84 300TD climate control troubleshoot: any tips from wise old owls?
 

this is type III system

The fan seems to blow well as it is supposed to on high, modulated, and low fan speed settings

Ducts do not open and close on que, only puny amount of air flows through any of them, it is warm but so little air flow its not heating well

The mono valve has a new plunger in it so it seems to permit flow of hot coolant as necessary,

I have verified the proper vacuum energy is being fed across the firewall to the proper tube feeding the complex of switchover valves

I have taken the ACC controller box apart and inspected the circuit boards, and do not see any broken solder joints

Since the switchover valves routing vacuum to duct actuators are controlled by electrical circuits, that seemed like the first thing to look for , defective vacuum delivery to the actuators would start there.

I am really really resisting taking the dashboard out . Big time. Seems like removing the dash is the only way to get good access to all those actuators

If it turns out you decide to remove the dash, send me your email address and I will be glad to send you free of charge a set of written instructions for the removal and installation of a W123 dash that I wrote several years ago.

BWhitmore2@earthlink.net

Thanks. Bill Whitmore

Pull the vacuum tubes off the solenoid block and use a mighty-vac to pump them up individually. They should pull vacuum and hold it. If they don't pull vacuum or if they leak down quickly, they need replacement.

The ACC controller box (silver box) is a common source of problems. There are several electrolytic capacitors in it that can leak and corrode the printed circuit board. Even if they haven't leaked, they can still drift massively in value and cause all sorts of unpredictable behavior.

OK, you mean the tubes that feed the vacuum to actuators.? , not the tubes that feed vacuum to the row of switchovers valves, you refer to them as solenoids, makes sense that is what they are. That under dash area is so freagin snarled with too many tubes its hard to tell what is what. Different topic, I repaired the connection to the "egg crate" vacuum reservoir which was leaky and retained zero vacuum.... and all the door locks came back to life. What a relief. Hoping to find a similar magic bullet for ACC. Using heat only .... here near the beach AC is not so important.

Since none of the vents seem to operate normally I thought first of checking on a system that would affect all of them.

The ACC controller box, in mine its black plastic, it has 5 buttons, the vertical roller to set temp, the fan speed buttons right? So if capacitors have failed that is new or rebuilt controller box time, right?



Thx for suggestions

The various solenoids (switchover valves) feed different vacuum elements to control different flaps in the HVAC box. You have 2 solenoids for the recirculation flap, one for the floor flaps, one for the center vent, and 2 for the defroster. You need to remove each individual line and pump up with the mighty-vac. If you have a big leak in one (especially the recirculation flap) it will affect the rest.

The box you messed with is the pushbutton unit. It more or less just has relays, pushbuttons, and some diodes in it. Behind your dash are 2 more boxes that control functions of the HVAC system. The blower is controlled by a black plastic box full of relays, the actual "brains" of the system are in a silver box with the word "Temperregler" or something like that written on it. The silver box is the one that tends to go flaky. I've repaired close to a dozen of them now, every single one has had damage to the printed circuit board inside.

OK thanks good job
 

Looks like that dash will have to come off

You can get to everything to test it without pulling the dash. If you have a dead vacuum pod, some can be accessed without pulling the dash, others require dash removal or a screw to prop the flap open. The monovalve is energized to close, so if the silver box is screwed up and running wild, you can expect the heat to be on pretty much full time.

Is the silver box repair simply reflowing solder joints?

Silver box repair is replacing every single electrolytic capacitor with new, cleaning any leaked electrolyte, and soldering jumper wires over any damaged/corroded traces. Some are in good enough shape that it's easy to repair. Some are so heavily damaged from leaked electrolyte that it becomes a challenge to repair satisfactorily. You won't know where you stand until you pull the cover off. I've had a very high success rate repairing the silver boxes, it just takes time and some soldering skills. Once repaired, they work very well, you'll wonder how you ever lived without it.