How to: A/C diagnostic & repair for Gen 1 Leggie

[91Beater]

I spent quite a bit of time repairing my A/C system. The most difficult part was improvising how to do certain tasks without professional tools. This guide is revised to reflect how I would do it in the future if I was to do it again.

Preface:
I acquired my 91 with busted A/C. Turns out that the car uses variable capacity compressor which is not easy to diagnose which caused extreme amounts of frustration.

ELECTRICAL Diagnostic
With the engine off, turn on A/C in the car. The A/C relay (in the box and its the one closest to the windshield) should click and the fans should turn on. There's another relay controlled by the ECU on the contact side of the relay to prevent compressor from engaging with the engine off. It also turns the AC off in response to TPS signal.

Can't get past it?
Check refrigerant pressure switch. It's on the passenger side on top of the filter dryer, which looks like a shiny aluminum bottle attached near the FR suspension. This usually doesn't go bad. If it's open, check refrigerant pressure.

Pull up the protective rubber boot from the sensor carefully and jump the blk/wht & red/wht (if colors do not agree you'll need to figure it out per below explanation) with alligator clip wires.

This bypasses low pressure safety. If it still doesn't engage, stick something into the back of connector at the evaporator switch and short the brown wire to ground. This bypasses the dash control with buttons on it. If it works at this point, you've got bad control unit like mine. If it still won't work with the two bypasses above, its your evaporator sensor switch.


If you can get the relay to click and fan to turn on but compressor won't come on, disconnect compressor clutch and connect it to + terminal on on the battery with a jumper wire. (10A fuse in line recommended to avoid accidents). No engage: replace compressor assembly. Engage: check thermal cut out and MFPI cut out.

On the sensor side harness:
I have
4^1
3_2

1 Black-white striped
2 Blue white striped
3 red blue striped
4 Red white striped

PER my diagram, blk/wht & red/wht is low pressure drop out switch. This disables the A/C system if the refrigerant pressure on high side drops below some point. It is OPEN with no pressure. It is CLOSED. This switch should not go open unless there is loss of refrigerant.

blue/wht and blue/red is usually open. When closed due to high refrigerant pressure, it activates the fan. I believe this is part of the normal cycle. This is not a high pressure cut out. There is a spring loaded relief valve at the back of the compressor that will vent refrigerant to prevent system component rupture in the event of a dangerous level of overpressure.

REFRIGERATION LOOP
Before the car is started, check refrigerant pressure. Either at low or high port. The numeric value in psi should be approximately numeric value in Fahrenheit ambient. If not, you have pretty much zero refrigerant left. The leak is likely at one of the O-rings or at the compressor. System is easy to tear down, so I recommend you replace every o-ring.

Remove & replace service port valves.
Pull a vacuum for 20 min and leave gauges attached for 20 minutes. If the vacuum is not holding, proceed with O-ring replacement

Buy the $6 O-ring kit from NAPA
Remove & replace o-rings at
Evaporator fitting (2)
Once you get the big line out, rub your finger inside the fat pipe that goes to evaporator. Inspect for bits of metal powder. If you find them, stop work here. Time to replace compressor. Compressor crapped the bed.

Condenser fitting (2)
Filter drier (2). use the rings that come with the new filter drier.
Compressor (2)
High pressure line - (1)... near the psgr side headlight. Leave it alone for now.

Leave in place for now:
TXV (3)

Leave the grille & glove box out for now.
Pull a vacuum for 20 minutes, then shut off the valve.
If vacuum is not holding, pressurize with shop air to 150 psi.
Check all the fittings that you just fitted with new orings for leaks using soapy water.
Fix and repeat the check.

If you're losing vacuum, but can't find the leak, its probably your compressor shaft seal, which requires compressor replacement.

Just buy a cheap vacuum pump and gauge set. If you're chasing down the leak, you'll likely use it 2-3 times. It's cheaper to buy them and dump then on CL than it is to go to a shop 2-3 times.

[91Beater]

You will need to replace the filter drier every time you open the system. If you're like me and buy a used compressor that doesn't work, you'll need to re-replace the filter drier. If you're not sure if you got the leak fixed, do all the leak testing with old one in place. Put the new one in right before the final vacuum.

Low side(expansion valve, evaporator and low side hose) needs to be immaculately clean inside or you'll wreck your new compressor. Though it should be clean, you can get away with a bit of a slack on high side as crap will end up in the filter bottle and it tends to go away from the compressor.

To get the compressor out, loosen the belt. Remove four bolts.
Crow bar between the bracket and bottom part.
Crow bar between the left side and bracket.

First time I got it out, I did it without the crowbar. It was hard. It's a cake with the crowbar. It's safe as long as you don't leverage it against the manifold, radiator, or anything fragile. Just don't let it touch anything but the bracket and the compressor.

to remove the condenser, loosen the radiator bracket, push radiator back, remove lines. Unbolt condenser.

You should be able to pull it straight out without removing the radiator.

you'll have to remove the grille to get to the condenser bolts.

Standard gauge pressure diagnosis is not as effective on variable displacement compressor like we have. Various difficulties are discussed in details here.

http://www.macs-tech.com/eng/board.php/ ... body&no=26

Here's my dead DCW-17 Diesel Kiki compressor.


The control valve does something in response to low side pressure change and move the swash plate. Pistons are attached to swash plate and this adjustment alters the compressor stroke.

When there's an increase in low side pressure, the feedback causes compressor to increase the stroke and try to bring the low side pressure to defined point. the fan is set to 1, evaporator load is low but pressure remains 30 psi. When the fan is put to 4, the low side remains the same, but increased evaporator load increases the pressure slightly and stroke is increased. So, the shaft load increases and high side goes up a bit.

With the proper charge, the system will still show bubbles in sight glass depending on what compressor is doing. At fan setting of 1, its foll of foam. At setting 4, its clear.

With a standard compressor, the compressor is a constant. It's either on or off and controlled by refrigerant pressure switches, evaporator sensor and the ECU. So, any restrictions will cause immediate symptoms on gauges. Variable compressor will destroke to maintain the same low side pressure to accommodate the restrictions so its elusive.


Evaporator core: my old one apparently failed catastrophically. I suspect that the system had a leak somewhere at one point and the previous owner(s) kept topping off the system with refrigerant until system was oil starved.

My tip: Don't bother with used compressors or in-house remanufactured ones you find on eBay.
Get a new OEM or reputable remanufactured like 4Seasons.

Evaporator. Outside and inside, both time consuming to clean. It's like $60 shipped on eBay. Don't overtighten that little side branch connection. You'll break the o-ring. Finger tight, then 1/2 turn or so.
Don't clean the old evaporator like I did. It's not worth the time and cost of solvent.


High side crap is more forgiveable because it will choke on filter drier or the expansion valve, but any debris in evaporator will get sucked into the compressor.

Mistake I made: I spent $30 of clean up solvent and two hours cleaning the evaporator. It works.. but if I was to do it all over, chuck it. It takes a lot of solvent and multiple flushes to clean it out. Don't waste your money and time cleaning out compressor's crap from inside and mold from outside.

TXV: that thing attached to the evaporator is called the expansion valve. If you found any glittery crap in the evaporator, this should be replaced as well. Pre-fit the evaporator/TXV assembly into the box to ensure proper fit before final tightening so it fits right. If you tighten it and have to untighten, you'll need to replace the o-ring again.

Low side hose: pull it from the car... and make sure you flush it out real good with Paslode Airtool cleaner.

Condenser: Remove connection at drier, then pour flush solvent through high port. push it out until you get a few ounces of liquid out at the drier connection. Capture into bottle... Move capture bottle to disconnected compressor connection and back flush it with shop air. Collect the waste solvent.

Try not to lose collected solvent, because you will let it sit and see whats settled out. This is important in knowing the conditions of your system.

The original condenser is serpentine type. If you are retrofitting to R134a, skip the flush and replace it with parallel flow type, which is also like $60 shipped on eBay. This helps minimize performance loss.

If the new compressor comes with mineral oil, keep it as is if you're using it with R12. If you're using R134a, drain and refill a few times with PAG or POE. It takes patience to get all the oil out. You should get about 5 oz out. Add 5oz in compressor, and add 1 oz in the drier bottle.

Use Paslode Air Tool cleaning spray from Home Depot to flush. It's like $7 and comes in a white spray can at tools section. That or proper AC clean up solvent. I've found that non-chlorinated brake cleaner swells seals in a way refrigerant doesn't and it doesn't dissolve mineral oil that well.

Use lots of shop air and expel the residual solvent the best you can, then park in the sun to keep it hot.
Vacuum for 25 minutes. Replace vacuum pump oil. You'll smell the solvent in it.
Vacuum for two additional hours on new vacuum pump oil.

Ensure that full vacuum holds for one hour after pump is turned off before adding refrigerant.

To add refrigerant...
Preferably, charge from high port as liquid with the car off initially. If you're using small cans, charge first can into high side and do it upside down. heat the can a bit with a hair dryer before you shut it off and remove the hose to push any liquid stuck in the hose. If you don't follow this, you'll lose some refrigerant which can throw off your charge level.

Turn compressor by hand, about a dozen times to clear the cylinders. If you skip this step, you risk hydolock damaging the compressor. This can be tough with a new tight compressor. With the belt still off, manually engage clutch by applying 12v, so you can use the pulley as a handle if needed.

Finally, add refrigerant to low side to specified weight with the A/C on.

[Legacy777]

Very nice write up!

I did a few things differently on my conversion, but yours is very detailed and helpful!!

Yeah these compressors are variable geometry....I've never seen one apart though. I talked with an AC guy about it, and he said don't bother buying a rebuilt one, the head end of the compressor isn't rebuilt and will end up failing. After 3 or used ones failing, I finally bought a new one from Subaru, and that one has been working great.

[Legacy777]

Added this thread to the sticky compilation thread.

[Alphius]

This is a great thread. Thank you very much!

I have one question: Are the later '94 factory R134A compressors still variable geometry? How would I be able to find out? And it is still a TXV style system like the earlier cars, right?

[Legacy777]

It probably still is a variable geometry design. If the compressor doesn't cycle, then yes it's a variable geometry design.

[Alphius]

My compressor does cycle like a non-variable system, and blows cold, but I don't know if I have a fault in the system causing that cycling or simply too low on refrigerant. I suppose I can just vac and measure a proper charge back in to rule that out.

My biggest issue is that the A/C only works in warm weather, and cycles like a non-variable compressor when it does work. Sometimes it just won't turn the compressor on, usually in colder weather. I think I have a bad evap temp switch, because wiggling the wire harness on the evap core under the dash always makes it click on when it is misbehaving. Even when misbehaving, low and high side pressures look normal. Low side goes a little too low, so maybe a new TXV, new drier and a vac/refill to spec would help that out. And I need to grab a new(used) evap switch from the junkyard and swap that out, since they are ungodly expensive from the dealer.

[Legacy777]

As noted in this thread, see if turning the fan all the way on to the 4 position does anything different.

I would tend to agree with you in that if wiggling the wire/temp switch causes it to work again, there may be an issue there.

[Alphius]

No, all functions work on all fan speeds with no issues. The climate control board, although not susceptible to cracking being a later board, also has all of it's connections reflowed.

The only problem I have at all is that when the ambient temp is cool, the compressor doesn't turn on, but wiggling the evap temp wires and connector makes it turn on and work for a few minutes. Hot weather (say 70+ ambient) it is 100% reliable. Pressures look good when it is running at any temperature.

I just wondered if the variable/non-variable compressor thing could shed some light on the issue if my compressor isn't supposed to cycle. It does cycle, and blows cold, so I think '94 R134A cars probably have a non-variable compressor. Maybe I should look in my FSM, it might say. I have the '94 supplement. I'll take a look tonight maybe.

[91Beater]

Look in the FSM and look at the color scheme for your evaporator temperature switch.
Bypass the switch to rule out other causes. Simply use a jumper wire.

[91Beater]

Alphius:
If it uses TXV, the receiver drier bottle is on the high side before it goes into evaporator.

If it uses an orifice, the bottle is called an accumulator and it will be on the low side between the evaporator and compressor.

To see which one you have, hook up the gauges, turn on the A/C and run the engine at 1500RPM.
Run the A/C with fan on high.
Change the fan to low and.
You'll see the low side remain near constant and high go down if it's a variable displacement. If it's a constant type, the low side will start dropping and compressor will cycle off.

low pressure switch: always on. Activates in case of charge loss.

Variable capacity is difficult to diagnose, but I think its better for car A/C use.
The compressor flow is determined by RPM x displacement.
I believe variable displacement offers modulation range of 100% to 10-15%.
If you're idling and heat load is at max, the low side pressure may shoot past 30psi, because the compressor can't provide enough flow even at maximum setting at that RPM.

As your RPMs increase, the flow will increase. So the torque it imposes on the engine decrease at higher RPMs. On Generation 1 legacy, you'd have to FLOOR the gas to get it to switch off.



Control side
+---(pressure switch)--(evap temp switch)--(A/C on switch that you push)--(relay coil)----ground


Contact side
+12v--(overheat switch)--(AC relay)--(clutch coil contacts)--gnd

+12v-(cut relay coil)---(ECU)------ground

SEQUENCE OF OPERATION, ELECTRICAL:
A/C on is activated by the user. If refrigerant is properly charged, the A/C relay is activated. Though not explicitly shown on diagrams I read, ECU senses this.

ECU cut relay's coil is connected to +12v. When A/C is turned on the ECU adds air flow through IACV, then After about 1/2 second of delay, ECU pulls the A/C cut relay coil's other leg to ground.

I simulated a switch chatter on control side by rapidly tapping on the AC button. I've found that this confuses ECU and causes A/C cut relay to activate long enough to cause clutch to jump but not long enough to fully light the test light.

I don't fully know what factors ECU consider in controlling A/C cut relay. So far I know that it uses TPS(floor it and it goes off), A/C control side status and engine on/off.

To diagnose contact chatter at evaporator switch, place a meter across the brown and green wires from back of the connector. Turn on the A/C. Put meter on 20 or 60v mode. It should read about 0v.

Put the meter in peak hold mode. If it picks up voltage when the A/C starts hunting rapidly you know that this contact is bouncing.

[91Beater]

Very nice write up!

I did a few things differently on my conversion, but yours is very detailed and helpful!!

Yeah these compressors are variable geometry....I've never seen one apart though. I talked with an AC guy about it, and he said don't bother buying a rebuilt one, the head end of the compressor isn't rebuilt and will end up failing. After 3 or used ones failing, I finally bought a new one from Subaru, and that one has been working great.

Were the "used" ones pulls or remanufactured? If the latter who rebuilt them?

One thing to think about. Returned cores do not yield 100%. When you look on eBay and the web, you will find a lot of in-house refurbished units sold as re-manufactured.

While the rebuild parts used maybe the same/similar, companies like 4 Seasons have far more extensive testing capability and specialized equipment needed for cleaning like vapor degreaser, ultrasonic washer, off vehicle test equipment, advanced leak tester etc. Quality control process is likely far more stringent and the ones that don't perform to OEM specs are likely not allowed to ship out.

Strict QC reduces production yield since those rejected in burn-in and the test process counts against the yield. Something with fault in non-rebuildable parts gets rejected.

Lack of burn-in and test eliminates the testing cost and pass on the burden to the customer. If it works, it works. If not,they replace until it works.

[Legacy777]

I don't recall the name, but they're in Florida and were near CV Axles.com in Ocala, FL.

In talking with the guy down there, he seemed to know what he was talking about.

[91Beater]

Ok, so I just jumpered the evaporator switch.

The compressor works to maintain the same low side pressure which directly controls the evaporator temperature. This will prevent freeze up as long as the evaporator load exceeds minimum compressor output.

If the RPM is high, (i.e. cruising at 80mph), cool day and you try to use AC on 1 setting... the compressor might destroke to minimum capacity and still have too much capacity to avoid freezing...

[Alphius]

To add a little more: I bypassed my evap temp switch a few days ago and the A/C now works perfectly. It appears that the factory '94 R134a system does still use a variable displacement compressor as well.

I do need an evap temp switch in the system however. I was running on defrost with the blower on setting 1 at ~75mph for a couple hours last weekend in 55 degree weather or so. The evaporator ices up and completely halts airflow within 10 minutes under these conditions. Normal driving around town does not appear to have this issue, it has only been apparent on sustained high RPM operation.

Too bad the evap temp switch is $85 and I have to evac and refill my system at the same time. I will also replace the drier at the same time.

[91Beater]

Too bad the evap temp switch is $85 and I have to evac and refill my system at the same time. I will also replace the drier at the same time.

You could abandon the old one. Loosen the old one off the bracket. Put new one on bracket. Drill a small hole in the evaporator housing somewhere and thread the new sensor in place. You just have to cram the sensor match head into one of the evaporator fins.

I would avoid splicing the new switch into the old sensor, because its a one unit part and its quite possible that sensor tip on new switch have a different calibration than the old one.