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Roadmaster A/C Compressor Clutch cycling too much?

589 Views 13 Replies 6 Participants Last post by  storm9c1
Hello, Have a 1994 roadmaster. A/C is still blowing nice and cold (today was 87 degrees) but on my old Auto Xray scanner it shows the A/C being requested/ not requested like every 2 seconds when in auto mode. Also states clutch is cycling on/off at same time. Is this normal for the Buick auto A/C system your do I need to hook some gauges to it to check for low freon? Also, when you first turn on the A/C the compressor stays engaged for like 10 minutes then starts the on/off thing. Thank you.
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Yeah , I would be having a look at pressures.
Could be , or low pressure cycling switch iffy.
Don't know if your scanner is telling you the request is changing or the clutch on signal is .

Are the fans running ?
At 87F could be high pressure shut off due to lack of air flow.

My point about request was , is your scanner reporting the request to the pcm or is it reporting the pcms output to the clutch relay ?
This I don't know.
Your thought of looking at real pressures is where I would start.
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Thanks, Number 1 fan was NOT on like it is supposed to be when A/C requested. Also, this I think is the first time running the A/C since I put in a 1994 9C1 Caprice PCM. Don't know if that would screw things up with the Buick automatic A/C. Don't know either about the request to the PCM vise-versa thing. It's a real simple scanner that just shows one item at a time.
Looked at it again. The number 1 fan IS coming on during initial A/C request. Fan shuts off going down highway. Is that normal or is the supposed to be on at all times if A/C is on?
Looked at it again. The number 1 fan IS coming on during initial A/C request. Fan shuts off going down highway.
Is that normal or is the supposed to be on at all times if A/C is on?
For your reference
Quoting Scott Mueller (yes, this is almost as old as IronBlock LT1 engines are):
"B- & D- car coolant fans operate under PCM control at the following coolant temperatures and AC system pressures:
Fan …….….…… Mode ….….… Temps ….….… AC Pressure
Primary (RH) .... ON ... ... 107°C / 225°F ... ... 225 psi
2ndary (LH) ...... ON ... ... 111°C / 232°F ... ... 248 psi

Primary (RH) ... OFF ... ... 103°C / 218°F ... ... 180 psi
2ndary (LH) ..... OFF ... ... 107°C / 225°F ... ... 203 psi

Additionally, the PCM will turn off the fans at higher vehicle speeds (above 48 MpH I believe), since running fans can actually impede airflow through the radiator at high speed.
Finally, certain Diagnostic Trouble Codes (DTCs) may cause the PCM to turn on one or both fans."

When AC works properly, so long as the AC system stays over 180psi, the primary fan will stay on.
However, once over 47MpH, the primary fan motor will stay off until the road speed gets under 45MpH.
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Thanks for the good info above. Haven't had time to install gauges yet but the compressor starts to short cycle 10 minutes or so after I command A/C. Still very cold air but maybe its low on freon. Guess I need to get some dye to find a potential leak.
Short cycling usually means low freon. Under a perfect charge and perfect conditions, it doesn't cycle much at all.

A clogged orifice tube (or other similar clog/restriction) will also cause a short cycling followed by high head pressure (both fans running with poor cooling performance).

Go to Harbor Freight and get a gauge set. I feel that it's required to diagnose/charge these systems correctly TBH. The FSM contains a table for pressure versus ambient temperature and humidity. While I've tried to follow the table to the letter, I've also noticed that there is more experience involved than reading the gauges and comparing to a chart. For example, the car needs to be fully warmed up. And when charging using the gauge set, having a strong supplemental fan blowing on the radiator is important to keep the readings stable. Even then, it's tricky to get perfect.

On most of my B-Bodies, I have LEDs wired up to monitor compressor and fans. It's really helpful to understand what these cars are doing.

Because of this, I've noticed that my fans turn off around 39MPH. Unless high head pressure, in this case, they shut off much higher, perhaps 70+ MPH based on the tables above. Never noticed that.

I tried to find the MPH in the FSM to no avail. Just a mention of VSS in the control stage:

Font Publication Paper Paper product Book

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This is taken from the Supplemental 3rd FSM book (the corrections), 1995.

Funny how every forum post I see seems to have some temperature or pressure listed that does not jive with the FSM. Granted anybody's car could also have a custom program. Granted the FSM could be wrong.
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Can you hear hissing from the vents? If so it’s probably low. check the valve on the high side port for a leak.
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So messed with the A/C again with gauge set. I do A/C service just enough to get good at it then I forget it all and have to relearn. The readings showed a little low on the low side with normal on the high port at 2000 rpm. Then I go driving and then my auto xrays "scanner" says the High pressure drops to below 130 and then the computer says the clutch is cycling off and on. Air is always cold. 40 degrees. When I pull in and park the compressor is engaged and the readings are normal at 2000 rpm. What is going on? How low can the high pressure side go during normal highway driving at 80 degrees outside if A/C system is working 100%? Maybe I am worrying about nothing? I would probably not suspect anything if I hadn't plugged in my scanner.
Ignore the high side, watch the low side. The low side controls all the cycling, not the high side (unless there's a problem). It will cycle the clutch off when the low side gets below ~35F, no need to rev it up, leave it at idle.

If it's hot/humid outside (or you leave the windows down), the compressor should run almost continuously when at idle.

Best case is to evacuate the system and fill by mass. You can put your jug on a postal scale that reads in ounces and do it that way if you don't have a full recovery machine (most people don't :)).
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Good advice. High side can be erratic once heat soaked and low air flow. Good air flow (like when the car is moving) will lower the high side pressure. And thus low side. Normal response.

The only additional advice I have to offer is that I always add a supplemental blower fan when I recharge AC. As you saw, at idle, the high side readings can be high once it heat soaks. The stock electric fans are barely enough to keep up.

The FSM mentions using supplemental cooling.

I use a fan out of an old furnace blower.

I can confirm that the readings are much more stable with supplemental air flow while recharging. So try that as well next time.
" good air flow "
Reminds me of first starting after the engine swap. WTF ??? THE HIGH SIDE IS ,,,,, Oh yeah I'll just slip that protective sheet of cardboard out of there :sneaky::sneaky:
" good air flow "
Reminds me of first starting after the engine swap. WTF ??? THE HIGH SIDE IS ,,,,, Oh yeah I'll just slip that protective sheet of cardboard out of there :sneaky::sneaky:
Oh I've been there done that! Don't want to scratch up that shiny new radiator. Albeit I learned to pull the rad during swaps. And/or the core support. And/or the entire front clip. LMAO. The firewall on the other hand, don't ask.

Back to topic... technically the FSM says "adequate supply of airflow". LOL! I just paraphrased.

Anyways, note the "high volume fan" reference in the 3rd paragraph.

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