W123 Heater control help needed
 

Not a new topic at all but I could use some brainstorming.

Just returned from a trip to Charleston SC, a total of 1250 miles. Drove the 1983 300CD. Car ran great as expected but ... froze our arses off on the Virginia to SC border leg.

For some time I had been having the common heater issues at highway speeds. Since I was using a cheap aftermarket monovalve solenoid, I fell for the urban legend about them "blowing closed" at high speed and just before leaving bought a high buck OE solenoid for about $130.00. After installing it there was absolutely no change in operation.

The system regulates perfectly at around-town velocities so I do not suspect the control pushbutton assembly or hose blockages. I tried pulling the connector completely off of the monovalve solenoid expecting it to default to full heat. It did not. There was no change, that is heat at low speed and no heat at high speed. The thermostat appears to be regulating the coolant temperature o.k., in the 82 to 84 degree range as expected this time of year..

I know there are a number of you out there with similar issues. Any discoveries to share?

#1 Question :

What temperature did the dashboard coolant temp read at highway speeds ? .

82 to 84 deg C. At one point in the trip, I even tried the old trucker trick of partially blocking the radiator with cardboard. No change.

For what its worth department, I repaired and replaced everything to get heat, with limited success, on an 84 300d. What finally fixed it permanently, was replacing the controller behind the glove compartment. As it turned out, that was the easiest repair I did on the car. I plan to take the old one apart one of these days to see if the board can be repaired.

Thanks. This one never occurred to me. I'll take a look after I try couple more things.

It occurs to me that, if your heat doesn't work with the monovalve unplugged, the problem cannot possibly lie with the temp controller.

Good point. Thanks.

Today removed the entire monovalve assembly took it totally apart and inspected it. I found a big, hard clot of calcification blocking the output side (lower outlet). I was excited about this since I thought I had found the problem.

The clog was so old and hard that I had to drill it out. Reassembled and installed the monovalve and guess what; NO DIFFERENCE. Still low speed heat regulation and cold air above 60 mph.

Still stumped. Can the aux pump block the flow if it is not in the circuit? Remember, it regulates fine at those speeds where the pump is supposed to work. When the pump is not working at higher speeds, can it block the flow?

If no answers from the brain trust, I'll test the pump to see if it is working correctly, then bypass it to see if that will allow flow.

You should have already tested the aux. pump and tried it again after the clean out with the monovalve unplugged .

Keep us posted and don't despair, you or someone will figure it out in due time .

Nate, I think the aux pump is probably o.k. since I have beaucoup heat at slow speed but it is worth a try.

I never give up.

That's The Spirit !
 

Neither do I, it's both a blessing and curse at different times....

Use it as a teaching lesson and persistence usually becomes a good thing .

A good heater in Jan./Feb. is a good thing too :P .

Can someone post either instructions or a diagram showing exactly how the monovalve solenoid is assembled into the valve body? I have had it apart so many times that I may have gotten components out of order. It can go together more than one way.

That part you drilled was a ball check valve , pops up with spring,test by lightly pushing
down and see or feel if pops back up from top side, FSM says to replace if not working.

Mono valve can become hydraulically locked if water gets on both sides of the rubber
diaphragm,even if new part. Test,remove mono valve repair part,pull back diaphragm
shake hard to remove water, if no water not the promlem.reinstall,if now works mono valve bad.

Test water heater hoses ,is temp.hot on both sides,in and out.

Is heater temp. nice and hot ,engine rev. to 2,500 parked,now cold when moving?
Check fresh air flaps.

What leads you to believe that the aux pump operation is speed dependent? Urban legend?

FSM description indicates that aux pump provides coolant flow at slow speeds.

I think that would be more of an explanation of purpose than it is a description of operation.

Yes. They mean slow engine speeds, not vehicle speeds. I just got a 1984 300TDt and it's got heater system issues in spades. Haven't gotten to probe what works and what doesn't. It's got a manual switch hooked up and a ball valve under hood but I didn't feel a lot of anything.

Anyway back to your issue. Can you sit in neutral at highway engine speed and get heat? Or better yet get hauling down a big hill on the highway and drop it in neutral and see if heat returns while coasting at speed(DONT HIT NEUTRAL BY MISTAKE!!!), rev match or wait till your slowed down alot to put it back in gear. Maybe a heater hose or component is internally failing and blocking flow under higher pressure when the water pump is running at highway rpm, the way a brake line can fail and become a check valve not allowing the brakes to release.

Sent from my SM-J737A using Tapatalk

Thanks. Will try engine-speed dependent operation as soon as possible. Will get back to you-all.

I think a hydro-locked solenoid is unlikely since I just installed an OE hi-$ one.

The operation of the aux pump is not speed-dependent. It isn't rpm dependent. It's function is to improve coolant flow at low rpm, but that doesn't mean that it operates only at low speed or rpm.

Don't jump to that conclusion too readily.

I meant (DONT HIT REVERSE) by accident when testing.

Sent from my SM-J737A using Tapatalk

Tried engine speed dependency and it is inconclusive. Cant coast long enough.

I tested my aux pump today and it is inop. I think I will bypass it to see if I get heat that way. If so, I need a new pump and I don't understand the system.

I replaced literally the entire HVAC system on my car, including several different monovalve housings and cores (OE and aftermarket) and never got it to work properly. Same problem, no heat at highway speeds. Do yourself a favor and search for "mercedes w123 monovalve eliminator" - There's a kit available that replaces the unit with a vacuum actuator, assuming your vacuum system is in working condition, the kit will dramatically improve the system. It's an easy one one night installation job. The kit also includes plumbing to bypass the auxiliary water pump, though that's not strictly necessary, I used it and haven't had any problems since.

Monovalve & Aux. Coolant Pump Eliminator
 

THANK YOU ! .

I found it easily, seems $pendy $125.00 but looks to be a good solution :

https://klimakit.com/product/monovalve-and-coolant-circulator-pump-eliminator-upgrade-kit/

Klima II Heat
 

As it turns out, it's rainy & cool in So. Cal. so I fired up my Coupe, rolled the temp. wheel to heat (not the MAX setting, just before the click) and headed out ~ in seven blocks the dash vents were softly blowing hot air, I use the 'EC' button and the low fan setting 99 % of the time as the fan noise bothers my wrecked hearing .

This upgrade _may_ be the co$t effective way to go.....

When the auxiliary water pump is dead, it drastically slows down the hot water flow to the heater core so by passing it might just be the solution if everything else is O.K. .

For a simple , NO CO$T test : block the radiator with cardboard and duplicate your highway speed no heat drive ~ if you get more heat, the thermostat isn't working well .

If you still get the same cool air there's a hot water flow to the heater core problem, monovalve or aux. pump .

I could be wrong and it may be different on the diesel, but I remember looking at my 450slc with a m117 v8 heating system and thinking that it was taking the hot coolant and returning it to a place where there was no positive pressure to move the coolant without help from the aux pump. Meaning that there wasn't any (or very little) change in the pressure from the main water pump in the in and out flow from the heater core. So the aux pump may be the only thing really moving a good amount of coolant through it. It's possible at higher rpm the heater core(and valves and bad pump)is just a more restrictive path and the hot coolant is just not going that way because it is taking the easier route. I have all of the heater control valves removed in my car and only have a blower and aux pump switch. It had the dreaded Chrysler garbage and it's a drift car so I only need defrost occasionally and to help cool it down. I can still feel a little heat at speed though. I've considered a ball valve to kill it in summer but haven't had to yet.

Sent from my SM-J737A using Tapatalk

In a previous post I noted that my aux pump is not working. My theory is that a BD pump blocks flow to the heater core.

Today, I bypassed the pump and the road test showed plenty of heat at both high and low vehicle speeds. It also seemed to regulate. I will replace the pump in the Spring but for now I am satisfied.

Auxiliary Water Pump
 

There are now inexpen$ive Chinese ones, I've not been able to find the original BOSCH or HELLA ones in a few years .

If you like to tinker, take the oil one off the car, invert it and ever so carefully pry off the delicate and crispy white cap n the motor end, it's a lubrication point and once you turn the impeller a little bit with a few drops of light oil in the now open bearing, it'll run when you power it up ~ the Dealer Mechanics were supposed to dribble in a drop or two of oil every so often, I doubt any ever did .

I'll try that nate. Thanks.

Bosch universal water pump E-bay $ 60.00 . Same pump?

?Who knows ? .

You didn't include a link for us to check.....

Bosch universal water pump,E-bay. (Google search).
E-bay seller Autodetus (656).
Listed as-Engine auxillary water pump bosch 0392020034
fits 00-01 cadillac catera.
Other pumps available at lower cost,quality unknown.:confused:

My aux pump ('82 300D) hasn't worked for decades. But my heat works fine at highway speeds. So there must be more to the story than the pump simply being inoperative.

Thanks TF007. I bypassed my pump and removed it to work on. I now get heat at highway speeds (I think). Haven't really cruised at sustained speed yet but so far it seems o.k.

Hard to know at this point how critical that pump is to normal operation.

The aux. water pump was installed because of complaints,low heat at slow
engine speeds.
Parts of aux.pump (broken parts) can lodge in water ways,slowing flow,sludge
can also block.
Yes! some cars seemed to heat ok with out this pump.

Here's the link : https://www.ebay.com/sch/i.html?_from=R40&_nkw=0392020034&_sacat=0&_pgn=1

Looks pretty close to me, you'll have to make sure the rotation is the same, measure the hose nipple size for match (20MM) and cobble up a connecting wire from the old pump, easy enough to do .