Funky Synergy between Aux Water Pump, CCU, and Monovalve - PeachParts Mercedes-Benz Forum

shertex · #1 12-02-2011, 05:37 PM

I had thought I knew all I needed to know about the auxiliary water pump: weak heat at idle, replace it. But then, subsequent to experiencing that tell-tale symptom, I began to have other heat issues: intermittent heat irrespective of engine speed. So I start wondering about the CCU and the monovalve. I swapped out the CCU with a known good one....same problem. So I conclude I must have a bad monovalve....even though it's only been in there for 22,000 miles.

BUT THEN....I start reading some posts suggesting that there may be a relationship between a failed AWP and CCU/monovalve functionality. So I wonder, "What would happen if I simply disconnect the power to the AWP?" It's still early, but it appears as if my intermittent heat issues have disappeared!

For some of you this may be old news....but it was news to me, so I thought I'd pass is along in case others are having weird heat issues.

Brian Carlton · #2 12-02-2011, 07:40 PM

You cannot make the conclusion that disconnecting power from a failed AWP will cause the performance of the system to improve.

The pump isn't rotating if it has failed. Disconnecting power from a failed pump doesn't make it rotate............sorry.

You've confused yourself with too many potential issues.

Disconnect power from the monovalve...........drive the vehicle...........report back with results.

shertex · #3 12-02-2011, 07:46 PM

Quote:

Originally Posted by Brian Carlton

Disconnect power from the monovalve...........drive the vehicle...........report back with results.

Disconnected power from monovalve during a time when cold air was blowing. Continued to blow cold air. Then reconnected power to monovalve and disconnected power to AWP. So far, heat functions normally (other than, obviously, weak heat at idle).

So what have we learned?

shertex · #4 12-02-2011, 07:50 PM

More generally, I'm wondering if and how a failed AWP can affect the rest of the climate control system. I'm wanting to ask these questions independently of whatever is going on in my particular situation.

Brian Carlton · #5 12-02-2011, 07:52 PM

Quote:

Originally Posted by shertex

Disconnected power from monovalve during a time when cold air was blowing. Continued to blow cold air. Then reconnected power to monovalve and disconnected power to AWP. So far, heat functions normally (other than, obviously, weak heat at idle).

So what have we learned?

We have learned nothing.

Something is missing from the story.

Brian Carlton · #6 12-02-2011, 07:53 PM

Quote:

Originally Posted by shertex

More generally, I'm wondering if and how a failed AWP can affect the rest of the climate control system. I'm wanting to ask these questions independently of whatever is going on in my particular situation.

The heat is weak at idle and at low speeds.

Nothing else is affected.

shertex · #7 12-02-2011, 08:02 PM

Quote:

Originally Posted by Brian Carlton

The heat is weak at idle and at low speeds.

Nothing else is affected.

In this thread on the relationship between AWP and monovalve...

Confirm this: AUX pump makes monovalve work better

you say, "There is some relationship between the pump and the monovalve. It has to do with the CCU in some fashion. When the pump draws too much current, the CCU does some funny things with the monovalve." (Post 3)

Do you still believe that's the case? This is what I'm attempting to explore.

Not trying to be snippy....got the idea from you, hoping you and others who are able to chime in can elaborate.

Thanks for your time.

shertex · #8 12-02-2011, 08:53 PM

Quote:

Originally Posted by Brian Carlton

We have learned nothing.

Something is missing from the story.

What's missing?

engatwork · #9 12-02-2011, 09:00 PM

I run the aux water pump disconnected on both my 95 E300's and the hvac system works great.

Brian Carlton · #10 12-02-2011, 10:03 PM

Quote:

Originally Posted by shertex

What's missing?

I wish I knew.

The only possible situation that I can fathom is if the seized AWP is drawing too much current from the CCU and is affecting it in some way. Disconnecting the seized AWP eliminates this condition and effectively unloads the CCU from the "problem".

This is strictly hypothetical and I'm really reaching here..............

Yak · #11 12-02-2011, 10:28 PM

According to the attached schematic, with the temp in MAX, the monovalve control signal is no longer routed through the control unit, but is sent through a series of resistors. The AWP does affect the current that is pulled through the monovalve in this case. It'll always see some in current in MAX, due to the direct path to ground via R3, but that would be in parallel to R5 + whatever is to the right of R5. But there's also a voltage at R1 that compounds the calculations.

Maybe it's an iffy in-car temp sensor or foam tube? That doesn't exactly make sense with unplugging the AWP and suddenly the monovalve appears to work correctly but that would also appear to affect the current through the monovalve. I'd check the tube, then try running it out of MAX and see what happens.

shertex · #12 12-02-2011, 10:42 PM

Quote:

Originally Posted by Brian Carlton

I wish I knew.

The only possible situation that I can fathom is if the seized AWP is drawing too much current from the CCU and is affecting it in some way. Disconnecting the seized AWP eliminates this condition and effectively unloads the CCU from the "problem".

This is strictly hypothetical and I'm really reaching here..............

That's kind of what I was wondering. I know the circuitry in my car designed in such a way that the AWP can't take out the CCU. But perhaps there's nonetheless some sort of adverse effect.

shertex · #13 12-02-2011, 10:43 PM

Quote:

Originally Posted by Yak

According to the attached schematic, with the temp in MAX, the monovalve control signal is no longer routed through the control unit, but is sent through a series of resistors. The AWP does affect the current that is pulled through the monovalve in this case. It'll always see some in current in MAX, due to the direct path to ground via R3, but that would be in parallel to R5 + whatever is to the right of R5. But there's also a voltage at R1 that compounds the calculations.

Maybe it's an iffy in-car temp sensor or foam tube? That doesn't exactly make sense with unplugging the AWP and suddenly the monovalve appears to work correctly but that would also appear to affect the current through the monovalve. I'd check the tube, then try running it out of MAX and see what happens.

As yet another variable to throw into the mix, my sampler blower does not appear to be working.

Brian Carlton · #14 12-02-2011, 11:01 PM

Quote:

Originally Posted by shertex

That's kind of what I was wondering. I know the circuitry in my car designed in such a way that the AWP can't take out the CCU. But perhaps there's nonetheless some sort of adverse effect.

In my engineering endeavors, we discount everything unless it can be positively duplicated.

So, replug the AWP and see if the adverse symptoms return.

Then disconnect it, again, and see if the the problem disappears.

Try to eliminate any of your preconceptions from the data.

shertex · #15 12-02-2011, 11:02 PM

In terms of how to proceed, I guess I'll live with and monitor the current set-up (disconnected AWP) for a while, see if I continue to experience good heat except at idle, then replace AWP and sampler blower and see what I'm left with.