A/C Servo

i apparently have to either repair or replace my A/C (heater?) servo. No idea how to get started. Can anyone help get me started off in the right direction? Thanks.

Some additional info. The system blows heat 90% of the time. The A/C is recently charged 134 and blows very cold abour 10% of the time. The diagnosis seems to be that the servo is bad (this being a 79SD and not possessing the monovalve). So, my Haynes doesn't cover the servo issue, and I am wondering how I might locate and tame the dreaded servo.

[GermanStar]

It's a simple matter to R&R the servo -- the part's readily visible and almost as big as a football. We currently have some in stock for $425. Better hurry -- they won't last at that price.

Ron
German Star

I dont know if you are a do it your selfer, but my experience with the heater servo was similar. I could not commit the 398.00 price tag on it so I decided to take it apart myself. The whole thing heater control valve, switches, and vacuum porting is controlled by a little motor. Mine (heater control) had been bypassed, so it did not leak, but would not position itself for heat (I assumed) because I had heat all the time with it bypassed. The motor is geared, and the grease in mine had solidified. I took it apart making note of the alignment of the gears, valve, and a feedback pot. I cleaned the whole thing and reassembled it. I can now hear it run when I turn the key and start moving the temeprature dial. I understand there is a lot more to these systems then that, but I had initially checked for voltage at the motor leads (removed 4 screws holding cover on red and black leads) and found voltage, but no motor running. I think it was an 8 volt motor. If you want more info let me know. I would be willing to walk you through the process.

PeterG

79 300SD 75K miles
96 Chrysler Concorde 120K miles
89 Jeep Cherokee 180K miles
69 Shelby GT350 Convt 54K miles

[burnan]

My 2 cents would be to check all the other components first. Like the temp sensors inside/outside, the amplifier, an temp dial. They should have a certain resistance and or output depending on the temperature and the potetiometer in the SERVO can be checked as well using this method. To me, it sounds like it could be the SERVO or vacuum element or door is stuck so no fresh air is being mixed in. Can you hear the SERVO turn when you turn the ignition off, going to the "park" posistion? I'm with PeterG it is a lot of money. I did what PeterG did but not to that extent. I just pulled the cover off the SERVO. The potentiometer is shaped like a bowtie with a white plastic phillips screw in the center. I marked the screw with a pen to see if it turned in response to changes in the temp dial. email me if you need any specs for what the resistances should be for the various electrical parts.

[jcd]

Larry Bible sent me some great ACC diagnostic information. It is very helpful in diagnosing specific problems in this very complex system. My experience, limited, is that you can throw alot of $ at ACC problems and not solve the ultimate problem. The diagnostic info helps a great deal.

I am beginning to feel like Larry Bible's administrative assistant,,,,,however, if you post or private message your fax number I'll be glad to fax it over.

JCD

TXBill
Here is what I know. When you take the top cover off (4 screws) there is a plate that has the motor mounted on it (motor hangs down into housing. I believe there are four more screws that hold this plate down. Before this I would make note like Burnan said of marking the pot, so you get it back in the same position when you put it back. Before proceeding. I would also check for voltage at the motor lead wires while manipulating the controls (about 8 volts). These are the only wires (Black and red I believe that were soldered to the main connector ). The shaft from the valve itself fits up into a center gear. This is what's tricky because the shaft has a washer with pins that can be interlocked with that gear in different positions. This is what I don't know. At what position is this assembled at from the factory. This would make it a lot easier. In other words you are timing the feedback pot, with the valve position, contacts, with the geared head. That is why it is important to mark these, because we don’t know where the valve stalled at so we have to put it back to where we found it. The connector has to be removed before you can lift it out, and make note at where that shaft is when lifting it out. This is where it gets a little difficult, when you lift the plate out (senior moment) I believe the contact assembly maybe stays behind. If it doesn’t it kind of stays in place, but be careful it does not fall off when transporting it. The contacts are spring loaded and can get lost. Look at the position of the contact assembly. this is where a camera may come in handy. This is where you are going to put when reinstalling it. I had my gear assembly completely apart. The grease was solidified, so I had no other choice. If you look real close you will find a hole at the bottom of the plate. This hole lines up with holes through the gears. I put a small drill through these for my lineup when I assembled it. This sets up the timing between the contact assembly, and the stops on the gears. My motor was sluggish and had to be exercised and lubricated. I removed the motor first to see if the gears turned by turning that pinion gear. It did not. If yours does I would see if the motor runs. I believe I measured 8 volts when it was on the car. As I posted. If you have access to a small power supply it would help. If you want to take the gears apart it let me know. I don’t think I have enough message length to go into that.