On a 1985 W123 300d, where does the power come from for the Auxiliary Pump. I have checked all fuses and wire connections. Would bypassing the pump for that year cause trouble with the heat controls. I have read some earlier post for different models, and others have no problem with bypassing their cars.

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Rick Keeling

I don't think i have one on my car. could you post a picture or two of the location and i'll take a look on mine for you

1985 300D turbo

I have bypassed the aux heat pump on my 1983. It is installed on OM617 diesels to improve heating at slow speeds. The reason I removed it and bypassed is because the pump was not working and as a result was getting little or no heat. I thought the non-working pump was impeding coolant flow.

So far I now have adequate heat but I intend either rebuilding or replacing it to restore nominal operation to the system.

If the aux pump jambs, which they are prone to do, it will trip the overload protection in the CC head unit. This will cause the whole system to stop working. There’s no need to bypass the flow path, unless it leaks. Just unplug the electrical connector and you will be good to go.

The 1981-1985 123, 126, and 107 chassis vehicles are well documented to have burned PC boards in the pushbutton unit with at least one documented vehicle that burned to the ground as a result of the Aux pump failing. Deleting, servicing, or fusing the Aux pump is a good idea on these year models.

The 1986+ 124 and 126 vehicles have a different pushbutton setup. There is no overload on board. If the Aux pump seizes or fails, it burns out a Darlington Transistor pack. Not really an overload device. The W201 may be different, but having been very intimate with the 1986-88 PBU and the 1988+ revision, I can saw with extreme confidence that there is no "internal overload" device present. The PBU is fed from a 5A fuse that should go if there is any sort of short circuit.

If concerned about the Aux pump, unplug it. If you live in a mild climate, delete it (heater flow is greater with it removed than with it present and not operating). If you want the Aux pump, but don't want to worry about it, an in-line fuse is cheap protection if it ever faults.

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Current stable:
1995 E320 157K (Nancy)
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1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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Remove or fuse. Its only function is to provide extra heat when a car is idling. At highway speeds it's redundant.

I removed mine on my 300d. Never noticed a difference.

Recommendation: install an in-line fuse in the wire that provides power to the auxiliary pump. As stated before, if the pump jams it will trip the climate control head stopping all HVAC function and resulting in replacement/rebuild of the CC push button unit. A 2 amp fuse is sufficient. A member of this forum (?) makes and sells an easily installed inline fuse for this purpose.

X2
There is at least one thread on putting a fuse into the Aux Pump Circuit.

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84 300D, 82 Volvo 244Gl Diesel

The CC head unit, is that the heater control unit above the radio? ( so the power comes for the CC head unit?) I only have air that blows from defroster, regardless of which button you push. I think the temp sensor is bad too because the gage stays on 40c. (

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Rick Keeling

Thanks you answered my question. I should have read all the post before I reposted to a post, if that makes sense. Power comes form the heater control unit, and may be burnt due to a bad auxiliary water pump. So I will look for a used or new heater control, install a fuse inline for the aux pump, and replace pump for better water flow. I live in TN. so we dont get real cold around here. Thanks again for your info.

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Rick Keeling

Thanks for all the reply's to my questions. time to stick my head under the hood and get to work.

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Rick Keeling