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random A/C function

3K views 41 replies 7 participants last post by  MBDiagMan 
#1 · (Edited)
Hi
I am new old 1987 300D owner. I've had a unimog
for 5 years, so a little familiar with MB stuff.
But...
My A/C is sometimes perfect ...sometimes only warm
air....or nothing at all. No blower..heat, anything. Just
when I think its completely dead...it works fine. Until I shut it off
and restart it...any idea where to begin?
The blower sometimes squeaks which I think means
It's near its end...but that would not explain the random
On off of the whole system...
Thanks
 
#2 ·
I think the randomness can be attributable to cold solder joints inside the pushbutton unit. You can attempt to resolder them, or buy a rebuilt unit. You might be able to relube the bearings in the blower fan, but as you mention, that probably isn't related to this issue.
 
#5 ·
I am doubting this as I don't touch the buttons much at all...between the intermittent changes nothing is done to the buttons, just the car is turned off and restarted....
but without much else to go on will look at it next...:surrender:
 
#3 ·
Cut a piece of paper 1 centimeter square, turn on the ignition switch and hold the paper against the small square grille next to the inside rear view mirror. If it does not stay there after turning loose of it, find the small sample fan and make it run, which will probably require a new fan.

When this fan fails the climate control unit gets no air sample and it will do weird things.

If the fan DOES hold the paper, then you can move on to other things. Since it's eratic, the sample fan is the most likely problem and the easiest to test.
 
#7 ·
Yes it is safe to assume that the sample fan is working.

There's nothing worse than troubleshooting an intermittent problem.

It might be worth doing some research and running the diagnostics at the diagnostic connector near the battery. This will tell you if you have a bad sensor, that is if the sensor is misbehaving at the time you run the diagnostic. A little research should give you the pins to use to check the a/c system. It can be done with an LED and a resistor. I haven't done this in ages, so I don't remember the details of the process.

Hope this helps,
 
#13 ·
Ok, I suppose as we peel away the onion, it raises the possibility that the KLIMA and OVP relays are potential contributors, as well. In fact, I think the last time I investigated a dodgy climate control on one of these, I replaced both those relays and re-soldered the PBU all at the same time. Not a very scientific approach to pinpointing the problem, but it narrows things down in a somewhat scattershot way.
 
#15 ·
I understand your desire to be scientific, I have the same desire myself. That said, never overlook the value of a good visual inspections and wiggle tests.

When a radar mechanic in the Army over 40 years ago, I fixed LOTS of problems by wiggling on coax connectors and thumping on vacuum tubes. Scientific? No. Effective? Yes. Don't limit your troubleshooting effectiveness in the name of science or pride.

My $0.02,
 
#16 ·
Speaking of wiggly stuff, since I own the same year car, I can tell you that the wiring harness in the engine compartment can get quite brittle, which potentially bares wires. I've found the compressor wiring to be especially vulnerable due I surmise to constant exposure to heat. So that's another potential culprit to be investigated.
 
#20 ·
Yes, there's nothing more difficult to pin down than an intermittent problem. When something smokes or quits, it's much easier to find.

Although it's not a scientific approach, I think you are at the step that Zeitgeist mentioned first. Reheat the solder joints in the control unit.

You say that when it is random it quits altogether. Is the compressor disengaging at that point?
 
#22 ·
Now I know this will not make any sense....
But last week I replaced the sunroof deflector springs....and the AC was working perfect.:confused:
for the last 5 days or so...
then today nothing....so I drop my son off at school, figured I would find which combo of settings might get it going again...nothing.
I start driving back and it comes on.....I pull over and stop it goes off. I put it neutral and rev the engine, nothing. I restart the car nothing. I drive away it begins to blow cold air..again i pull over it stops....

So it appears to now only work when the car is moving...i don't have much hair left...i am pulling it all out....:surrender:

What the F$%%#@ is going on...?????
 
#24 ·
LOL...no shooting going on here...
would that keep the system from heating as well? if i try the heater it does not come on either...

and since my last post it will not work even while moving now...
I will test the pressure today and get it charged if need be...
 
#26 ·
update..i think.
two days ago I noticed my coolant light illuminate. yesterday I topped it off and noticed the AC was working, even when stopped.
I now remember reading about a sensor in the cooling system that could have been the or a cause of my problems.
But...now I may have other issues if i am loosing coolant. I see i have the often maligned #14 head. what can I do other than checking coolant fluid levels?:rolleyes:
 
#27 · (Edited)
Check for white smoke out the back. Have a helper run the engine up to about 2500 and check the exhaust. The romp on the throttle a few times fully depressing the pedal, you should see only more black smoke. No blue or white smoke should be visible.

I would also pull all the glow plugs and see if there any signs of coolant stains on the plug body or tips.

Lastly, check the coolant reservoir. It should be at the right level and the fluid should be yellow in color, indicating that genuine MB or Zerex G-5 is the only coolant being used.

How many miles on the car??? And are you allowing the turbo to spin down before shutting down the engine, preferably 60-90 seconds?

The #14 has a bad rep, but can be longlived if you let the engine cool down for a good minute or so before shutting it down.
 
#31 ·
That's too low. Ideal temp should be around 87-90C.

How old is the thermostat and are you using a genuine MB, Behr or Wahler thermostat?

How old is the rest of your cooling system components, such as radiator, water pump, etc.?
 
#34 · (Edited)
doc,

Yes a blower motor can sometimes get intermittent when it's worn out. There is friction that keeps it from running, but maybe bouncing around or something might get it turning. Once it gets to speed it will usually stay there.

The CCU can sometimes be fixed be heating the solder joints, but IIRC this is more common in the 123 cars.

Do you have the wiring diagrams? You really need to trace it down. except for the blower motor and regulator itself, it's not dead simple enough that you can work your way through it easily without them.

If you're not afraid to tackle a big job, you can remove some of the cowling, the windshield wiper motor and dig your way into the blower motor where you can test the motor and controller.

I re-read the thread to get my head back in it. I think baert is pretty close in suggesting CCU, motor or motor controller. Don't, however, let yourself fall into the trap of replacing all three of these items without some smart measuring and troubleshooting. See if the regulator is getting power, if not then it's probably the CCU but IT ALSO COULD BE a wiring issue between the two units. Power goes from CCU to regulator to motor. Find out which link in the chain is broken.

If voltage is making it into the controller (regulator) then see if it's getting from there to the motor. If you're getting voltage to the motor with no rotation, then you know the motor is shot.

Hope this helps,
 
#39 ·
Progress...it WAS the blower motor...and it was confirmed by Roy at peachparts..when I got it apart there was a lot of friction, I could turn it by hand but it was very tight..i lubed it up and it works great now.
Still need to figureout the rear end wobble at speed..
leaning towards the multilink rear end and or the subframe bushings....
:crybaby2:
 
#37 ·
hamrt,

I really appreciate you posting the service manual links. I have the CD's at home, but I'm trying to look at a subsection which is Engine/Cruise Control/Clutch Switch. When I get to the clutch switch, it pops up a screen with no URL and the screen quickly goes away.

Have you seen something like this before?
 
#40 ·
Glad you found it. I hate to tell you this, but lubing it will probably only be a temporary fix.

The rear end wobble at speed as you describe it is most likely one of the dog bones gone bad. Safely get underneath and look at all of the rubber ends on them. If the rubber is pooched out on one or more, replace that particular dog bone. When you get underneath the back, it will be glaringly obvious what I am referring to as dogbones.

They are simple to replace and will not require alignment afterwards. The ones that they call tie rods have an eccentric on one end for adjustment. If you have to replace one of those, spray the eccentric with paint so you can see exactly where it goes when you put it back together in case you disturb it.

Hope this helps
 
#41 ·
i realize it's a temporary fix....i will order a new one shortly. just glad
i know what it is.
i will look at the dog bones this weekend. i briefly looked last week and
thought i observed at least one rubber bushing end oblonged
out....near the midline veiewed from the rear..is thjs the dog bone
?? what is the proper nane?...surely mercedes does not call them
dogbones...??? thank you btw....
 
#42 ·
I understand your relief. Knowing exactly the source of the problem is the biggest part of correcting it.

The proper names are determined by their position. In the past Performance Products among others, had very good parts diagrams with parts numbered. You then referred to the listing below the diagram with the number to determine correct nomenclature.

Hope this helps.
 
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