91 B2600i...A/C Compressor Cycling...**Updated with PICS**

Hello,

I have a 91 b2600i, with what appears to be original A/C system that has been converted over to R-134. I do have a slow leak in the system which usually requires me to add a 12oz can of R134 prior to summer usage. I did just recently start using the A/C system again and have noticed a few things which I have some questions about.

1st, and again bear in mind this is an r-12 system converted to r134 usage, it takes approx 4-5 min of steady driving, with the a/c on highest speed and air recirc. for the temperature at my vents to reach what I have deemed to be coldest possible temperature, which is around 55-60 degrees at the vents. Is this delay normal?

2nd, and perhaps more important is while I am driving with the a/c on, I will hear a click coming from underneath my dash board on passenger side followed by the compressor shutting off for usually about 30 seconds or longer and then kicking back on, accompanied by another click under dash. I've noticed this almost always happens when my vent thermometer is reading about 50 degrees or colder.

At first I thought it was normal for compressor to cycle on and off due to the evaporator getting too cold, although I must say, 50 degree vent temps are not what I thought the system would consider 'too cold'.

But even if that theory is correct, and for some reason the system senses that I've reached optimal coldness and shuts the compressor off temporarily, the temperatures at the vents raise fairly quickly when this happens, to the point I can feel the air coming out warming quickly. Not something I want to deal with in 110 degree temps outside soon. I assumed if this compressor cycling was a normal process due to the evaporator about to freeze up, then even when this happens, during that 30-40 seconds the compressor isn't engaged, the air coming out of my vents should still remain cold...not warm up to over 60 degrees.

Again, I do hear a click under the dash on passenger side everytime this cycling happens, and vent temps increase quickly during this time the compressor isn't operating. And...based on my coldest vent temps being about 50-60 degrees on highest fan setting with recirc. mode on, it's hard for me to believe the evaporator is to the point where it's about to freeze over.

Any ideas? Suggestions? comments? Could this all possibly be due to system overcharge? Undercharge? Bad electrical component? I tend to believe it has something to do with the pressure in the system due to the rapid temperature increases when the compressor cycles off, and the delay in my air becoming cold when I start using the system.

Thanks in advance for any replies.

The click you here is the low pressure switch shutting off the compressor. Your vent temps should be a bit cooler than that even though it's converted to 134-a. The whole reason your compressor is cycling like that is because there isn't enough charge to keep the pressure up high enough to not trip the switch. I am not a big fan of 134-a being I used to do the conversions from r-12 to r134-a back in the 90s. When we did the conversions the vent temps never rivaled the old r-12 systems. My 89 B22 runs the outlawed r-12 that was made by DuPont, my vent temps drop to the low 40's on a 90 degree day. I am not sure what method you are using to put the r-134a in your system but I have seen small cans of it that come with a gauge that are reasonably easy to use if you don't have a set of gauges.

1. That delay does seem to take a while, especially if that is in the morning when your truck is cold. But in the afternoon, if it's been baking in the sun all day, it's going to take longer because all of the duct work is going to be hot in addition to the cabin air that your are trying to cool down. So even if the evaporator is getting to it's coldest temp in short order, it's going to take a while to get that vent temp down.

2. The click under the passenger side dash is the defrost switch cycling. That is a good sign if it's still calibrated correctly. I think it is set to cycle around 35 degrees F to prevent the evaporator from icing up. So it could be out of calibration and shutting of the compressor too soon, or it may in fact be accurate, and your are loosing your cool air somewhere else. If you drop the bottom half of the evap case (see below), you might rig your own thermostat in there temporarily to how cold it is when the switch cycles. If it's not 35-ish, then it probably needs to be replaced.

So here are a few other thoughts on your system.

1 Air Flow
There are several placed that you could be loosing airflow. The first is the evaporator. If it hasn't been cleaned in a long time, there is a good chance that it's clogged with dust, debris (often leaves), mold and or mildew. It's usually on the intake side. You can drop the lower have of the case and clean it without opening up the AC system. Just pop the little clamps off... there are also one or two on the back that you won't be able to see, but you can get to them with a flat-head screw driver. If your vent's don't feel like they're blowing as hard as the should, the you probably have some blockage at the evaporator.

Also, there a couple of band style couplers that connect the evaporator case to the blower housing and the air handler/heater core assembly. They originally had foam on the inside of these bands. Unless it's been replaced, the foam has probably deteriorated away causing air leaks. It might not look like a big air leak, but it can be. So I would check those items out to make sure you're getting the maximum flow across the evaporator and aren't loosing any airflow. There are several other foam seals on the duct work and system, but these areas the block or loose the most air flow.

2. Even with R134, I'm able to get into the low 40s in the low 90s ambient temperature. But if you are in in 110 degree heat, then 50 might be all you can get.

3. You should consider getting the leak fixed if and when you can. Even using the cans that come with the gauge, you really don't know how much refrigerant is in the system. You could be overcharging the system (and over stressing the compressor) even though the low pressure gauge on the can is in "the green."

4. If you do get it fixed, I would consider adding a binary / 2 way pressure switch. It will turn the compressor off if the system is to low or too high. Especially if you are running 134 through your old small R12 condenser. Your current system (if original) only has a low pressure switch. By adding in the high pressure switch, it will protect the rest of the system (seals) and the compressor in the event you get some kind of blockage and the pressure spikes. I have an electric fan on my B2600i, and when the controller for is on the fritz, the fan won't come on. In idle traffic in warm weather, it only take about 20 seconds for the system pressure to spike to 400-500 psi and trigger the high pressure switch.

5. It probably won't make much difference in highway traffic where you have plenty of airflow, but if you spend a lot of time in slow city traffic, you might consider adding a larger parallel flow condenser. If you look at cars that were built with R134 systems, you will see that they all have much larger condenser... some the size of radiators. The newer style parallel flow condensers are supposed to be 30% more efficient than the older tube-n-fin style & serpintine condensers. That will help bring down the temperature and the pressure of your system and probably get you some more cooling in hot city traffic. It would cost around $100 plus the expense of having somebody fit it to your existing lines. Keep in mind that IF your current system is cycling (at the defrost switch) at the proper temperature, then you won't have any extra cooling to gain since the evaporator temp is already getting to 35-ish F. But if you're not cycling in hot city traffic, then another option would be to add a condenser fan if you can find one small enough to fit. That would probably be cheaper than having a condenser custom fit.

Thanks for the speedy replies. A few questions. If what i've read is correct, you guys are saying that there is no high pressure switch and therefore even if I was overcharged, there wouldn't be a high pressure shut off of the compressor? I did notice while trying to get my amount of r134 right, when i've added more than a couple 12oz cans, the compressor starts slipping and then won't engage at all until i've let some of the freon back out. Is this because someone has already installed an aftermarket high pressure switch or is it normal for the compressor to just start failing when it's way to overcharged?

Second, assuming I only have a low pressure shut off switch, the cycling could be due to low charge, or as someone else mentioned, the evaporator reaching optimal temperature of about 35 degrees F?

And finally, would it be of any help if I were to post what my high and low side pressure readings are? Figure if some of you guys know what kind of pressure I should be getting, those numbers would help determining the probable cause of the cycling correct?

Thanks again for any and all responses.

No, there is/was no high pressure switch on a stock oem system. You can replace the low pressure switch with a high / low pressure switch. They don't look any different.

If somebody had installed a hi/low pressure switch, the switch would cut (interrupt) to the power to the compressor clutch. So you would be seeing the clutch disengage. Since you said your clutch was "slipping," then you were probably over charging it and the clutch was slipping. The pressures can easily spike to well over 500 PSI if you are not careful. As I said before, having a high pressure switch will help protect your compressor and the rest of your system. Most modern cars have them now.

Also, you mentioned adding "more than a couple of cans" of refrigerant. Your factory system was designed to hold 28 ounces of R12 refrigerant. When you run 134 in an old R12 system, you usually run a lower amount because the pressure and temps will run higher using the older smaller condensers. If you search the internet, you will find recommendations ranging from running 80% to 95% of the original system capacity. When I converted mine, I used 2 12 ounce cans of 134. So 24 ounces works out to about 85%. So if you're adding more than two cans, you are likely overcharging your system.

If you can hear a distinct click in the passenger dashboard, that is the defrost switch cycling. It doesn't reset until the temperature warms enough.... probably 40 degrees or more. This will usually take 20 or 30 seconds or more depending on the conditions and fan setting.

If your low pressure switch is cycling, it will sound more likes it coming from the engine bay and not the dash. Also, when it cycles off, the low pressure side will rise very quickly and it will kick on again... probably in a few seconds... not 20 or 30. And then it will kick off again in a few seconds. On/off, on/off, ect. If your low pressure switch was cycling, I don't think you would ever get the 50 degree temps you are getting at the vents.

I think you can now tell why it's cycling with the info above. But if you are still not sure, you can put a multimeter on the low pressure switch itself. If the low pressure switch is in fact cycling, then you will have 12 volts on one side of the switch, but not the other, when it has cycled the compressor off. If the defrost has cycled, you won't get 12 volts to the low pressure switch, so you won't have voltage on either side!

Putting some gauges on them would definitely be a good idea so you can see how high you are running on the high and low side. I don't remember off hand what the high pressure should be running at... it will depend on the ambient temperature and the amount of refrigerant in the system.

The only way to truly know how much is in the system is to evacuate it and weigh it or just simply evacuate it and recharge it with the proper amount.

If you add to it in the near future, you might try adding a refrigerant with dye in it or just adding the dye by itself. Then you can use a small UV flashlight (available at your local autoparts store or amazon) to look for leaks. Hopefully, it's just an o-ring somewhere and you don't need any of the hard components. That is another reason to drop the lower evaporator case for cleaning so you can inspect for leaks there too. Even if you find a leak somewhere, you want to look over the whole system to make sure there aren't others. Otherwise, you'll be going through the process all over again.

Checked the pressures right now, keeping in mind it's about 70 degrees outside...

Low side ranged from 20-25 psi and high side was 150-175 psi. When the compressor did click off, did notice low side pressure climb a bit for a while, then compressor would engage and pressure dropped.

So, based on that, possible i'm right where I should be as far as r134 goes and it's just the evaporator getting too cold and the defrost switch is turning compressor off?

I still don't understand why vent temps would be increasing so much during that 20-30 seconds the compressor stays off.

20-25 is too low on the low side if I'm not mistaken. You should have something in the neighborhood of 50 psi on the low side. I don't remember the OEM spec, but think 28 is a typical low side cut-off. You are going to see the pressure rise immediately when the compressor stops regardless if it low on refrigerant or if the defrost has switch has cycled.

So as much as I thought your problems was the defrost switch based on your clicking in the dash, I think you are in fact low on 134. So I would get that looked at first. If you can add a can and get through the summer, that's not too bad. But if it's doesn't last long, then I would add a can with dye and try to locate the leak and then get it fixed.

Also, being low might cause it to ice up also. So when you get the system properly serviced, your clicking in the dash may become less frequent.

I'm also in Arizona, but still run R-12 in my B2200, and it works great.

1. You admit there is a leak, you need to have that fixed. "Always fix first what you KNOW is bad".

2. Conversion to R134a is not my recommended practice in a super hot area like Arizona or Nevada. Stuff like the condenser was engineered for R-12.

3. Parking in the sun means that your seats, dashboard, etc., will all be like 140F, that heat needs to be dissipated.

4. Make sure to drive for 10 minutes with the Recirculation/Maximum off, because the hot air outside is actually cooler than the hot air inside the truck.

5. Make sure that there's no heat being added, like that the heater flap isn't sealing off completely. Similarly make sure that when you do switch to Recirculation mode that the outside air is mostly diverted off.

6. Yes, the electrical switch clicking on/off under the dashboard is the evaporator de-icer switch, senses temperature. There is no adjustment of temperature on the factory switches.

7. The receiver-drier has a low-pressure shut-off switch.

8. The compressor on the later models has a high-pressure relief valve on its rear side.

9. When the R134a was first added, was the system evacuated first, and then the R134a added? This would be the most-correct way, after you fix the leak.

10. Window tint will also help.

Mr. Scotch,

you say my low side pressure is too low...I know I mentioned this in the post with the data, but the pressure test was conducted with the outside temperature being about 70 degrees, and rpm's of engine were held at 2k...which I was told to do for an accurate pressure reading.

So, with that information in mind, the 20-25psi is still too low?

Think I need to add another can? And you think the shut off of the compressor is due to the pressure dropping too low, not the defroster switch?

Thanks.

I'm not sure what the cut-out pressure was for the original switches. But I know that most of the 134 switches cutout in the 25-30 range.

The way to know for sure why it is cutting out is to get a multi meter and check for voltage at the low pressure switch while it has cycled off. Power flows from the AC Relay, to the defrost switch, then to the low pressure switch, then to the compressor. So, while the compressor has cycled off, check for voltage at the input to the low pressure switch. If you aren't sure which one it is, check both. If you don't have power at either side of the low pressure switch, then the defrost hasd cut the power to it. If you have voltage on one side of the low pressure switch, BUT NOT THE OTHER, then the low pressure switch has cycled (off). So that is the best way to tell. You can also just listen closely in the passenger floorboard and see if you hear the defrost switch cycling. It's possible both switches are cycling at different time.

I think you are probably low, but I wouldn't add any until I confirmed that it is the low pressure switch cycling.