For what it's worth the W123 auxiliary water pump interchanges with aux water pumps found on many other Mercedes(part numbers below). The consensus with the aux hot water pump appears to be either unplug it to protect the climate control unit or replace it with an elbow or some other variation of "bah humbug on that pump". I wonder if that's really what the majority of owners are doing though. Lots of hits on threads about adding a 1A fuse to the pump so clearly not everyone is ditching it. It's definitely one of the cheaper and easier parts to replace. Another factor for many owners is that other people often drive the car and sometimes non-mechanics lose interest if a car has too many asterisks to operate it. If it was just me I'd leave it but if I can keep an always-shivering-wife 'on board' with little things like seamless heat for $30 then it's a no brainer. I think initially it definitely provides more heat to the cabin sooner during engine warm up compared to no pump. In the UK they call it a water pump for the parking heater. I guess if you're going to go parking you'll have better odds of getting past first base if your parking heater is in order.

When I was looking for another pump there were very few used options and they were all as old or older than my dead pump. After some digging I chose a pump based on dimensions, configuration, and the plug. The interchanges were worked out from there after the install verified everything. Interchange numbers get you a lot more choices and more importantly much newer pumps. From what I can tell the only difference is the plug location. The W123's plug into a pigtail coming off of the pump while the others plug-in at the body of the pump. It's all the same connector and so far the same polarity + / -. The pump I got was an exact fit. It quietly draws .250 amps and is 16 years newer.

You can cut/paste this into an eBay search window and sort the results from lowest to highest price and click to include descriptions in the search. It is set to exclude some of the lower quality brands. The first two numbers are my original pump's part numbers from Bosch and Mercedes. The rest are interchanges. I'm sure you can cull out a few more.

(0008357064, 0130002071, 0392020026, 0130002070, 0008356964, 0008358064, 0018351064, 0018351164, 0018351294, 0018351364, 0392020002, "3050-293593", 3050293593, "0133-1803329", 01331803329) -(mtc, uro, vollig, motorking)

Note that a seller might be selling multiple pumps and get part numbers mixed up so just make sure there are only two hose fittings and the 2-pin male plug-in. The odd pumps have a third slender hose fitting which does not apply. I suspect that third fitting on those pumps feeds a coil inside the washer fluid tank. Also, based on the volute end these will only operate correctly one way so just make sure that the brown wire goes to the negative terminal(clearly marked on the pump) or to the other brown wire if you're connecting to a pigtail wire.

For those thinking of replacing their pump with an elbow I can attest that a clear inlet/outlet chamber already acts like an elbow. While moving lines over to the replacement pump for a brief moment there was an open inlet on one pump and an open outlet on the other and coolant flowed surprisingly well from both.

Some vehicles may actually *NEED* the Aux pump, some don't. The 126 with the OM603 sure doesn't. My pump was seized and I ran the car last year with it unplugged but still in the circuit. It was capable of heating the car to a comfortable temperature and the vent temps were "pretty warm".

This year I decided to try unseizing the pump. Worked for 2 days before the shaft seal went belly up. With the Aux pump working, you could just about cook on the air coming out of the vents - VERY hot air with the temp wheel cranked to "MAX".

Before shelling out for a new pump, I tried replacing the pump with a simple brass elbow. In my car, it works nearly as well as with the Aux pump functional. The air is ultra-hot and still heating fine at idle/sitting - quite a noticeable difference from the Aux pump installed but non-functional. Makes sense that the impeller isn't blocking partial flow.

I won't be replacing the Aux pump since it works fine without it - no need to shell out money for a new one and far less to go wrong.

Some other engine/chassis designs may not be so lucky, but the 126/603 combination seems to be a winner when the Aux pump is replaced with an elbow.

__________________
Current stable:
1995 E320 157K (Nancy)
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Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
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Yes I think it is later models that use the pump to circulate coolant for a certain period after shut down to "eliminate hot spots" in the engine. That's actually an interesting idea if someone is dealing with such an issue. I like that with everything off I can make the pump run with the key on position 2 and the climate control settings calling for heat.

Someone had also mentioned using the pump after a coolant change/system service to help remove air from the system without having to run the engine if that is an issue or a concern at the time. Makes me wonder if this electric coolant pump could get you to the next exit if you're dealing with fan issues or a failing main water pump, a working temp gauge of course, and the heater on max to circulate coolant to help dissipate heat. Keeping this pump in the system just might have other uses on the W123 beyond extra heat while idling even if it has to be run on demand. Having that much control of the pump might even be an advantage. On other systems it mostly runs off of sensors. If it's $30 every 10 years I will likely always maintain an auxiliary coolant pump.

A nice link, thanx .

I didn't see one cheaper than $44, still not bad .

I try to keep all the little fiddly bits working and so have replaced my aux. water pumps but in the South West they're a maybe thing ~ I get lots of blistering heat @ 40° F in the wee hours of Desert travel .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

When I do the search at the time stamp of the posting there are exactly 39 pumps under $44 including shipping. Darn good. I won't post links but here are some eBay item numbers. Just cut/paste them. Check your search settings to figure out why you are missing 39 search results. You might be missing a lot of results in general.

262228776033 ($24.49)

172841060588 ($25)

232540090690 ($25 obo)

Thank you again kind Sir ! .

I'm not overly computer literate so I accept help gratefully .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better