Ac blows cold from center vents and hot from side vents

[ltpowers]

I have this problem only on very hot days, over 95, while driving in traffic or after parked for awhile. The AC will be working fine then suddenly extremely hot air comes out of the side vents while the center vents continue to blow cold air. I found that by switching the control to the next AC button and putting the fan on high, after a few minutes I can hear and feel the air flow change and then I switch to the regular AC button and everything is fine. Could it be that by increasing the airflow through the evaporator thus causing a pressure change in the system and then it returns to normal operation? It has only happened on very hot days. Also if the fan happens to be on the low setting it will happen more often. Aux fans are working. Maybe viscous fan clutch problem, but the engine temperature doesn't rise.

 

[gsxr]

Sounds like a monovalve issue to me (or, a flaky pushbutton unit). Is this a US-spec car with automatic climate control?

 

[ltpowers]

It is a USA car, 1994 E500

 

[gerryvz]

I would also check function of all of the vacuum pods to make sure they are functioning correctly. You have to pull the airbag and perhaps passenger side center console side carpet to do this, but a few minutes with a Mityvac will tell you all you need to know, and you can eliminate pods as an issue.

If pods have never been done on ANYONE'S car, it's more likely than not at this age, that at least one vac pod is "bad" and has failed. And most of the pods on the 124 require dashboard removal to access (and one pod is no longer available from MB).

You know that $5,000 GVZ "deferred maintenance" tax that is talked about so much here? Well, if you have a bad pod, it's likely that replacing all of your pods will burn through close to halfof that $5K. Just for this one job.

 

[gsxr]

FYI... the vac pods control where air blows out the vents, but it does not control temperature. If air is blowing out the wrong place (i.e., no cold air out the center dash vents, or no hot air out the footwell vents) then you have a vacuum/pod issue.

If there is heated air present at the same time as cooled air, most likely the monovalve is not receiving a single, or the monovalve isn't working right, or the pushbutton unit (brain box) is not sending the correct signals.

I've never experienced this particular failure mode on a W124, but on my W210 it was the duovalve...

 

[ltpowers]

I see your thread on replacing heater valve on non v8 cars. How is it on the 94 E500?

 

[ltpowers]

I like the idea of replacing the monovalve rather than dealing with the pods. Definitely heated air coming into the cabin. Can I get it aftermarket or does it need to be MB part?

 

[gerryvz]

Now that I re-read the question, I believe you are 100% correct Dave. Sorry for the confusion in my earlier response.

As I remember it now, I had an issue on a certain white 1995 E320 wagon that mirrored this one a few years back ... exactly the same. A replacement of the monovalve heater valve fixed the issue. That valve wasn't cheap, either.

See this tread for the same issue: HOW-TO: Replacing Heater Valve on Late W124 (non-V8 models) E-Class cars | Heating and Cooling

Note though, that the 500E and E500 maintained the "early" style W124 heater valve system, whereby the components (aux coolant pump and monovalve) were separate.

I believe that with advent of the 1992 model year, or thereabouts (latter part of calendar year 1991), MB "integrated" the monovalve and aux coolant pump into a single unit. Again, this "integrated" unit DOES NOT APPLY to the 500E and E500, which kept the separate components.

I believe that for the 500E and E500, at least the aux water pump is available as a separate OEM component (Bosch), for about $135 from the usual vendors. MB list price is over $300 for this auxiliary water pump component of the system (part number 001 835 13 64).

The actual "monovalve" (water valve) component should be something like part number 001 830 20 84 and would be around $125 from the normal suspects. GSXR EDIT: See post #17 below with more info on the 2 different monovalves, and the VIN break.

By the way, for optimal system operation, I would STILL check the operation of all vacuum pods. The rubber diaphragms in the pods aren't getting any younger, and there's still a high likelihood that at least ONE of the pods is not working correctly. Just sayin'

Cheers,
Gerry

Auxiliary water (coolant) pump component:




Monovalve (water valve) component:

 

[ltpowers]

Thanks Gerry for the quick response. I will look around and order valve.
Now for the $5000 ongoing maintenance....I need to do some front end work, struts, undercarriage, etc.

 

[mistermiata]

I had the exact same symptoms on my 94 E500. Monovalve fixed it. Pods still ok (knocking on wood with fingers crossed).

 

[samiam44]

If you need a monovalve-

INspect the old part first. EPC parts break is wrong for this part... THe earlier ones are 3 hose connections and later were 4.

 

[ltpowers]

The heater control valve has 4 hose connections. Looking around for prices, Amazon $335, Parts Geek $445, Ebay $494, eEuroparts $92 (but no picture). I will call MB tomorrow.

 

[gsxr]

I'm seeing p/n 001-830-25-84 in the EPC for chassis as of C096887. $474 MSRP from Mercedes, around $375 wholesale. Doesn't appear to be available aftermarket.

EDIT: See post #17 below with more info on the VIN break. Also, this late-style valve is NLA from Mercedes.

1958-1993 Mercedes-Benz Valve 001-830-20-84 | MB OEM Parts

1958-1993 Mercedes-Benz part # 001-830-20-84 - Valve

www.mboemparts.com

 

[samiam44]

I thought there was someone on the list said you could use the guts of the early valve to rebuild the late.

I don't think he wrote it up. Could be mixing them up.

M

 

[gsxr]

Michael, I'm not sure, but that is a good idea. The early valve (to chassis C096886) is p/n 001-830-20-84 which is only $128 MSRP.

EDIT: See post #17 below with more info on the VIN break.

 

[samiam44]

Not me to credit Dave.

Another member..


M

 

[gsxr]

BTW - Michael is right, the EPC chassis break is incorrect. The late-style expensive monovalve was phased in earlier than the break specified in the EPC. My one car (C0870xx) has the late valve and it's not supposed to.

C0036xx has early valve
C0581xx has early valve
C0607xx has early valve (October 1993 build)
C0870xx has late valve (December 1993 build)

C096886 = EPC last chassis with early valve
C096887 = EPC first chassis with late valve

 

[ltpowers]

I ordered one from Lionel at Gainsville MB. $360. A rebuild sounds like a good idea and if I had extra time I would like to explore making a hybrid valve.
I car has been getting loud, so I checked out the exhaust and I have a rotted middle muffler ( acouple of holes on the bottom and perferated inlet pipes. I will ask the for options on a new thread.

 

[IanAtkins]

BTW - Michael is right, the EPC chassis break is incorrect. The late-style expensive monovalve was phased in earlier than the break specified in the EPC. My one car (C0870xx) has the late valve and it's not supposed to.

C0036xx has early valve
C0581xx has early valve
C0607xx has early valve (October 1993 build)
C0870xx has late valve (December 1993 build)

C096886 = EPC last chassis with early valve
C096887 = EPC first chassis with late valve

Hi GSXR - sorry to be dumb, but my chassis number WDB1240361C162922 how does that relate to these numbers above for the valve? Is it the letters after C which count? So as mine is C162922, obviously later than the 'first chassis with late valve' so am I right in thinking that it's the late valve I need?

 

[IanAtkins]

by the way, my superheated air coming through the side vents is an infrequent intermittent fault at the moment, summer and winter. My pods have been checked so the mech thought I needed a new control unit. Sourced one, and now the intermittent hot air problem is less frequent than before, it's still there.. what do you think?

 

[IanAtkins]

so some journeys I don't have the problem at all, mostly after about an hour, the superhot air comes through the side vents. it can disappear after 5 mins, or it can last for 30 mins and won't right itself until the car has been switched off, then on again...

 

[gsxr]

Hi GSXR - sorry to be dumb, but my chassis number WDB1240361C162922 how does that relate to these numbers above for the valve? Is it the letters after C which count? So as mine is C162922, obviously later than the 'first chassis with late valve' so am I right in thinking that it's the late valve I need?

You are correct. Your chassis (last 6 digits after the "C") is well beyond the break point, and should use the "late" style valve.

by the way, my superheated air coming through the side vents is an infrequent intermittent fault at the moment, summer and winter. My pods have been checked so the mech thought I needed a new control unit. Sourced one, and now the intermittent hot air problem is less frequent than before, it's still there.. what do you think?

My first thought would be to swap the control unit, just to see if anything changes. It's interesting that the problem is still present, but now is slightly different.

Before shelling out $$$ on a new monovalve, have you tried unplugging the auxiliary coolant pump located ahead of the CAN box? Sometimes, if this pump is drawing excess current, it can put the control unit into a safety/fault mode. If the pump is old/original, and the problem goes away if you disconnect it, the pump is likely bad. This is a long shot, but the experiment is free...

 

[IanAtkins]

Thanks for that. Yeah, it's strange that the new control unit (from a 53k-miler) has only partially solved the problem. The issue with your suggestion to see if the problem goes away if the pump is disconnected, is that it's a rarely-occuring intermittent fault. Can I test this without needing to run the car for hours to see if the problem doesn't recur?

 

[gsxr]

Can I test this without needing to run the car for hours to see if the problem doesn't recur?

Intermittent faults are the worst. If it's not something you can easily reproduce, most likely you'd have to run the car for a while (possibly hours) to see what happens. I'm really not sure what to suggest in your situation. For grins you could check for fault codes from the HVAC unit with a blink code reader, but if there are none, you're back to square one.

 

[luckymike]

Ian, I had this same problem on our '95 E420. Super hot air from the sides. Several times turning the car off and back on would cure it temporarily.

The final answer for me was to remove the monovalve and clean it up. It had some debris on it and I guess wasn't moving as it should. Just four screws as I recall. Anyway, no more issues.

 

[IanAtkins]

Thanks for that guys - the monovalve was the thing that nobody had mentioned, from the early posts in this thread it seems like it could be that.. I'll see if it's dirty and hoping that could be the issue. If cleaning it up makes any difference (fewer faults/less frequent) I'll replace it; if not - square 1...

Much appreciated.. will let you know.

 

[Bill Walsh]

so some journeys I don't have the problem at all, mostly after about an hour, the superhot air comes through the side vents. it can disappear after 5 mins, or it can last for 30 mins and won't right itself until the car has been switched off, then on again...

I'm glad I found this thread. I also have had this problem for awhile. This happened when I stopped for a few minutes to get coffee and then upon start up the hot air would come blasting out the side vents. I just had a new mono valve installed and so far so good.