After 22 yrs (19yrs of ownership) the AC system in my 93 is finally starting to act up, blowing warm air. Let me start by saying, in the past 19yrs I have never touched/open ANY part of the AC system. I've never even removed the high and low side caps where you connect the pressure gauges.

Recently it started blowing warmer than normal, still cool but not as cold. So I popped the hood and with the engine/AC on, I started feeling hoses for temperature difference and hoping to see a oily residue somewhere. Low side hose wasn't as cold and no oily residue anywhere.

Took it to the shop, they injected a few ounces of leak dye, I then went to Autozone, bought couple cans (20oz) of freon and put it into the system. Drove around for a few days, went back to the shop, used ultraviolet light and could not detect a definitive freon leak. Used a electronic freon gas detector, no definitive leak under the hood. Went inside the car to check the air coming out the vents and the meter might occasionally sound then not sound. When it did sound, it was a weak sound. The gas meter had to be held in the right position inside the car for it to sound.

Ultimately, I need to hook up a pressure gauge to see what's going on. I'm guessing I might have a bad compressor and possible evap core leak. The car has 229,000 miles.

 

Off topic, but you still have a great looking car.

 

Your thermo control amp should almost certainly rank very high on the list of suspects. I replaced it on my blue VE probably two years ago and it's already screwing up again.

When your system is blowing lukewarm, is your compressor staying engaged or no?

Aside from the thermo control amps on the 93 and 94 models, the third gen AC systems have proven to be incredibly and inordinately reliable. It's pretty impressive really.

 

My compressor (and cond motors) are not staying on while blowing lukewarm. I can vouch for the reliability of the AC system. When I first notice the lukewarm air, it felt like someone reach down and turned off the AC/clutch button while I was driving.

This thermo control amp you speak of makes sense. I have to admit, I've never heard of it and I have no idea where it's at or what it does. But based on the name I can suspect what it does.

My under hood inspection wasn't concluding anything and the in-car freon leak detector wasn't making sense either.

What sucks if it is the thermo control amp, I've overcharged my system by 20oz

 

Is this similar to our Maxima's ?

http://www.clubfrontier.org/forums/f...rontier-72291/

 

Autozone will loan you virtually everything you need to do the job properly. Manifold gauges, vacuum pump and leak kit. Doesn't sound like you evacuated the system, or checked your pressures. You should replace the receiver drier as well.

Bottom line is, you nor anyone else can diagnose the problem without analyzing the pressures in the system.

 

Yep. Unfortunately for us, we can't technically replace the thermo control amp without removing the evaporator which means having to discharge the system and then recharge again. The reason I say that is that we can't remove the lower evaporator box cover with it in place.

HOWEVER - there is a bit of a workaround if you are adventurous. You can cut the bottom of the evap box off with a dremel, then replace the thermo control amp, then glue the bottom of the evap box back on. I did this on mine a few years ago since I didn't want to have to discharge/recharge the system AGAIN. And also our evaporators can't come out without removing (or at least loosening and moving out of the way) the dashboard so it's a huge hassle.

Another thing to consider is that the "bulb" in the thermostat almost surely hasn't gone bad. What goes bad is the "brain" box of the thermo control amp. This brain box is on the outside of the evaporator box down below the glove box (facing the front passenger seat). It's held on by a 10mm bolt/screw. Technically what you should be able to do is to leave the evap sealed up, and cut off th brain box leaving the sensor wire down in the evaporator. Then solder on your new brain box to that. This way you get the same outcome but without having to rip your dash out and recharge the whole system.

Another test you can do is to jump the thermo control amp next time your compressor cycles off for too long a stretch (this giving you the lukewarm air). Just unplug the switch, and jump it with a paper clip or something. If the compressor then immediately re-engages then you've confirmed the culprit as the thermo control amp.

I have typically always gotten false positives when using those sniffer leak detectors inside cars at the dash vents. Not saying it's not possible that you have a tiny leak at the evaporator or something, but I'd lean toward it being so small to be negligible. If I were you I would also fully discharge and recharge by weight. If you really did add a full 20 oz you are likely pretty significantly overcharged now. Unless you start fresh by weight you have no idea exactly how much refrigerant is in your system.

 

I came across your AC post which showed where you cut the evap box with the dremel. Also someone had posted a link to the TSB for 93 Maxima's where it says to re-position the sensor higher on the evap itself.

Was it the "brain box" you replaced on your blue VE a couple years ago that's now going bad?

I agree with everything you're saying, the false positives, the bulb being good, and discharging then charge by weight.

 

When I say "brain box" that's all part of the same part I'm talking about - the thermo control amp. The thermo control amp is made up of a little "brain box" and then has a 6" or so length of wire sticking out of it with a sensor bulb on the end of it. The sensor bulb is what fits down into the evaporator box and clips onto the evaporator itself. The "brain box" part sits OUTSIDE of the evaporator box and screws onto the outside case with the 10mm bolt/screw.

All I meant is that if you didn't want to have to cut apart your evaporator, and didn't want to have to totally remove the evaporator, that you could just leave your existing sensor bulb in place, cut the wire, and just wire on a new "brain box" section. Hopefully that makes sense.

I already have my new thermo control amp and am leaning toward just soldering the new "brain" section to the existing bulb sensor wiring. But then again I already know for 100% certain that my sensor bulb is in the correct spot per that TSB whereas you don't necessarily have that luxury.

I would venture to say that if your system functioned perfectly fine for 19 years though that you almost certainly AREN'T on the wrong end of that TSB.

*edit* and yes, the thermo control amp was already replaced OEM brand new a couple of years ago on my blue VE, and yes that is the part that I am suspecting is failing (again). When I would drive it this summer, it would annoyingly cycle off until sometimes 70 degree vent temps before cycling back on again (when really it should cycle between off at about 40 degree center vent temp, and cycle back ON at about 45 degree center vent temp).

 

I got it, it's the "brain box"

Just like our door lock timer and probably many other high amperage electrical issues, it's a bad solder joint in the brain.

I just went out to the parking lot, found the brain box, 8mm socket, removed the brain from the evap box (keep in mind the brain is still connected to the bulb so you can't pull it out completely) I cracked open the brain box, start the eng/ac and every time I twist the harness I can hear the clutch cycle on and off.

Bad solder joint

And it appears this is a common issue with other Nissans. Over engineering at it's best

 

Ahh, so it's an 8mm - thought it was a 10mm. Well you knew what I meant

Are you going to try to repair the solder joint? Seems like I remember reading about some guy on another Nissan forum repairing his that way. I think it was on maybe one of the hardbody trucks have it in a more easily accessible area.

 

Oh yes, that is the plan. When I finally re-solder my door lock timer correctly, it has been working beautifully since then. And I anticipate my AC to work the same after I re-solder the brain.

The 20oz of freon, I'll fix that

 

Problem solved, AC is once again working as it has for the past 19yrs

Re-soldering the "brain box" worked perfectly. There were three broken solder joints on the bottom of the board. This is where the harness plugs into the circuit board.

Once you separate the brain box from the evap housing, there's just enough slack in the harness where you'll be able to re-solder the joints inside the car. No need to cut and splice the harness as I was originally thinking of doing



I also used the pressure gauge and removed enough freon to get the pressures at a proper reading base on ambient temp.

 

Nicely done. I have a couple of spare thermo control amps, I'll have to open them up and inspect the circuit board. I remember the original one out of my blue car, I didn't notice any obvious solder flaws on the circuit board but then again I didn't really inspect it closely either.

Did you entirely de-solder and re-solder or did you just add some solder "on top" so to speak?

 

Nice work! It's kind of annoying it roots down to electrical stuff but par for the course.

I figure just leave everything in here section wise vs creating a new one. It's not as busy as it was in it's hay day. I cleaned up a bunch of the stickies/combined them, and one of these days will go through and further reorganize them/verify links.

 

I added on top just to get me going again. Notice I didn't post a after pic because it wasn't the prettiest solder

I used a old-school blunt tip pencil solder. A pin tip pencil solder would have worked better.

EDIT: My plan is to pick up a couple thermo control amps from the salvage yard by cutting the harness for the resistor (that clips onto the evap) Properly re-solder those and have them ready in case my crappy solder false, then cut and splice into my car.

 

guilty
Yeah but the larger tip lets you work much faster

 

Anyone got a parts car laying around? Looks like my crappy re-solder has failed again. I tried re-soldering it again this past week, but to now avail it's failing again.