[zitobenz]

I have been trying to diagnose and repair an HVAC issue on my 1988 300SEL. The problem is that the heat stays on all the time at idle or slow speeds. At this point I have given up and all I want to do is disable the heat to the cabin.

Does anyone know of a way to completely disable the heat?

 

[bondavi]

Did you replace the monovalve as a first step? A 5 minute job.

Photo credit to MBdieseldiy.tripod.com

 

[zitobenz]

Yes, replaced it, unplugged it and still no change even though the old monovalve is torn.

 

[fanboy]

You can completely bypass the heater core. Just get some longer coolant hose. My 300SEL has this "mod," as in our climate the heater is not necessary.

Note: It is wise to note that the heater core is part of a car's cooling system, and by bypassing it, you are diminishing the cooling system's capacity to throw off heat. I have never had an overheating problem with any of my cars that have had the heater core bypassed, but if engine overheating is a concern for you, you may not want to aggravate it with a heater core bypass.

 

[torrem3]

I remember once I was stuck in horrible traffic and it was the middle of summer and my dad taught me a trick incase the car started to overheat, where you turn on the defroster use the heater core as a spare radiator, and it helped me get out of a tough jam.

 

[MBL87560SEC]

OK, replacement mono valve kit installed… aftermarket I am guessing…

Anyways, when you remove the power feed… it will be ON FULL always until they electrics are re-connected… Disconnect the power wires = FULL heat…. The aftermarket inserts prove faulty of most members … research the threads…. I have MB replacement ($$$ yes) and it works perfectly since last December & now spring… parking lot speed to over 100 mph earlier this week.

With respect to the kind advice given above….. your situation / needs are different…

Bypass the core connections with the long piece of heater hose.

Then you have time to sort it out, should you choose…

*Yes guys, using the heater as an extended heat exchanger DOES work well… but not in normal circumstances is the heater core used for this. His is a different issue.

MBL