Driving the 300 TD today for about 125 miles. as we approached the destination, I noticed the heat was blowing cool air. made a stop at a food store, for about 15 minutes, and the heat came back. until about 15 minutes later inthe trip. finished the drive with little heat. Any Idea's ? I just replaced the thermostat, which helped a overheating problem. but now the car is running on the warm side as well or the gage was reading about 90 C. and the outside air temp was in the low teens. How do I test the water pump? It is not leaking, and the coolant is not low...???
Jason

__________________
46 WDX Power Wagon

84 300TDT daily driver

heat/no heat cycle could mean failure of monovalve. I had similar symptoms several years ago. I would get no heat until I drove a couple miles, then shut engine off. Waited about 10 seconds and restarted. I would get heat until the cabin temperature reached the setpoint, then it would shutoff. Watch for similar symptoms. If this happens in your case, i would strongly suspect the monovalve. It's easy to replace, and a "repair kit" is available that is easy to do and saves a lot of money

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

What Pete said.........

And, the engine is running right at the proper temperature if the new thermostat results in a gauge reading of 90C. The water pump is probably fine.

I have the same exact thing happen from time to time, the heat just "goes away" and only comes back if I come to a stop, turn the ACC on and off, wait a second, then go on my way. I will have to look into a monovalve rebuild kit soon, mine keeps giving me very sparing amounts of heat, which is no fun when its 5-10 deg's out around here so often, its warm right now though! 21!!

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Could be worse. My CD.. the monovalve is touchy, plus the blower motor is dying. At least I live in NorthWest Florida where it doesn't get too cold.

__________________
'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.

So I will replace the monovalve with a spare off the parts car and see what the difference is. Thanks for the help.

jason

__________________
46 WDX Power Wagon

84 300TDT daily driver

BTW what does the monovalve do, other than fail and need to be replaced? It is the plastic doohickey on the firewall, with heater hose and an electrical connection on it?

__________________
46 WDX Power Wagon

84 300TDT daily driver

The monovalve controls heated water flow through the heat exchanger in the cabin.

It is opened and closed by the ACC control module. When the ACC calls for maximum temperature, it is open. When the ACC calls for cooling, it is closed. In between, it is cycled on and off.

Coupled with fan speed, varying the open/closed time results in a very good control of heat exchanger temperature, and thus control of cabin temperature.

Best Regards,
Jim

The symptoms you describe do sound like a bad monovalve.

As I recall, the monovalve repair kit costs only about $35.00....if a used one works ok, then go for it.....however, depending on the failure mode of a monovalve, it could fry the push button unit...a lot more expensive to replace, therefore, I would recommend a new one. Up there in northern Vermont, you really want your MBZ diesel "winterized". The monovalve repair kit is one of those things I replace every about 3 years (along with battery, snow tires, coolant, thermostat and glow plugs) to ensure a trouble-free winter (along with fingers crossed!!).

Good luck up there in the beautiful country,

Mark

__________________
1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

Thanks for the explanation Jim, I will repair my existing unit..
Jason

__________________
46 WDX Power Wagon

84 300TDT daily driver

check the water level first.

I've had similar problems, turned out to be the radiator was low.

The real issue was a slow leak, the loss of heat cycle and eng temp cycles were only a symptom of the real problem.

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5

My monovalve was replaced and I'm still not getting my heater to come on for about 30 minutes.

What is engine temperature, if thermostat is bad = no heat until engine reaches 30C.
Have you done a 10 mile+ Italian tune up since the mono valve?
Oops, out of time.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Well it won't kick in at that time. My monovalve was just replaced about 3 weeks ago. It didn't help it at all. If it's cold outside, say 25 degrees the heater will not come on for a good while say like 20 miles of driving. If it's say like 48 degrees, heater will come on in about 5 miles of driving. In any case, I know when the heater will start to work. Lots of clicking noise in the console to the right of the CC unit. You can actually feel the clicking by laying your hand on the side and under the dash near the CC. I'd guess it's a relay. No heat blows out the middle, just sides. No heat will blow out the Defrost vents on the top, when the heat actually comes on. Just always out the side. Could be several issues. I'm just not sure where to start. My indy would do it, but man, it'd probably set there for 4 or 5 hours just troubleshooting the thing. That's costly.

You need to check the connections on the ACC pushbutton unit, the suction motor for the interior temp sensor, and squirt some contact cleaner on the temp wheel rheostat on the pushbutton unit if you are getting constant clicking of the switchover valve.

You have vac in the right places if you are getting floor heat. It's normal for no warm air to come out the center vents, they are set up to delivery AC only.

Pull the plug on the monovalve, this should give you full heat immediately. If not, either the monovalve is stuck (corrosion in the solenoid -- replace it if this is the case) or the heater core is full of air or there is a blockage somewhere.

You will get only very slow blower speed until the engine reaches 50C unless you press the "high" blower button. If you have a bad thermostat, you may never get to that temp!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!