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1995-2002 Range Rover P38A
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Discussion Starter #1
Folks, I' ve read the threads i could find and looked through Rave - wonder if I might get some help hear.

Heat works (auto or manual). My control unit accurately shows outside air temp.

I do get the "book display", I used RSW software to download messages from HEVAC and see some messages about sensors "to ground"

My compressor is not engaging - When I think it should be (hot day, controls set to LO) I took a voltage meter to the connector at the compressor and am only reading millivolts.

So it seems that there is insufficient voltage going to the compressor. Is this a correct assumption?

Should I try and connect a spare car battery to the compressor side of the connector and see if the clutch engages?

Should I keep downloading HEVAC messages and see which ones keep coming up?

I've become a little skeptical about immediately reacting to every message i get - I've notice on my EAS system some error messages come once - never to be seen again without doing anything.

Any other ideas?

Aside from the compressor I did take a pressure gauge to the lo pressure side of system and it is charged to 40 psi which is in the green range.
 

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Super Moderator
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Feed for the compressor comes from the HEVAC via the multiway connector behind the RH footwell kick panel and then via the Trinary switch before it reaches the compressor. If you are seeing only millivolts, corrosion in the multiway connector is the most likely problem.
 

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2019-2021 Range Rover Sport
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471 Posts
I’ve got the same situation. I tested the clutch by temporarily hooking it up to the battery and it engaged. I looked at the multiway connector and didn’t see anything obvious. Is there an easy way to check the trinary switch?
 

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1995-2002 Range Rover P38A
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Discussion Starter #4
well that gives me a place to start. thanks . the RH kickpanel? - is that on the right side of the right footwell, or left side of right footwell
 

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LEGACY VENDOR
1995-2002 Range Rover P38A
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4,224 Posts
RH footwell, on the outer side of the vehicle. There's an access panel in there for the inertia cutoff switch, but one screw and then it will come out with a bit of persuasion as it's also held in with a couple of spring clips.

Also, one other thing... is the system charged with the correct amount of refrigerant? If not, then it won't try to turn the AC compressor on.

Another thing worth checking is what part number the HEVAC controller is.... if someone has swapped it previously for an older model, then it won't run the AC properly due to changed in the way the AC compressor clutch is controlled across the production years.
 

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1995-2002 Range Rover P38A
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Discussion Starter #6
Thanks, the unit shows 45 psi on low pressure side so it has that much refrigerant in it.

Using my VIN, Microcat it shows part number JCF 102550 for the control unit.

I don't know if someone might have put an older unit in the vehicle before I bought it.

I have a spare unit with that part number that I could swap in and see if that works if the inspection of the plug under the kick panel doesn't fix it.

thanks you gents.
 

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1995-2002 Range Rover P38A
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Kind of odd that you happen to have a spare......Did it come with the vehicle?
If so, it may indicate that PO did indeed change the control head.
My Borrego had a "Bad Air Con" when I bought it as well as a "Spare" control head tossed in with the deal......I checked charge, replaced Trinary, and tried a couple other things.
Found the replacement controller to be wrong (Old style) but had a good display.
Proper controller, the "Spare" was correct for 2002, and worked fine but had a bad display, so I took display from old controller and mated with correct electronics. Problem solved!

Check for the proper control head before tearing too far into things.
 

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1995-2002 Range Rover P38A
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Discussion Starter #8
I got behind kick panel this AM and yes there was lt blue powder evident on the larger white multi-connector. Took all three apart and sprayed electric contact cleaner, reached in with a Qtip where I could and reassembled but it did not solve the problem.

Regarding Bolt's question my rig came with all sorts of spare (some used) parts in the rear including a spare controller. I sent the spare in and had the pixels fixed and installed it. What I did not do was confirm the part no before I put it in. But the AC was not working before and after I changed the HEVAC controller so it would seem not to be HEVAC head related. I can tell you the one I took out had the right part no JCF 102550, and the AC was not working with that unit.

Also regarding old/new HEVAC control units is there anyway to tell them apart at a glance(without seeing part no)? I ask because on my microcat the different part no's relate to whether the unit has ATC (auto temp control) and heated seats. So it would seem that as long as the unit you have installed is driving your heated seats and the AUTO setting is working then it must be the right HEVAC control unit? yes?/no?

Also where is this Trinary switch?
 

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There's different HEVAC part numbers because some markets didn't have heated seats, some didn't have heated windscreens, etc so the HEVAC for those markets won't have buttons for things it didn't have. So as long as you have the required number of buttons, then it is right. Early ones drove the AC compressor directly (via the connector and trinary switch as mentioned) the later ones drove an external relay which applied power to the compressor via the same route as before. On an early one a break or bad connection between the HEVAC and compressor will result in the book symbol as insufficient current is being drawn. On a later one, you won't get the book symbol for this fault as it would normally only be driving a relay anyway. A later HEVAC will work on an earlier car but not the other way round.

Trinary switch is high up behind the LH headlamp (Rave shows the location in the ETM, Z5 page 32, connector C0844) and can be accessed from underneath. There's two pressure switches in that area, one with two wires and the trinary switch with 4 wires. As your car is a 2000 model it will have the external relay which is in the under bonnet fusebox as Relay 3 and is supplied with power by F27. The HEVAC sends it's power via the connector you have found in the footwell to the fusebox where it operates Relay 3. Power from F27 is on one switch pin of this relay so when the relay is operated power flows to the Trinary switch and out to the compressor. Check that the relay is fitted and operating. If it is check the voltage arriving on the Green/White wire at the Trinary switch connector. As a temporary, very temporary, measure you can link the Green/White wire and White/Light Green wire at the trinary switch connector. If you find 12V at the compressor and the clutch engages, this will tell you if the trinary switch is faulty (unlikely), if the connector is dirty (slightly more likely) or if the gas pressure is too low (most common). Before getting to the compressor, the power goes from the trinary switch through a big round multiway connector underneath the coolant header tank but this on one of the few that is fully waterproofed so rarely gives problems. But the ETM tells you all of this, Section K4 page 10.
 

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1995-2002 Range Rover P38A
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Discussion Starter #10
That's a lot of info thank you Richard
 

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1995-2002 Range Rover P38A
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Please allow me to speculate:
Aircon has issues and does not work, or is intermittent, so PO buys used control head with good display.....Old style , so it still does not work......Problem was really corrosion, which you have remedied, so now the issue has switched to it being the wrong head unit so it still is not working, but for a different reason?
I know it is a PITA as I have done several, but now that you have had practice, you can probably change the controller in 15 min. Tops!
I would confirm proper part number.
You can also jump the contact terminals for the compressor relay to see if this runs the compressor. Oh, and try swapping in a different relay as well......
 

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1995-2002 Range Rover P38A
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Discussion Starter #12
OK, I replaced relay #3, based on what Richard G. says (right buttons) I have the right controller, I cleaned multi connectors behind kick panel as best as i could.

Pressure on LOW side of AC system is 40 psi.

I jumped the green/white and white/lt green wires at the Trinary switch - no change, the compressor clutch does NOT engage.

I applied 12V from a spare battery to the connector at the compressor and the clutch DOES engages - so it seems if provided 12V the compressor WILL engage and run.

Any further suggestions what to try?

Any ides
 

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Is relay 3 operating? Does the compressor clutch engage if you put a jumper in place of relay 3?
 

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Sorry I missed the smoking gun in your first post........
40 Psi is an virtually empty system. R-134-a at 77degF or 25degC would be about 82Psi.....
If it happens to be 45 DegF under the bonnet while you are taking the static pressure reading, then you would see 40psi....... (Of course, then you would be wanting the heater instead!)
So, you need probably need a recharge:

"Most systems will have a low pressure cut off switch that turns the system off at approximately 20 psi. The compressor will not function again until the pressure reaches approximately 45 psi. So, In most cases, you will need a static pressure of at least 45 psi before you begin to see the compressor operate."

Thus, if the compressor did try to run, it would do so for a fraction of a second before the pressure went below the cut off.
You can confirm this by putting the gauges back on, and jumping compressor again, pressure should plunge "like a stone through a wet paper bag!" Oh, and the air will not get cold......
Do not run the compressor for long without gas, as the gas recirculates the oil for the compressor.
 

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Discussion Starter #15
I just tried the R3 jumper and no change - no clutch engagement.
 

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Discussion Starter #16
I also tried jumping the trinary switch while jumping R3 = but the compressor still did not engage.

Just got these messages from the HEVAC
hevac.JPG
 

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1995-2002 Range Rover P38A
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Discussion Starter #17 (Edited)
Could those error messages mean its as simple as someone forgot to plug these sensors in when they were behind the panel last time - like when the heater pipe o-rings got changed last?

I know when I swapped the HEVAC control unit a couple weeks ago I noticed three large holes drilled in the plastic shrouding on the right side of the heating/cooling assembly- I seem to remember this was some kind of work-around for getting to the blending motors. Maybe someone forgot to plug sensors back in?
 

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Ignore the error messages for the time being, they may be red herrings anyway (you also don't know if they are current or historic faults) and the holes cut in the plastic side panel are to give access to the screw that holds the O rings in place. Do you have permanent power at fuse 27? Is fuse 27 good? This feeds the contact side of relay 3, do you have power at relay 3? Jumper relay 3 and check you have power on the Green/White wire to the trinary switch. Jumper the green/white to the White/Light Green, do you have power at the compressor clutch? Does it engage? I suspect that somewhere along this line is where the fault lies but I must admit I'm with Bolt on this and you simply don't have enough refrigerant in there.
 

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Discussion Starter #19
These are new messages - I have the RSW software to read and clear these.

Fuses are good but I will check the permanent power at f27 and relay 3.

I thought that jumping the trinary switch (white/green to lit gr/white) fooled it into thinking it had enough refrigerant and or eliminated the switch itself as the culprit.

Will keep trying.
 

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I thought that jumping the trinary switch (white/green to lit gr/white) fooled it into thinking it had enough refrigerant and or eliminated the switch itself as the culprit.
It does but if there is no power getting that far, it won't matter. The switch has a fairly wide tolerance and will show insufficient refrigerant at anything down to 35psi so your measured 40psi is probably right on the limit.
 
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