Aux. Water Pump: To what extent does it help when NOT at idle? - PeachParts Mercedes-Benz Forum

shertex · #1 01-10-2010, 07:45 AM

Since I have heat but very weak heat at idle, I know I have a bad auxiliary water pump (91 300D). However, the heat when I'm moving, while pretty good, does not seem as hot as it should be.

My question: does the auxiliary water pump contribute to any significant degree to the system's ability to heat the cabin when the car is NOT idling?

Eric · #2 01-10-2010, 08:05 AM

it restricts it more than helps when above idle.

shertex · #3 01-10-2010, 08:10 AM

Quote:

Originally Posted by Eric

it restricts it more than helps when above idle.

So are you saying that a bad AWP would have no negative impact on heat while above idle? Just want to make sure I'm understanding you.

Eric · #4 01-10-2010, 08:14 AM

a bad pump will choke water flow at any speed. a good working pump only helps heat at idle.

shertex · #5 01-10-2010, 08:21 AM

Quote:

Originally Posted by Eric

a bad pump will choke water flow at any speed. a good working pump only helps heat at idle.

So then it does make sense that, while a bad AWP will be MOST noticeable at idle, it can also be manifested in diminished heating ability above idle. Is this correct?

Eric · #6 01-10-2010, 08:24 AM

yep

scottmcphee · #7 01-10-2010, 10:37 AM

Not quite a full yup.

"while a bad AWP will be MOST noticeable at idle", Yup, that's true. That's because the water pump on the diesels are a low pressure at idle, by design. Diesels like to run hotter and would otherwise be over-cooled at idle if the main pump was different. We suffer with that design by having low cabin heat at the stop light.

Invention born: aux water pump.

"it can also be manifested in diminished heating ability above idle", No, I don't think so.

At some point the belt driven water pump begins providing sufficient flow through the heater core, such that your interior fan couldn't take more heat even if it were offered. What the aux pump is doing at this point (being on or off) is irrelevant.

The aux pump is an impeller style thrust pump, which allows flow-through. If the input pressure is higher than what the aux pump could produce on its own, that pressure is flowing through and appears on the output side. Yes minus some flow losses, because even a straight pipe degrades flow. If the impeller is spinning in these conditions, it will at least "remove itself" from being a contributor of that problem.

I have two aux pumps on my car, for different reasons. So I've become very familiar with their operation.

I've actually tinkered with the idea of cutting power to aux pump based on an RPM threshold. They're a brushed motor design, so it would extend their useful life.

I *think* the stock aux pump is either coolant temp switched, or climate controlled, so it will cut itself off when conditions are right (not needed)... I'd have to look at the schematics again.

kerry · #8 01-10-2010, 10:44 AM

When I unplugged the auxilliary water pump on my 85TD while awaiting the delivery of the fuse harness I noticed that there was a noticeable reduction in temperature of the air blowing thru the heater. It wasn't cold but it definitely was not as warm as it was when the pump was functioning. I didn't use a thermometer to check the difference but it was noticeable on my hand. I'm guessing that the difference would become bothersome at sub zero temperatures and just a nuisance at higher temps.

Brian Carlton · #9 01-10-2010, 11:08 AM

Quote:

Originally Posted by scottmcphee

"it can also be manifested in diminished heating ability above idle", No, I don't think so.

At some point the belt driven water pump begins providing sufficient flow through the heater core, such that your interior fan couldn't take more heat even if it were offered. What the aux pump is doing at this point (being on or off) is irrelevant.

The aux pump is an impeller style thrust pump, which allows flow-through. If the input pressure is higher than what the aux pump could produce on its own, that pressure is flowing through and appears on the output side. Yes minus some flow losses, because even a straight pipe degrades flow. If the impeller is spinning in these conditions, it will at least "remove itself" from being a contributor of that problem.

I agree with the caveat that the auxiliary pump must be running. As Kerry mentioned, if it's present and not turning, it provides an impediment to flow.

scottmcphee · #10 01-10-2010, 01:19 PM

Quote:

Originally Posted by Brian Carlton

I agree with the caveat that the auxiliary pump must be running. As Kerry mentioned, if it's present and not turning, it provides an impediment to flow.

So long as the aux pump rotor wasn't seized or blades built up with crud, I don't think unplugging it would be a true impediment to heating the cabin once the main pump is pushing hard enough. The aux pump would free-wheel under sufficient pressure. Yes the little devil would impede some in this condition but again, it wouldn't matter to cabin heat.

At what RPM the main pump becomes sufficient, I don't know. It also depends on the condition of the blades in your main water pump. Ambient temps, condition of your cooling system, your mono valve operation, and yada yada yada...

I've driven in nearly -40 degrees this year already, and routinely at -35 C, and we know cold here. I feel the effects of aux pump stopping and starting, I've experimented with it. Ever had to scrape the inside of your windshield while driving just to break even on visibility?

shertex · #11 01-10-2010, 05:47 PM

Quote:

Originally Posted by kerry

When I unplugged the auxilliary water pump on my 85TD while awaiting the delivery of the fuse harness I noticed that there was a noticeable reduction in temperature of the air blowing thru the heater. It wasn't cold but it definitely was not as warm as it was when the pump was functioning. I didn't use a thermometer to check the difference but it was noticeable on my hand. I'm guessing that the difference would become bothersome at sub zero temperatures and just a nuisance at higher temps.

I assume you mean at speeds other than idle?

kerry · #12 01-10-2010, 06:23 PM

Quote:

Originally Posted by shertex

I assume you mean at speeds other than idle?

My recollection is that the difference was the same at idle and at speed.

scottmcphee · #13 01-12-2010, 12:06 PM

You sure your mono-valve is OK?
And thermostat?

Graplr · #14 01-12-2010, 12:15 PM

I have my aux pump hooked up to a manual switch.

Things I've noticed-
At idle with the pump off, there is a NOTICEABLE drop in vent temps in any outside air temp.
At highway speeds-
When above about 15-20F outside air temps, not much noticeable vent temp difference with the pump on or off.
When below about 15F outside air temps the vent will be cooler with the pump off. With it on a bit warmer.

howards@howards · #15 01-16-2010, 10:49 AM

I have done it all. The pump burns then melts the heater circuit board then the fuse. And also the blower moter melts the fuse holder ten times.

Put the new aux water pump and the blower motor wired direct on a relay that is activated by a toggle switch.

The toggle switch gets it's power from the old pos blower lead.

At idel or running down the road I can melt the snow on the side of the road.

It will eventually cool the motor off at idle below 10 deg outside