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  #1  
Old 10-30-2007, 05:59 PM
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Climate control on and off rapidly

The climate control on my 87 300D has been cycling on and off rapidly under certain conditions. When it is warm out and I put on the a/c everything is fine, but if it is cool out the system powers on and off sometimes as fast as 2 or 3 times per second. At first I thought only the blower and recirc function were going on and off, but looking under the hood it appears that the auxiliary water pump under the washer fluid reservoir iscycling on and off in the same pattern. The a/c compressor seems to be doing it also. To summarize the symptoms:
1.system powers off and on rapidly when temps are cool outside
2.this occurs regardless of which pushbutton is selected (e/c, defrost, etc.)
3.This occurs regardless of the fan speed setting.
4.The rapid cycling occurs less once the car is up to operating temperature.
What I have done so far:
1.Put piece of paper over the inside temperature sensor to insure aspirator motor and tube are ok. Paper sticks, so no problem there.
2.Checked draw of aux. Water pump, pump draws .8 to .9 amps which I believe is ok.
3.Removed pushbutton unit and resoldered pushbutton connections and rheostat connections and fan pushbutton connections. Nothing.
4.Removed and inspected all 3 boards from PBU, inspected and found no evidence of burned circuits or cracked solder joints. Checked under magnification. All appears ok.
5.Tried jiggling PBU and its buttons and connectors: no effect on cycling of climate control.
6.Tried turning ignition key counterclockwise slightly and also tried jiggling it. Nothing.
7. Tried contact cleaner sprayed into rheostats. No effect.
What do I do next? I have scoured the posts and don't know where to go from here. Can the blower motor regulator being faulty cause the whole system to shut off and on, including the aux water pump and the a/c compressor? The blower motor is new and runs great with direct power. I have bypassed the low pressure switch near my a/c accumulator because it is faulty, but I don't see how that should affect anything. Could the problem be my outside temp sensor? I did rear end someone last winter and damaged the front of the car, could I have busted the sensor or shorted a wire?

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Last edited by ramjensen; 10-30-2007 at 06:19 PM.
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  #2  
Old 10-30-2007, 06:19 PM
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Klima relay?
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  #3  
Old 10-30-2007, 07:24 PM
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Disconnect the auxiliary water pump and report back with the results.
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Old 10-31-2007, 10:01 PM
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Auxiliary water pump was the culprit?

Unplugged the aux water pump as you said, Brian. Problem stopped immediately. I assume that my ammeter readings were somehow incorrect and the aux water pump is in fact faulty. It must be drawing too many amps and activating some sort of protection circuit that shuts down the system to avoid frying the pushbutton unit? Why did the problem happen only with cool outside temps? Does the aux pump come into play only when it's cool outside? I assume replacing the aux pump is the thing to do, unless this test reveals something else is at fault? Thanks for the help.
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Last edited by ramjensen; 11-14-2007 at 08:46 PM.
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Old 10-31-2007, 11:16 PM
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Quote:
Originally Posted by ramjensen View Post
Unplugged the aux water pump as you said, Brian. Problem stopped immediately. I assume that my ammeter readings were somehow incorrect and the aux water pump is in fact faulty. It must be drawing too many amps and activating some sort of protection circuit that shuts down the system to avoid frying the pushbutton unit? Why did the problem happen only with cool outside temps? Does the aux fan come into play only when it's cool outside? I assume replacing the aux fan is the thing to do, unless this test reveals something else is at fault? Thanks for the help.
Your ammeter readings were correct.

There is no "protection circuit".

The problem happened with cool temperatures because the monovalve started to cycle.

Now, as to the real culprit, I cannot provide a solid explanation because the SD does the exact same thing. A brand new auxiliary water pump (aftermarket) lasted about six months before the same symptoms were manifested.

My guess is that the pump is still drawing too much current.........even though it's below 1A........and the system is not at all pleased with the result.

A properly running OE pump won't draw more than .5A.
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Old 11-01-2007, 06:04 PM
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Auxiliary water pump: to replace or not to replace?

Since I am cheap I will probably try removing the aux water pump and replacing it with an elbow, then I will tear into my pump to see if I can get it to run at a lower amperage draw. Thanks for the help.
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Old 11-01-2007, 07:26 PM
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Is it also possible that the monovalve is drawing too much, but with the aux. pump removed from the circuit the control unit is able to handle the monovalve overcurrent?
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Old 11-02-2007, 01:01 AM
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Originally Posted by ramjensen View Post
Since I am cheap I will probably try removing the aux water pump and replacing it with an elbow, then I will tear into my pump to see if I can get it to run at a lower amperage draw. Thanks for the help.
Please report back with your results with the elbow. I'm very curious as to whether the pump restricts flow when not running and whether an elbow will result in sufficient heat when it's 10F. The pump, not running but installed, doesn't create any issues until it gets down below 20F. I'd like to find out that it can be bypassed permanently.
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Old 11-02-2007, 01:02 AM
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Originally Posted by babymog View Post
Is it also possible that the monovalve is drawing too much, but with the aux. pump removed from the circuit the control unit is able to handle the monovalve overcurrent?
This is a definite possibility, since they both come from the same source.

Don't know the specs on the monovalve, however.........???
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Old 11-13-2007, 09:22 PM
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Aux fan not the culprit

With my aux fan unplugged all was fine for a week or so. Well, this morning about 2 minutes into my drive to work the climate control system cycled on and off again just like before, clicking sound in the dash, recirc light on and off, blower on and off. The one difference this time was that it kept happening even when I shut the system off completely by selecting "0" on the pushbutton unit. I'm not sure how the monovalve works, but I wonder if it cycles even when the climate control system is off? Also, isn't there another piece to the climate control system? A black box of electrical bits that lives under the hood? Or is that black box only involved in A/C?
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Old 11-13-2007, 09:46 PM
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Monovalve?

Just read some monovalve posts. Evidently the monovalve's unpowered postion is open, allowing coolant to flow thru the heater core. When engergized the monovalve closes, stopping the coolant from circulating thru the heater core. Therefore if the monovalve is the problem: If the temperature wheel is on max or the defrost setting is selected, shouldn't that stop the cycling on and off or at least lengthen the intervals between the ACC system cycling on and off? I'm going to try this tomorrow morning. The problem happened only in the am today, when it was about 30 deg F. It didn't happen this afternoon when it was about 60 deg F. Actually, by the above logic, one would think that the colder it is outside, the longer it would take for the ACC to tell the monovalve to close and stop pushing hot coolant through the heater core, wouldn't it? Does any of this make sense?
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Old 11-13-2007, 10:21 PM
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Short story:
I bought my current '87 300D in LA, drove it home to MI. It was fairly cool weather so I selected the economy setting, no A/C. Periodically the A/C compressor would come on, and I'd get cold air out of the vents. I'd push the off button and then back to economy, the compressor would stay off. Eventually the compressor would come on again and I'd repeat the process.
At one point I decided to turn off the climate control, it was night and I had the windows open. To my surprise, the A/C would still come on occasionally, fan and all, I'd have to cycle through the on and then off again to kill the A/C temporarily.

When I returned home, I installed a NEW CCU, all was well. I also unplugged the aux. pump to avoid possibly damaging the new CCU until I could test it (it was seized I found out later).

Anyway, your symptom is slightly different than mine was, but similar in that it turns on and off or seems posessed. Probably a bad CCU, unfortunately. Finding what made it bad is another concern of course, but it sounds like you have found a bad aux. pump which might have damaged your CCU.
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  #13  
Old 11-14-2007, 10:37 PM
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No problems today

I had no problems from my ACC this morning. Looking back, I don't see how the monovalve drawing too much current would be causing my problem. When I had the on/off thing again yesterday, it was about 2 mins into my ride on a cold day. The temp couldn't possibly have gotten up to the 22 deg C that I had on the rheostat wheel in that short time, so the monovalve should not have been grounded and drawing voltage at that point. So how could it have caused my problem? Removing the aux water pump did reduce the frequency of the ACC on/off phenomenon. When does the aux water pump come on? Is it only at low rpm's to enhance water flow?
I am wondering if the PBU might be the problem after all, although it looks fine on inspection. What else would cause the whole ACC system to power on and off, even with the PBU set to "0"? Is my "0" button intermittently cutting power off? And then yesterday the first time I couldn't stop the cycling on/off of the system by pushing the "0" button means the "0" button was unable to cut power off? I have another PBU from another car, but it is not an exact match. I am unsure of the year it comes from. It has one more pin terminal where the plugs hook up than my existing pbu, but otherwise it looks very similar. It also has a rheostat marked in deg F instead of deg C like my existing unit. I suppose it would be stupid to try hooking up this slightly different PBU? What if I scavenged the "0" button from it and installed it on my existing PBU?
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  #14  
Old 11-15-2007, 12:16 AM
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It's time to replace the CCU. You've covered every other possibility.

You can go the cheap route and do it again in six months to a year.........or get the rebuilt that will probably last five years or more.
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Old 11-15-2007, 07:06 PM
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PBU = CCU.

The CCU is likely bad as Brian and others have mentioned. Replacing it with a different model is a bad idea IMO.

The electronics in the unit turn things on & off based on the buttons and other input, the 0 button does not act directly.

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