heater controls not working - '87 300E - PeachParts Mercedes-Benz Forum

raysea390 · #1 10-26-2007, 03:50 PM

its been raining a lot the last few days and my windshield has been fogging up a lot. I knew my AC wasn't putting out any cold air when i got the car and also the on/off switch was messed up - the fan stays on all the time. But none of the other switches seem to make any difference to which way the air is directed - the only air coming out is on the side vents no matter what button i push. the fan speed button works.

Here's what i discovered to day - if i hold in the on/off button, the fan turns off; when i release the button the fan blows air on the windshield for about a second and then blows out the side vents as usual.

it's still in the 80's here so my temp setting is at its lowest. when i turn the car off, after about 30 seconds i think i can hear the heater duct direction flaps drop down.

Anyone know where i should start troubleshooting?

Thanks as always...I hope there is no limit to the number of questions i can ask, I've got a long way to go with this car!

raysea390 · #2 10-27-2007, 09:02 AM

No replies yet so maybe no-one has had this problem before. I want to remove the wooden panel that houses the heater controls and see if theres anything obviously wrong with the switches. How do i remove that wood facia withouout wrecking the lacquer? I see 2 screws above the radio - anything else?

psfred · #3 10-27-2007, 03:22 PM

Lots of possibilities here, from a bad pushbutton control unit to a bad temp sensor. Do a search, you will find all sorts of help on diagnosis.

To remove the cover, find the two screws on the bottom just above the radio. Remove them and pivot the panel up until you can get the hooks out from the top.

Re-soldering the pushbutton unit often fixes problems like this, but you really need to check all the temperature sensors and various actuators too. The first thing I would do is unplug the aux. water pump on the right front fender -- this is often drawing way too much current, and will cause the output circuits to intermetantly shut off on the control unit. It can also fry one, sadly.

Peter

raysea390 · #4 10-27-2007, 03:38 PM

Thanks Peter. Auxiliary water pump on right front fender (passenger side on US car?) - what does that look like? It must have hoses running to and from it, right? Unplug it then try the controls, and if all is good then plug it back in again? If not then replace the aux pump?

I'll try this first and pull off the switch cover next. Thanks again!

psfred · #5 10-27-2007, 05:14 PM

Passenger side. Two hoses, fairly obvious.

If it works with the pump unplugged, leave it unplugged, the pump motor is drawing too much juice. A new on is $150 or so.

You won't see anything by pulling the pushbutton unit, it is completely covered. You can check the temp sensors though. Proper testing requires a special harness so you can check functions with the unit on, which I don't have.

Peter

raysea390 · #6 10-28-2007, 08:25 AM

You were right Peter - the pump is very obvious! But the connector seems to be corroded together - i couldn't get it apart (the plastic parts separated leaving the pins exposed). I'm going to shoot some PB blaster on it but if that doesn't free it up, I'll have to cut the wires. What are the chances this is the source of the problem before I make the cut?

psfred · #7 10-28-2007, 10:39 AM

Almost certainly it's A problem, but disconnecting may not cure all of them.

See if you can twist one connector loose (the pins are on the pump side, I think, the female part on the wire harness). Once you get one side undone, it's out of the circuit.

Mine is stuck solid on one side, but the other came off and now it's disconnected until I feel a strong urge to get another pump. The only effect of not having it (and you probably don't now anyway) is that temp control will be erratic at idle in cold weather. The rate we are going now, that isn't gonna be a problem except for a a couple days in January....

By the way, how far are you from DC? My sister lives in Alexandria.

Peter

raysea390 · #8 10-28-2007, 12:09 PM

Drove the car for about 30 minutes this morning - air was coming out of the windshield ducts (and the side vents) for a while then it went back to just the side vents. This may have been the case all along but I only noticed the problem last week when the windshield fogged up. With the engine running I felt the aux pump and I do not think it is running. Anyway, I've only ever had the temp control at its lowest setting so i moved it to its hottest setting and air started coming out of the windshield vents no matter what i did with the controls. If this is what i have to do to keep my windshield from fogging up i can live with this for as long as it stays that way. For what i paid for the car, its not worth messing with.

I checked out the price of a pump online - just as you said, around $150 - it is supposed to help the heater work better at low rpms. I can live with cold feet for that kind of money.

I'm in Portsmouth, near Virginia Beach, about 3.5 hours from Alexandria when the I-95 isn't a parking lot. The company i work for is based in Alexandria and i was supposed to relocate there but instead they let me work out of my home. Olde Towne Alexandria is a beautiful city but sooooo expensive.

Since buying the 300E in September I've found a few MB garages in and around town that sell used parts...haven't needed anything yet but it's good to know.

psfred · #9 10-28-2007, 02:18 PM

Get the aux pump disconnected -- a short (or too much current) will cause the PBCU to shut off outputs randomly, and this can cause funky air flow.

This is an automatic climate control -- set for the temp you want and it will hold it, switching air flow, heat, and AC to match conditions. Quite nice when it works!. Sounds like yours is suffering from several problems.

The pushbuttons should work as follows: O for Off should shut off the fan and all temperature control. You may still get airflow out the defrost vents while driving. next button to the left is economy -- normal operation without AC compressor running. Middle button is full automatic with air out the appropriate vents. Next button directs flow out all the vents most of the time (nice in the heat as some cold air will flow out the floor vents front and rear). The leftmost button is defrost -- should run the AC compressor above 32F, full heat, and full fan directed to the windshield only.

Set the temp wheel at 75 or 22 (depending on whether it's F or C calibrated) and see what happens. If the AC never comes on, check for charge and a bad KLIMA relay. At this time of year and the weather you are having, airflow out the side vents in normal, about half fan speed when you start the car and slowing down as the temp controls. Set to minimum, you should get full speed fan and flow out the center vents -- if not, you have a pushbutton control unit faulty.

Peter

xtreme barton · #10 10-28-2007, 02:43 PM

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brand new bosch aux water pump for $85.15 and free shipping ********az is the shizz when looking for new MB parts in my opinion

raysea390 · #11 10-28-2007, 10:26 PM

Thats a good price for the aux pump compared to what i found.

Thanks Peter for taking the time to explain those heater controls to me - I'm sure its a nice set-up when it works but its certainly not as intuitive as the controls on older american cars, of which i have had many. I will definitely get that aux pump disconnected (somehow - i hate cutting wires) but I'm pretty sure my AC has lost all its freon - I was going to treat myself to a charge next springtime if nothing major has happened to the car. i think i saw that KLIMA relay when i was looking for the OVP fuse - protected by a plastic cover behind the the battery, right?

raysea390 · #12 10-30-2007, 08:48 PM

So i got the aux pump connector apart and all sorts of good things happened on a short drive out tonight. The heater controls all seem to do what they are supposed to, the recirc switch is now working and much to my surprise, the high beam position now works on the combo switch! Wow!

I didn't check out the AC yet and my glove compt light still comes on with the ignition switch but that's no biggee - i now have a much safer car. Thanks Peter!