A/C clutch not coming on

[Nhaz]

Howdy peps. After waiting a year to fix the A/C on my truck, today I looked into the problem.

I found a hole in the front rad. what ever its called. replaced it from my parts truck. vacuumed out the system and put in new gas.

but the clutch does not engage on the pump. I jumped the low side pressure sensor still nothing. I then applied power directly to the A/C clutch and got it to engage and the system to produce cold air.

So what should i be looking for to find out why my clutch doesn't engage.

 

[BRUTUS_T_HOG]

if you have it properly charged, it is most likely the low side pressure switch. good news though. you don't have to recover the system to replace this, it unscrews and has its own schrader valve

 

[Nhaz]

The pressure in the system is a tad low according to the dial. about 25 pounds on the low side when the system is running and 30 when its not. I cant see the high side I dont have the correct connector for the high side connection.

I had by passed the low side pressure switch by pulling the plug off and stuck a wire in each hole to jump the circuit. The pump still would not come on. I forced it to come on by applying power directly to it. at this point the system is producing cold air.

tomorrow I am getting another small can of coolant to fill the system up.

But if I had by passed the low pressure switch should the pump have come on? If it refuses to and I know it works. what should I be looking for next?

 

[BRUTUS_T_HOG]

The pressure in the system is a tad low according to the dial. about 25 pounds on the low side when the system is running and 30 when its not. I cant see the high side I dont have the correct connector for the high side connection.

I had by passed the low side pressure switch by pulling the plug off and stuck a wire in each hole to jump the circuit. The pump still would not come on. I forced it to come on by applying power directly to it. at this point the system is producing cold air.

tomorrow I am getting another small can of coolant to fill the system up.

But if I had by passed the low pressure switch should the pump have come on? If it refuses to and I know it works. what should I be looking for next?

check for a blown fuse, check for power to one of the wires of the low pressure switch with the key on, and a/c on. if there is power then its the switch, if not then you need to check the power supply.

30lbs is very low when not running. how long are you waiting after you shut it off?

you are properly charged when low side or high side pressure is about 5-10PSI above ambient temperature with the system off.

for example if its 70 degrees F., your low side pressure should be at least 70PSI.

 

[Nhaz]

the pressure switch had power. I jumped the switch and the clutch didn't engage. I had to power the clutch right off the battery to get it to come on.

I ran out of coolant which is why it was low. I only had 1 can and the left overs of another. (they are small cans). I am getting another tomorrow.

outside temp when i was doing this was 30 degrees Celsius which is 86F degrees of that other temp measuring system =>

I currently am not finding a blown fuse.(yet! nothings labeled!) the blower works and the A/C light on the push button/ blower speeds switch comes on.

is there a separate relay I dont know about between the pressure switch and the clutch on the compressor?

BTW first time ever playing with A/C ive got a book here thats not helping me much other then to explain how to put the stuff in.

 

[kimcrwbr1]

you need a schematic to find out how to power up the clutch there are a few reasons why it wont enguage pressure switch temp switch and heater A/C control switch. You need 35-40 psi refrigerant to test the pressure switch and see if you have continuity if the switch is closed then it is elsewhere. check the other two switches if you have a temp switch. It sounds like a wiring thing to me. If your getting cold out of it by jumping the compressor. You probably have a hi-low clutch switch when below 35 the switch is open and whatever the high pressure is it opens the clutch switch so you dont freeze the coil in the cab. You need to get a wiring diagram to show you how to test for power to the clutch.

 

[kimcrwbr1]

On a 1990 there is a WOT cutout relay that is controlled by the ecu that turns off the compressor with your foot heavy on the skinny pedal.

 

[BRUTUS_T_HOG]

the pressure switch had power. I jumped the switch and the clutch didn't engage. I had to power the clutch right off the battery to get it to come on.

I ran out of coolant which is why it was low. I only had 1 can and the left overs of another. (they are small cans). I am getting another tomorrow.

outside temp when i was doing this was 30 degrees Celsius which is 86F degrees of that other temp measuring system =>

I currently am not finding a blown fuse.(yet! nothings labeled!) the blower works and the A/C light on the push button/ blower speeds switch comes on.

is there a separate relay I dont know about between the pressure switch and the clutch on the compressor?

BTW first time ever playing with A/C ive got a book here thats not helping me much other then to explain how to put the stuff in.

you put it in on the low side only, if it won't take it then do it with the system running.

i don't know exactly how its wired, but there is a the cycling switch (low pressure swithc), high pressure switch on some systems, evaporator pressure switch, and the WOT relay.

 

[Nhaz]

Today I finished putting the correct amount of coolant into the system. When i force the pump to run I am getting a steady 50 pounds on the low side. when its off it will jump to 110.

this did not get the pump to run on its own. I still need to force the pump to run via a jumper cable. Checking voltage at the pressure switch it gives me 12 in and 12 out.

So I now in need to the locations of those other relays on the vehicle. I have tried searching for them online and what not. but cannot pin down any location.

I have located the fuel relay and the ecc-iv relay. thats it. there are not any others on that fender that I can find.

So if anyone would be kinda enough to point out where they are I can proceed to test them.

If not I am going to go redneck fix soon and put a toggle on my dash!

 

[Nhaz]

After searching my truck inside and out carefully for several hours today I simply could not find anything resembling the WOT relay. In any case I put a switch on my dash and put it inline with the pressure switch.

Everything appears to work. I just need to remember to flip off the AC if I want to seriously open it up. which I almost never do.

 

[BRUTUS_T_HOG]

Today I finished putting the correct amount of coolant into the system. When i force the pump to run I am getting a steady 50 pounds on the low side. when its off it will jump to 110.

this did not get the pump to run on its own. I still need to force the pump to run via a jumper cable. Checking voltage at the pressure switch it gives me 12 in and 12 out.

So I now in need to the locations of those other relays on the vehicle. I have tried searching for them online and what not. but cannot pin down any location.

I have located the fuel relay and the ecc-iv relay. thats it. there are not any others on that fender that I can find.

So if anyone would be kinda enough to point out where they are I can proceed to test them.

If not I am going to go redneck fix soon and put a toggle on my dash!

50psi is too high, 110 is too high so you've overcharged the system.

i just checked my wiring diagram, there is a cycling switch and a WOT relay.. since you have power to the cycling switch this means its the relay. the relay might be missing, it should be under the hood somewhere

 

[kimcrwbr1]

did you try turning on the defroster it will cycle the compressor to remove moisture from the cab I am guessing it is one of the switches or a wire or connector you need to get a wiring diagram and start checking wires and switches dont rely on the redneck fix you could burn up the compressor and yea at 50 psi you will probably freeze up the evaporator in the cab get it down to 40 if you are going to cycle it by hand and dont run it too long that way or you may be changing the compressor too.

 

[BRUTUS_T_HOG]

did you try turning on the defroster it will cycle the compressor to remove moisture from the cab I am guessing it is one of the switches or a wire or connector you need to get a wiring diagram and start checking wires and switches dont rely on the redneck fix you could burn up the compressor and yea at 50 psi you will probably freeze up the evaporator in the cab get it down to 40 if you are going to cycle it by hand and dont run it too long that way or you may be changing the compressor too.

he said he has power to the cycling switch, this means the switch in the HVAC controls are ok, this leaves the WOT relay which might be missing.

pressure=temperature, so while i don't disagree with you.. i just wanted to correct this information: 50psi will equal about 50 degrees F. and won't freeze the evaporator

low side pressure should run between 30-40 degrees. keeping in mind that evaporator freeze up happens at 32 degrees F., 0 degrees C.

 

[kimcrwbr1]

he said he has power to the cycling switch, this means the switch in the HVAC controls are ok, this leaves the WOT relay which might be missing.

pressure=temperature, so while i don't disagree with you.. i just wanted to correct this information: 50psi will equal about 50 degrees F. and won't freeze the evaporator

low side pressure should run between 30-40 degrees. keeping in mind that evaporator freeze up happens at 32 degrees F., 0 degrees C.

While I have little experience with auto refrigeration I did receive my certification in refrigeration so I doubt your 50 = 50 is correct I will check tho. I am just going by a post I read that says the reason there is a high pressure switch in a system is to avoid freezing of the moisture on the evaporator in the cab 50 degrees might be the temp of the air coming out of the vents but the refrigerant going thru the system is much colder and if the fan is not moving enough air thru the coil the water it produces will freeze.

 

[BRUTUS_T_HOG]

While I have little experience with auto refrigeration I did receive my certification in refrigeration so I doubt your 50 = 50 is correct I will check tho. I am just going by a post I read that says the reason there is a high pressure switch in a system is to avoid freezing of the moisture on the evaporator in the cab 50 degrees might be the temp of the air coming out of the vents but the refrigerant going thru the system is much colder and if the fan is not moving enough air thru the coil the water it produces will freeze.

the first sentence says it all. im not trying to say you don't know anything.. i just don't want misinformation to spread. i work on these systems all the time

pressure=temperature.. is very true. more so with R-12 where this concept started... try looking at a pressure/temperature chart for R-12 and R-134a and you'll see that within the operating pressure range of the evaporator temperature is equal to pressure.

the HIGH pressure switch is installed on the high side of the system to turn off the compressor when pressure is too high.. as happens when the cooling fan fails or the system is over charged. this switch was introduced to eliminate the high pressure blow out valve so refrigerant is not vented to the atmosphere.

the low pressure switch A.K.A. cycling switch is a switch that is installed into the low side of the system to turn off the compressor when pressure gets too low.. say anything below 30 degrees... because at THIS pressure the surface of the evaporator will be 30 degrees give or take, and any water on the surface will turn into ice and restrict air flow.

if you hook up a set of gauges to the system and your gauge says 50PSI on the low side .. you can be very sure that its NOT working as well as it should, of COURSE it will feel cool to your hand when its 85 degrees outside. sometimes when ambient temperature is high you can't get down to 30-40 PSI.. in this case the general rule is 30degrees below ambient temp. is a good working system

maybe certification is too easy to get, you said you are going off the information you read in somebody's post? you should know these things if you're certified.