ACC Amplifier

[jbrasile]

Great Jon!

You know, I am starting to suspect my servo is acting up because I still get "over control" from the ACC, it is either too hot or too cold... I have calibrated the temp wheel as per the manual, checked all the sensors and everything is within spec. I shall try a servo adjustment to see if it makes any difference, you can play a little with the adjusting points.

Tks,

Joe

[1980sdga]

It's doing the same thing *sigh*

The amp will get warm and I'll lose system response. I can unplug the amp to let it cool down and all will be well again. Then it'll lose response again...

I guess I need to check all the connections and resistance values along the sensor chain.  I sure hope I didn't get a bum servo 

Did you recheck the foam sniffer tube Joe?  Mine had slipped off and caused problems similar to yours.  My system works great but it won't continue to operate for more than an hour.

[koan]

Knowing nothing about ACC, servos or amps but just from electronic knowledge if there were burnt resistors in your old amp and it was running hot it sounds like something external causing excessive current draw, possibly a motor at the end of its travel or driving a stiff mechanism, or maybe there is a connection to earth somewhere that shouldn't be.

Your new amp doing the same thing and needing to cool down before it works again tends to confirm that.

As you mention it could be a sensor reading driving the motor hard one way.

Take care fault finding, might end up with a blown amp if you leave it running too long.

koan

[jbrasile]

Hmmm..... I would suspect an iffy servo.... you did mention it was kind of noisy so that could very well be the problem.

Yes, my sensor tube is actually repaired with a/c foam tubing and tested ok so I am leaning towards the servo overdoing its adjustments.

Tks,

Joe

[1980sdga]

This is the "sensor chain"



Hey Koan, the temp selector wheel is what Joe calibrated. The in-car sensor is a thermistor that has the foam tube running to it which picks up air from the blower. As far as I can tell the only adjustments are the temp selector wheel and the pot in the servo assembly.

The motor drives a clockwork mechanism which switches the blower speed and controls the duct flaps. It also drives a water valve which seems to be the sore spot in this system.

I'm doing further testing with the old amp I have which is working fine on a "test servo" I put together that doesn't drive the water valve.  So far It's looking good!

I read through my troubleshooting stuff and it said sniffer tube problems can cause the system to run too cold on warm days Joe. Does yours seem to run too hot as well? I've checked my components and checked the calibration of the temp selector and the pot in the servo and would like to hear more about what you're doing!

[koan]

Just had a look at the M-B HVAC manual and was reminded how much the of a monster the ACC is.

Reading back through your posts I can't see what your problem actually is.

Hot amplifier is mentioned, Joe mentions overcontrol, hot or cold and nothing inbetween, is this your problem?

koan

[1980sdga]

'Tis a complicated thing.  I think I discovered that my problem (Amp getting hot and stopping and then sometimes "fixing" itself when it cooled) was what you described. The water valve was binding the servo motor and overheating the amp. I hope...

It sounds like Joes problem is that it "overshoots" the temperature set point and either goes to "too hot" or "too cold"  I mainly posted the picture to clarify what Joe was talking about.

I'm interested in hearing what you find Joe.

You really need ACC Koan  You can get the cool bumpers with it as well 

[jbrasile]

Jon,

Since I have already checked the sensor chain and everything is in spec, I will try and play a little bit more with the servo potentiometer. I did calibrate mine slightly but there is a range you can use so I am curious if that will change the system's sensitivity.

And Ken, Jon described my problem accurately, the ACC is always overshooting either too hot or too cold. I also have to replace two vacuum actuators which are preventing correct air distribution, that may be affecting temp accuracy since the center vents are supposed to close in certain conditions but since I have mine locked, perhaps the side vents are warmer than they should be, the cold air that is supposed to come out the side vents comes out the center and the whole thing is out of whack.

Tks,

Joe

Tks,

Joe

[koan]

The water valve was binding the servo motor and overheating the amp. I hope...

Reading the book I got the idea the water valve was operated by a relay from a contact in the servo, it's part of the servo is it?

I'll read some more, see if I can find anything on overshooting.

You really need ACC Koan  You can get the cool bumpers with it as well 

Don't mention the bumpers! I let it slip once but I think I got a way with it

I like complicated mechanical ways of doing things, especially the challenge of fixing when they fail, the ubiquitous microprocessor has spoiled all that. The ACC does seem to have a few problems, being of non M-B origin maybe it's not engineered as well as it should be.

koan

[1980sdga]

I like complicated mechanical ways of doing things, especially the challenge of fixing when they fail, the ubiquitous microprocessor has spoiled all that. The ACC does seem to have a few problems, being of non M-B origin maybe it's not engineered as well as it should be.

koan

The servo:

http://www.youtube.com/watch?v=hHJW_nGMMm4


The water valve is a screw type affair driven by a "lead screw"arrangement.  I've rebuilt some German clocks so I can see how this Chrysler monstrosity would appeal to them   

[koan]

The servo:

http://www.youtube.com/watch?v=hHJW_nGMMm4

The water valve is a screw type affair driven by a "lead screw"arrangement.

OK, I see how the water valve works.

The servo in the video appears to work as it should. Does the amp get hot driving the servo without the water valve attached?

How are you syncing up the water valve to the rest of the servo?

I would think the water valve needs to be set fully open (or closed) and the servo moved to hot (or cold) and assembled. Then tested to see the valve move through its range from hot to cold.

If the water valve is not binding over its range maybe something is bent or warped, or there is some distortion when the valve is assembled to the servo.

Well I read more of the ACC book and to be honest the book isn't as good as the the recently posted pdf, contains lots of tests to perform with the special tester and what to do if the ACC doesn't pass a particular test but no list of faults and possible causes.

The electronics are pretty simple, the complication is in the servo vacuum switch,  vacuum pushbutton control unit and the associated vacuum switches and actuators.

Joe, can you detail the over compensation problem?

On a particular setting of the thumb wheel does the output air temp oscillate from hot to cold, or do you get full cool or full heat when moving the thumb wheel?

Quite like the ACC, wish I had one to play with.

koan

[jbrasile]

Ken,

This is what happens:

Say outside temp is 90F and you set your temp at 75 for example, turn Auto-Lo, fan comes on at its highest Auto-Lo speed (it has 6 steps in this setting), chilled air coming out of all 4 vents. As the interior cools down fan speed decreases and as the temp gets close to 75 air coming out of the vents is supposed to be less cold and when the system finally stabilizes you should have air temp and fan speed in such a way that ambient temp is 75. If outside or interior temp changes fan will increase or decrease and again you will get cooler or warmer air.

Now, here is  my problem:

Everything works as it should including fan speed decreasing as the interior temp gets close to 75, BUT air temp will remain at its coldest and all of a sudden you will feel warm air starting to come out of all vents and the floor flaps open as if the system going into heat mode,that's the over compensating symptom we are talking about, then as the interior temp increased cold air will again start to come out of all vents until the cycle repeats itself.

A properly functional ACC is actually quite accurate in maintaining interior temp and  very pleasant and smooth in its temp/air control.

Since my servo and amp seem to be ok and the sensor chain checks out, I am suspecting that because I have my center vent and defrost actuator lines plugged and flaps locked, the colder air that is supposed to be sent to the side and floor vents when the system closes the center is not being directed correctly,  therefore giving the impression of overshooting. Then because the interior temp rises too much, the system "feels" it  has to go back to full cold to compensate. What I am trying to say is that the center vent being open all the time is keeping colder air from being directed to side and floor when max cooling is not required.

Tks,

Joe

[1980sdga]

Today I drove about 450 km with the "semiservo" and the bottom half of an aluminum servo body for a manual valve.  The amp and servo both did fine and I manipulated the controls a lot. The amp got a little warm but not hot like before.

I'm pretty happy with the outcome. At least I'm pretty sure that the valve was the problem.

I'm still working on the water valve part. There is a guy over here that rebuilds these so I'm going to have him build one proper but this is fun to mess with and maybe some folks will get some ideas.

The traditional fix is to change to an aftermarket aluminum bodied unit that won't crack, leak or bind up. The OEM units are made of plastic like distributor caps but it seems to crack from the heat and lead to problems.

I've studied the geartrain to try to find a "trigger" for something like a solenoid on the valve but there's nothing easy to access. The friggin water valve gear (the one with the little "ears" in the vid) turns 20 or so times from open to close!

I spent several full days troubleshooting my vacuum system  and yea, it's complicated but not impossible.  I actually got that booklet AFTER I straightened it out but it would have been a good help.

I kind of liken the ACC system to the vacuum door locks. Kind of silent and creepy, almost alive.  It's really cool to me the way the vents sloowly open and close and the blower speeds change.  You'd dig it 

Joe, it sounds like your center vent may be causing the trouble. I'm not sure, but a leak at that element may be causing another leak that's leading to your flip to heat.  I had my water valve open slightly this morning and I used both the defrost and bi-level modes and noticed that the "blending" of the hot/cold air seemed to be different from the defroster vents to the side vents so it sounds like your analysis is dead on.  Are you sure it's a bad element or do you perhaps have a vacuum line loose somewhere?  Sure would make it easier!

[koan]

What you describe happening and your reasoning sounds plausible.

The book diagrams three stages of "LO" cooling operation, I assume that's AUTO-LO, the three stages are different blower speeds as the servo responds to the changing in car temp (sensor) as the set thumb wheel temperature is approached, exactly as you described.

Only blower speed should change, no flaps move, and that raises a suspicion about your reasoning. I can't see what would prompt such a sudden change in inputs (sensor chain resistances) to the amplifier to drive the servo into heat mode. There's no change in air flow where cold air is suddenly blown over one of the sensors because the flap positions don't/shouldn't change.

Do you get the same behaviour at different temp settings, say less than 70 or greater that 80?

Just wondering if there's a crusty area on the servos feedback potentiometer, where the track is worn from constantly moving around that area, when the pot gets to near where it should be the resistance in the sensor chain (feed back pot is in the chain) suddenly increases which the amp sees and goes to heat mode. An increase in resistance of the sensor chain is same as if thumb wheel was moved towards max temp setting.

Does moving the thumb wheel to a slightly higher temp stop the oscillations once they have started at 75 setting?

Measuring the Volts between pin 2 of the amp and ground might be useful, an instant change in Volts just before the overshoot occurs would suggest a problem with the feedback pot. A slower, smooth change in Volts would indicate something else is causing the problem and the servo is just responding to what the amp is (incorrectly) telling it to do.

Just to be clear about what I wrote above about the flaps not moving, I realise the flaps do change position in LO depending if mode changes from cool to heat. Please correct my understanding of operation if I've got it wrong.

All what I write could be total BS of course, never having seen one and just coming at it from circuits and books.

koan

[jbrasile]

Jon,

I wish.... unfortunately the defrost and center vent pods are leaking, I tested both with the vacuum pump,  so what I did was to plug the lines that go into them and I locked the center vent so it doesn't close on its own,  it 's actuator has a return spring built in and vacuum is what pulls it open.

Ken, you are getting pretty good at this, hehehe..... the problem happens no matter what temp you select but I noticed that on cold days the system works a lot smoother which tells me that the overshooting is being caused by the center vent being open all the time and preventing cold enough to the sides when max cooling is not required.

Tks

Joe