1995 KZJ78 Prado (RHD) Air-Conditioning Issues (1 Viewer)

[mikehuntt3]

Bought vehicle in winter time and never tried the aircon. Shipped vehicle to the USA and tried aircon this summer. Noticed that the clutch would NOT engage no matter what. Had a half can of R134a and decided to put some in. All of a sudden, the clutch kicked on while parked in my garage. I put another can in (I don't know amount). Air-conditioning blows very cold, its awesome!

The issue is this, as soon as I put the truck into drive, the clutch disengages and the vents blow warmer air. As soon as I put the truck back into park, the clutch re-engages and the air gets cold again. At first, because I wasn't sure how much Freon I put into the system, I though maybe the under pressure or over pressure sensors were doing their job and sensing too little, or too much and shutting the clutch off.

I took the truck to a shop and had them pull a vacuum on the system and then refill the system with 900 grams (1.98 pounds) of R134a. Just got the call from the shop and they told me they pulled a vacuum and the system only had .9 pounds in the system. They then told me they serviced the system with 1.9 pounds (861 grams) and the aircon is doing the same thing (disengaging ac clutch when truck is put into drive).

Does anyone have any ideas of what might be the cause?

 

[GTSSportCoupe]

Having chased A/C issues on my LJ78 a few times now, I can say the best way to do it is to un-plug the A/C amplifier main plug, and measure each circuit at a time per the FSM manual. You'll be measuring voltages/resistances one at a time. This should help you hone in on the issue.

Recently I had an issue where a A/C coolant temp sensor had a bad connection (had broken) and was causing the system to not work as it thought the engine was too hot. Once I learned the technique for testing at the A/C amplifier, it was really easy to find the issue.

The system usually also cuts out if your throttle is open beyond a certain point; and the ECU does this. KZJ78's are known to have leaky capacitors in the ECU which cause all sort off grief as they ruin the circuit board. It's possible your ECU is not working right and is shutting down the A/C. Usually the ECU's can be repaired though. Might be worth removing, opening and inspecting it.

Here is the part of the FSM RM183E that helped me. Not sure if the KZJ78 uses the same A/C amplifier or not though....

 

[GTSSportCoupe]

@mikehuntt3 I added a picture from the manual to illustrate what I was talking about.

 

[pithicus]

On petrol 2f/3f's a/c clutch engagement is rpm related. So are you saying once in drive the a/c won't engage in drive even when the revs go up?
In other words is it the lowering rpm's from the engine [that occurs when you put it in drive] cutting off the a/c clutch? Does it engage again when you rev it in drive?

 

[mikehuntt3]

@mikehuntt3 I added a picture from the manual to illustrate what I was talking about.

Thanks for the advice. I will check this out. Is this AC amplifier behind the glovebox?

On petrol 2f/3f's a/c clutch engagement is rpm related. So are you saying once in drive the a/c won't engage in drive even when the revs go up?
In other words is it the lowering rpm's from the engine [that occurs when you put it in drive] cutting off the a/c clutch? Does it engage again when you rev it in drive?

So when I put it into drive, the clutch just disengages. No matter how high I rev the engine in drive, the clutch never rengages and blows cool air. I can reverify but I'm almost certain it doesn't engage. If it were to renegade with rpms, I should feel it getting cooler while driving though.

Any idea if 900 grams freon is the right capacity?

Thanks for your reply.

 

[pithicus]

Any idea if 900 grams freon is the right capacity?

No idea sorry. but that sounds like a lot.

A/c's don't like being overcharged. It won't blow cold and the compressor will get really hot quickly .

 

[GTSSportCoupe]

Thanks for the advice. I will check this out. Is this AC amplifier behind the glovebox?

So when I put it into drive, the clutch just disengages. No matter how high I rev the engine in drive, the clutch never rengages and blows cool air. I can reverify but I'm almost certain it doesn't engage. If it were to renegade with rpms, I should feel it getting cooler while driving though.

Any idea if 900 grams freon is the right capacity?

Thanks for your reply.

Haha, I'm trying to remember now. I do recall it was easy to get at. I think you're right that it is accessed after removing the glove box.

Another thought, you can unplug the A/C compressor single wire connector. 12V there turns on the compressor. This will help you test whether the issue is with the compressor, or somewhere else. Disconnect the connector. Test for 12V at the loom side of that connector with the engine idling vs. revving (should remain 12V in either case). You can also jumper 12V direct to the A/C side of the connector, and rev the engine. If the compressor clutch is ok, it should keep operating even when the engine revs. If it disengages even with 12V, then the compressor clutch is toast.

 

[jblueridge]

On mine it is just to the right of the glovebox. Get plastic glovebox out.

I bypassed the little black box and wired my compressor clutch to a dash switch.

 

[kgrove]

Can one of you describe how your AC switch works? Mine might be broken, but I've never been in another KZJ78 or frankly any 70 series for comparison. The fan speed switch has defined "click stops" for various fan speeds. The AC dial underneath that rotates from Off to Hi, but there are no stops or intervals, almost like it's a dial that's not attached to anything. Does your ac dial have tactile evidence it's on? Why is it a dial and not a binary switch? Since there is a separate temperature mixing lever and fan speed switch, what difference does it make if you rotate the AC dial from low to high? I'm not sure I understand what this thing is supposed to be doing as ac systems I'm familiar with have a compressor that is either on or off... no varying degrees of freon being compressed.

Separately, I know I'm low or even empty on freon. Unfortunately the manifold gauges I ordered won't connect to my fittings, so I need to get a different one (the quick connect fittings were too bulky and being blocked by the shape of the compressor). I know I have multiple problems preventing my ac from working, so I need to pursue possible freon leaks in parallel with verifying the switch works. With any luck, it's just a 25 yr old system that never got recharged, the switch works just fine, and I'll live happily ever after in a gumdrop house on Lollipop Lane.

 

[pithicus]

Does your ac dial have tactile evidence it's on?

No. it' just varies the temp that the compressor kicks in. I usually set the dial to the top [max] and the compressor cycles more often and keeps the cabin colder, it's like a thermostat. And you are right, it's a very smooth dial, very little tactile resistance.

 

[jblueridge]

The ac dial is a variable resistor that works in series with a thermistor ( temp sensitive ) resistor that is mounted on the evaporator. As Pithicus rightly says, it is your thermostat.
There is a pressure sensitive switch in the system that prevents the compressor from running if you are low on refrigerant.

 

[kgrove]

Does anybody have either the specs for how many grams / ounces of r134a an empty KZJ78 ac system should take? PAG oil capacity? Also helpful would be a chart, if Toyota ever published one, of appropriate high and low side pressures (with temp / humidity adjustments)?

Refrigerant was cheap enough I decided to pull a vacuum and go ahead and charge the system to see if I had functionality... which I do, at least for now. Lacking capacities or a Prado specific pressure chart, I found charts for a couple other cars and used those as a proxy, but I'd love to cross check that against what I was supposed to have done to make sure I didn't overcharge. I've seen a few of these charts that can be different by as much as 50psi at the same temperature for what they consider a normal high side pressure.

 

[Roastchestnuts]

Can one of you describe how your AC switch works? Mine might be broken, but I've never been in another KZJ78 or frankly any 70 series for comparison. The fan speed switch has defined "click stops" for various fan speeds. The AC dial underneath that rotates from Off to Hi, but there are no stops or intervals, almost like it's a dial that's not attached to anything. Does your ac dial have tactile evidence it's on? Why is it a dial and not a binary switch? Since there is a separate temperature mixing lever and fan speed switch, what difference does it make if you rotate the AC dial from low to high? I'm not sure I understand what this thing is supposed to be doing as ac systems I'm familiar with have a compressor that is either on or off... no varying degrees of freon being compressed.

Separately, I know I'm low or even empty on freon. Unfortunately the manifold gauges I ordered won't connect to my fittings, so I need to get a different one (the quick connect fittings were too bulky and being blocked by the shape of the compressor). I know I have multiple problems preventing my ac from working, so I need to pursue possible freon leaks in parallel with verifying the switch works. With any luck, it's just a 25 yr old system that never got recharged, the switch works just fine, and I'll live happily ever after in a gumdrop house on Lollipop Lane.

I just added Freeon from those quick fix cans to mine and it started working again.

 

[pithicus]

Does anybody have either the specs for how many grams / ounces of r134a an empty KZJ78 ac system should take? PAG oil capacity?

I saw on another forum 900 grams was approx right for this truck. Sorry, that's all I could find. No charts. 120 ml of pag should be sufficient if there is no oil at all in the system. If it's never been flushed and there's an unknown amount of oil left in the system, then it gets tricky. Pulling a vacuum won't remove the old oil.
I had a slow leak. fixed it, pulled a vac and added 20 ml pag to hopefully replace what might have leaked out. Seemed to work ok, but it's a guessing game if the system is not flushed.

 

[kgrove]

I'm shooting from the hip, I guess. From what I read, you can only flush the lines and condenser, not the compressor and receiver / dryer... I'm not sure about the evaporator. So if that's true, then I'd never know how much oil is in there. So... I added a couple oz (50-60ml) because I felt like it. I think I added a little too much refrigerant as the majority of the charts say for my ambient temperatures I should have a high side reading in the 170 area (it was about 75F in my garage) and I went up to 225psi. I turned the last bottle upside down for a bit and the pressure took off on me a little faster than intended... should have been more patient. That, and some of the earlier info I got wasn't temperature adjusted and I didn't realize how drastically different the recommended high pressure was as temperatures changes (e.g., 75F should be 150-170psi but 90F jumps all the way to 250-270psi. My summer temperatures routinely exceed 110F and the high pressure recommendation goes all the way to 345psi, so if I'm over pressured at 75F, I would think I could run into some problems when I get to summer.

I'm not using ac at all right now other than for testing purposes, so I'll probably leave it alone for a couple weeks and come back to test it again later.... see if anything has leaked out. If so, I'll have to take it to somebody who knows what they're doing. If it's still over-pressurized, I'll have to figure out how to legally bleed off some pressure. I know some mechanics will discharge your entire system for free. Do you think they'd bleed off just enough to get the high pressure back in spec? My guess is they would want to drain everything so they get paid to put all new stuff in.

 

[pithicus]

I know yer not supposed to vent r134a . It's a bit perplexing when computer duster / freeze spray etc is r134a, and is used and vented to the atmosphere when used obviously. It's even used as propellant in some inhalers . It's not toxic, but it is a greenhouse gas. You can always get a dedicated container/bottle for refrigerant , pull a vacuum on the container and vent it into that.
Better to have undercharged a bit rather than overcharged IMO.

 

[ATL Cruiser]

Thanks for the advice. I will check this out. Is this AC amplifier behind the glovebox?

So when I put it into drive, the clutch just disengages. No matter how high I rev the engine in drive, the clutch never rengages and blows cool air. I can reverify but I'm almost certain it doesn't engage. If it were to renegade with rpms, I should feel it getting cooler while driving though.

Any idea if 900 grams freon is the right capacity?

Thanks for your reply.

900g (2lbs) is the correct freon refrigerant fill spec for an R12 system, which is what all the old manuals are referencing. R134a is a lower fill. The actual formula is:

R12 (in pounds) * 0.9 - 0.25

So in this case, the correct amount would be

2 lbs * 0.9 - 0.25 = 1.55 lbs or 700 grams of R134a

I just charged mine to this. Pressures are perfect. Sight glass shows zero bubbles. I hope this helps someone in the future.

 

[WheelyApplegate]

Does anyone know the location of the AC relay on a KZJ78? According to Megazip, it's a round, 4 prong relay, 88630-14030.