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Auxiliary Coolant Pump Replacement
 
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Pelican Technical Article:

Auxiliary Coolant Pump Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$90 to $560

Talent:

**

Tools:

Flathead screwdriver, large bucket

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Auxiliary pump, coolant

Hot Tip:

Get a really big bucket

Performance Gain:

Proper cooling

Complementary Modification:

Coolant flush

The W123 has an auxiliary coolant pump that circulates the coolant through the heater, head and block after the vehicle has been shut off. This pump insures that there will be no hot spots in the head caused by hot coolant siting stationary. Hot spots like these can lead to a head gasket failure, so if your pump stops working make sure to replace it right away.

A couple of safety precautions/instructions you may want to observe before beginning:

Allow cooling system to cool down to a coolant temperature of less than 90°C. Open the cap of cooling system slowly; turn a conventional coolant cap as far as the first detent and turn a screwed coolant cap approx. 1/2 turn and release the pressure. Wear protective gloves, protective clothing and eye protection. Always dispose of used coolant in accordance with the regulations in your region; NEVER pour coolant down a drain, into the ground or a sewer.

Remember your car may have been serviced before and parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve as they grow older. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

Begin by safely jacking up and supporting your car. Please see our article on these procedures for additional help.

The pump is located on the heater lines below the expansion tank reservoir (red arrow).
Figure 1

The pump is located on the heater lines below the expansion tank reservoir (red arrow).

The pump is at one of the lowest places in the system so you will need to drain the coolant to replace it.
Figure 2

The pump is at one of the lowest places in the system so you will need to drain the coolant to replace it. Please see our article on coolant flush and replacement for additional assistance.

You will need to remove the expansion tank reservoir to access the pump please see our article on expansion tank replacement for further information.
Figure 3

You will need to remove the expansion tank reservoir to access the pump please see our article on expansion tank replacement for further information.

With the tank removed you can access the pump and the heater hoses attached to it (red arrow).
Figure 4

With the tank removed you can access the pump and the heater hoses attached to it (red arrow).

I have moved the air filter housing out of the way for photographic purpose only; you do not need to remove it to perform this job.
Figure 5

I have moved the air filter housing out of the way for photographic purpose only; you do not need to remove it to perform this job. Remove the connector from its bracket and unplug the connection, it simply pulls apart (red arrow).

Use a flathead screwdriver and remove the clamps and two hoses connected to the auxiliary pump (red arrows).
Figure 6

Use a flathead screwdriver and remove the clamps and two hoses connected to the auxiliary pump (red arrows). Even though you have drained the cooling system there will be a substantial amount of coolant that is still in the heating system that will now escape. Be prepared to catch it and either reuse it or dispose of it properly. (Now is a really good time to inspect your hoses and replace if needed.)

The pump sits in a rubber mount to isolate it from vibrations.
Figure 7

The pump sits in a rubber mount to isolate it from vibrations. The pump can get corroded over time so you may have to use a small flathead screwdriver or pick to help break the seal between the mount and pump and then push the pump back and out from the mount (red arrow).

You can now remove the pump from the vehicle.
Figure 8

You can now remove the pump from the vehicle. Installation is the reverse of removal and do not forget to fill and bleed the cooling system.

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Comments and Suggestions:
clearstoker Comments: On at least 83 w123 cars the aux heating pump has nothing to do with the cooling of the engine. It is only in operation when the engine is running and the climate control system calls for heat. Per the Mercedes workshop manual control of the aux heat pump is through the climate control system. There is no provision in place in the electrical system that would be able to run the pump after the engine is shut down till temps in the head decrease. I think it would be a neat thing to have but it is as I mentioned not present in the 83 and earlier cars. I have 5 83 300D types including a C two D sedans and 3 TD station wagons. I also have two SDs an 81 and an 82 neither of which have any means to run the aux heat pump after the engine shuts down.

Is it possible that this was a function that was added with the 85 California version which has the trap oxidizer?
May 16, 2016
  Followup from the Pelican Staff: Not sure, I don't have info that far back. I will check with the author and have the article updated if needed. - Nick at Pelican Parts  

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