I changed the coolant in my 93 300SD last week,while it was warming up so I could top it off I turned on the heater and got nothing but cold air. I let the car run for about a half hour,the temp gauge was reading around 80 and still no heat. The heater was working earlier this year (I know this because the thermostat stuck and I used it to help cool it down) but nothing now. The heater hoses are getting warm so I'm thinking it may be the heater control valve.Is the heater control valve located under the dash and is it vaccum or electric? It may be something else any suggestions or ideas? Thanks alot.

Whenever I've had that problem it has always been the mono-valve. Very easy to fix.

X2 on mono valve & if not that it could be the electric coolant pump.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

PM me- I have pulled many working monovalves from the yards here- $10 to your door in the USA. I've helped about a dozen folks with this- close to enough after fuel, yard entry fees, parts cost and shipping to buy the family a pizza. It's more of a help to members than a effort to make money.

+1 on MTU's monovalve. I got one from him and it is in great shape!

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AJ

1985 300D (SOLD)

The last monovalve I repaired was for an '81 w123. Perhaps the procedure is different for your car. For the '81 it was less than 10 minutes to repair once the valve was out of the car.

If the process is the same for you then it would be better to use your existing valve and replace the diaphragm than to replace with a used valve. The used valve at some time will require the diaphragm to be changed as well.

As I recall about every 4 years I replaced the plunger/diaphragm assembly. So whether you use your old valve or purchase some other used valve you'll still be replacing diaphragms.

There is no reason to buy a new valve as only a small rubber piece is broken.

Can someone confirm that when unplugged, the monovalve on this car defaults to full heat, like the w123? If so, unplug it and see if it gives you heat. If not, the problem could be elsewhere, or might need bled out.

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1989 300E 144K

Thanks for all the replies,it seems that the mono valve is my problem from all your answers. I assume it is located under the dash. Is it located on the pass. side behind the glove box or somewhere else? Thanks again.

Under the hood, passenger-side firewall. Wait...w126, or w140?

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1989 300E 144K

A '93 300SD would be a W140. And it would have a have a much more complex heater valve than a W126.

Can anyone tell me where the valve is located on my car?? And yes it's a W140,I did do some looking around under the pass side of the dash when the blower motor quit. Is there a vaccum line in that area that I may have inadvertantly disconnected that could cause this?

Right side, towards the back of the engine mine is I think

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed