Meh... joining the AC issue crowd... - PeachParts Mercedes-Benz Forum

cphilip · #1 05-28-2008, 05:28 PM

Running along nicely cool today and all the sudden she started dumping heated air in. Has been running nicely and it was real sudden. Like perhaps the Door slammed open. This was after a 45 mile run one way and was about half way back and it happened pretty much suddenly. Fan speeded up too.

Compressor runs and cycles as if its doing its thing just fine. Was putting out real nice cold air yesterday (and for a few months now even) and today up until it just suddenly started dumping real hot heated air. In every setting. If you shut it all down and turn it back on a few minutes later. it seems to try and put cool air for a minute but is quickly over taken by Heated air. Even dumps heated air when should be pumping in Outside (cooler than this) air on the EC mode.

Belts all there. Other Vacuum stuff running fine. Locks and Shut off etc. Fuses all look fine.

It's like the deflector Door lost its Vacuum to me? Like it defaulted to Heat or something.

Where to look... besides me heading off to do a search, as I seem to recall seeing this discussed a few times...

This is a converted (by PO) 134r system but works well... until a real sudden change on that ride today. It is the 123 in my signature.

Search showing possible culprits... cracked Vacuum line, Monovalve falure and perhaps cracked control dial.

techguy512 · #2 05-28-2008, 06:25 PM

My bet is busted monovalve.

cphilip · #3 05-28-2008, 06:41 PM

It might be my imagination... but looking back... I seem to recall hearing a thump... like some road debris... Not sure where it came from. Happened perhaps two three minutes back down the road from when it suddenly started to dump hot air and fan speed up.

Now... I don't want to put too much into that because there was a lot of traffic and nothing else is missing or wrong with the car. Runs good.

But wanted to mention a possible symptom that I might have disregarded but don't even think maybe is related. Could maybe not have been from my car. Could have been something sliding around in the trunk as we were hauling back some music gear back there.

Where is this Monovalve located? Is is also called the Heater control valve in some cases? Or is that different?

cphilip · #4 05-29-2008, 10:34 AM

Oh I have had an epiphany on this... just this morning... after having already ordered a Monovalve rebuild kit last night (but no big deal as having one on hand is always a good thing)...

I have had issues that I have not yet had time to resolve with my battery. It appears my Alternator is not putting out and I purchased a new Voltage regulator that I have not had time to install to see if that fixes it or if I am going to need to put in another entire Alternator. Well... I have been charging the battery periodically to get by. And normally go a week of normal use before it gets sluggish. But this period saw the holidays (Had charged it up on Sunday last) and an increased number of starts and stops occurred until yesterdays longer trip. What was different about this trip (that didn't dawn on me until this morning) was that yesterdays trip was in a light rain. Causing me to have to use my Headlights for most of the way over there while burning the AC all the way. This is the first time since the Alternator quit putting out (about three weeks ago) and second time I have recharged the battery, that I have had to burn the lights too. Making the period of time between needed charges less, I suspect. It did get a number of starts and stops that would equal a week (in hindsight) PLUS a run with lights on, in a three day period (I estimate now)

And so (chime in if this theory holds water) what I now think happened was that on the way back it reached a point where the voltage to the Monovalve dropped to the point that it allowed the Monovalve to open. It was not getting enough voltage to hold it closed.

I conclude this because when I went out to start the car this morning, dead (well very low) battery to the point it would not crank over. So the problem is probably not the Monovalve but the inoperable Alternator and resulting low voltage from the battery. It was enough to run the fan and the lights but probably not 100% but at some point could not supply the CCU enough to put out 12V to hold the Monovalve closed.

Will not know for certain until I recharge the battery all the way but I think I am on to something.

Sound logical?

#5 05-29-2008, 10:48 AM

Quote:

Originally Posted by cphilip

Sound logical?

Yes - it sounds like you are onto something. Fortunate as battery/alternator/voltage regulator replacement (even all three!) are certainly less budget-busting than what would be a true A/C refurb.

Good luck.

babymog · #6 05-29-2008, 11:04 AM

I had the alternator go on my '91, fan sped up, couldn't turn off the climate control when battery voltage dropped.

cphilip · #7 05-29-2008, 11:08 AM

Well... I sort of dismissed the AC itself because I could turn the Climate system off and then wait a bit and turn it back on and feel, for a few brief seconds, cool air. Then the Heat would return and the fan speed up and hot air overwhelm the cold. Plus I could feel and hear the AC compressor kick on. And the AC system has been redone by the PO and had been working well. And it was a sudden loss and quick burst of Heat accompanied shortly there after by a increase in fan speed (indicating the CCU and Sensor saw the condition and was trying to correct it). So I had both clues that the AC was working fine, and so was the CCU and Sensor. So that left me with the conclusion that the Monovalve was not stopping the hot water coming through the heater core. And that could have meant failure of the Monovalve by either voltage not applied or rupture or even sticking. But it failed to dawn on me that it could be just because it was not getting enough voltage from the battery because, at that point, I thought I had plenty of battery. Now... it seems not. The final clue was not evident to me until I tried to start the car this morning.

I imagine, that if I was to have been able to see my headlights yesterday on the way back., I would have seen they were dim. I bet they were indeed.

I knew I had a Alternator, Regulator issue already but I did not connect the two immediately because I thought I still had plenty of battery left. But... this morning I found out different. And then it dawned on me. The Monovalve needed 12V and might not have gotten it.