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I purchased my 2004 545i in the summer and as soon as the temps dropped I tested the "REST" feature and found that it blew cold air after a couple seconds. I also had the KLIMA 9C77 code that pointed to a bad auxiliary water pump. This wasn't a big deal to me, but I noticed that a few times my heater started blowing cooler air while I was stopped at a light on a really cold day. That was a bigger deal to me. So I started shopping for a replacement pump. I know there are many who insist on OEM parts, but I'm hopelessly cheap. I found our pump on eBay for $50, but also found one that looked identical with a different part number and the addition of a rubber mounting bracket. The listing said it was compatible with the E70, F15 and others, and it was only $36. I had to go for the $36 pump just to add a little more intrigue to the job.

Last weekend I received the pump and did the swap. This aux pump is on the driver's side of the engine bay, just below and to the left of the air box (if you're standing in front of the car looking under the hood). I did not drain the coolant prior to removing the old one. It drained a little less than a gallon of coolant when I released the hoses, but nothing that a shoebox-sized tub couldn't catch. The two hoses that connect to the pump use spring clips to secure them. I just used a flathead screwdriver to pop the clips out and then gently pried the hoses off. The pump easily slides up and out of the rubber mount and the electrical connector comes off with a simple squeeze and pull. I then took the rubber mount off the new pump, slid the pump in the old mount, and reconnected everything. The hardest part was maintaining enough pressure on the hoses while I pushed the spring clips back in to lock them in place.

After adding coolant I went for a test drive and confirmed that nothing was leaking, and when I got home I tried that “REST” feature again and what do you know? I now have heat for minutes! I really don’t see myself using this that much, but hopefully this gives me more consistent heat while driving as well. This job did not require anything to be disassembled in order to get at the pump. It’s fairly accessible. And I can confirm that the updated pump (that I found cheaper) is compatible. The part numbers for the pump I used, according to the eBay listing, are 6411919708, 64 11 9 197 085, 64119197085, 41523E, and 89040. I just wanted to throw this info out there in case it helps anyone else. I’ve attached a picture of the aux pump installed (taken by user kskane) and of the one I purchased.
 

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very first repair i did when i got the 550
 
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