I'm going to start with the big dumb question: Where is it? I've found several threads on here that suggest it's under the washer fluid reservoir but my FSM and the EPC suggest it is behind the coolant reservoir. There are items in each location that look like they could be the aux pump, although it's very hard to see the one under the washer reservoir.
Next questions: 1) If I turn the key to the 2 position and turn on the defrost at max, the aux pump should make noise, correct? The one under the washer tank does not, so either that's not it or it's not working. 2) Does my car have a fuse for the aux pump or not? The fuse listings suggest fuse #7 covers the pump, but I've read several threads suggesting the addition of a 1-amp fuse to the power input. I'd like to add one if it is not already protected.
Thanks ... busy with the wiper motor for now, but figured I'd pick up an aux pump at the dealer while I'm there to get the wiper motor, if I can determine the pump is failed.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

It's under the windshield washer reservoir. The monovalve is between the battery and coolant reservoir.

I don't think there's a separate fuse for the aux pump. Fuse 7 is for the ACC system. I could be mixing this up with another car.

Sixto
87 300D

Thanks ... that was my original impression. What's the thing behind the coolant reservoir?

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

I edited my first response.

There's a buzz under the hood of my car when the key is one click between off and glow. I always thought it was the aux pump.

Sixto
87 300D

The pumps go bad, then they wipe out the contacts on your heater control. I Bought an aftermarket pump installed a switch on the dash, a relay and a circuit breaker. it's great you can cook a Turkey in the front seat when that pumps working. I did the same thing with the blower motor.

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1983 300 D 256K Turbo drain, vacuum pump

Be a bit careful in making this claim for a W124.

It's not applicable to the second generation W126, and I suspect that the W124 might be similar.

I stand corrected , i got excited

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1983 300 D 256K Turbo drain, vacuum pump

The point is that folks need to be sure of their statements prior to posting. Just because it occurs with your vehicle does not necessarily mean that it occurs with a different model of Mercedes.

You might be correct...........might not...........personally, I know nothing about the W124 and I always defer to those who own them.

Brian, you are correct. Although not separately fused (as Sixto notes), the aux pump in the W124 model is powered by the climate control electronics, which automatically shuts off if the pump draws too much current.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Thanks Jeremy.

Good luck trying to hear it, as usually the engine is running when you're making use of it. Only reliable way I know to tell if it's operating is to pull the washer reservoir out of the way, and reach down and feel it when the climate control is off, then again when you have it turned on for full heat - you can feel the difference.

If it isn't operating after you've done a drain and refill of the coolant system, you should hear air gurgling thru the heater circuit inside the cab when you call for full heat - without the aux pump, apparently the engine doesn't generate enough flow thru the heater circuit at idle to purge the air out - this happened on the blue 87 when I got it, had a failed aux pump - once I installed the good pump out of the wrecked 87, that problem went away.

For that matter, it seemed bloody near impossible to get all the air out of the heater circuit, even with revving the engine repeatedly, until I gave up and changed out the aux pump.

Another indication that the aux pump and monovalve are healthy and doing their job - on a cold morning, after the coolant temp is up to some semblance of hot at idle, turn on the climate control for max heat - within a half-minute, you should see a sudden drop of around 5 to 10 C on the coolant temp gauge, as the cold water from the circuit inside the cab is flushed into the engine and past the temp probe. If after a couple minutes or so you don't see any noticeable drop, or have to rev the engine to get this reaction, then something's fubar.

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Just say "NO" to Ethanol - Drive Diesel

Mitchell Oates
Mooresville, NC
'87 300D 212K miles
'87 300D 151K miles - R.I.P. 12/08
'05 Jeep Liberty CRD 67K miles
Grumpy Old Diesel Owners Club

If that aux pump is blowing fuses you're lucky. It will often take out the CCU. Whenever the aux pump is suspect, rig up some jumpers from the battery and series in an ammeter to test the current draw of the pump while in place and cooling system filled. If it draws more than 1.2 Amps replace it. If it is drawing more current than this it can easily take out the CCU.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

Larry, you're thinking of the W123 CCU. The W124 CCU is internally protected and will shut down in the event of an overload in any of the devices that are powered from it.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

My experience has been much different. I replaced the CCU on my 300E and a slip of paper even came with it warning that if the aux pump was drawing more than 1.2 Amps, the warranty would not be honored. I checked the pump and sure enough it was drawing almost 2 Amps. I ordered the pump and the car was down a little longer because I was not going to risk the CCU.

Now, from what you say, if this was the case on mine, then maybe it shut down with the bad pump and when I replaced the pump it allowed the new one to work. If that's the case, however, why would they deny claim in such a situation?

My 124 car is an '88 model. Maybe others have a different set up.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

Maybe you're both right. The pump might blow a CCU internal fuse which has to be replaced before the CCU can function again. Less destructive than frying the CCU.

Sixto
87 300D