A/C Control Unit - Ideas - Alfa Romeo Bulletin Board & Forums
 
LinkBack Thread Tools Display Modes
post #1 of 7 (permalink) Old 06-19-2006, 05:03 AM Thread Starter
Registered User
 
Join Date: Mar 2004
Posts: 91
Send a message via AIM to altmand
A/C Control Unit - Ideas

Hope this isn't a breach of etiquette, but I'm beginning a new thread for this issue because the thread I posted last week contained too many unrelated issues (my own fault), and I've definitely seen some questions about this mysterious black box.

Why the heck does my A/C compressor cycle on and off frequently, making it take forever to cool the car in 90+ degree FL heat. The compressor is new as is the dryer, and, when it runs, it blows freezing cold (R134).

It seems that the series three spiders, prior to the Motronic years, lacked any sort of pressure switch in the A/C system. The on/off signal (12v+) to the compressor relay is controlled by the A/C Control unit, which, as I understand it, takes its cues from the rheostat on the A/C switch in the cabin and a temp sensor on the evaporator. I think the A/C fan/temp control switch is working correctly because the compressor responds to a reduction or increase in the "cold" setting on the dial; however, I'm wondering if the temp sensor on the evaporator has gone bad. According to the car's second owner, the A/C had suffered from this condition from the time he first purchased it (1987!). I've had the car for the last several years.

Just a thought - If I can isolate the wires from the evaporator sensor that run to the control unit, I think I could trigger an "always-on" condition by jumping the wires (or severing the wires as the case may be - I'm not sure yet). On the connector, the wires are: 1-B, 2-R, 3-Bl, 4-B, 5-P, 6-Y, 7-W, 8-W. The two white wires run to a two wire connector that appears to lead toward the evaporator, and I am guessing that these are the sensor wires; however, I'm not sure and my shop manual (purchased on ebay years back) lacks the A/C section and accompanying wiring diagrams. If these wires were simply jumped (or disconnected...one or the other), the only other factor affecting whether the control unit sent an "on" signal to the compressor would come from the A/C control switch on the console.

Thoughts?
like...what's the possibility that the A/C freezes up if the compressor is allowed to run incessantly? I have also considered creating a "max A/C" type button that bypasses the sensor when I need the car to cool down quickly; thus, I can simply hit the button and reactivate the sensor (and bring the on/off cycling back) once the car cools down.
altmand is offline  
Sponsored Links
Advertisement
 
post #2 of 7 (permalink) Old 06-19-2006, 06:01 AM
Registered User
 
santoli3's Avatar
 
Join Date: Aug 2004
Posts: 112
The temperature sensor on the evaporator serves the same function as a low-pressure cutout would. This is because the refrigerant in the evaporator is at saturation, and so pressure is directly proportional to temperature. Overriding the sensor would be a particularly bad thing to do. It would likely cause evaporator freeze-up in short order.
You say it blows freezing cold? How cold? With the system at max on a warm day what is the temperature coming out of the vent? What is the cycle time in this condition (length of time compressor is on, and interval between cycles)?
Just a guess; but short cycling is generally caused by insufficient refrigerant charge.
santoli3 is offline  
post #3 of 7 (permalink) Old 06-19-2006, 10:09 AM Thread Starter
Registered User
 
Join Date: Mar 2004
Posts: 91
Send a message via AIM to altmand
Don't know exact temperature coming out of the vents, but it is cold. Probably in the 50 degree range, but this is just a guess: I haven't put my A/C thermometer in the vents. The cycling occurs frequently even with a known full charge. The compressor is almost guaranteed to shut off under acceleration, once the revs get up above 3,000 rpm. If the car is left to idle, the cycling is less frequent; however, this does nothing to cool the car when I'm driving in normal in-town stop and go traffic.
altmand is offline  
Sponsored Links
Advertisement
 
post #4 of 7 (permalink) Old 06-19-2006, 01:55 PM
Registered User
 
Join Date: Apr 2006
Location: NW Arkansas(yes we have paved roads)
Posts: 84
If the evap sensor is the probe type we sometimes pull the probe from between the fins on a balky conversion. Changing over from R12 to 134A can sometimes be a great pain. I have converted some European units(BMW) where you will have to evacuate the system a second time and recharge the old fashioned way by holding onto the metal part of the suction line as it leaves the evaporator and charge till you feel the cold slug hit your hand. This should be done at 12-1500 R.P.M. with a good shop fan blowing across the condenser. The vent should be blowing around 38-40 degrees @1500 RPM with the car sitting in the shade (Medium fan speed).
If your tech gives you the standard line about 134A can't cool as good as R12 try a different tech. It's all in how you charge the system.

Dave
CaverDave is offline  
post #5 of 7 (permalink) Old 06-19-2006, 02:02 PM Thread Starter
Registered User
 
Join Date: Mar 2004
Posts: 91
Send a message via AIM to altmand
Dave,

Are you saying that pulling the temp probe, or otherwise bypassing it, is a viable option? (i.e., that doing so won't cause the system to freeze up easily?).

In terms of overall charge, when the compressor is turning, the air coming from the vents is nice and cold.

Drew
altmand is offline  
post #6 of 7 (permalink) Old 06-19-2006, 05:23 PM
Registered User
 
santoli3's Avatar
 
Join Date: Aug 2004
Posts: 112
Quote:
Originally Posted by altmand
In terms of overall charge, when the compressor is turning, the air coming from the vents is nice and cold.
Yes. Seems counter-intuitive doesn't it? With low charge, the evaporator pressure (and temperature) quickly drops to the low trip-point under load. You need enough Freon in there (enough heat capacity) to maintain constant saturated conditions.
Steve
santoli3 is offline  
post #7 of 7 (permalink) Old 06-19-2006, 10:13 PM
Registered User
 
Join Date: Apr 2006
Location: NW Arkansas(yes we have paved roads)
Posts: 84
Older Chryslers and most European systems use a non adjustable thermal probe to cycle the compressor and prevent freeze up of the evaporator. Most of those I've worked on cycle around 50-55 degrees not a good thing if you live in the South. You can try pulling the probe 1/2-3/4 of the way out of the evaporator and see if that helps. Sometimes you can move the probe closer to the air inlet side of the evaporator and get the desired results. If the system has been properly charged and the condenser has a good flow of air you should be able to get the outlet temp of the vents 38-42 degrees before the compressor cycles.

Dave
CaverDave is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Alfa Romeo Bulletin Board & Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome