Help with air conditioning

Hi all. This is an appeal to the experts. I've been battling with my a/c for well over two years now. I still don't have it working despite taking it to three different shops, four times and spending $$$$s.

The problems are:
1. When I turn the a/c on, it dumps out load of piping hot air.
2. After the hot air, A/C works great for about 15 minutes. Then shuts off, won't come back for hours.

I've done a lot of the simple stuff like replaced the receiver/dryer, switch, heater control valve, messed with the relays in the passenger footwell, checked the condenser fan comes on. One of the last shops I took it to, reckoned it was the potentiometer for the a/c temp control that was at fault. Testing it with a multimeter confirmed that it was a little scratchy and intermittent, so I replaced that, but the problem remains. I've included some of the diagnoses from one of the shops as there is other info there.
Does anyone have any ideas?
Thanks

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The compressor clutch gets a signal to engage the compressor, if you rig a wire up to the clutch supply and bring that into the car, connect to a volt meter. That will tell you when the system is calling for cold air and the clutch is getting a steady 12 volt plus supply.
When the car is ticking over with clutch engaged, put you hand both sides of the filter drier and the 2 pipes should be the same temp, if the outlet is colder than the inlet then the drier is dirty. The pipe to the drier should be about 5C above the outside air temp, this will tell you the condenser is working.
Try that and see how you go

 

First thing to do here is make sure that the HVAC heater control valve plumbing is hooked up properly. See the little "arrow" molded on the nipple near your pinky finger in the photo = that nipple must be connected to the hot water coming from the engine. If it's hooked up backwards = hot water will always be flowing in the heater core.
Second thing to do is measure the voltage between the two wires connected to the HVAC heater control valve = should always be +12V when the AC system is "off" (whenever the key is "on" or the engine is running).

 

First: test the heater valve works correctly.
There is a relay cascade, when the AC relay is ON, the heater relay is ON too, making the heater valve to close, ignoring the HVAC ecu.

Second: set a direct 12v to the compressor, so you bypass all ECU, Relays, etc, etc, and you should get a strong cold air from it.
On my car I can clearly hear the compressor engaging (a strong click) that helps to know when its actually called to work.

 

The shop analysis looks complete. They identified that the system works fine on full cold.....is that true or not true?

Pressures are okay so the issue most likely is the compressor either has a cycling issue (it cycles based on demand and pressure) or the internal air vents are not opening or closing properly. Does the air coming out of the vents change in velocity as well as temperature?

Did you take it back to them?

 

Okay, thanks for the tip. I will try that. The 12V going to the compressor is steady for a while, then there is zero voltage to the compressor. With that, my lay person's brain assumed the problem is not the compressor.

Thanks Steve. Ah, yes. I installed the heater control valve exactly the same as the old one I took off, but it's perfectly possible that one was rigged incorrectly (I replaced it thinking the symptoms were related to a faulty original part). I'll poke around in that area and also check the voltage as you suggest.

I will do that.

Yes, when the a/c is working, it makes all the right noises and sensations.

Yes, that is true. It works, then it stops working.

No. I was disappointed that when I got the car back, there was a mess of scratches on the rear flank. The shop works on priceless supercars all day long, so I assume they figured a mere Mondial is not worth the trouble of being careful. Rather than fight them over it, I just won't use them again.

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When the clutch is engaged and you have cold air, the 2 connections from the compressor in the engine bay. One will be hot and the other should be cool or cold. Are you getting any frosting on the cold pipe ? If the evaporator is dirty or you are not getting enough airflow over it, you will get frosting or the compressor clutch could disengage due to low suction pressure to the compressor.

 

I had a similar issue years ago and the culprit was the little relay controlling the compressor clutch power inside the ECU. Replacing it fixed the problem.

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Thanks for the replies, guys.

Hi Steve. I can confirm the heater control valve is plumbed in correctly and does indeed receive +12V when the AC system is "off". I noticed that the connector is not keyed, so I tried plugging it in the other way, but the results were just the same. I assume it is working because the air temp does change with the temp control. There's just an initial blast of roasting hot air when a/c is engaged.

 

Wow, good job. Is that unit just for the HVAC? Is it in the passenger footwell? Did you replace just the relay or the whole thing?

 

That's almost too bad as those would be the easy fixes. There is no polarity requirement on the hot water valve connector so either way is OK. As long as it has 12V between the two terminals = the hot water valve should be fully closed (and the water inside the heater core should remain cold). However, that "initial blast of roasting hot air when a/c is engaged" is still indicative that hot water is (wrongly) flowing in the heater core when it shouldn't be. With having the system "off" (after running the engine to operating temp), you can probably just feel the hoses going to the hot water valve = they should both be relatively cold. If hot to the touch = definite problem.

 

Yes, that unit is the HVAC ecu and in the passenger footwell and I just replaced the relay. It's a standard part.

To remove the heater from the system during testing, just hook up 12V and a ground directly to the heater valve. It will be closed.

If you directly hook up 12V to the compressor clutch it must engage and remain so. You should get cold air if your system works.

If the clutch doesn't hold with direct power, you will have to change its coil.

Once you have established that your basic AC works with the control systems removed, you can start trouble shooting the control system.

The wiring diagrams do exist

 

Here’s a pic of the temp my aircon puts out just to keep you motivated to sort the issue

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I have a wiring diagram for a european 3.2 if that helps

 

Thanks for the replies, guys.

Thanks for the info, Urs. Is that in the wiring diagram I got from you? (I don't have it to hand).

 

Thanks for your reply, Steve. This is the diagram in the owner's manual. If the top inlet of the heater control valve depicted here has the little arrow pointing into it, would this diagram be correct?

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Yes. The hot water coming from the engine (the green line) should go into the upper (top) inlet on the hot water valve as noted before. That matches the flow direction arrow on the valve itself. (The blue line is the return flow going back to the "cold" side of the radiator.)

 

Okay, thank you. Well that is peculiar as that is definitely how it is rigged up in the car. I wonder what is wrong.

 

If you measured for the 12V at the hot water valve just in the unplugged harness connector (i.e. no current load) = this can give a false result (if there is a high, but not infinite, upstream resistance). Try remaking this voltage measurement with the connector plugged in (you may need to do something like wrap some test wires around each male pin to go to your voltmeter and then plug the connector partially on). If that gives a lower voltage result = the hot water valve may not be closing well.

 

Interesting. Okay, thanks. I'll try that.

 

Re the heater water circuit: In my RHD car, the hot water hose from the engine comes out the sill, goes directly into the heater core, then exits it and gets regulated through the heater valve on the way out the firewall towards the return circuit of the engine coolant.

The brass fitting is where the inlet hose from the engine water hose gets connected to

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The arrow on the heater valve is where the outlet hose of the heater core is attached to:

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You can see the exit down pipe disappearing into the firewall.

I don't think the position before or after the heater core would change anything in its function as long as the internal flow of direction is correct.

 

Did you replace number 11? This is an important line temp sensor and cycles the fan on and off, it has probably failed and your compressor fan is not going on, the line overheats, pressures rise and the pressure sensor to the compressor shuts down to protect the system. You can test this by jumping the two wires that connect to it, your ac compressor fan should work all the time when cooling is called for. I assume your compessor fan is also actually working.

 

I jumped this one so it runs when the AC is on. It sort of pre-cools the condenser between the cycles.

 

Once the system cools down, the ac will work again. Seems to be the exact symptom you are having.

The sensor is mainly to shut the fan off when you are at highway speeds, when the air blast is sufficient for condensor heat to be dissipated. At eg. idle at a stop light, the fan will kick on when the sensor/switch detects the line heat. It may cycle on and off if you are at idle for a long time. If you jump the sensor, the fan will always be on, at any speed (but only when you actually turn the ac on). The sensor/switch is really only trying to increase the fan motor life by running only when the supplementary fan cooling is needed. Some people in hot climates just bypass the switch, as the fan is on all of the time anyways.