87 300td heat/vent air issues - PeachParts Mercedes-Benz Forum

callocco · #1 04-06-2012, 08:58 AM

Heat comes out of the vents at times when on cool air. It just decides on its own that it will turn on the heat even if the temp outside is 90 degrees F.
ac has not worked for some time but even when it worked it would change to heat on occasion.
thanks carmen

Bio300TDTdriver · #2 04-06-2012, 09:29 AM

Quote:

Originally Posted by callocco

Heat comes out of the vents at times when on cool air. It just decides on its own that it will turn on the heat even if the temp outside is 90 degrees F.
ac has not worked for some time but even when it worked it would change to heat on occasion.
thanks carmen

The PBU is known to malfunction. Lots of threads on that subject if you search. When the pods wear out and leak the default position is air to the defrost openings. Again tons of threads on this and on replacing the pods. Check the wiki as well.

lt1fire · #3 04-06-2012, 08:16 PM

Would this work for the 87 300sdl w126 also??

callocco · #4 04-07-2012, 04:52 AM

What great pictures on removal of the pod and dash Sixto is unbelievable!
However is the dealer the only place to find the center vent pod?
ca

Bio300TDTdriver · #5 04-07-2012, 07:05 AM

Click on buy parts at the top of the page. Email Phil and see if he can get it. It wasn't a a dealer only part 5 years ago when I bought new ones.

Jeremy5848 · #6 04-07-2012, 10:56 AM

Quote:

Originally Posted by callocco

Heat comes out of the vents at times when on cool air. It just decides on its own that it will turn on the heat even if the temp outside is 90 degrees F.
ac has not worked for some time but even when it worked it would change to heat on occasion.
thanks carmen

Temperature problems are generally electrical in nature (but see below). Check the cabin temperature sensor, located either in the center of the dash or in the overhead with the dome light/sunroof switch assembly. If the sensor becomes disconnected or fails (less likely), the PBU will switch to heat unless the temperature dial is in the click-stop all the way at the end of the cold side. A loose connection or short-circuit in the sensor wires might cause the intermittent problem you are seeing.

A failing or failed monovalve can stick open (open is the default position) and supply hot coolant to the heater core even when the PBU is calling for cold air. You can put 12 Volts across the monovalve terminals (+ & - doesn't matter) with the electric cable unplugged; in a quiet environment you should be able to hear and feel a "click" or "thud" as the valve operates -- it's a form of solenoid valve. The monovalve can be taken apart and cleaned; a rebuild kit used to be available for some versions (W123) but is now NLA -- you have to buy the whole thing.

The center vents in these older cars (before the mid-90s, say) are intended to deliver cold air only. Heat is supplied to the left and right dash vents, the floor vents, and the defrost vents. Getting air to come out of the correct vents is usually a job of repairing or replacing pods.

The PBU itself can fail due to a number of reasons. The W123 PBUs are not short-circuit proof; a frozen auxiliary coolant pump will draw excess current and fry PC board leads. Short-circuited wires can cause a similar problem. Later models (W124) like yours have protection built in so that this no longer occurs. You can remove the PBU from the dash and make sure the two big electrical connectors are seated properly and all of the connections clean and shiny. Dirt and corrosion or a loose pin or wire could cause the problems you are seeing.

All of this usually applies to W126 and W201 models (with which I admit to zero experience) as well as W123 and W124 models like yours.

Jeremy

germanv8 · #7 05-20-2013, 12:17 PM

This is great. My 85 500SE won't stop putting out heat, and I like the list of possibilities you list.

The vent change dial is also very hard to turn and really doesn't want to go "up" to defrost. Do you think that's related?

sixto · #8 05-20-2013, 04:13 PM

An '85 500SE with a vent selector dial has two cabin thermostat valves rather than a monovalve on cars of that vintage with ACC. Find the valves and determine if they're getting a steady ground signal along with constant +12V to close the valves. If not, there's a problem in the signal to cut off heat. If the signal is good, you probably need new cartridges or new valves if cartridges aren't available.

The stiff vent selection dial is unrelated to temperature control. There's a cable in a sheath of 'positionable' rigid sections between the dial and the air box just behind it. Either the cable is binding or the levers and flaps it moves are binding. Maybe something fell into the defrost vents and is preventing proper flap movement.

Sixto
87 300D

germanv8 · #9 05-20-2013, 04:18 PM

That's awesome. Thanks!

jay_bob · #10 05-20-2013, 10:52 PM

Try the simple stuff first, my 300TD started acting weird a month ago.

Make sure all the fuses are good and tight. My fuse #7 was intermittent and that was causing the PBU to shut down and the system to go into safe mode (max heat and defrost).

Also I lost vacuum to the pod switcher device behind the glove box. I had just replaced the vacuum lines in proximity to the engine and forgot to hook the "green" hose back up between the small port on the vacuum pump (with the 5 way splitter) and the joint just beside the master cylinder. That too is a one way ticket to safe mode.

germanv8 · #11 05-20-2013, 10:57 PM

Thanks, Jay Bob. Did not know that. Very good to know.