Anybody knows why I get a UNDERVOLTAGE error every time I use 'AUTO' button on my climate control for a few minutes?

I have not use "Auto" for about a year or so since I had the same error--
no problems whatsoever when not in use.

Some threads/posts states that it may possibly be the battery/alternator/regulator.

Why only with the "Auto" button on? Radio not turned on even.

It's a pain in the a$$ to get stranded. A battery change maybe necessary.

Had my battery & alternator checked during my last oil change & it was all OK.

Thanks.

 

How old is the battery? Check that regulator!

 

I'm guessing it's the HVAC control module. Especially if you have lights/radio/AC on and it's OK, but 'Auto' on only trips the message.

 

Don't really know how old. It's a 03 c240 bought the car in 2006 with a non-OEM battery in it already. Has not been changed yet.

 

Pardon my ignorance...HVAC module is unknown to me. SOunds expensive.

 

Heating, venting, air conditioning. It's the box in the center dash console that has the buttons on it. I have the digital HVAC module and when I checked on the price it was about $300. Easy to replace. Yours should be less expensive.
Much cheaper on ebay. I'm guessing that module because putting it in "Auto" mode should be minimal/next to nothing additional power drain if it's functioning properly. All it's doing is telling a microprocessor to monitor the sensors and adjust accordingly. How long does it take to trip the message after you push 'Auto'?

 

If it were me, I'd look at the battery first. What is the voltage of the battery when the car is not running? I haven't had a new battery last more than 3 years in over a decade now, and I've paid for some supposedly real good ones. I'm amazed at the OEM battery in the c230 (still the original one in my '03), and want another of the same type.

If you're hitting the 'auto' button and kicking in the A/C, that would cause a big current surge, but if your hitting it and the A/C's not kicking in it might be the module itself. If you have some jumper cables you could try hooking up to another car battery (leave the one in the benz hooked up too) and try running the car and hitting the 'auto' button. If you still get the undervoltage warning it's probably not the battery...

If the voltage on the battery with the car running is ~14 volts, the charging system is fine.

 

The batteries in the W203 seem extremely robust. I've read on here that they last about 7 years, granted nothing is wrong that drains it.

 

Takes about at least 5 minutes or so to trip.

 

I was on my way home from my bro's house-- a 20min drive. 1 hr later after I got home & on my way back out I turned the AC full blast & within 10 seconds error came on WITHOUT engaging the 'auto'. Had to change cars.

Anyway I bought a new battery from Advanced autoparts & installed it.
So far No errors from disconnecting.

Found out the battery was from april 2005...also an Advanced autoparts brand.

Hopefully it was just the battery...I'll find out tomorrow if it still trips.

Anybody tried to listen to their AC (full blast)while outside the car? Mine seems to be loud. Or is it my imagination.

Next thing I need checked is a leak. AC seems to take forever to cool. Had it charged about a year or so ago.

Thanks all.

 

I disconnected my battery 3 times before my rear SAM blew - I wish you the best of luck.

Out of curiosity... if you enable every electrical consumer
(audio, all lamps, rear defrost, blower fan, wipers, etc) what voltage is
observed at the battery terminals, at the +12V alternator - engine block,
and the +12V alternator - battery ground? If it can be reproduced by
electrical load through other means, I would suspect the regulator.
I don't know your skill levels, so my apologies if this is too much or too little info.

I replaced the regulator on my Valeo alternator around 85K (~30k miles ago)
without removing it... and was rather surprised to see such a strange wear
pattern on the brushes. They seemed to be worn at an angle instead of
straight down. (I might still have the Bosch regulator I purchased before
discovering it was the wrong brand if anybody might interested.)

Are you hearing the engine cooling fan and not the AC compressor?

Best of luck.

 

I'm a dummy when it comes to cars .I understand what you are saying but when it comes to the procedure I know so little.

I sounds like the big cooling fan & I'm not sure it's normal to be that loud.

A/C does not get cold as fast as I want it to be...Usually after 10 min of steady driving that it starts to cool.
I will probably get it recharged soon & see what happens...Summer in FL is brutal!

So far the undervoltage error has not come back after the battery change.
Knock on wood...

 

Allow me to assist:
Start the engine, turn on the headlights, radio, and blower fan to a high speed.
Pull the lever for the hood release; open the hood.
Remove the cabin filter housing over the battery and set aside.
Now, grab your trusty digital multimeter and set it to DC auto (or 20V range).
Touch the red (+) probe to the positive battery terminal, and the black probe
to the negative battery terminal.
Expected observed voltage there should be +13.8VDC - +14.4VDC.
What is the observed reading on the meter?

PS - don't forget to re-install the cabin filter housing, close the hood, turn
off those electrical consumers and the engine when done.

 

Damn UNDERVOLTAGE error came on again. Even after a new battery.
It came on about 3 minutes after using the 'AUTO" button so I switched it back to 'regular' & the error was gone after a minute.

I will check the voltage this weekend like DarthBudice suggested and see.

 

Hey there, i am having similar problems. Have you been able to fix your problem yet?

 

It is normal for it to be loud outside the car when on full blast. Mine sounds like an airplane!

I can't think of anything other than voltage regulator or possibly alternator that could cause this problem.

 

You need to check the volts of the car when its running... should be 14+ like 14.4 If its not your alternator its the regulator.. But you really need to check the volts... Report back when you do.

 

The voltage measurement across the battery with the engine running is for the alternator. It should be 13.5 volts + The 'regulator' is nothing more than a diode bridge on the back of the alternator and you would get failures at most loads if it were bad. If you turn all the electric users on and the message doesn't appear until you go to "Auto" mode with the HVAC I would be more suspect of the HVAC control unit.

 

Well said

 

Finally found the time to check the voltage. It is 13.8. Now the undervoltage warning ALSO comes on if AC is in full blast...and not just in AUTO mode.

Additionally it only blows satisfying cold air when the car is moving...man more repairs ahead this car is taking me to the poor house.

 

will it turn on if you put the heated seats on? rear defrost?

or is it ONLY when you use the hvac?

if so my guess would be that the hvac is at fault

 

ONLY when i use the HVAC!

 

then your problem lies in the hvac control unit

just when ac is on or heat too?