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1988 560SL (California Model)
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Discussion Starter · #1 ·
Need some advice/ideas on by-passing the heater core on a 560SL. Problem is I get warm (not hot) air when running in the EC (economy) mode.
I did replace the monovalve a couple of years ago due to a torn rubber piece and today I re-checked it (all was fine) and also checked to make sure I had voltage to it (12+ volts) with the engine running.

I’m guessing in the pics that the hose marked with an A is the intake or feed and the one marked B is the output or return. Is it as simple as splicing in a new length of heater hose in-between A and B? Another thought would be a valve inserted into hose A? I’m in uncharted territory here so any advice is most welcome…





 

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Need some advice/ideas on by-passing the heater core on a 560SL. Problem is I get warm (not hot) air when running in the EC (economy) mode.
I did replace the monovalve a couple of years ago due to a torn rubber piece and today I re-checked it (all was fine) and also checked to make sure I had voltage to it (12+ volts) with the engine running.

I’m guessing in the pics that the hose marked with an A is the intake or feed and the one marked B is the output or return. Is it as simple as splicing in a new length of heater hose in-between A and B? Another thought would be a valve inserted into hose A? I’m in uncharted territory here so any advice is most welcome…


yes
 

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Outstanding Contributor, Vintage Moderator
450slc5.0cab 280sl5sp 280se4.5 500seAMG +250seStkW108 350sl4spdX3 500secEuro 300sel5spd R+C107galore
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I know you don't often need heat, but while making the change, I wonder if it would be worth the time and effort to put in some type of T connection that would allow you to bypass the heater core, and send to heater core with a twist of a valve (or two).
 

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1988 560SL (California Model)
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5,281 Posts
Discussion Starter · #4 ·
Thanks Dave. Mike I had considered that...may go that route. I really don't remember if I had the heat on in the car last "winter".
 

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99' E320, 00' E320, 03' E320T, 05' C230K
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MB engineering can be counterintuitive sometimes. i.e. voltage to mono valve means its off, no voltage means its on.

I believe B is the inlet line to the heater core and A is the outlet. (If I'm wrong, it wouldn't be the first time:smile) Would a inline on/off valve on line B work then?
 

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Outstanding Contributor, Vintage Moderator
450slc5.0cab 280sl5sp 280se4.5 500seAMG +250seStkW108 350sl4spdX3 500secEuro 300sel5spd R+C107galore
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24,300 Posts
Thanks Dave. Mike I had considered that...may go that route. I really don't remember if I had the heat on in the car last "winter".

The only reason I could ever see you needing heat out there would be if the car was overheating and it needed a little extra cooling power.
 

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Outstanding Contributor, Vintage Moderator
450slc5.0cab 280sl5sp 280se4.5 500seAMG +250seStkW108 350sl4spdX3 500secEuro 300sel5spd R+C107galore
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And oh... One other thing... I wonder if it would be better to protect your heater core to have it filled with coolant, or if it is left to dry. Most of this only applies if your car ever ventures in to colder climates, like maybe a benzapalooza or if it moved to a colder climate for some reason. (That's me, just thoughts, no action.)
 

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Monika '74 450SL and her lil sis BrownHilda '79 280SL
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Couple points...

Is the air lukewarm or hot? The air will always pick up some heat as it passes through the system just from heat soak through the firewall.

If you want to unplug the aux water pump that will slow down any coolant flow into the core as a quick check before reply bing the system.

Mono valves wear our pretty quickly. Since you already replaced it once, take it apart and check to make sure that it is intact and moving freely.

Good luck :grin
 

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1984 380SL, 1973 450SLC
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2,174 Posts
I would install an inline heater hose shut-off valve like this one, and then keep it open just a pinch to keep flow through the heater core and prevent it from corroding.. :)
 

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99' E320, 00' E320, 03' E320T, 05' C230K
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I would install an inline heater hose shut-off valve like this one, and then keep it open just a pinch to keep flow through the heater core and prevent it from corroding.. :)
Make sure its rated for hot water!
 

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1988 560SL (California Model)
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Discussion Starter · #11 ·
Thanks all. Overall solution is pretty easy by just connecting hoses A and B. Now to think about the best and cleanest way to do it...
 

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1973 450 sl
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185 Posts
MB engineering can be counterintuitive sometimes. i.e. voltage to mono valve means its off, no voltage means its on.

I believe B is the inlet line to the heater core and A is the outlet. (If I'm wrong, it wouldn't be the first time:smile) Would a inline on/off valve on line B work then?
I saw somewhere on the sight that electrically hooking up the mono valve that way is a safety feature. If you loose power in the system, hot water is still available for defrosters and heat. Anyone remember that?
 

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'85 380SL ;-)
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12 Posts
I would remind you to disable / disconnect the Aux Coolant Pump (Recirculating Pump) if you bypass the heater core...

Otherwise anytime the climate control calls for heat, pressure may build up (or weirder things happen).
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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13,038 Posts
This is an old post, but I will add that if the monovalve insert looks good but it not a genuine MB or Bosch it probably is bad. But I'm not sure I understand the OPs question. It seems he wants to bypass the heater core because its not getting hot enough. I really think he means to bypass the monovalve. Also you will still need the Aux water pump to get heat at low speed, if the mono valve is bypassed.
 
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