A/C Clutch not engaging

[DennisG01]

Noticed my AC wasn't blowing cold (this was the first time I had it on this season), so I pulled over and saw that the clutch is not engaging. When I got home I removed the 2-wire plug (on the AC), cleaned the connections and re-plugged the connector back on (actually unplugged and plugged a few times). Still nothing.

I disconnected the plug and measured volts at the plug with the engine running and the dash button "on". The black wire measured a solid 14.2 millivolts, while the green wire's reading fluctuated wildly... between 300mv and 1.3v (the best that I could tell, anyway, as it was fluctuating rapidly).

I can move the clutch by hand, fairly easily. All battery cables/connections are clean and tight.

Am I correct in thinking that I should trace the wires (black and green)? Does anyone know right where they go? Chilton's does not cover the AC system.

Any other advice?

 

[btfarm]

If you're ranging in the Mv category then you aren't getting 12 volts.
Obvious question alert!
Did you check the fuse?

 

[DennisG01]

Obvious? Nope! Slipped right past me! :rolleyes5:

The fuse on the side of the dash is fine. But, that led me to check the relay under the hood. Here's what I found (NOTE: these numbers are with the A/C dash button on or off)...

Pin #85 = 65mV (with A/C dash button "off", only goes to about 50mV)
Pin #86 = 13.7V
Pin #87 = 14.0V
Pin #30 = 8mV If I'm reading the schematic (printed on the relay) correctly, that would be expected as that is the "out" feed to the compressor?

I don't know how "sound" this practice is, but while the engine was running (with A/C button "on") I plugged the relay in and out a few times. I DID NOT feel any type of "click" (didn't hear anything, but I doubt I would over the engine noise). Should I feel a click?

Can I jump 12v to Pin #30 to see if that activates the compressor clutch? This is, of course, assuming that Pin #30 is the correct one and that it is safe to do so. I won't do this until I get the "A-OK".

 

[btfarm]

If it was me, I'd try to jump with 12V (with a 15 or 20a inline fuse) direct to the clutch hot lead with everything else off. Might have to jumper a ground to make that work.

 

[DennisG01]

The wires going to the clutch look to be only 16 or 18 gauge. I didn't bother with a fuse (figured the worst that would happen is I burn up the 18g wires I used for jumpers). The clutch engaged and disengaged perfectly by jumping from the battery.

But, unless I'm mistaken we need to figure out why 12V isn't making it's way to the clutch?

 

[THEFERMANATOR]

You need to go in and get a couple of jumper wires and bypass you safety switches is where I would start. The one on the dryer is the low pressure switch and is the most common one to fail. Then you have the one either in the back of the compressor or the high side line before the condenser9there will also be one after the condenser, but it is for the recirc door over ride). Just unplug the connectors and jumper them with a short piece of wire and see if the clutch engages. It does you have a switych problem, if it doesn't you have a wiring problem. I'm not sure how the 98's are wired, so I can't tell you how it triggers the relay for one.

 

[WarWagon]

Systems tend to leak down over the winter. Do an ohm test on the sensor for the low side - it's on the silver can, accumulator. It should be shorted and opens about 40 PSI. If it is open it has gone bad or more likely you have lost the freon charge out of the system.

Running the system with the switch bypassed for anything other than testing will lock up the compressor from lack of oil. No freon = no oil going to the compressor.

So you are into servicing the AC charge, looking for big leaks like a blown hose, vac the system, and charge it up. If you miss the leak you have just paid for shop labor in freon alone: 1st charge $40.00, and second charge of freon after the leak repair $40.00 is 1 hour of shop time...

 

[DennisG01]

OK, just to make sure I've got this straight...

I do see the two switches you guys are talking about (one on the high pressure line, right behind the compressor and the second on that silver can). So, I can jumper the pins of the plug and/or ohm test the pins on the connector (the piece the plug "plugs" in to). That's easy enough, I'll do both.

I think I have one of those cheapie pressure gauges that came with a can of freon (from parts store). I can hook that up, too?

 

[DennisG01]

I ohmed each switch - both returned an "infinity" reading (open). I jumped both switches and pressed the "AC On" button - clutch did not engage. I used that cheapie pressure guage - it connects to the fitting on the low side tube. It read about 9psi. The "low range" on the dial goes to 25psi, then the "full range" goes 25psi to 45psi. Then there is an "alert" and a "danger" range above that.

Is it safe to assume I need a charge? Is it worth while buying a can at the parts store... or just take to a shop?

I just remembered that I had it in to a shop last summer and all it needed was a charge.

 

[btfarm]

I'd say you need to get it charged Dennis. I'd also try real hard to find the leak and get it fixed so you won't have to deal with that again.

 

[schiker]

I would try and recharge and see how long it lasts. A can of freon is relatively cheap once a year or 2 and might reseal. Some have conditioner in them and dye to help find leak.

My understanding is ....
It pumps til low side is down to a minimum psi maybe 30 ish psi and there is a safety of 20 psi minimum low side pressure once under the compressor won't cycle on (freon is too low).

Someone want to tell us how to add freon I need to do this too. Which port you say the low side is on the silver can correct.

Shade tree ....

I believe you just turn on the system set to cold. Hook up a can and slowly add a little freon till compressor comes on between 30 40 psi turn can off. Let the system cycle and repeat until the low side pumps down to what about 30 something psi and cuts off and will come on by itself. And feel if the air is cold. Suppose to also hook up the high side so the system is not overcharged but adding a little at the time and feeling it might get you by.

I think if you can equate the dash air temp in degrees F to the psi on the low side when the pump turns off the system is working correctly you are ok close to correct charge ????

Don't overcharge the low side ( help me here 45 psi???) because if compressor doesn't cycle on Freon being low is not your problem.

 

[DennisG01]

Updating... Seems like charging it up is working. I've got two cans in it and it is engaging and blowing a little bit cold. I'm going to get another can tonight for tomorrow (PSI is hanging around 30, so far). Shiker - thanks for the PM's and the step-by-steps!

Thanks guys! I never would of guessed it was as simple as just needing a charge. Now, the next step would be to find out why it's taking 3 cans! Course, if it lasts this way for at least a few months, I'm fine with charging it up when I need to. One of the cans I bought was for "high mileage" vehicles with extra "stop leak" in it. I'll buy another one of those.

I also don't really use the A/C much, so that might part of the problem. If I use it more, maybe the seals will stay in better condition.

 

[THEFERMANATOR]

The SUBURBANA/C will hold about 4 pounds for a dual air set-up, which is just over 5 cans. The most common leak point is the high side valve at the compressor.

 

[WarWagon]

Cans are 12 ounces and a pound is 16 ounces. With that pressure reading your system was more or less empty, gas only. So add up the cans and get to the exact weight your system needs. Usually on a sticker on the silver can. Undercharge starves the compressor of oil and overcharge doesn't cool well.

Run the system often as the oil keeps the compressor seal from leaking - no matter the season.

 

[Acesneights1]

Cans are 12 ounces and a pound is 16 ounces. With that pressure reading your system was more or less empty, gas only. So add up the cans and get to the exact weight your system needs. Usually on a sticker on the silver can. Undercharge starves the compressor of oil and overcharge doesn't cool well.

Funny how many don't know that. Well put.

 

[schiker]

I had the same problem the compressor clutch would not engage. There are safety's built into the system like mentioned above. When refrigerant is low and the low pressure side drops below ~20 ish psi (compressor running) the low pressure switch opens the circuit to disable the compressor. It does this so the compressor won't suck in any air and moisture I believe. If moisture (air or other contaminants get into the system acids form and moisture freezes causing problems and or moisture robs refrig of lubricity and effeciency etc - Refrigerant is hydroscopic - absorbs water).

My '97 pickup holds 2 lbs = 32 oz. I added an 18 oz can (with oil charge mixed) and operated it several days. It blew fairly cool; almost cold at first then warmed up after days. I just added a second can 12 oz and its blowing much colder. 18 oz + 12 is 30 oz. I figure there was some residual in there but not all of the product comes out of each can so I am guessing 20-25 ??? oz of refrigerant was added overall. I hope it just takes some circulation and run time for the system to lube itself and seal up.

Its not always this easy and there is more to it than just adding refrigerant but at 200K miles and 13-14 yrs old truck ...... $30-40 for refrigerant every yr or 2 is ok with me for now. Note look for refrigerant on sale in fall or early spring and save it for next summer. As long as I can do this in the driveway and it doesn't inconvienance me too much its not too bad of gamble. If it warms up I might add one more can to finish a try and maybe leave fresh residue from a leak for the shop to find with leak detector.

In my case if it were a younger truck or the sytem had been opened I would say take it to vac down the system and have it properly charged/checked by a more knowledgeable person. But I just really dislike taking my truck to anyone to work on it without at least trying to fix it myself.

 

[DennisG01]

OK, so I've been waiting to post back until I drove it for about a week. I don't recall seeing the pressure gauge "cycle" up and down before (but maybe I didn't take notice)... Yesterday I observed it, with the engine running, the gauge would read about 20PSI with the clutch engaged and when it disengaged it would creep up to about 48PSI. This continued to happen as long as the engine was running and the clutch engaging/disengaging. After the engine was off, it crept up to about 52PSI.

The ?condenser? (big silver can) says the system holds 3lbs 8ozs. I've put 36ozs of freon (not counting oil/lube - just freon) in, which means I've got 20ozs to go until I would have put in the whole 3lbs 8ozs.

The air is blowing cold (not super cold, though). I'm not sure how to interpret the gauge readings (when the clutch is engaged or disengaged). Should I put more freon in?

 

[WarWagon]

OK, so I've been waiting to post back until I drove it for about a week. I don't recall seeing the pressure gauge "cycle" up and down before (but maybe I didn't take notice)... Yesterday I observed it, with the engine running, the gauge would read about 20PSI with the clutch engaged and when it disengaged it would creep up to about 48PSI. This continued to happen as long as the engine was running and the clutch engaging/disengaging. After the engine was off, it crept up to about 52PSI.

The ?condenser? (big silver can) says the system holds 3lbs 8ozs. I've put 36ozs of freon (not counting oil/lube - just freon) in, which means I've got 20ozs to go until I would have put in the whole 3lbs 8ozs.

The air is blowing cold (not super cold, though). I'm not sure how to interpret the gauge readings (when the clutch is engaged or disengaged). Should I put more freon in?

The gauge clearly shows the system is low, because you know what you put in and what the capacity is you know exactly how low. (Unless it is cool/cold out.) With the blower on high engine at idle the clutch should not cycle or on cool days 30 seconds plus.

When you first started by the pressure reading and the fact your AC clutch would not turn on, yes - put the entire charge in the system. The only thing left in the system with that pressure/ no clutch was gas - no liquid freon. The weight of the gas in the system is a rounding error it is so small.

The "pressure" is dependent on some liquid in the system and varies by temperature. The low pressure cut out switch doubles to keep the system from running with no freon in it. it's real purpose is to keep the evaporator above 32 degrees and not allowing it to ice up and plug airflow. It does this by pressure and pressure indicates temperature of the freon. Only the temperature and general diagnostic of the system can be obtained from pressure readings.

The only way to charge the system is by weight. As your system was "empty" with only residual pressure left in it you will need the full charge. The silver can will hold some extra freon. You want this as close as possible for the system to work properly. Err on the overcharge side but not more than a few oz. Point of fact a slightly low charge will have colder vent air than a properly charged system - the low charge starving the compressor of oil.

Again you should have pulled a hard vacuum on the system, checked for leaks, filled the system by weight and rechecked for leaks. As the system had pressure no water or air leaked in so you can get away with what you are doing.

If the system had liquid freon in it - well you would never get the system perfect. Temperature and the way this system works do not allow a good pressure reading to determine the proper charge. Condenser has hotspots etc to make it impossible.

 

[DennisG01]

Thanks for the details! I'm going to re-read that a few times.

But, just so I'm clear, I can fill the system (using the gauge) based on the reading when the clutch is engaged? I'll also do my best to estimate 20ozs.... The cans I've been buying have 11oz in them, so that shouldn't be too hard.

 

[DennisG01]

UPDATE:

Well, it's still leaking out somewhere. Not from any place that I can readily see, though. I need some some front end suspension items replaced that it's going into the shop for, so I'll have them take a look at the A/C, as well.