Coolant pump surging?

[mayagman]

Hello, I tried searching for this, but its a little unique. Here goes-

My 97 C280 has a bizzare thumping noise coming from the engine bay. I noticed it last summer, and it only seems to happen when the car has warmed up. I looked under the hood to see where it was coming from, and I noticed that a rubber hose coming off of what I believe is a coolant pump of some kind seems to shake every time it makes that thumping noice. It happens at a constant interval regardless of engine speed, its a deep thump about 5 seconds apart. Audible from within the cabin. The device the hose is connected to is a small black cylindrical thing which is mounted to the passenger side wheel well area, under and to the rear of the air box. It looks like some kind of electrical pump.
Any ideas what this wierdness could be?

[Renn 208]

is your heater/defroster on when this happens? If so, does it happen even when it's off?

[Rocky]

happens to me too. Try this out, turn off the car prest rest. open the hood and listen for that whinign sound..it's coming from this black thing that looks like a small pump. I'm not sure why it goes thump thump thump either liek every 8 seconds or so.

[stox]

Let your mother in law out from under the hood.

[Sorry, couldn't resist ]

[benzfan]

The thumping isn't coming from the pump. It is coming from the duo-valve. The pump is shaking because of the pressure transient created when the duo valve briefly pops open (solenoid actuated) then abruptly closes. The coolant pump exists to circulate coolant at idle more effectively (through the climate control circuit), so that you don't end up with a sudden surge of hot air as you bring the engine off idle that the A/C then has to counteract. The heater valve is popping open briefly every few seconds to keep a constant heater core temperature. This will be more evident in spring/fall weather. The coolant pump also functions when you press the 'rest' button on your climate control unit after you shut the ignition off to circulate warm coolant through the heater core for 30min in order to maintain the interior of the car warm for you. You will also hear the duo-valve operating depending on ambient air temperature in this mode. Occasionally you may also hear a gurgling sound inside the dash area when the duo-valve operates. If it cycles quickly enough, very little fluid is allowed through the heater core each cycle, causing this gurgling. All sounds are normal for the C280/C36. If your 'rest' button produces no result, your pump is bad.

[benzaddict]

Originally posted by benzfan
The thumping isn't coming from the pump. It is coming from the duo-valve. The pump is shaking because of the pressure transient created when the duo valve briefly pops open (solenoid actuated) then abruptly closes. The coolant pump exists to circulate coolant at idle more effectively (through the climate control circuit), so that you don't end up with a sudden surge of hot air as you bring the engine off idle that the A/C then has to counteract. The heater valve is popping open briefly every few seconds to keep a constant heater core temperature. This will be more evident in spring/fall weather. The coolant pump also functions when you press the 'rest' button on your climate control unit after you shut the ignition off to circulate warm coolant through the heater core for 30min in order to maintain the interior of the car warm for you. You will also hear the duo-valve operating depending on ambient air temperature in this mode. Occasionally you may also hear a gurgling sound inside the dash area when the duo-valve operates. If it cycles quickly enough, very little fluid is allowed through the heater core each cycle, causing this gurgling. All sounds are normal for the C280/C36. If your 'rest' button produces no result, your pump is bad.

Benzfan hit it right on the head, it's the duovalve, I see it quite a bit here at the dealer. They're all a little noisy but as they wear they get really noisy. The duovalve is located near the module box area in the passenger side of the engine bay, near the windshield. It has a few coolant hoses going to it, you can't miss it.

[benzfan]

The failure mode of both solenoids of the duo-valve is open. Later cars, and higher models, came with dual-zone climate control, but not the 96/97 202. However, the only difference was that each of the solenoids was controlled by a different circuit for dual zone. For the 96/97 202, both solenoids are controlled simultaneously from the same control signal. What does this mean?
The heater core on the 96/97 202, and indeed most models of that era, is actually 2 separate cores, each supplying heating for one side of the car, split evenly at the middle of the center vent. In a failure of the duo-valve, typically one solenoid will exhibit failure symptoms first, causing a differential heat output from driver to passenger side vents, seemingly split exactly at the middle of the center vent.
Because the no-current mode of the solenoid valves is 'open', the car will warm up nicely, then as the valves begin to cycle, and one is not functioning properly (they are sent a brief pulse, which cannot be seen by a digital meter - the dampening typical of a digital voltmeter will prevent the fast +12V pulse from registering), one side of the car will start to see cooler air from the vents, and the other will still see hot air from the vents. This gets worse as the side that is still functioning will try to compensate for the one that will not respond properly, so one side gets cooler and cooler. Replacement of the duo-valve will solve this problem that can sometimes erroneously be attributed to air flap actuators.

[mayagman]

Wow, I wasn't expecting such a great response. I thought this would turn out to be a bigger problem. Thanks for not only telling me what is, but why it is. I'll check out the valve tomorrow when the sun comes out. Once again, I'm extremely greatful for all your help, in this and all matters. I'm very impressed ^^

[benzfan]

You're welcome. As the thumping sound is normal, don't replace the duo-valve unless it is actually exhibiting failure signs. It isn't cheap, even though replcement is a simple DIY, and new ones make some noise too.

[96C280]

This past Friday as I was waiting for the light, with a/c on, very weird noises came from the general area of the center air vents. It sounded to me as if the vacuum diaphragms for the a/c vents were failing then turning into as if something was bouncing around in that area. I noticed then that the temp shot up to over 120 degrees and touched the red. It stayed there, I turned off the a/c and it cooled off slightly. Just as I was about to turn off the car, the coolant warning light went on. In the end, little or no coolant was lost. Water pump looked fine, belts looked fine and there was no coolant in the oil and vice versa. Could a failure of the duo valve been the result of the symptoms and to the point of almost overheating the car?

Thanks for any thoughts on this.

By the way, it sounds as if one wanted to convert to a dual climate control they could? Not necessarily economical but couldn't one basically plug and play a new climate control module and get dual climates since the other hardware is there already?

[benzfan]

Are you sure your water pump is OK internally? A climate control failure cannot cause engine overheating, but an impeller problem can make the sounds you describe and cause overheating without losing coolant. I have already done the water pump on my 96 c280, and am anticipating doing the one on my 97 c36 soon too.

[96C280]

Everything seems fine right now. Temp is steady, no weird noises. Someone on Mercedesshop pointed out this post and it did sound as described on this post. I initially thought it was the a/c vacuum actuator by the center air vent that was making the noises. I had the water pump replaced almost exactly 2 years ago. From the outside, the pump looks normal, no moisture, leakage from weep hole, etc. Can a hiccup in the duo valve prevent flow of coolant? It seems as if its part of the pathway. The duo valve also is closer to where the sound seemed like it was coming from.

[benzfan]

The duo valve only controls coolant flow through the heater core to: a) get the heater core up to temp on cold days, or b) moderate the A/C on semi-warm days when full A/C is too much. How's your thermostat?

[96C280]

before and after the episode it's been at the (i believe) correct 87 degrees.

[benzfan]

Hmmm. It does sound like it stuck on that one occasion, though.