2nd Gen Non-Engine/Transmission HVAC Blend Door Part 2 TRW Automotive

[RBallard]

Back some time ago I posted about a problem with the HVAC blend door. It must be a rare occurrence because so far I have only found one other person with the same problem. Anyhow I have replaced the heater controls as well as the blend door servo (separate occasions). In each case I followed Chrysler’s instructions to disconnect the battery and turn the control dial to the mid point. When I replaced the heater controls I didn't see any difference. When I replaced the blend door actuator I did see a difference. When you turn the temperature dial it would advance accordingly, with in a certain range. It is as if it is out of calibration. I just want to know how the thing works, I thought it may be a stepper motor but when I took it apart today I found a small circuit board, 4 plastic gears and a small worm drive motor made in china. Hard to believe the dealer wants $106 for this thing. Anyhow I tracked the part down to TRW Automotive, Automotive controls division. I got shuffled around to three different locations before I actually got to speak to someone. Unfortunately he was a sales guy who told me they are not allowed to discuss the part due to liability. It must be handled by the dealer. I explained that the dealer has not been successful and I would like to speak with a technical person to help me trouble shoot. I then got the friendly, "I feel your pain but my hands are tied, sorry. " My last job being a R&D manager for a fire protection firm, I never turned down a customer either for technical advice or even parts. Heck I would located parts, charge them out as samples, just to keep the customer going. I wasn't looking for a hand out, just an explanation as to how it works. Who was the guy that came up with the phrase, "what comes around goes around". Perhaps that only works for bad things.



Anyhow does anyone on the list have or know a technical contact at TRW Automotive?



Thanks,



Bob

 

[Ethdee]

Bob, I have a similar problem.



I was operating the temperature knob and heard a snap. It was the sacrificial coupling giving way.



My servo does not respond properly to the voltage input from the controller.



I removed the heater control and the servo and put them on a test bench, so I could eliminate any wiring issues.



The heater control is a simple voltage divider. It puts out nominally 12v through 0v in 0. 5 volt steps. There are about 24 detents in its rotation.



I put a voltmeter on the position input to the servo, and ran the switch through its range. My servo would lose its ability to figure out where it should be according to the remote set point (the knob), and start rotating endlessly. The first time this happened it caused the snapping sound I referred to earlier.



I ordered a replacement from a local Dodge dealer, and paid more than you did. They must vary the price by area.



The new one exhibited odd behavior. It worked fine from cold (12v) through a little bit after mid-range (5v). Moving the knob toward hot from that position brought the voltage down to 0v as it should, but again, the servo started spinning. As soon as it was restored to 6v or greater, the servo started tracking properly again.



I took it back to the dealer and told them that I was quite surprised that a brand-new part would be bad, but then again, it *did* say "Lucas" on it.



They got another replacement and it arrived today. This one also exhibits anomalous behaviour, but it's different. Below mid-point, it seeks to a position about sixty degrees farther than the end-point of travel, which would no doubt break the coupling, and, in any case, doesn't give me the control I need.



I haven't yet returned to the dealer, as I'm not quite calmed down enough to do so.



I have a question, though. You referred to the device seeking the ends of travel and calibrating itself. I've taken it apart, and I see no method whereby it would sense the increased torque at end-of-travel. The feedback through the worm gear would be minimal, and add cost and complexity that I'm not sure is necessary. How did you determine that it does that upon start-up? The manual says to put it mid-position, but doesn't refer to any auto-calibration.



I am looking for some other reason than two bad new parts in a row, and appreciate any information you may have.

 

[WestTN]

servo problems

I think your temperature control is bad. A servo controls by balancing a bridge circuit. If your divider is broken, you will see strange operation with complere loss of control in one direction.

If you put another replacement part in it will break it too.

 

[Ethdee]

When I bench tested the temperature control, I measured the resistance at each detent, and saw that the change was approximately linear. I then hooked it up to a power supply and checked the output voltage while it was hooked up to the servo. The voltage dropped from 12v to 0v in nominally half-volt steps, and there were no glitches.



This control is a voltage-divider circuit. The "pot" gets 12v at one end, ground at the other, and the wiper delivers the remote set point signal.



After benching it, I checked it while installed in the vehicle, and observed the same results.



I've been scratching my head to figure out how it could be bad, and haven't come up with anything.



I also eliminated the temperature control, and used a variable power supply (while it was on the bench) and got the same results from the servo.



The first replacement had problems in the 5v - 0v range. The second had problems in the 12v - 8v range.



Can you see any reason why the servo would contine to run past its 0% or 100% points? It seems to me that regardless of the input, once it gets to the end of its travel, it should stop; whether the input voltage is 0v or 12v.



I'm stumped here...



Ethdee

 

[RBallard]

I have a neighbor that is a Chrysler mechanic and he told me that they auto calibrate and that if I still had problems he would bring the DRB home and program the end stops.



Ethdee, I am with you. I don't see any method of feed back coming from the servo actuator. I have 2 temperature controls and 2 servos. Actually, I have 3 temperature controls, the third is from a 99 which still had the cable and I may go back to that design if I can't figure this out.



I played with the unit yesterday and found that going to the hot side, my door was at it's extreme where the dial had about another 30 degrees of motion. I am thinking that if I move the temperature controller to the hot side, and move the blend door to the extreme hot side, then take the servo actuator apart and manually change the gearing so that everything lines up and then see what happens.



About the coupling, when I first took mine my servo actuator off I found broken plastic. I made a metal key and put it back together and haven't had another problem. When my door stops, the motor stalls until it the signal stops.



I will play with it tonight and let you know what I find.

 

[Ethdee]

I must admit I know next to nothing about OBD-II.



I hadn't thought that there was any OBD connections to this device. I wonder how the PCM communicates with the servo? Is there a signal superimposed on the system wiring?



I noted that the replacement servos have four pins on them, and my original only had three, and there are three wires on the connector to the servo. They go to fused ignition, ground, and the temperature selector. I theorized that the fourth pin might be related to OBD, but I have no clue what it does. I'll look at that today.



I keep reminding myself that it's a heater control. The original design, a cable, was quite simple and worked every time.



Ethdee

 

[Ethdee]

I put it back on the bench and put a crescent wrench on the shaft so I could stop rotation. After mine went to the physical limits, rotating the dial back a couple of clicks started the appropriate motion at the servo.



It worked perfectly after that. I installed it in my truck, and nothing broke.



I over-analyzed the situation. Had I just installed it, I never would have learned as much as I did.



I don't think there's any OBD communication, but I believe it is self-calibrating. I don't think re-positioning the shaft will help, as the wipers affixed to the gear will tell the electronics the relative gear position.



I am interested to see how yours works out. Keep us posted.



Ethdee

 

[ntillm01]

My two cents: There are heater control servos that do communicate with the CCD buss and are programmed to learn the "stops" of a replacement servo.



I don't see anything in my 2001 service information to indicate that the Dodge truck ones are (or could) be programmed with a DRB. (still wish I owned one though )



However, I believe the servo itself "learns" its physical extremes. Since the replacement procedure for the HVAC control doesn't say anything about putting the temp control in it's mid position when it's replaced, the "learning" isn't happening there.



The service procedure DOES have you put the temp control in the mid position when the Servo is replaced, though. hmmmm...



Based on Ethdee's experiments, it does seem that the "learning" takes place at the servo itself. The circuit board may contain some sort of physical resistance logic that remembers where the end of travel is, based on increased motor current or something. I wonder if this is a one-time learn, preventing swapping one from another truck?



For those still having issues after replacement, I would check for a strong Ground at the servo harness. All too often, the power side looks good, so we think all is well (I've done it myself). For this bridge thingy to work right, it will need power and ground to be good. Try juicing up a headlight buy connecting one side to the servo ground and the other side to a test lead running to a battery + underhood.

 

[Boondocker]

faulty blend switch

I too have a faulty blend switch. It works to the extent that it is full hot or full cold. I've learned to compensate using the fan speed and vent controls. I just don't want to bother with the dealer for this. I sure wish there was a mechanical option. How much trouble is it to replace the unit?



Good luck,

Neil

 

[RBallard]

I have played around with both servo actuators and got the same results so now I am turning back to the temperature controller itself. The actuator does communicate its position. It must be located under the white gear pressed onto the shaft which turns the blend door. I played with the gearing and motor. The motor will run continuously when not attached to the gearing. The direction is dependent to the temperature selector. It also appears that the motor is switched on/off by the torque of the motor. I took pictures of the internals. When the door reaches it limit and the motor will twist loosing its contact from the supply voltage thus stopping the motor and I assume once power is lost the motor returns to its position making contact again.



I suspect my problem is with the temperature controller because the actuator motor will advance as supposed to until you get within 15 degrees of either extreme and then it just turns until it hits an extreme. I will play some more later today.

 

[Ethdee]

I have some piano-wire probes that I can stick into the connnector while it's attached to the servo. You might try something like that while it's in the vehicle and checking your voltages. If you have three wires going to the servo, it's the middle wire. My experience was that at full cold, it's 12v, and after a couple of detents, starts dropping at approximately half-volt steps until it hits 0v, which is a click or two away from full hot. Okay, maybe it was 12. 5v, and 0. 6 volt steps, but the actual number isn't as important as the roughly even steps you should see while turning the dial. Roughly means give or take 0. 2 - 0. 4v.



There are wipers under the the white gear, and a corresponding resistance band built into the pc board.



I didn't notice that motor torque reaction disconnect. Thanks for that tip; I think I'll take my old one apart again over the weekend and study it.



The switch replaces easily. However, you have to buy the whole unit. There are two different ones, depending whether you have elelctric heated mirrors. It's a little over a hundred dollars for the "without" one. The dash facia is held on with clips similar to door panel clips. I used a door tool to pry it away from the dash. I started at the bottom right and worked around. After you get that off, there's four phillips screws that hold the switch assembly in. Unplug three wire connectors and remove the jam nuts and the pneumatic connector and you're done. Tilt the wheel down so you have more room to work.



The servo seems to calibrate itself every time it's used, or when it feels like it. Power to it is cut off completely when the ignition is off.



I sure don't recommend having the dealer replace the servo. They charge according to the factory service manual, which has you drain the cooling system, evacuate the air conditioner, remove the dash, remove the heating duct, and replace the servo.



I used a right-angle ratcheting screwdriver to reach the rear screw (after peeling back the carpet so I had a bit of room to maneuver), and used scotch tape to tape the screw into the servo boss when I replaced it, as I couldn't put the screw back in with the servo in the way (It's right against the sound padding on the floorboard). Once the servo was in place with the tape holding the screw there, it was easy to stick the screwdriver through the tape and tighten the screw. The screw closer to you is trivial. Fifteen minutes R&R versus around $800 for the dealer to do it.



That's an excellent point about checking the ground.



As far as the mid-position, it's the only spot I've found where the servo will always seek to no matter where it starts. Maybe that's related to the reason they recommend that.



Ethdee

 

[Cliffman]

Originally posted by Ethdee



I used a right-angle ratcheting screwdriver to reach the rear screw (after peeling back the carpet so I had a bit of room to maneuver), and used scotch tape to tape the screw into the servo boss when I replaced it, as I couldn't put the screw back in with the servo in the way (It's right against the sound padding on the floorboard). Once the servo was in place with the tape holding the screw there, it was easy to stick the screwdriver through the tape and tighten the screw. The screw closer to you is trivial. Fifteen minutes R&R versus around $800 for the dealer to do it.

Ethdee

Ethdee



Are you saying one can remove the servo without removing the dash and can you get to the blend door using this method?

 

[Ethdee]

You can R&R the servo and the coupling this way. Sadly, you can not get to the blend door.



Ethdee

 

[RBallard]

Here is my latest, hooked the DRBIII up to it yesterday and went through all of the menus and searched for modules. There is nothing to control it other than the controls in the dash and the servo actuator. I put the original back in an now it seems to swing the door from one extreme to the other but only using 75% of the control switch? I hooked up a meter to my other HVAC controller thinking I would find dead spots or something but it was smooth ranging from 0 to about 10. 5k ohms. I removed the rheostat and blew it out with air. Next time I decide to fool with I will try it out. At least now it seems manageable. I bought a new plastic coupling ($12), but plan to pull the dimensions and make a new one out of brass.



Concerning the DRBIII, about the only thing neat thing I was able to do was turn on the 3 cylinder option.