Aux Water Pump Question - PeachParts Mercedes-Benz Forum

VA-Merc · #1 02-08-2013, 09:10 AM

Hey guys, I replaced my leaky aux water pump on my new to me 83 300sd. I did do the fuse mod as well that I read about. The question is...the pump seems to be working, but after the car has been running for a few minutes, I can hear the pump cycling up and down every 2 seconds or so. Is this normal? I have never had the aux pump working in this car so I have no reference to know how it's supposed to behave. While it is cycling the heat seems to still be working fine. Any thoughts, or am I just being paranoid? I can hear the noise on the inside of the car as I am idling with the blower on the low setting. Thanks for any help!

Simpler=Better · #2 02-08-2013, 09:19 AM

No idea, but a possible fix would be to wire it up to it's own switch

cho · #3 02-08-2013, 09:23 AM

Quote:

Originally Posted by VA-Merc

Hey guys, I replaced my leaky aux water pump on my new to me 83 300sd. I did do the fuse mod as well that I read about. The question is...the pump seems to be working, but after the car has been running for a few minutes, I can hear the pump cycling up and down every 2 seconds or so. Is this normal? I have never had the aux pump working in this car so I have no reference to know how it's supposed to behave. While it is cycling the heat seems to still be working fine. Any thoughts, or am I just being paranoid? I can hear the noise on the inside of the car as I am idling with the blower on the low setting. Thanks for any help!

.

I never heard noise by aux pump... so maybe improper mounting comes to mind
is the sound "bubble-ish"...what about coolant lost in process..?
or maybe pump positioned too high....

edit: is it used pump,...did you check it before installation?

.

shertex · #4 02-08-2013, 10:00 AM

I assumed that it's supposed to run constantly.

Zulfiqar · #5 02-08-2013, 10:04 AM

I read somewhere that the pump is commanded by the CCU when the heat goes weak (idling at a red light)

VA-Merc · #6 02-08-2013, 10:18 AM

Thanks for the quick replies! It was a new (and not used) pump I bought off of ebay. It does not make a bubbling kind of noise...more like an electric motor whine (similar to when you have a noisy electric fuel pump on a gas powered car). You are right though, I should double check the mounting. I did put it back in the rubber-insulated mounting sleeve, but maybe it is sitting a little too close to the firewall. It still seems odd to me that it sounds like it is speeding up and slowing down. Would a good check for the ccu control be to put a voltmeter on the pins in the plug while the pump is running and see if it is getting a constant 12 volts? I guess I can't rule out a faulty new pump...I just wasn't sure if it was normal for these...I had thought they were either completely on or completely off.

rscurtis · #7 02-08-2013, 10:35 AM

The cycling of the monovalve will affect the speed of the pump as its water flow starts and stops. Temporarily disconnect the monovalve and see if the pump runs at a steady speed.

VA-Merc · #8 02-08-2013, 11:05 AM

Great idea! I didn't even think of the monovalve in the equation. I will test that during my lunch break. Is it normal for the monovalve to cycle that frequently if that is it?

uberwasser · #9 02-08-2013, 11:15 AM

Yes, when not fully open or fully closed the monovalve cycles open and closed quickly to let measured amounts of coolant through. That's what allows for specific temp control via the CCU.

MB-Owner-in-ind · #10 02-08-2013, 11:20 AM

Boy. I have tested the aux pump out of the car and barely heard it. I can't for the life of me figure how you can hear it inside the car over the sound of the engine and insulation

Jeremy5848 · #11 02-08-2013, 01:22 PM

The aux pump should come on and run continuously whenever the climate control system is "on" in any mode (any button pressed except "O" Off). However, there could be differences between models of Mercedes and I'm not familiar with the W126. I agree that it should not be possible to hear the pump over normal engine noise. You could put a voltmeter across the pump terminals to confirm that it gets 12 volts whenever the SSU is "on." The brand-new factory aux pump just installed in my '95 E300 (W124, OM606NA) made a lot of noise until the air bubble was pushed out of the way after the thermostat opened. It screamed like a small pump running dry (duh!).

Jeremy

Zulfiqar · #12 02-08-2013, 03:46 PM

Quote:

Originally Posted by Jeremy5848

The aux pump should come on and run continuously whenever the climate control system is "on" in any mode (any button pressed except "O" Off). However, there could be differences between models of Mercedes and I'm not familiar with the W126. I agree that it should not be possible to hear the pump over normal engine noise. You could put a voltmeter across the pump terminals to confirm that it gets 12 volts whenever the SSU is "on." The brand-new factory aux pump just installed in my '95 E300 (W124, OM606NA) made a lot of noise until the air bubble was pushed out of the way after the thermostat opened. It screamed like a small pump running dry (duh!).

Jeremy

how painful was the check? last I checked MB sell the entire unit for this car north of 500 dollars. Or did you just get a pump and screwed it on your old base.

cho · #13 02-08-2013, 04:14 PM

Quote:

Originally Posted by VA-Merc

Thanks for the quick replies! It was a new (and not used) pump I bought off of ebay. It does not make a bubbling kind of noise...more like an electric motor whine (similar to when you have a noisy electric fuel pump on a gas powered car). You are right though, I should double check the mounting. I did put it back in the rubber-insulated mounting sleeve, but maybe it is sitting a little too close to the firewall. It still seems odd to me that it sounds like it is speeding up and slowing down. Would a good check for the ccu control be to put a voltmeter on the pins in the plug while the pump is running and see if it is getting a constant 12 volts? I guess I can't rule out a faulty new pump...I just wasn't sure if it was normal for these...I had thought they were either completely on or completely off.

Correct!

.

Jeremy5848 · #14 02-08-2013, 04:35 PM

Quote:

Originally Posted by Zulfiqar

how painful was the check? last I checked MB sell the entire unit for this car north of 500 dollars. Or did you just get a pump and screwed it on your old base.

In the case of my '95 E300, the plastic base was cracked and leaking so I had to replace the entire pump and monovalve (this car and a couple of others have a combined unit) for ~US$500!

Several other cars use a similar combined pump/valve that sells for a lot less money but that did me no good. I saved the old unit as its mechanical parts are still good.

Jeremy

VA-Merc · #15 02-08-2013, 07:51 PM

Well...turns out my e-bay new pump wasn't such a good deal. It was making a lot of noise, and when I checked after work it hard started leaking coolant out of the casing. I will have to buy an OEM one this time.