Hi All,

my P38 1995 Air Con works fine as long as it's not a really hot day.
Hot days it fails.

My good Man at the garage ( with good Range Rover experience ) suggests re gassing. My A/C does have weaker cold than it should be when working. Can low gas stop a working A/C when it gets too hot?

Also, I have an LED light on the output of the HEVAC to tell me when the HEVAC sends the signal to the DUAL pressure switch to run the compressor and this LED light does not come on when the A/C is failing in hot weather. As far as I can read in the ETM the only thing that might stop this signal from the HEVAC is the Evap Temp Sensor , sensing ice on the evaporator. I gather if this sensor is faulty it could stop the A/C working. What do you think?

I'm not sure that I have ever seen my condenser fans run. If there is a fault here can this stop the HEVAC from sending the signal to start the compressor? What do you think?

RR Brains trust can you please come to my rescue 😀

cheers,
Paul.

Are you in the UK, if not where are you and what is your climate like? What kind of ambient temp counts as a hot day for you?

The evap sensor is used constantly to cycle the system - not just for icing. Ideally you'd need diagnostics to see what is going on with that sensor, the aspirator (internal cabin temp) and the outside air temp sensors. All three will be used to determine as and when to run the compressor. The only other thing that gets involved is the engine ECU - the HEVAC has to 'request' whether it can run the AC compressor output, and if the engine ECU says no (overheating, max throttle/revs etc) then the HEVAC complies and does not run it.

If you're not getting the +12v output from the HEVAC to the compressor clutch, then its likely a sensor fault or the request signal is being denied for some reason. Does the outside air temp read properly? That's the easy one to check as it will be on the display.

Same post as on the dark side as paul500. Australian flag. It doesn't get much hotter.

Aha, since the "upgrade" to the forum I can't be arsed looking there anymore.

In that kind of climate, while stationary you might get the electric fans to come on at low speed, but it is quite normal for them to rarely/never run if your viscous fan is working well - as that pulls a lot of air over the whole cooling pack even when the coupling isn't locked up. But if you're not even getting a compressor clutch output, I'd say something else is at fault first. Honestly off the top of my head, I can't remember if the HEVAC a) has any way of detecting faults with the fan relay control and b) if it did, if that would disable the clutch control. Diagnostics again would be your friend here.

The LED you have on your HEVAC output - where is that wired in? Actually physically on/by the HEVAC, or elsewhere in the car's wiring loom?

One of my condenser fans seized years ago so they never ran and I recently noticed the other has now seized (probably due to lack of use) and it hasn't affected anything else.

One thing that will cause the compressor clutch to not kick in on hot days is if the clutch air gap is too wide. It should be between 16 and 30 thou but mine was nearer 40. It would work perfectly up to an ambient of around 22 degrees but as soon as it got hotter than that, just when it was needed, it wouldn't. Set the AC on Lo which should force it to come on, then tap the end of the clutch with a screwdriver handle, if it kicks in, that is the problem.

I was going to mention the clutch air gap - but if no +12v from the HEVAC to the electromagnet, it isn't going to do anything. But I'm working on the assumption that +12v is being measured (or lack of) on the connector on the back of the HEVAC, if its somewhere else on the loom, then it could be a break or the usual footwell connectors etc.

Hi Sloth, and All,

thanks for your replies. Much appreciated.

The LED is wired at connector C202 C102, pin 7. It's behind the kick board RHS footwell. It's electrically between the HEVAC and the dual pressure switch. And then subsequently the compressor. I put the LED in the dash so I can see it go on when the HEVAC requests the compressor to turn on. It stopped going on when it was quite hot on a long trip. Yes I am in Australia, although it was not one of our scorchers, it was only about 34 celsius. The outdoor ambient temp sensor reads quite accurately BTW.

The problem is not the clutch as that would still show the LED lighting up. I fixed that one several years back and installed the LED to help with that problem at the time.

I noticed yesterday that of the 2 coolant lines in the engine bay, none of them are suitably COLD.
This is days after the hot day trip when the AC is working again as it's not under heat stress. Compressor going no problem.

Without a really cold coolant line I suspect that the coolant needs a re fill.

From other things I read on the forums I suspect that the EVAP temp sensor may be reading that the AC is VERY ineffective due to low coolant and the HEVAC is NOT HAPPY and is withdrawing the compressor signal.
Sloth said "The evap sensor is used constantly to cycle the system - not just for icing. "
I'm suspecting that poor temp readings by the evap sensor due to other faults ( eg low coolant ) might be able to shutdown the AC.
Does this sound plausible to others?

RE GAS, top up, is booked for Friday.
Sunday is meant to be 36 degrees and nice and hot enough to go for a drive and test it.

Cheers,
Paul.

When working correctly, one of the refrigerant lines should be too hot to touch while the other should be very cold and may even have condensation forming on it. If you can touch both then it isn't doing anything. However, even if low on refrigerant you would still get the signal from the HEVAC to engage the clutch, it just wouldn't get as far as the compressor due to the pressure switch. So it does sound as though something is stopping the HEVAC from engaging it. Ideally you need diagnostics that can show you what values the various other sensors are giving.

Thanks GilbertD.

Makes sense to me.
The mechanic just wants to blindly re gas as a first step.
sigh...
Hopefully he intends to check with the diagnostics.

Look, the signal from the HEVAC does go further than the pressure switch and the AC works reasonably 90% of the time.
On HOT days the signal does not come from the HEVAC. That is the problem.

ps I like after 3 rangies you have: "96 4.6HSE Ascot because you never know when you might need a spare" 😀
I agree. You can never have enough rangies 😉

Paul.

The '96 has been used mostly by my other half to carry building materials for a house we have been renovating. Tools in mine (the '98 ex-plod, now with 443,000 on the clock) and sheets of plasterboard, plywood, wall tiles, etc in that one. A lot more practical than her Merc SLK280 and quicker than two trips in mine.....

It depends what sort of a mechanic he is. Some are fully skilled parts swappers, the better ones will try to diagnose the problem first. However, with AC, virtually everyone will go for a regas first, providing that is done properly (recover, pressure test, vac out, refill with the correct amount of R134a). As far as sensors go that could be inhibiting it, you've got the aspirator (the cabin temperature sensor in the dash) but if that was to go open circuit it would detect a cabin temperature of -40 degrees C so wouldn't engage the AC compressor as it thinks it is cold already. You've got the heater core sensor which, again, if open circuit tells the HEVAC it is at -40 degrees C although I think that will only stop it from heating not cooling if the cabin sensor says it needs it. Then there is your suspect, the evaporator sensor which will shut it off if it is getting too cold and likely to freeze but it wouldn't prevent it engaging the compressor, it would engage, get very cold then disengage as the temperature gets low. But you need diagnostics to be able to see what the various sensors are reporting.

However, thinking it through, you've got an early car which drives the compressor clutch directly and not via a relay like the later Thor ones do. In the case of an early car, if it tries to engage the clutch and the pressure switch is open due to a lack of refrigerant, the HEVAC detects that it isn't drawing sufficient current so logs the fault, brings the book symbol on and doesn't try to engage it again until it is reset the next time you start the engine. So you would get a very brief flash of your LED the first time it tries to engage it then after that it won't try again. So the regas may just be all it needs.

I'd be willing to bet an open circuit evap sensor would stop the compressor engaging if it defaults to -40 like the others. I haven't confirmed that on a P38 though - but otherwise the HEVAC has no way of knowing how to cycle the compressor. Normally it cuts out at about 4c and back in at 7c, regardless of how you have the temperature set on the HEVAC itself. If you have both sides set to 'Lo' it will still only cycle in that window - otherwise it would risk a freeze up and flood liquid refrigerant back to the compressor and likely cause a big bang as the compressor chugs something it can't squash any more :)

Being an early one as you say though, it may genuinely just be a regas but eh, if it works reasonably well most of the time in his climate then I'd think charge should be okay. The other possibility is that on a properly hot day... if its overcharged, the high side pressure spikes and the high limit is cutting it off. That could be hindered by a dead condenser fan(s) or faulty pressure switch not bringing them on at all or at high speed. The high side limit is then hit, the pressure switch goes open circuit and the HEVAC will see it as a fault and disable the output, so no LED.

Because I have no viscous fan, here in the UK I have had my fans come on at high speed on a hot (35ish) day when first cooling the interior down - had to wonder what the hell the noise was to start with.

Does it work at all to start with on a hot day Paul?

If not... I'd personally be suspecting a wiring issue especially down by that footwell connector. The annoying thing is, that kind of issue will also raise it as a fault that disables the output... Perhaps a regas is the easiest place to start in this instance without diagnostics to ensure a proper charge - and then perhaps look at bypassing that connector for the clutch wiring to rule it out.

Sorry to throw more possibilities out there... always difficult to diagnose something over the internet.

That said if you want to fly me out to you, I'll happily bring diagnostics and recovery/evac/recharge kit with me to diagnose and regas it once we find the problem. No charge, just cover my travel costs :P

Sloth wrote:

That said if you want to fly me out to you, I'll happily bring diagnostics and recovery/evac/recharge kit with me to diagnose and regas it once we find the problem. No charge, just cover my travel costs :P

It would be interesting to see what airport check in would make of a 10kg cylinder of R134a and an empty recovery cylinder too. I was once stopped from boarding a plane because I had a pair of gas shocks in my hand luggage. Then again I was also stopped once when my luggage tested positive for Semtex which turned out to be two dead Duracells in the remote for a satellite receiver I was taking over for a mate......

I wouldn't be surprised if they're fine with the cylinders and my fgas - probably detain me for a tube of toothpaste though!

I know Duracells are pretty good... but they're going to pretty sketchy lengths to pack that much power into them...

Hi Guys!

sorry for late reply. Been busy, including working on the Rangie.

Ok been to the mechanic. The thermometers on the diagnostics ALL look ok. I have a pic for you but can't post it here for some reason. Won't let me.
He did the whole re gas procedure. Properly. (recover, pressure test, vac out, refill) It had about 390 something in there and should have had 1200 according to him. So needed a REGAS. Still can't see why a total shutdown of the AC occurred in 35 degree day, but maybe low gas was to blame. Maybe the HEVAC got jack of trying to cool the interior when it was so hard with little GAS.

I tried to test it Sunday but it only got to 30 degrees. So 1 hours drive in 30 degrees is probably not hot enough to see if the original fault is fixed. It used to work on 30 degrees on low gas but 35+ would normally mean "pack it in". I'm hoping that the REGAS will be all that is required to keep it running on super hot days.

However, Sloth made an interesting comment about first start up on a hot day. The rangie will try to quickly cool the interior by turning on the condenser fans.
I normally only get the test book symbol on these hot days , a few minutes after start up and I never notice my condenser fans going.
Hmmmmmmm !

I was keen to diagnose my fans. I tested all fuses and relays. All good. Then I removed RL18 and shorted the switch holes. Both fans worked! I tested the output of the ECM and it was hi. ie +12V which will keep RL18 OFF. I checked the input to the ECM and it was also high +12V. The only stuff on the input to the ECM is the press switch 1 and the HEVAC. I checked the HEVAC and it's line was 0v. Obviously pressure switch was open. But where was the +12V coming from? It could have been from the un loaded coil of RL18. So I pulled out R18 and the +12v was still there. So it is coming from the ECM. More importantly it's coming out of BOTH sides of the ECM. Input and Output. Which is interesting because it means the ECM is actually signalling the HEVAC. You would think it's the other way around. PS the diesel P38 wires directly from the HEVAC to RL18 but the petrol P38 goes via the ECM. I then bypassed the single pressure switch and put that +12v from the ECM to the HEVAC and straight away the HEVAC found some sink for that and the input to the ECM went to 0v and the fans came on. I think that HEVAC senses briefly that 12V from the ECM before it brings it low. That way the HEVAC KNOWS that the pressure switch has closed. On very hot days it expects it to close and if it doesn't briefly see the +12v from the ECM it lights the testbook. Not wanting to have to convince the mechanic to replace the press switch and also not necessarily wanting to pay for that I put a new "pressure" switch in my dash board. ;) . Now I can start my condenser fans when I want. Yesterday was about 25 degrees. Not that hot I know, but I went for a drive and switched on the fans. 1/2 hour later still no test book and very cool interior.

Could consider doing something similar for the press switch 2 which sends the fans to parallel and fast mode. But no rush.

Regards,
Paul.

At 390 grams it's a wonder it worked at all but a regas was definitely needed so should sort out the problem. When it was working, what sort of temperature were you getting coming out of the vents? When working correctly you should be getting really cold air coming out. In the UK at an ambient of maybe 20 degrees, air at down to 3-4 degrees is the norm but the higher the ambient temperature the higher the output, so I would expect around 10 ish degrees. You would have been getting the book symbol on hot days when it tries to engage the compressor clutch but it doesn't engage (so doesn't draw sufficient current) almost certainly due to the lack of pressure meaning the pressure switch is open.

ECM does signal the HEVAC, it tells it that the engine is running and the engine temperature is normal so it will allow the AC to be engaged. HEVAC also signals the ECM to tell it when it engages the AC compressor so it knows to expect the revs to drop at idle and to raise them. On a hot day as soon as you start the car it will try to cool the interior as quickly as it can by bringing on the AC and interior fans but not necessarily the condenser fans. As Sloth also says, with a properly working viscous fan it doesn't need them (they don't do much at the best of times). As long as they come on when the ECM knows the engine is getting hot, that is all that really matters.

Hi Gilbert,

• when working before it was reasonably cool but not as cool as now. Especially when condenser fans on.
• I do not get test book from failing compressor clutch, this is not a problem, ie working.
• HEVAC signalling ECM for compressor clutch is a different signal to the condenser fan comms to the ECM.

Working beautifully now but I'd like to test in in a 35+ day.
Only a few weeks of summer left and we have El nino this year and the summer is mild.
If it was El nina we'd be getting our fair share of 40 or 42+ days.
Happy for El Nino. 😁 Making the most of it...

Regards,
Paul.

REDP38 wrote:

• I do not get test book from failing compressor clutch, this is not a problem, ie working.

You should do, unless the HEVAC has been swapped for a later one intended to drive a relay, in which case the error flagged when not drawing sufficient current isn't enabled. It won't make a difference, a late one is still capable of supplying enough current, it's just the firmware is different so it doesn't bring the book on.

Hi redp38,
one thing I don't think anyone has mentioned is the thermal overload switch on the compressor. With the small charge you had in the system there would be no cooling gasses getting back to the compressor to keep it cool therefore the discharge temperature would go sky high and trigger the thermal overload switch that would cut the power to the clutch. If the climate control module was expecting to see a load on that circuit and it did not see that load it could trigger a fault. Also if the system only had 300 odd grams of refrigerant in it I would be looking for were it got out. Condenser, evaporator, discharge hose ETC. Am I sounding like a TAFE teacher ?
Regards,
Alan

Hi Guys,

thanks for replies.

Some communication wires somehow got crossed here though.
I can't seem to say it clearly enough 😀
There is NOTHING wrong with the compressor in my P38.
IT WORKS WORKS WORKS perfectly.
Nothing wrong with it.

I don't know how this topic got side tracked on a non existent fault,
so thought I'd better remake the point.

The only reason the compressor was not functioning ( on the hot days )
was that it was no longer being ASKED TO FUNCTION by the HEVAC.
As said I have LED indicators on my dash to show me what the HEVAC
is asking for. That's how I know. 😀

cheers,
Paul.

I know this is a closed issue and I apologize to REDP38 for a partial hijack, but since the topic is interesting and relevant, and I do not value my question important enough to warrant another thread ...

So, the topic of the A/C request and A/C grant between ECM and HeVAC has been mentioned a few times here, and is clear before "switching on", the HeVAC ECU exhanges request/grant status with the ECM.

The ETM describes both signals as "active low", but my question is ... what are they exactly? Plus? Earth?
In the ETM I see both in Section K4 page 8, pins 13 and 14 on connector C0793 ... is there any way to override those, giving the HeVAC ECU an "always grant" signal and thus controlling the activation of the compressor from the driver?
You may certainly ask "why would you want to do that?" And I would answer "I like to be in control of the operation of my machinery, thank you" ... :-)