Cold A/C but only in the back seat

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Ragierer
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Cold A/C but only in the back seat

Post by Ragierer »

The car is a 1988 635 with rear air. I have had the car about 60 days. I recently had the air retrofitted with an R134a system. It is blowing cold but only in the back seat.

I will pick the car up tomorrow and try to sort this out. Any ideas you folks may have is appreciated.

Robin
1988 635CSI
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dwcains
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Re: Cold A/C but only in the back seat

Post by dwcains »

Usually an indication of a low charge in the system - throw a set of gauges on it to check. Unfortunately, it also often indicates a leak in the system, especially if it was working fine previously.
Dean
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Ragierer
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Re: Cold A/C but only in the back seat

Post by Ragierer »

This is after compressor has been replaced and system vacuumed and recharged. I think charge is good now.
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Da_Hose
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Re: Cold A/C but only in the back seat

Post by Da_Hose »

Assuming charge is indeed good to go, you describe pretty classic symptoms of a bad expansion valve up front.

Jose
1987 M6 - My dream car
Hakeem43020
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Re: Cold A/C but only in the back seat

Post by Hakeem43020 »

On later e24s the rear charges before the front. It could be undercharge, a leak or bad expansion valve.

Unfortunately I also have the same issue. Tested and mine has no leaks. Everything on my system is new but i still haven't been able to figure it out.

My high side is at 200 with r152. Get 40 degrees in the back, 80 up front.. Tried purging but temps went up instead. Giving mine a break for now. Good luck, hope you figure it out.
Ragierer
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Re: Cold A/C but only in the back seat

Post by Ragierer »

I picked the car up from the mechanic working on the A/C and while driving home on the highway, it started blowing cold air from the front.

I'm not sure what this means but I think I am going to watch it for a bit.

Thanks for the help
R
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dwcains
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Re: Cold A/C but only in the back seat

Post by dwcains »

I think that front/rear proportioning valve (voltage regulator as realoem calls it), just from what I've been reading from guys with dual-a/c cars, is the root of all the trouble you guys seem to be having. I certainly can't get the airflow you guys get in my single-a/c car, but the simplicity of the system seems a fair trade-off.
Dean
Lutz, FL

'85 635 CSi Euro #9402254
'87 Spider Veloce
'92 Spider Veloce
'08 350Z

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Da_Hose
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Re: Cold A/C but only in the back seat

Post by Da_Hose »

Hey folks,

I have done a fair bit of digging and put what I have learned into this thread.

viewtopic.php?t=21047

The regulator in the dual AC cars doesn't serve a balancing function between the two circuits. It balances the output from the dual circuits to the pump. My understanding is that since the two different exchangers could create differences in pressure, the regulator makes sure the pump side gets one consistent number. It's the expansion valves that do the "balancing" by opening when there is enough flow/pressure. The rear one seems more robust, and gets preference when refrigerant is low. It's also easily rebuildable.

It's the front expansion valve that is a bugger to deal with. You have to take the dash apart to get at it, and it is not rebuildable. You can only replace. I have to replace a lever in my HVAC controls, as well as get my dash re-covered (87 with original, shrunken leather dash). When I take all that apart, I am replacing my front expansion valve and putting new seals in the airbox, and on the flapper.

All said, I think the problem for the OP is that dreaded forward expansion valve. Several folks have said that when they replaced that, they started getting water/ice out the front vents because it works so well.

Jose
1987 M6 - My dream car
Hakeem43020
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Re: Cold A/C but only in the back seat

Post by Hakeem43020 »

Da_Hose wrote:Hey folks,

I have done a fair bit of digging and put what I have learned into this thread.

viewtopic.php?t=21047

The regulator in the dual AC cars doesn't serve a balancing function between the two circuits. It balances the output from the dual circuits to the pump. My understanding is that since the two different exchangers could create differences in pressure, the regulator makes sure the pump side gets one consistent number. It's the expansion valves that do the "balancing" by opening when there is enough flow/pressure. The rear one seems more robust, and gets preference when refrigerant is low. It's also easily rebuildable.

It's the front expansion valve that is a bugger to deal with. You have to take the dash apart to get at it, and it is not rebuildable. You can only replace. I have to replace a lever in my HVAC controls, as well as get my dash re-covered (87 with original, shrunken leather dash). When I take all that apart, I am replacing my front expansion valve and putting new seals in the airbox, and on the flapper.

All said, I think the problem for the OP is that dreaded forward expansion valve. Several folks have said that when they replaced that, they started getting water/ice out the front vents because it works so well.

Jose
Thanks for the link Jose, I'll take a look and I imagine the valve could be my issue. Also a huge note to anyone with a rear AC car, if you've never replaced the front expansion valve on the rear AC cars although they might look similar they do use a completely different valve than all other E24's(Probably due to it being a dual air system?). Later ones use a different part number which is rarely documented, prior to finding out I've tried 5 or 6 different valves, valves from earlier cars, valves from autohauz, bavauto, fcpeuro, autozone etc and none of them fit. I think even RealOEM has the wrong valve and p/n listed. If the valve is listed at less than $100+ I imagine it being the wrong valve.

If I recall correctly the front has a bigger expansion valve connection than all the previous cars. You could use an adapter(I did and it was the culprit of a small leak so I sourced one from BMW of South Alanta). Afaik they are the only one that has the correct one for the later cars. It's not an OEM valve but a denso valve that bolts right up. I've replaced mine a few months ago but I've tempered with it alot so it might be possible that it went bad.

The valve is $120 or so if I recall correctly. I haven't found another source for later cars and hope someone out there can find a cheaper alternative.

I'm sure I could dig up the p/n and grab the link if anyones interested.
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Da_Hose
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Re: Cold A/C but only in the back seat

Post by Da_Hose »

Hi Hakeem,

Yes, please post the p/n for us. This kind of thing can be a HUGE thorn in your side when the system is open. If you have to wait a month to get something from the factory, you just bought a new dryer as well, so getting parts right the first time is really important when working with your AC.

Jose
1987 M6 - My dream car
Hakeem43020
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Re: Cold A/C but only in the back seat

Post by Hakeem43020 »

Da_Hose wrote:Hi Hakeem,

Yes, please post the p/n for us. This kind of thing can be a HUGE thorn in your side when the system is open. If you have to wait a month to get something from the factory, you just bought a new dryer as well, so getting parts right the first time is really important when working with your AC.

Jose
Sure thing. I had to dig up my thread on bimmer forums. The p/n for the front expansion valve in later cars is 64 50 1 380 658 . I've been at fixing mine for a year and it's been a long road and huge thorn in your side is an understatement. I did manage to get 38 or so blowing from both vents once upon a time.

Now for the most part I've been cleaning up other people's mistakes. I've had to deal with people charging with pressurized air to test for leaks and leaving it in over night(probably why my expansion valve went out?), telling me to only pull vacuum from one side, advising me to mix refrigerant oils(PAG and Ester oil), convincing me to mix refrigerants, leaving my drier out for months and trying to reuse the old drier, telling me not to pull a vacuum more than 5 minutes etc.

Also when replacing the front expansion valves, it might be worthwhile to clean your flapper doors and ensure they move freely. Mine would only open half way this restricting air flow and after cleaning it the vent above the radio now blows close to the same amount of air as the top vents.

Makes it a lot more bearable since I can point the vents on me rather than using the top vents to just cool the cabin.
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Da_Hose
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Re: Cold A/C but only in the back seat

Post by Da_Hose »

That part number comes up on the Pelican and FCPEuro sites for $140. This is great information to have in this thread.

Thanks.

Jose
1987 M6 - My dream car
Ragierer
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Re: Cold A/C but only in the back seat

Post by Ragierer »

Just FYI
Driving the car home from the shop I had AC in the front vents kick in and has continued since.
I'm keeping my fingers crossed

Thanks
Robin
baders
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Re: Cold A/C but only in the back seat

Post by baders »

For reference, that part number does come up on RealOEM;

http://www.realoem.com/bmw/enUS/partxref?q=64501380658
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FewofM
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Re: Cold A/C but only in the back seat

Post by FewofM »

Guys, I'm new here, "jeffcfp1" on BF and similarly have gone through years of suffering with cold air from only the rear unit. Upon replacing both front and rear expansion valves with my nearly complete a/c rebuild (including line/system flush and Sanden 508 Compressor) and determining that the mystery "Voltage Regulator" on the inside right fender was still available, I decided to dissect it and determined that the one plus inch piston was gummed up and frozen solid/inoperable. Link below. I'm getting 39 rear/40 front on R-12 after removing it and cleaning it. You'll see in the link that there's a blow off/bypass that goes down to the front expansion valve on the front rear systems. If the piston is frozen, you don't have proper communication between it and the front expansion valve. https://www.bimmerforums.com/forum/show ... -Regulator!

Also, know that upon replacing the front TXP on the dual setup (all setups for that matter, that the "bulb", must be clamped properly at 11 o'clock only to the low pressure "return". This includes ensuring direct contact with low side and not strapping high and low lines together.
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