This is my 1st time on the site. It looks great. If I'm doing anything wrong in asking my question please let me know.

I have a 1985 MB 300 turbo diesel. The Climate Control fan only seems to operate when the AC is on or when I switch from AC to a non-AC setting. Even then it will shut off within a minute or two.
What is the problem and how do I fix it.

The climate control unit is a piece of work in the old benz..


It is suposed to be automatic and vary the air flow and temp. It sould run continuously if it is set to the max air flow... the button on the upper right with a fan on it. If it does not stay on then you may have a problem.

There is alot to go wrong with the system. If that keeps it running full time and all of the vents are functional (center most specifically) then you should be ok.


check that stuff out and get back to us



bob

Hi Bob-
Thanks for reply.

"It sould run continuously if it is set to the max air flow... the button on the upper right with a fan on it. If it does not stay on then you may have a problem."

It does not come on. At best it may come on for a minute or two, but it will not stay on.

well drawde.

It sounds like you may have a problem...

It could be the control unit $220
It could be a sensor that goes to the control unit
It could be the ever popular vacuum problem
It could be a lot of things.

How do we diagnose such a problem.

If you have a friend that has a working control unit swap them out and see if it cures your problem.

When the fan does go on does your central vents work. ( a common symptom of a vacuum problem).

Does your air conditioner work (does the compressor turn on and auxillary fan go on)

Does your defroster work continously (IF it does it might be good news for your control unit.

It is possible to take the car to the dealer and have them diagnose the problem but before you do change out all of your fuses to make sure that is not the cause. (Not a bad idea anyway)

The control units are prone to failure so dont be surprised if it need replacement.



Good luck with it and get back to me.

bob

Hi Bob-
I believe the defroster works fine. I don't have anyone I can swap parts with. Are there any sensor, relays or even the switches (buttons) that could be the problem.

I have a '85 300D that I resurrected the a/c system in. I replaced the compressor, dryer and pushbutton unit with new parts in addition to the vacuum actuator that opens/closes the "recirc" door behind the glovebox. Everything works as it should EXCEPT if I just want the vent fan on when the "off" pushbutton is mashed I have to mash any button EXCEPT the vent fan pushbutton and then I mash the vent fan button and it works fine. Speed varies based on temp setting, etc... If I go from off to vent fan pushbutton it will work for a few minutes and then turn off. I just live with it and mash one of the other buttons and then the vent pushbutton. The reason I had to replace the pushbutton unit is because it was not putting any power to the protection circuit starting at the evaporator freeze protection switch then to the pressure switch on the dryer and then to the Klima relay. There is a relay underneath the glove box on the right hand side that has a fuse on top of it. You may want to try to check it out too. My guess is that you need to replace the pushbutton unit.
I have found that the best troubleshooting manual for this car is the Mercedes-Benz manual titled "Electrical Troubleshooting Manual". It is around $35 from the 1-800 Mercedes number.

__________________
Jim

drawde

I know that I said defroster but might have ment defogger. It is the left most button on the control unit. It should blow out hot air to the windshield and side vents.


also did the system ever work properly before.

bob

The pushbutton climate control unit in this series of cars is notoriously finicky. They seem to last 10 or 12 years and then quit, so if you dig around through others' posts on this board, you'll see many other people with similar problems. (Actually, that goes for almost all the questions one could come up with for a 300Dt).

There's a company in California that rebuilds these climate control units, as well as the cruise control computers & actuators. Others on these forums have had sucess with them, but I've never dealt with them.

General Development Laboratories

Also, take a look at the Diesel Discussion forum. A large percentage of the regulars on that forum have 123 series 300Ds, 240Ds, and their kin, so any questions pertaining to your engine or virtually anything else about your car will be met with a host of valuable information and insight.

- Nathan
'83 240D "Steiner"
'00 New Beetle TDI

Drawde,

I would also tend to suspect the pushbutton control unit. Mine went out last summer and the main symptom was intermittant cutting out of the fan for about 10 seconds and then everything would work fine for a while. I got a rebuilt one from my local independent mechanic. For the warranty to be valid, you have to test several parameters with a multimeter, to make sure that another electrical fault such as a malfunctioning aux. water pump isn't causing the pushbutton unit to go bad.

Also, since you asked for feedback about your posting, you will tend to get more replies if your subject line is more specific. For example, you might use a subject line like "Malfunctioning climate control in W123" or some such, since a lot of listers not interested in diesels would pass your post by, assuming your question to be specific to diesels. . . Just a thought. . .

__________________
Steve
'93 400E

Thank you for replies. Steve, thanks for advise.
Well, I don't have a multimeter nor do I know how to use one.My problem is I don't know which component is causing the fan to cut out in manual mode after 40 seconds. The fact is I can't even get the fan to start in manual mode unless I first turn on the A/C. I took out the climate control unit and inspected the push button unit and the circuit board. I noticed that the circuit board was soiled with a black soot which was caked on the green side of the board. Also, a copper circuit was exposed where the green covering had melted or somehow moved to expose the circuit. I checked the sensor under the hood near the fuse box. It seemed ok. Also, I checked the sensor under the glove box. The fuse attached to it was fine. My question is, how do I figure out what to replace?

Sounds like your auxilliary water pump has frozen up and damaged the circuit board in the pushbutton controller. Was the damaged trace in the circuit board the one which runs right along the very back of the board, from one side to the other? If so, this is the one which drives the aux water pump.

The aux water pump is a small electric pump connected to the engine cooling system. It feeds a constant stream of hot water (coolant, really) towards the cars heater. The intent is to feed a healthy quantity of hot water to the heater even when the engine is running at low speed (such as a traffic light). At such low speeds the engine's own mechanically driver water pump doesn't move enough hot water to keep the interior toasty warm.

You can unplug the aux water pump to see if the controller behaves any better. The pump is located under the hood, in the front, passenger side corner of the car. Look for some water piping coming straight over from the front of the engine. It connects to the pump, which is a black, about 6 inches long, and has an electrical connection. From the pump water piping heads straight back towards the passenger compartment. There is an electrical plug at the aux water pump - just disconnect it.

If you live in a climate where winter is not too harsh, you can just do without the aux water pump. However, if you live somewhere seriously cold, it makes life a bit nicer. The partsshop on this site has them for $138 (which surprisingly isn't a competitive price - they usually do much better.)

- Jim

jcyuhn
Jim-
Thanks for the help. I checked the aux water pump. I disconnected it. The fan still would not come on unless it was with the A/C. Did notice that when disconnecting the aux water pump the connection was difficult to pull apart. In fact, I had to take the connectors apart in order to separate them. I don't know how to determine if the pump is frozen. I bet it is. Also, I imagine that the damage has already been done to the circuit board. So it didn't matter that I disconnected the aux. pump... So what do I do from here?
Ed

you could take the p.b. unit to a electrical repair store, some radio shacks will work on used electronics. They can re-solder the board and maybe repair or at least check the circuts.

you have found the problem! it is the pushbutton controller you could attempt to resolder splice the burnt section 50 percent success rate if you know what you are doing, Or get a remanufactured one from one of the many benz parts specialists.. look in the mercedes section of HEMMINGS MOTOR NEWS
GOOD LUCK whichever way you choose to go