photos of the brass solenoid valve that I installed in place of my mono valve. For the top connection I used a street elbow and a thread to hose barb adapter and on the bottom just an adapter. I used tie wraps to hold the valve up wrapped around the original valve mount which is a little rigged. The first photo is from the ebay listing for the valve from a seller named "duda diesel" (I'm not kidding). Seems like a fine company! Valve is brass body, buna-n rubber, 1/2", 12VDC, and normally open.

Attached Thumbnails

       

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1984 300CD (sold)
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
1988 300CE (sold)
1997 E420 (Totaled)
1998 E320 Wagon