Coolant circulation pump function
Thread Starter
MBWorld Fanatic!
Joined: May 2015
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From: Allentown, PA
'08 GL 320 CDI
Coolant circulation pump function
I'm all of sudden getting very little heat. Coolant level is good. There is also a heavier clicking sound coming from under the hood that isn't normal. It was 9:30PM and it was already a long day by the time I got home so I didn't really feel like investigating too far. BUT, turning the HVAC off would also cause the clicking to stop. That was the extent of my diagnostics for the night! 
For now, just a quick question that hopefully someone will know... Is the coolant circ pump (the one near the coolant resevoir) SOLELY responsible for sending coolant to the heater core? Meaning, in a "normal" car
the engine circulation pump (belt driven) would send coolant through the engine AND the heater core. But in the Mercedes, does the engine circ pump ONLY take care of the engine while the electric coolant circ pump takes care of the heater core?
I imagine I can start tracing coolant hoses tomorrow afternoon and find the heater core and then figure it out?
For now, just a quick question that hopefully someone will know... Is the coolant circ pump (the one near the coolant resevoir) SOLELY responsible for sending coolant to the heater core? Meaning, in a "normal" car
the engine circulation pump (belt driven) would send coolant through the engine AND the heater core. But in the Mercedes, does the engine circ pump ONLY take care of the engine while the electric coolant circ pump takes care of the heater core?I imagine I can start tracing coolant hoses tomorrow afternoon and find the heater core and then figure it out?
Thread Starter
MBWorld Fanatic!
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From: Allentown, PA
'08 GL 320 CDI
Just adding info...
The "clicking noise" is better described (now that's it's not 9:30 at night, 25* and 25mph winds) as a slow/lazy slap, slap, slap. Roughly, 2 slaps per second. But as soon as I turn the HVAC off, it goes away. It also goes away with very slight increase in RPM (maybe just a 100 or so RPM) from the throttle pedal.
The "clicking noise" is better described (now that's it's not 9:30 at night, 25* and 25mph winds) as a slow/lazy slap, slap, slap. Roughly, 2 slaps per second. But as soon as I turn the HVAC off, it goes away. It also goes away with very slight increase in RPM (maybe just a 100 or so RPM) from the throttle pedal.
Last edited by DennisG01; Mar 5, 2021 at 08:42 PM.
Thread Starter
MBWorld Fanatic!
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From: Allentown, PA
'08 GL 320 CDI
Well, then it should fix itself once it starts feeling a little better? Woud be nice...
36*F this morning, on my way to work I watched the engine temp. It climbed up to 90*C within about 7 or 8 minutes (guesstimate) - or at least a timeframe that felt very reasonable. I was "hoping" to see that temp stay low, meaning the culprit would have just been an old t-stat not fully closing (or lodged open from some kind of grit/gunk).
36*F this morning, on my way to work I watched the engine temp. It climbed up to 90*C within about 7 or 8 minutes (guesstimate) - or at least a timeframe that felt very reasonable. I was "hoping" to see that temp stay low, meaning the culprit would have just been an old t-stat not fully closing (or lodged open from some kind of grit/gunk).
Senior Member
Joined: Oct 2017
Posts: 302
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From: Saint Louis
2010 GL550 | 2020 S560
Dennis-
This is clearly not a forever fix however, the dealer stated my pump was bad too. When my heater was on, I would only get cold air.
There was a was weeks in which it was between 0-5°F.
I did a little experimenting. When I turned the heat on the rear on, the front would blow out hot heat.
Maybe yours will work the same?
This is clearly not a forever fix however, the dealer stated my pump was bad too. When my heater was on, I would only get cold air.
There was a was weeks in which it was between 0-5°F.
I did a little experimenting. When I turned the heat on the rear on, the front would blow out hot heat.
Maybe yours will work the same?
Thread Starter
MBWorld Fanatic!
Joined: May 2015
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From: Allentown, PA
'08 GL 320 CDI
Thanks for the input, Yup. Unfortunately, no go - front heat stays somewhat warm (even though I had it on manual and turned all the way up), but the rear heat was blowing cool. The only positive is that we've got at least a week or two of warmer (not frigid, anyways) weather coming up.
Edit: Just realized I should easily be able to tell if the aux circ pump is operating by selecting "rest" after turning the engine off - and/or checking for V at the plug - and/or jumping 12V directly to it.
Edit: Just realized I should easily be able to tell if the aux circ pump is operating by selecting "rest" after turning the engine off - and/or checking for V at the plug - and/or jumping 12V directly to it.
Last edited by DennisG01; Mar 8, 2021 at 08:01 PM.
Out Of Control!!
Joined: Sep 2018
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From: unbegrenzt
2017 GLE350 4MATIC
Posts on this site suggest the circulation pump is a “helper pump” or “booster pump” with the main engine driven pump to achieve proper engine cooling at low engine rpm such as when idling. And it performs the REST function, if the vehicle has REST. My GLE does not have REST, and does have a circulation pump.
The circulation pumps are cheap commodity garbage and a component shared across many MB models. They fail at somewhat early age/mileage and are not terribly expensive. However the labor time needed to replace the pump drives the dealer service price for this very high. Most circulation pumps are buried in the bowels of the engine bay. Mine is sandwiched between the left headlight and left front tire.
The network of small diameter cooling lines associated with the circulation pump leads to the potential of needing to replace several clamps or hoses or both.
Summary- these pumps fail, they are not that expensive, they assist with engine cooling and provide the REST function and are a pain to replace.
The circulation pumps are cheap commodity garbage and a component shared across many MB models. They fail at somewhat early age/mileage and are not terribly expensive. However the labor time needed to replace the pump drives the dealer service price for this very high. Most circulation pumps are buried in the bowels of the engine bay. Mine is sandwiched between the left headlight and left front tire.
The network of small diameter cooling lines associated with the circulation pump leads to the potential of needing to replace several clamps or hoses or both.
Summary- these pumps fail, they are not that expensive, they assist with engine cooling and provide the REST function and are a pain to replace.
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Thread Starter
MBWorld Fanatic!
Joined: May 2015
Posts: 1,932
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From: Allentown, PA
'08 GL 320 CDI
Yes, it seems that an extra function of this aux pump is to keep running after engine shutdown if the engine is hot to help cool down "hot spots". I suspect this is more important in the gassers than the diesel I have, but none-the-less it is a function.
I selected "rest" when I got home today. The pump continued to run (I could hear it) and I could "feel" the coolant being pushed through the hose that goes through the firewall to the heater core. With this info, I suspect there is a partial blockage somewhere in the heater core(s) and/or hoses. I was planning on doing a complete flush in the Spring, anyways
I selected "rest" when I got home today. The pump continued to run (I could hear it) and I could "feel" the coolant being pushed through the hose that goes through the firewall to the heater core. With this info, I suspect there is a partial blockage somewhere in the heater core(s) and/or hoses. I was planning on doing a complete flush in the Spring, anyways
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From: Emmett, ID, USA
2007 GL450
Don't forget, the last guy to do a coolant system flush ended up needing a new motor. Tread carefully.
Seriously, though, I bet there are many reasons besides blockage that could cause the heater to perform sub par. I'd look into the architecture. Perhaps the aux pump is weak but not failed completely?
By the way, I found that a bit of 3/8" ID vinyl tubing can be connected to the radiator drain, so the coolant can be easily reclaimed.
Last edited by eric_in_sd; Mar 10, 2021 at 01:53 PM.
