AC Advice

[robc311]

I’m looking for recommendations on where to start with troubleshooting my AC. All I know is that when I push the AC button nothing happens. I don’t hear the compressor kick on and the air doesn’t get cold. I haven’t had the truck long so I don’t know where to start with it. It’s a 95 with 207k miles so what should I check first?

[StarquestMan]

Im not sure how much has changed since the switch over to R134a but on my 87' there is a pressure switch that you can test for continuity behind the grill. To save you the hassle of taking off the grille the wiring harness for this switch runs near the battery that can be disconnected to test for continuity on the switch side. If you don't have continuity it means you are low on refrigerant or there is a slight possibility that the switch has gone bad. My guess is that the system is low on charge but there is always the possibility of an electrical issue. You will need some A/C equipment if you decide to try charging it yourself.

[geezer101]

Do the basic checks first - it could be a simple fault (fuses + relays). If they appear to be good, test the low pressure switch theory (on the dryer unit) - with the engine running and the A/C switched on, disconnect the dryer switch and bridge the 2 connections together. The compressor clutch should trip and the A/C should power up. Some A/C's have both low and high pressure side switches. It's important to note that the A/C shouldn't be run any longer than necessary to test it as it could damage the system if it's low on oil/gas refrigerant.

[robc311]

I finally got around to troubleshooting the AC and I hooked up a pressure gauge and it went up into the red so it appears that it’s not leaking and it’s overcharged...not sure what to do about that.

[StarquestMan]

What kind of pressure gauge? If you mean one of those ones that is attached to the can that attaches to the low side it may not be giving you the full picture. That and if I remember correctly they want you to check when the AC compressor is cycled on which will cause the gauge to drop down. If all this is true it sounds like you have an electrical problem. Did you try jumping the two terminals on the low pressure switch? this should tell you if the electrical side of things are working or not.

[StarquestMan]

Check out post 6 of this post

http://www.mightyram50.net/vbulletin/showthread.php/5821-Wiring-to-A-C-Compressor-after-rat-problem

In the diagram posted it looks like the R134a systems use a dual pressure switch, instead of a low pressure only switch like in my 87'. If your system is overcharged it would keep the AC from starting just like if it didn't have enough refrigerant.

[robc311]

I have an update...I discovered that the fuse hidden under the hood was blown so I replaced it and the compressor came right on. I took another pressure reading using the gauge that came with the refrigerant and now it read very low. I put in a can of refrigerant that had
uv dye and leak sealer and it got cold. I checked for leaks with the uv light over several days and didn’t find anything. The refrigerant level was still reading pretty low and it wasn’t blowing ice cold so I added more...the sticker on the inside of the hood says it holds 27oz and the two cans I had were 12oz and 14oz but they aren’t 100% refrigerant...and I didn’t add all of the second can. It still reads kind of low and I don’t want to overcharge it so I stopped. It’s still not blowing as cold as I think it should but it would probable get a bit cooler if I was actually driving it rather than just idling it. Does anyone have any other tips on why it’s not getting really cold? I’d say it’s cool.

[StarquestMan]

Try sitting in the truck while giving it a bit of throttle for a minute or two that should make the ac a bit colder than at idle. It is possible that the fuse went and the truck was driven without ac by the PO and the refrigerant simply leaked out over time. what did this fuse go to by the way, is there a specific component that it powers or is it for the whole system?

[StarquestMan]

I just looked over that diagram i mentioned earlier and it looks like other than the main fusible links there is just one 10A fuse for the clutch and clutch relay. Im guessing this is the fuse that blew? either way it sounds like your ac is working and now you just need to get the charge level correct;)

[pennyman1]

get a thermometer that you can stick into a vent, like a cooking one that reads down to 20 - 30 degrees F. run the a/c and see how cold it is getting. 40 or below is working properly.

[robc311]

I took it for a drive and it didn’t get any cooler. The fuse I replaced was the 10A one. I’m torn between getting a pressure gauge that measures both the high and low sides (about $40) to see if that tells me anything, or getting the system evacuated and replacing the accumulator/dryer due to the age and possibility of contaminates and moisture, or just taking it to a shop. The PO told me they never used the air and they had it 20 years so who knows what’s in the system.

[StarquestMan]

I was able to fix up my ac myself but you will have to decide whats best for you. If you do decide to do it yourself you will most defiantly need a proper gauge set and a vacuum pump to pull out any moisture. The tools will probably pay for themselves but you may want to shop around and see what the shops will do and wont do. It sounds like you are on the home stretch though to getting your AC sorted out though, you may not even need to replace anything if the system had some pressure when you first worked on it because if it had any pressure at all no moisture could make its way into the system. I would say get the gauge set and charge it up to the pressures the factory service manual says and try it out. If it doesn't hold a charge for a while you may then need to find or have someone find the leak. Once it is repaired you will already have the tools to keep it charged. Oh yea and yes you will want one of those thermometers pennyman1 mentioned they are about $8-$9 at an auto parts store. I'm surprised the PO went so long in Florida without AC lol.

[StarquestMan]

One thing I forgot to mention is the thermistor. There is a variable resistor called a thermistor in the dash that keeps the evaporator from freezing. I bypassed mine for a while to get the ac working and my truck was like a freezer on a hot day I had like 20 degree vent temps and it was great! However the other day I decided to replace the thermistor so the ac could better self regulate but now im back to 50 degree vent temps like the original thermistor. I wanted to mention this to you because if your ac is only cooling down to a certain point and then the clutch cycles off then you may actually have a problem with the thermistor or the box that it attaches to. Its unlikely but its certainly possible.

[robc311]

I took it to a shop today...they had a special on AC checks. They ended up checking it out and evacuating the system...refilled it to spec and it still had issues...something about erratic high pressure readings and problems with the low pressure side too. They basically told me to replace just about everything...compressor, drier, expansion valve and recommended the evaporator too. The quote was of course insane...as much as I paid for the truck...but the guy was cool and knew that...he basically told me if I’m handy to just do it myself since I can get the parts a lot cheaper and it’s not that hard of a job. I appreciated his honesty and the hours they spent with the truck for the $9.99 I paid for the diagnosis. The guys at the shop loved the truck and that made me feel great despite the outcome. I can get all the parts on rock auto for about $300...I think the thing I’m most worried about is th evaporator and expansion valve due to their location...does the whole dash have to come out? I took the blower out during my cigarette smell removal project and it wasn’t too bad but I don’t know if the ac stuff is as easy. Any advice? Should I go for it?

[2nup350]

you do not need an evaporator or condenser or hoses. you may need a compressor and the thermister and expansion valve, basically the things that control the flow of freon. if there were a problem with the condenser/evaporator/hoses it would be a leak. the entire system really is simple and replacing these few things will probably make everything work right again.

[robc311]

Can you tell me what the thermister is and where to get one? Thats one piece I’m not familiar with. I think they recommended the evaporator because they said it’s located by the expansion valve and said I might as well do it while I’m in there...not sure if it really is located by the expansion valve or not to be honest.

[StarquestMan]

You can test the thermistor with an ohm meter to see if it is within specs. If it needs to be replaced they are on ebay their part number is MB276616. there is a chart listing the resistance specs according to ambient temperature, I wana say it varies from 8k ohm to 2k ohm. The thermistor and expansion valve are the two cheaper parts but are the hardest to replace since they are in the evaporator housing so the dash has to come out. If it were me i would test the thermistor and replace the expansion valve first then replace the compressor if necessary since it is the most expensive part of the system and is easier to get to afterwards. Just my opinion though. I will see if I can find that chart.

[StarquestMan]

I cant seem to find a manual for a 95 Mighty Max but I can give you the specs for an 87' which SHOULD be the same. The thermistor connector is behind the glove box and is a two wire connector with yellow wires and a blue jacket going into the evaporator housing. here is the resistance numbers from the 87' SM

40 deg - 9k ohm
50 deg - 5k ohm
68 deg - 3k ohm
86 deg - 1k ohm

[dash]

There is a variable resistor called a thermistor in the dash that keeps the evaporator from freezing. I bypassed mine for a while to get the ac working and my truck was like a freezer on a hot day I had like 20 degree vent temps and it was great! However the other day I decided to replace the thermistor so the ac could better self regulate but now im back to 50 degree vent temps like the original thermistor.
obviously plays a critical role in AC performance.
How did you "bypass" yours ?

I remember my ac buddy servicing my old rwd toyota corolla. When he connected the gauges, he recommended installing a 10 inch fan on the condensor for traffic. While fiddling under the dash he pulled out this'AC cxontrol box'? He opened it up and turned a trimpot slightly with a small screwdriver then put it all back together..... and said u should b good to go
That hatchback with no tint on the glass, would freeze you in midday miami florida summertime sun
I'm wondering if that trimpot varied the thermistor's functioning ?
None of my other rollas blew ice until I adjusted that trimpot (like he did 1st car). 10" condensor fan made a difference in traffic

[geezer101]

...I'm wondering if that trimpot varied the thermistor's functioning ?
None of my other rollas blew ice until I adjusted that trimpot

Short answer - yes. It's a variable resistor. Less resistance in the circuit = shorter recovery time.

[StarquestMan]

obviously plays a critical role in AC performance.
How did you "bypass" yours ?

I remember my ac buddy servicing my old rwd toyota corolla. When he connected the gauges, he recommended installing a 10 inch fan on the condensor for traffic. While fiddling under the dash he pulled out this'AC cxontrol box'? He opened it up and turned a trimpot slightly with a small screwdriver then put it all back together..... and said u should b good to go
That hatchback with no tint on the glass, would freeze you in midday miami florida summertime sun
I'm wondering if that trimpot varied the thermistor's functioning ?
None of my other rollas blew ice until I adjusted that trimpot (like he did 1st car). 10" condensor fan made a difference in traffic

I bypassed mine with a jumper wire on the thermo control side connector.

[dash]

sounds like sending power straight to the AC compressor clutch then ?
Since I'd think u can't simply put jumper accross the thermistor sensor terminals itself (making 0 ohms).... as you'd do pressure switch

[StarquestMan]

sounds like sending power straight to the AC compressor clutch then ?
Since I'd think u can't simply put jumper accross the thermistor sensor terminals itself (making 0 ohms).... as you'd do pressure switch

Yep it basically takes the freeze up control out of the equation but keeps the low pressure switch and engine temp switch in circuit for protection.

[StarquestMan]

sounds like sending power straight to the AC compressor clutch then ?
Since I'd think u can't simply put jumper accross the thermistor sensor terminals itself (making 0 ohms).... as you'd do pressure switch

I just re read your post and realized I didn't interpret it correctly the first time. Actually that's exactly what I did. The module Is simply looking for a resistance between 0 and about 10k ohms so a jumper wire simply makes that module think its say 120 degrees outside and turns on the compressor. The module itself is about the size of an average relay (at least on the 87-89 90 and newer were different). The circuit is a very basic one with one 8 pin IC and 2 transistors and some passive components it isn't smart enough to really figure out why the resistance is so low.

[dash]

Thank you! for that detailed explanation
I'm wondering if mitsu oem thermistor setting/calibration responsible for 'weakening' ac optimal performance, much like toyota definitely did on the old rwd corollas

[pennyman1]

the thermistor on these trucks on some is used to set the temp in the cabin on the adjustable models - the others are for the freeze up issue when there is no adjustment. I have had both types - my 85 Royal would freeze you out even in 100 degree weather, but it is on or off.

[StarquestMan]

could be. I replaced mine and it just doesn't get cold enough for me. I guess that the electronic thermo controller may have drifted internally but there aren't any trimpots inside to adjust so you would have to find either a resistor that drifted in value or replace one with a different value.