A/C blows cold...most of the time

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Nov 19, 2006
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I recently had the A/C compressor replaced on my wife's 1996 and the A/C blows cold all the time. Very happy with it.

On my 1995 however, the A/C works almost as well but every once in a while it completely stops working. I've been paying attention to this and notice it only happens on warm/hot days when the engine has been running a while.

Also, when the A/C stops working, I can tell the compressor stop cycling on and off and I don't hear the clicking noise inside when the clutch engages. This leads me to believe I have an electrical problem as the clutch itself was replaced not that long ago and the rest of the mechanical side seems fine..

I imagine there is an interior temp sensor somewhere and a relay to engage the clutch. Is there a link to a wiring diagram of the electronic side of this A/C system available? Do the climate control panels on the dash ever go bad? In my case, when it fails, it doesn't matter what A/C settings I have, it just doesn't work.
 
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every once in a while it completely stops working. I've been paying attention to this and notice it only happens on warm/hot days when the engine has been running a while.

AC shuts off when the engine reaches or passes 226F, which is hot. The dash gauge is a dummy light, which only moves above center/normal when it’s already starting to overheat, unfortunately. Hopefully it is something else and you don’t have a slow overheating problem, but..

Do you know the state of the cooling system? Age and/or condition of radiator, thermostat, water pump, hoses, coolant, fan etc
 
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AC shuts off when the engine reaches or passes 226F, which is hot. The dash gauge is a dummy light, which only moves above center/normal when it’s already starting to overheat, unfortunately. Hopefully it is something else and you don’t have a slow overheating problem, but..

Do you know the state of the cooling system? Age and/or condition of radiator, thermostat, water pump, hoses, coolant, fan etc
Coolant was just replaced with the Toyota red stuff this spring as part of routine maintenance. I don't know of any coolant leaks in the system. All the coolant components could still be original but they all look to be in good shape. Since you mention it, I will try and verify with a laser temp gun. Is there a good spot I can aim for to get an accurate engine coolant temperature? From my experience the engine doesn't seem too hot and seems the same as the 96. It could be a bad temp sensor however. Is this the source of the engine temp I would read from an OBD II scan?
 

flintknapper

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Coolant was just replaced with the Toyota red stuff this spring as part of routine maintenance. I don't know of any coolant leaks in the system. All the coolant components could still be original but they all look to be in good shape. Since you mention it, I will try and verify with a laser temp gun. Is there a good spot I can aim for to get an accurate engine coolant temperature? From my experience the engine doesn't seem too hot and seems the same as the 96. It could be a bad temp sensor however. Is this the source of the engine temp I would read from an OBD II scan?

It does indeed sound as if the temp 'cut out' switch is disabling your compressor. As @SpenserAK alluded to....there is a cut out switch that will break the circuit to your compressor at a temp of approximately 226°F and will come back on at about 216°F. It is a feature designed to help prevent further overheating of the engine.

The factory dash gauge is next to useless...as it has a large 'dead spot' in it and by the time it registers in the red you are already well on your way to overheating. Many of us monitor coolant temperatures via a 'scangauge' or something similar.

ScanGauge temp.jpg


Your engine has a coolant temperature sensor (which is what generates the temp you would see on a OBDII scan) AND a cut-out switch/sensor who's only purpose is to disable the compressor. See photo below for sensor locations:

sensors temp.jpg


There are a number of other reasons why your A/C would fail to cool intermittently (such as the evaporator freezing up) but none would cause the compressor to stop running (save for the binary switch which would not allow it to run at all if working properly). The binary switch senses system 'pressure' and would not allow the compressor to run if too high or low. But it is an unlikely culprit here.

Rather than try to take temps via an IR gun....I would recommend installing an external monitoring device that will let you look at various parameters real time (under actual driving conditions). Some of them are very reasonably priced.

IF you are overheating....look first to your fan clutch and be sure it is turning your engine fan well. Also look at your A/C condenser and Radiator to see if they are clean. Lastly....if your coolant thermostat is old...you might consider replacing it.
 
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Not sure about the LCs, but on my 97 LX I believe there is an AC amplifier electronic box.
 
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I'm also having a similar problem on my 94. When I'm on the freeway the a/c blows ice cold but as soon as I get off or traffic slows down, the a/c won't entirely cut out but it'll get warmer & as soon as I'm going 50+, it'll be cold again. Would that also be the engine getting too warm?
 
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In both cases above there could be a handful of reasons the AC isn't working as desired. The issues could be in the engine cooling system, AC system, etc.. I would absolutely be taking the following steps on both rigs ASAP if they aren't in place already, even though they may not end up being direct fixes:
  • get a newly tuned fan clutch (valve opening temp set, 15k or heavier oil in a new or confirmed good blue hub)
  • get setup to monitor actual coolant temps in real time via a new sender/gauge or odb2 (if available)
  • clean, inspect, test and update cooling system as needed including installing shroud, foam between core support and rad, checking t-stat, etc.
Every case is different but if you don't know that you have the above covered you are rolling the dice on overheating the engine and there is some evidence that you may be pushing high temps already. Running the engine too hot just accelerates wear, can cause HG failure and other damage, etc. None of the above is particularly hard or expensive and even if you find that the engine is already running cool and that there were other reasons for warm AC you'll have peace of mind worth the cost of confirming the cooling system is in spec.
 
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Had a problems when in traffic, mostly solved by refilling my relatively low mileage cooling fan clutch with heavier cst oil.
 
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Update: I borrowed a bluetooth scanner to sync with my iPhone and have been driving around monitoring the engine temp through the device. I am making the assumption that the reported temp is accurate but I have no reason to suspect otherwise.

One thing I do see is the reported scanner temp does work differently than the temp gauge on the dash. From a cold start, the scanner temp moved up to about 180 *F fairly quickly. After that it slowly moves up to about 190-195*F while driving. I did park in once and then started it again and it was at 210*F but then worked its way back down to 195*F as soon as I got on the road again. Also, during this time that I have been monitoring, I haven't experienced the A/C cutting out either. As always, reproducing the problem is half the battle. I will keep monitoring and seeing if I can get this to happen again and provide another update at that time.
 
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Doesn’t appear to be extreme, but something in your cooling system is not quite sufficient IMO, allowing you to get too warm at rare times. Those casual-driving temps could be a little better, but not necessarily a problem in & of themselves... however, I can see how it could get too warm once in a a while. I’d say the truck is telling you something, before it becomes more serious. And, there’s a reason your AC would cut out.

Sounds like your tstat reacts in general, and you have fresh coolant, but you said the parts could be original. If it were me and I wanted to keep the truck, I’d replace or at least physically test everything. Just part of baselining - the cooling system being a big priority. I replaced everything on mine & now run on the cool end of factory spec, dropped 9-10° average.
 
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Well it took a few weeks but I finally reproduced the problem. I parked the truck outside last night and left it until late on a hot (85 *F) afternoon. It was hot inside when I started it. The AC never really got going and I had to roll the windows down just to keep cool. I know I did hear the compressor cycling during this time and the OBDII scanner was reporting the max temp in the 190-200 range while driving. It seems heat might have something to do with it but not engine heat. Not sure what to look at next but it seems like an electronic problem in nature.
 
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@e21pilot , I have two '95 80s. One runs reliably at 176°F (230k miles and mostly OEM cooling system) and the other one won't run below 194°F (310k miles and entirely new OEM cooling system, with the exception of a larger brass/copper radiator, and a newly reconditioned head). The hotter running engine will reach 200°F in traffic even on an 85°F day, when stopped. IME at least, the increased engine temperatures are not affecting the A/C performance, both systems in both 80s cool the same and operate similarly, if not identically. I doubt your engine cooling is your problem.

I measure both temperatures using a bluetooth OBDII reader and Torque (which you can't use with your IPhone). You can use an IR thermometer at the inlet (next to the distributor) and outlet (at the thermostat) coolant pipes to the radiator, but not when you're driving, obviously. These measurements will not be your engine coolant temperatures, but they will tell you if the radiator is cooling and, IME, are reliably 10°F lower than the coolant temperature, measured by the ECT sensor in the head.

I wouldn't spend the time or money installing a scangauge, just to diagnose a problem, but that's just me. I have nothing bad to say about them, but there are less expensive and time consuming single use tools.

But I think you have two, unrelated problems.

The A/C problem, as others have stated, is difficult to find. However, if your compressor is cycling on/off, and you know the clutch/harness connection is good (use the FSM to verify that), I would look at the amplifier. Unfortunately if you have a defective board or board level component, it'll be hard to diagnose, without swapping a known good component. It's fairly easy to do, since it's accessible. The climate controls do fail with age, and you could have a bad switch, but again, that's going to be hard to verify, for the same reason as the amp.

The other possibility is the ECT cut switch connector. When I reconditioned the newer 80's head, I had to replace several connectors and sensors, and the cut switch was one of them. In my case, one of the wires in the connector housing had broken, meaning the switch could not function to send a temp signal to the ECU. However, it can short. The problem is that, with head on the engine, it's really hard to even see the connector. You would have to unplug everything from the DS side of the engine and remove the upper intake to get a good look at it. And you'll very likely lose the knock sensors in the process. I did. It is possible to assess the wiring without doing this, but you still have to remove the oil filter, and the DS wheel, to do that.

Regarding your engine temperatures, I suspect my newer 80 is suffering from early onset clutch failure, causing the AT temperatures to rise and thus heating the radiator. The AT fluid is cooled by running it through the lower radiator tank and the AT cooler in front of the radiator. If the AT fluid is heated beyond normal temperatures, it will raise the engine coolant temperature. A sure sign this is your problem is higher than normal (above 2k) engine rpm at low load when shifting from 1st to drive. I don't mean stomping on the gas at a light and running the rpms above 3k, I mean accelerating slowly until the transmission has run through the gears, especially uphill from a standing start.
 
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This topic has been open awhile, but a quick comment would be to check your A/C belt condition closely. I had a case where the belt failed unusually, which caused the belt to crack internally and slip when it got hot from friction. Finally, it expanded enough that it wouldn't grip the A/C Compressor Pulley with enough force to turn it. The belt would turn with the engine, but the compressor pulley wouldn't, reliably.

Grabbing the belt and tugging when cold, it felt fine. When hot, it had lots of play. Replacing it with OEM belt (*and doing the Alt-Water Pump belts at the same time) fixed it for good.

Just a suggestion to start with the simple, easy, and cheap things first. Good luck,
 
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This is exactly what was occurring with my 94, and it was resolved by putting a high weight oil in my fan clutch. Ever since that change it's ice cold even when stopped and 100+ outside.
 
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Update: I had purchased an extra A/C compressor and fan clutch from my local dealer a while back thinking I would replace them someday but since I had them on the shelf anyway and my A/C system was having problems I decided to have my local inde install them and maybe get some free trouble-shooting while they were at it. They installed the new components (turns out the A/C compressor had already been replaced by a PO and it was a Napa branded unit), recharged the system and it worked perfectly. I had told them about the intermittent problem and they kept it a few days but could not get it to fail. Indeed I picked it up from the shop and the A/C was as cool as it had ever been. The next day however, after leaving it in the parking lot at work all day, it did the same thing -- failed because the clutch would not engage to turn the compressor.

At this point, I think I can rule out the clutch and the compressor and probably the engine cooling system and the belt (has a new OE one) as well. It really seems like something is getting hot inside the vehicle has more to do with it than anything. Is there a wiring diagram available for the A/C clutch circuit? If I could just figure out if it is getting any signal or where the signal is being lost, I could probably solve this.

I should also mention that this problem is not that big a deal for me as I often drive with the windows down anyway. When I take the family however, that is when I need the A/C so I do want to fix it.
 
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See my post just above yours. Mine pretty much only worked while driving until I replaced the oil in my fan clutch. It actually turned out that my fan clutch was almost empty, too.
 
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The shop installed the new fan clutch along with the compressor about two weeks ago. I could check it but I still don't think that's my problem. I hope to have time to dig into it in a week or two. A good wiring diagram of the A/C components would help a lot.
 

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