W124 Auxilary Water Circ Pump

Philbask

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Hmmmm, puzzled and hope someone can shed a little light?
The auxilary water circulating pump is not running, resulting in warm (ish) cabin temps! What I have found so far:

I have a spare pump and the fault is there with both, pretty much eliminating the pump.

Voltage checked at plug (unplugged from pump) > 12V, plug connector onto pump , it will not run, unplug and check voltage again, 0V ! At first I thought I was not getting a good connection as after fiddling around with the meter probes, >12V. However, I connected the plug to the pump and measured the voltage this way, 0V, meter still connected I unplugged the pump and after 4 or 5 seconds the voltage came back to > 12V ??? :confused:

Any ideas? I'm now guessing the pump is not simply fixed 12V supply but there are other things, it's more like a current switch rather than voltage?

Thanks in advance for any input
 

grober

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In some setups the pump is supplied from fuse 1 =16 amp and its is controlled by a switched earth from the heater/aircon control unit. First try cleaning the contacts of the fuse and the fuse ends [ can suffer a build up of corrosion which might give enough contact to register 12v on a voltmeter but not sustain any current. Test the pump by supplying it with an independent 12v and earth supply--- one or both heater duo valves should be open for this -- its why the pump can't run free without a free flow of water through the heater -- the control unit won't supply the pump with an "earth" to complete the circuit otherwise.
 

bolide

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The only useful measurement is the voltage across the pump when it's running. If it drops to zero it does suggest a bad earth or bad feed, assuming it's a straight DC feed and not switched like that for the Duovalve

Is this a diesel or petrol car?

Nick Froome
 
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Philbask

Philbask

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Hi Nick, it is petrol, I believe the dou valves are working, I have checked voltage to both and they both switch from the temp setting controls in the car, also if controls are turned down the air goes cold, set high the air warms but not hot. The 12V to the pump comes on immediately the ignition is turned on, without starting the engine. The thing puzzling me is why it takes 4-5 seconds for the voltage to come back up, and it is a jump up, not creeping. I would have thought without load the voltage would restore immediately. When I have thawed my fingers I will look at the earth / supply connections as advised.
Thanks, Phil...
 
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Philbask

Philbask

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Thanks for the info, will look into this, I have tried the pump without being in the pipework so no resitance to flow. I need to delve into the darkest corner of the garage loft to seek out my DC power supply unit!!! When you say the control unit won'y supply an earth, what / where is switching / connected to provide the earth path?
Phil......
 

grober

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Thanks for the info, will look into this, I have tried the pump without being in the pipework so no resitance to flow. I need to delve into the darkest corner of the garage loft to seek out my DC power supply unit!!! When you say the control unit won'y supply an earth, what / where is switching / connected to provide the earth path?
Phil......

Its a solid state switch inside the heater control unit- unfortunately MB merely depict these units as a box with a transistor symbol on them on their circuit diagrams. There's no internal circuitry depicted just input and output connections :(
 
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Philbask

Philbask

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Ah, now that makes sense, I thought it looked a bit (semi-conductor like), voltage but no current. I can see myself ending up there, there being? (behind the panel housing the temp control / fan / vane controls?)

Thanks for the V useful info Graeme,
Phil....
 
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Philbask

Philbask

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Well, snow has stopped play today, (garage full of c*p, so outside!). Have done some more searching and it appears the controller does have a safety cut-off in the event of a short. This looks exactly like what I am seeing. I have tried the circulating pump with a 12V power supply, I can't remember but seem to thing the power supply is around 2A max, this is not enough to run the pump! is this right? does anyone know what current it should draw?

Phil.....
 

krimat2005

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Hi, have you checked your pump for continuity? if not, get it done and post the results. My pump is dead and is disconected at the moment and it's not causing any probs with heating, turn temp. down it goes cold, turn it up and you can have a sauna. this pump is only there to help with water circulation when in slow traffic/stationary, but it will not affect heating in a way you desribe. So I think you might have a problem with duo valves sticking or controls on dash itself.
 

kth286

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I was always of the impression that the auxilliary water pumps were there for when the engine had switched off after a journey, to avoid hot spots within the coolant circuit - nothing to do with the cabin heater control.
 
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Philbask

Philbask

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Have heard that before, so.... current status:
Pump removed and pipe bypass in place, both duo valves disconnected, no change!

It's unlikely the duo valves have failed closed, esp unlikely they both failed this way at the same time, I guess I need to bypass these to prove though!

Other than that it is looking like a blockage?? Does amyone know if there is another valve / device that interrupts the water flow? Initially thought possible air-lock but I have partially drained and re-tilled so many times? Also I did break the union where the flow is taken from the engine (highest point) and full of coolant.
 

grober

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I guess the heat exchanger radiator in the heater could be choked. You could try back flushing it from the duo valve side to the inlet side single pipe from the cylinder head.
 

grober

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I guess the heat exchanger radiator in the heater could be choked. You could try back flushing it from the duo valve side [ one exit at a time]
to the inlet side single pipe from the cylinder head.
 
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Philbask

Philbask

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I have heat !!!
Today removed the duo valves and tested on the bench, both switching ok and open fully. (there is a non-return valve built into these!). So as it was I had no pump fitted (bypassed) and known working and open duo valves but no heat. I then flushed the heater cuircuit from the feed pipe at the cylinder head through the valves out to the pump pipe, flow was good, all clean coolant too! I then put everything back together and flushed again until a good flow was seen coming from the cyclinder head pipe, remade this joint and it appears it was an air-lock all along! Proof will be tomorrow on a proper run but just idling on the drive it was toastie :) Thanks guys for all the input, I have learnt much! One final point, does anyone know when / under what conditions the pump will run?
Cheers, Phil........
 

trapperjohn

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Ace fix Phil. Sorry cant answer when the pump will run. But Im sure someone will be along who can.

CHEERS
 

grober

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AFAIK it should run when either of the duo valves is open with the ignition on. On some later cars the pump is incorporated as part of the duo valve assembly. Silver Cylindrical object to right of the duo valves.
 
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