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‘01 AC issues (can’t add enough weight of refrigerant)
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Whridlsoncestood
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PostPosted: Tue Mar 30, 2021 6:50 pm    Post subject: ‘01 AC issues (can’t add enough weight of refrigerant) Reply with quote

Found out the AC also doesn’t work in the van I bought this winter. Tough to test when it’s single digits out and your buying it.

Vcds scan show code 00792 AC Pressure switch (F129)

First thing I did was check fan operation. On tiring on AC just the driver side radiator fan kicks on. I’d assume both should be running and that’s probably a shot fan resistor I will tackle later. But I at least know the message is going to kick the fans on. Then I checked pressure. With a top up fan from my LAPS it read 0 on low pressure side. I added a little from the can and got it to 45 psi. Now one of these cans doesn’t hold but maybe a little more than half of what this system requires so that didn’t add up to me. It’s since held 45 psi for the week. I did order a new pressure switch which arrived today. I’ve installed that and still no ac function. The compressor isn’t engaging.

In VCDS after I’ve cleared the fault and have ac supposedly on this is what I see.
Image may have been reduced in size. Click image to view fullscreen.


It appears to think the ac is on and compressor is running. I believe the hvac system needs to see a fault with the ac like 10 times or something before it shows an error code again. Compressor is clearly not running even though it thinks it is.

What’s my next step? Look for 6v I believe at ac clutch connector? If I have 6v there just a bad clutch or would I be more likely looking at a bad compressor? What other paths do I need to be looking down?
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Last edited by Whridlsoncestood on Sun May 23, 2021 5:41 pm; edited 3 times in total
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Whridlsoncestood
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PostPosted: Tue Mar 30, 2021 6:52 pm    Post subject: Re: ‘01 AC issues Reply with quote

If it matters this is a tin top MV. So it has the rear AC
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PostPosted: Wed Mar 31, 2021 4:02 am    Post subject: Re: ‘01 AC issues Reply with quote

Low side pressure is meaningless to determine charge. If you didn’t have charge, evacuate the system and see how long it holds vacuum to see if you have a leak. You can also use a sniffer if you have a bit of Freon on the system, but putting charge into a leaking system to test is both bad and illegal.

You need the vacuum to properly charge fir repairs,so get it for testing! About $100 from Amazon or harbor freight
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Whridlsoncestood
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PostPosted: Wed Mar 31, 2021 5:21 am    Post subject: Re: ‘01 AC issues Reply with quote

I assumed the low pressure side was just trapped pressure. Until the compressor kicks on it wouldn’t show me anything useful really.

Am I safe to assume both fans should be running? And if that’s the case I have a likely bad resistor on the passenger side one? Putting together another fcp order for my other cars too to pick up later this week and I’ll see if they have them as well.
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Whridlsoncestood
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PostPosted: Wed Apr 07, 2021 8:15 am    Post subject: Re: ‘01 AC issues Reply with quote

As I wait for my manifold gauges and vac pump to arrive I’m thinking about the order of operation. What is the exact order of operation before power is sent to ac clutch?

AC turned on. Relay sends power pressure switch. If that isn’t tripped it sends signal to fan control module? Fans turn on then prove and send signal to ac clutch via another relay? Is there anything else I’m missing in line? Is this the proper order? Fans are last line before clutch gets power? If only one fan runs will that satisfy the parameters and send power to clutch or do both fans need to prove for clutch to get power?

Fan resistors are on order as well since likely one is shot as only drivers side fan runs. Need to find the fuse block for fans too. Is it one fuse for each fan or one fuse for both?
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Whridlsoncestood
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PostPosted: Mon Apr 12, 2021 10:47 pm    Post subject: Re: ‘01 AC issues Reply with quote

Ac gauges came in. Static pressure with Van off. Maybe 55 degrees out.


Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Tue Apr 13, 2021 4:09 am    Post subject: Re: ‘01 AC issues Reply with quote

Looking at your readings your pretty low especially if the engine is warm. Static pressure should be equal to air temp around the system
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PostPosted: Tue Apr 13, 2021 4:45 am    Post subject: Re: ‘01 AC issues Reply with quote

A bit low but that is an iffy test.

To clarify, static system off pressure in PSI is approximately the same as system temperature in Fahrenheit.

You had zero pressure at start? You will need about half charge to get the system to kick the low pressure switch and turn on the compressor. Best way to do this is evacuate, pump down , test for leaks, then attach the can t o the empty System. Whoosh, it’s half full.

If you are in western CT let me know. I’m commuting via Bridgeport every week so driving through.
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PostPosted: Tue Apr 13, 2021 4:45 am    Post subject: Re: ‘01 AC issues Reply with quote

Engine was stone cold. Hadn’t been run in 24 hours.

Low maybe but not empty or low enough that the compressor wouldn’t at least try.
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Whridlsoncestood
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PostPosted: Tue Apr 13, 2021 5:27 am    Post subject: Re: ‘01 AC issues Reply with quote

Abscate wrote:
A bit low but that is an iffy test.

To clarify, static system off pressure in PSI is approximately the same as system temperature in Fahrenheit.

You had zero pressure at start? You will need about half charge to get the system to kick the low pressure switch and turn on the compressor. Best way to do this is evacuate, pump down , test for leaks, then attach the can t o the empty System. Whoosh, it’s half full.

If you are in western CT let me know. I’m commuting via Bridgeport every week so driving through.


Before I replaced the pressure switch I had one of those recharge cans. Forget how much is in one but it’s maybe half of what the tin top needs for a full full which I think is like 1100 grams? I hooked it up to check pressure. Digital gauge read zero. I pumped refrigerant in for maybe 10-20 seconds. Gauge read 45-50 psi I think. 65 degree day. Fully warmed engine. Which if there was nothing in the system I don’t feel like I added nearly enough to basically fully charge a system from empty if it really was. That was a few weeks ago at this point and doesn’t seem like much or anything has been lost with my current cooler ambient temp and cold engine. After that I switched out pressure switch as that was the vcds fault code. That didn’t change anything besides the fault code not coming back. Single fan kicks on and vcds says says compressor is “on”. Fan fuses are good so I’m guessing it’s a shot resistor. Those are on order and not sure when the arrival date is. I know if fans don’t run compressor won’t turn on but I’m curious if only one runs it would allow the compressor to run.

Also if at least one fan kicks on I know is passing a series of pre checks already correct? Pressure being one of them I believe?

I think I need to get underneath and check for voltage at compressor soon and see if this is possibly a clutch issue. If so I guess I’d be looking at a new compressor anyways.

I’d hate to evacuate the whole system and do a whole new charge just to find it wasn’t my issue and it was something else electronically or a shot compressor. Hate to evacuate/recharge and then have to evacuate again....Don’t mind doing it if I know it needs to be done.

I’m East of Bridgeport. Probably 15 minutes up rt 15 in Orange.
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PostPosted: Sat May 01, 2021 6:00 pm    Post subject: Re: ‘01 AC issues Reply with quote

Replaced fan resistors today. Was waiting for all my pets for rest of service items to come in so I only have to put it up on stands once.

Both radiator fans now run when ac is turned on. Still no compressor running though. I pulled the connector to the compressor to check for voltage across the two pins. Nothing there. So I’m not getting power to the compressor. I suppose that’s better than getting power and the clutch not engaging.

So what has final say in sending the 12v to the clutch? I believe that the pressure switch is reading what it wants to as fans kick on and I think that the pressure switch proves first. Then fans? So far my understanding of operation is ac switched on. Pressure switch checks out. If that’s ok. Fans kick on. If those kick on then clutch can finally engage?

Weird part is vcds says the compressor is on in that previous screen shot. It can’t be as simple as a fuse at this point right? I swear I check all those prior when I had door wiring shorts. And replaced a bunch after that repair. Doesn’t the ac fuse share a line with a few other things too?

Any more pointers? Time to lay in bed and dig out the Bentley and see if it leads me anywhere else.
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PostPosted: Sun May 02, 2021 4:49 am    Post subject: Re: ‘01 AC issues Reply with quote

It’s hellishly complicated in Manual AC, I don’t know Jack about climatronic

Power from the fan blower switch routes to the ac switch , which then goes through the ac pressure switch and the thermal switch , then back to the Ac cutoff relay 147 and finally to the ac relay 140, and then the compressor clutch

Wait I left off the Evap switch , that is between the ac switch and the ac pressure switch

Pm sent
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PostPosted: Tue May 04, 2021 5:39 am    Post subject: Re: ‘01 AC issues Reply with quote

Does anyone have schematics for the 24v. Obviously the Bentley only
Covers the 12v and I’m trying to figure out what coolant sensors or what and the wiring schematics so I can jump two of the 4 wires on any of these to see if I get 12v to compressor. I replaced the main cts in tstat housing with a new green top. There is a second gree 4 pin cts next to it in the housing and then another blue too cts in the lines going to heater core up high by the firewall. Also whats the easiest way to get to the evap temp sensor under dash?
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PostPosted: Thu May 06, 2021 7:18 am    Post subject: Re: ‘01 AC issues Reply with quote

So spent the night under the car with a DMM and the schematics and traced 12v. Had it to ac pressure switch and had continuity through pressure switch. Traced that back down to cts and had 12v there and continuity through that as well. That in turn should trigger relay 140 which then sends 12v right to clutch. Turned car off to look over schematic again. Fired it back up to test some more. oil pressure light starts flashing. No tachometer. No gear selector in cluster. Found 10 amp s19 fuse blow. Of course that is the line for the clutch, afterrun coolant pump and 3 wire fan control sensor. Unplugged fan sensor and afterrun pumps. Replaced fuse and fired up the van and ac clutch engages! Put 3 speed radiator fan sensor back in and left afterrun pump unplugged as this is what the connector looks like.

Image may have been reduced in size. Click image to view fullscreen.


I just find it odd how the afterrun pump was stopping the clutch from getting power but didn’t blow a fuse until last night.
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PostPosted: Thu May 13, 2021 5:17 am    Post subject: Re: ‘01 AC issues Reply with quote

Just because I was able to get the compressor to work doesn’t mean the AC works!

No cold air in front or rear systems. If I feel the high pressure side right after the TXV before it enters the firewall it’s cold. The low pressure return side at TXV is not though.

These are my pressure readings. Outside temps around 55 degrees. Static pressure ac off about 65 psi on both sides.
Image may have been reduced in size. Click image to view fullscreen.


When running the low side drops to 30. Great. But high side barely goes up. 75 psi maybe.
Image may have been reduced in size. Click image to view fullscreen.


Obviously somethings not right. The contents of the system truly are unknown at this point. I have pressure but who knows if anything besides refrigerant and oil are in the system. One option is to evac at a shop and then vac/recharge at home and be sure the system is full with proper weight of refrigerant.

Talking with abscate we are wondering if the compressor is starting to fail or there is a bad TXV valve. With the Tintop split system there is two TXV valves and neither front or back are cooling. Not sure if one bad TXV valve would make it so neither front or back properly cools. Anyone else an AC guru here? I don’t mind doing the work I’m just hoping to narrow this down so I don’t end up evac and recharging the system 3 times chasing the problem. If I had easy access to a proper evac machine it would be one thing, but I don’t.
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Whridlsoncestood
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PostPosted: Sat May 15, 2021 5:09 pm    Post subject: Re: ‘01 AC issues (interpreting pressure readings) Reply with quote

Anyone have any thoughts?
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PostPosted: Sun May 16, 2021 5:34 am    Post subject: Re: ‘01 AC issues (interpreting pressure readings) Reply with quote

Probably best to evacuate, change by weight.
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PostPosted: Sun May 16, 2021 6:22 am    Post subject: Re: ‘01 AC issues (interpreting pressure readings) Reply with quote

subdewd wrote:
Probably best to evacuate, change by weight.


Thanks. It’s what I’m leaning towards. Seems like it would be such a simple solution and as far as me with cars with AC issues it’s never been that way in 20 years. Wink
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PostPosted: Sun May 16, 2021 12:46 pm    Post subject: Re: ‘01 AC issues (interpreting pressure readings) Reply with quote

I’m driving through new Fairfield Ct tomorrow morning en route to Smallbany. Can drop in and look befuddled at it with you.
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PostPosted: Sun May 16, 2021 3:18 pm    Post subject: Re: ‘01 AC issues (interpreting pressure readings) Reply with quote

On the road back from Maine in a few. Long ride ahead. Survived the ride up. I’ll be exhausted by morning. Another time we’ll cross paths and scratch our heads over this.
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