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Thread: AC not as cold when accelerating

  1. #1
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    AC not as cold when accelerating

    So when I got my 99 740 about 5 years ago the AC was ice cold within a mile of starting to drive. The past couple of years it's not been doing as well, and since it was 105 here today, I decided to look at some things. Before I did anything, the car would get cool eventually, I'd say dash temps in the low 60's. However, when you accelerated, even mildly (not full throttle, just easing getting up to speed) the temp at the vents would go up.

    So today I threw in a can of 134, and the temps at the dash lowered considerably. When I first put the gauge/filler on, the pressure on the low side was reading high, but as I added in the cold juice it lowered to a more reasonable pressure. Electric fan speed increased dramatically as I was putting in the can, so some sensor was being made happier. However, when I took it for a test drive I am still having the increase in temp on acceleration.

    On other cars I've had, this is due to a lack of vacuum or a bad vacuum actuator somewhere in the system causing the temp blend door to close partially. Is there anything like this in the e38 HVAC system? Just to answer the questions that are going to come up, the electric fan is working, and the water pump and viscous fan coupling have both been replaced by me. Belt is also in good condition.

  2. #2
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    While I'm not having your issue, I did "top off" my R134a today. And I noticed the same as you, the pressure went DOWN and the AC got colder. Every car I've ever owned more RPMs means more cold! On the ancient R12 system on my 951 (been converted to R134) I have to rev if I'm sitting to keep it somewhat cool. Same with my Suburban.

    I MAY be wrong, but it's my understanding all the HVAC is ran off servo's, but I have a theory for ya.... something I HAVE seen....

    If the control arm for your mix door has come loose, then it could be moving by physics.... In other words acceleration moves it to backwards to heat.... less G's and it moves back to cool.... just a thought.....
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  3. #3
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    When you put the pedal down in most cars, the ECU turns off the A/C compressor. It isn't safe to operate at high RPMs, it may throw a belt or damage something inside.

  4. #4
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    Quote Originally Posted by Kingtal0n View Post
    When you put the pedal down in most cars, the ECU turns off the A/C compressor. It isn't safe to operate at high RPMs, it may throw a belt or damage something inside.
    Actually, it's more a increased power thing, AC compressors are designed to handle the max RPM of the engine. Furthermore, if it really was a RPM problem, it would cut out at a certain RPM, not through a accelerator pedal cut out. However, to reiterate in my case, the AC is getting warm with mild acceleration, not when I'm at full throttle. Steady cruise - cold air. Tip in the throttle just a bit, it starts getting warmer. Also, doesn't seem to be RPM related, more engine vacuum related.

  5. #5
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    Bmw has never used vacuum in there climate control, might I suggest checking your fan clutch first. A/C is extremely dependent on air flow through condenser, also will replicate problem when parked

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    If you hit the pedal switch the A/C and alternator cut out to increase power. Based on the fact that you have covered the fans I would take a look at the heater valve. If it is worn and most are it may leak by and when you step on the pedal the flow is increased. Most cars with climate control will completely cut out the heat and the bottom temp setting and cut out the A/C at the top. You could run it down to 60 and shoot the heater hoses and see if you are getting some bypass from the valves?

    To get into the nitty gritty of "it's just not being cold enough" you will need a full recovery, evacuation and proper charge. If you shoot cans in from the auto store something colder is about what you will get.
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  7. #7
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    I would also look into the heater valve diagnostics. an always open heater valve will make the cabin hotter as the engine coolant gets warm. that was how i solved my intermittent AC problem when i first got my car. hotter when moving, cooler when stopper or slowing down.

  8. #8
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    With the drivers side temperature set to the lowest (16C) the water valves should be closed - and the heater pipes cold - that is quite easy to check....
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  9. #9
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    Quote Originally Posted by Sundaycruzer View Post
    Bmw has never used vacuum in there climate control, might I suggest checking your fan clutch first. A/C is extremely dependent on air flow through condenser, also will replicate problem when parked
    I get cold air when sitting at idle.

  10. #10
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    Quote Originally Posted by acspecialists View Post
    If you hit the pedal switch the A/C and alternator cut out to increase power. Based on the fact that you have covered the fans I would take a look at the heater valve. If it is worn and most are it may leak by and when you step on the pedal the flow is increased. Most cars with climate control will completely cut out the heat and the bottom temp setting and cut out the A/C at the top. You could run it down to 60 and shoot the heater hoses and see if you are getting some bypass from the valves?.
    I think we may have a winner here. It's significantly cooler here today, only 100 with 40% humidity, ha! So while I was changing the rear brake pads on my 07 RAV4 V6 (nearly identical HP as the 740, 276 hp) I let my 740 run at idle. Here are the specs from my temp gun, all in degrees F: Upper radiator hose - 140. All heater hoses 186. Air coming out center dash vent - 50. So maybe a partial opening of the valves that lets more though as the RPM's increases coolant flow. I'll order a repair kit and see if I can rebuild the stock unit before I throw down $500 on a new one.

    Now onto my 88 F-250 which has both a leak from the compressor and a definite vacuum issue (all the air diverts to defrost on acceleration.)
    Last edited by ConvGeorge; 06-20-2018 at 03:46 PM.

  11. #11
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    Quote Originally Posted by acspecialists View Post
    If you hit the pedal ... the A/C and alternator cut out to increase power.
    Ditto this...the IHKA and ECU/DME communicate and if the compressor is running, under certain conditions power gets cut to the compressor (which is one reason why I have ??? for those who HATE that the HVAC system always turns itself on & they think that it is sucking engine power reserves...when it actually doesn't...if you need to accelerate, the AC compressor can get the "axe" for a few minutes)




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  12. #12
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    I think that is a red herring, notice that the 'turn-off' conditions are: Less than 10 MPH AND Full throttle (not OR) - the off conditions are going to last a very few seconds - the car will be doing 62 MPH in 7 seconds or so on a bad day!

    As the heater hoses are hot a water valve is stuck open.....
    Last edited by Timm; 06-20-2018 at 05:00 PM.
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  13. #13
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    This might be totally unrelated but I had a 1999 735il and previous owner had replaced the dash control unit regassed etc spent thousands but I was not convinced it was cold enough especially for an West Australian summer. Found the in cabin filters completely blocked so I washed them out and hey presto problem solved.
    So I always order a set of replacement filters when I get a new car and the 750il is getting a birthday when I get back to Australia in a couple of weeks

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  14. #14
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    Quote Originally Posted by westaussie323i View Post
    This might be totally unrelated but I had a 1999 735il and previous owner had replaced the dash control unit regassed etc spent thousands but I was not convinced it was cold enough especially for an West Australian summer. Found the in cabin filters completely blocked so I washed them out and hey presto problem solved.
    So I always order a set of replacement filters when I get a new car and the 750il is getting a birthday when I get back to Australia in a couple of weeks

    Sent from my CPH1611 using Tapatalk
    I did the interior cabin filters earlier this year. They weren't really that bad, even though I'm sure they had never been changed just the under hood ones (I changed those a few years ago.)

  15. #15
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    AC not as cold when accelerating

    Quote Originally Posted by ConvGeorge View Post
    I get cold air when sitting at idle.
    It’s not always about idle. If the fan clutch can’t keep its connection when you rev up, there will be insufficient air flow. The a/c cut out is for full throttle acceleration, when you smash the gas petal to the floor. if what your saying that you lose cold air at slight acceleration you need to look at something simple...like air flow.
    Last edited by Sundaycruzer; 06-21-2018 at 09:11 AM.

  16. #16
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    Definitely check the heater valve, but how do you know it is not overfilled? If the system is overfilled, the compressor will be pushing against too much pressure in the condenser and won't be able to suck the low side down as much. That directly relates to your vent temps. This would become a bigger issue as the RPMs increased and the compressor built up more high side pressure. Just a thought. Do you have a set of manifold gauges to look at low side AND high side pressures?

    May be worth a recovery and recharge to the right amount.
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  17. #17
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    Quote Originally Posted by racer2086 View Post
    Definitely check the heater valve, but how do you know it is not overfilled? If the system is overfilled, the compressor will be pushing against too much pressure in the condenser and won't be able to suck the low side down as much. That directly relates to your vent temps. This would become a bigger issue as the RPMs increased and the compressor built up more high side pressure. Just a thought.

    May be worth a recovery and recharge to the right amount.
    Good one. Totally agree

  18. #18
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    Quote Originally Posted by Sundaycruzer View Post
    It’s not always about idle. If the fan clutch can’t keep its connection when you rev up, there will be insufficient air flow. The a/c cut out is for full throttle acceleration, when you smash the gas petal to the floor. if what your saying that you lose cold air at slight acceleration you need to look at something simple...like air flow.
    I have a new electric fan up front, and a new viscous fan coupling a couple of years ago when I redid my entire cooling system. I can assure you, there are no air flow issues present.

  19. #19
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    I tried to rebuild my heater valve without success. I tore the whole thing apart to find an over engineered magnetic coupling for the pump. The pump was shot and with the stationary impeller in the way it reduced the flow from the engine. I got a heater valve off of an X5 in the junk yard with no pump and it has been working fine.

    As to an overcharge. The accumulator/dryer stores excess liquid. When the system is running it might be 1/2 full this allows for refrigerant loss over time because the truth is all systems leak. If you overcharge the system it would store the excess with no effect or if you really went overboard the high pressure switch would cut out the compressor. If you really, really went overboard the safety valve on the compressor would vent the excess refrigerant.

    If you look at the logic involved in the above post you can understand how it could be several things. For example if the engine is to hot. Remember it does not have to be overheating just above the desired set point for the logic.
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  20. #20
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    Quote Originally Posted by acspecialists View Post
    I tried to rebuild my heater valve without success. I tore the whole thing apart to find an over engineered magnetic coupling for the pump. The pump was shot and with the stationary impeller in the way it reduced the flow from the engine. I got a heater valve off of an X5 in the junk yard with no pump and it has been working fine.

    As to an overcharge. The accumulator/dryer stores excess liquid. When the system is running it might be 1/2 full this allows for refrigerant loss over time because the truth is all systems leak. If you overcharge the system it would store the excess with no effect or if you really went overboard the high pressure switch would cut out the compressor. If you really, really went overboard the safety valve on the compressor would vent the excess refrigerant.

    If you look at the logic involved in the above post you can understand how it could be several things. For example if the engine is to hot. Remember it does not have to be overheating just above the desired set point for the logic.
    Due to the way it reacted when I added the can, I'm pretty sure overcharging isn't the issue. If anything, it may need another, but I'm going to wait until I check the vent temps after I try to repair the heater valve. I'll disassemble it first before I crack open the repair kit, I can always return that (I got it from Amazon.)

    Heater valve is a strong suspect, though, because now that I remember winter driving, the heat is kinda variable there, too. I'm pretty sure the electric pump is OK, because it worked when I drained and filled the cooling system (multiple times for multiple reasons.) Good to know one from a e39 or x5 without the pump will work, however, there are plenty of those in my local junkyard, and they are significantly cheaper even if I go new (like 1/4 the price.)

    What you say about the refrigerant level makes a lot of sense, you usually don't notice that you're a little low, by the time it needs a recharge, it generally needs at least 2 cans. In my previous BMW (e36), the AC didn't work when I bought it, I tossed in 2 cans on my way home and it worked fine for the next 5 years I owned it.
    Last edited by ConvGeorge; 06-21-2018 at 04:25 PM.

  21. #21
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    Quote Originally Posted by ConvGeorge View Post
    I have a new electric fan up front, and a new viscous fan coupling a couple of years ago when I redid my entire cooling system. I can assure you, there are no air flow issues present.
    Have you ever put the system into a vacuum using proper gauges, checked for leaks and then filled with exact weight of refrigerant? Obviously not after reading more of your thread you know nothing about what your doing..... with that said you can’t tell anybody on this forum that you know how much refrigerant you have in your system. Take it to a pro or buy the right tools for the job.

  22. #22
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    Quote Originally Posted by Sundaycruzer View Post
    Have you ever put the system into a vacuum using proper gauges, checked for leaks and then filled with exact weight of refrigerant? Obviously not after reading more of your thread you know nothing about what your doing..... with that said you can’t tell anybody on this forum that you know how much refrigerant you have in your system. Take it to a pro or buy the right tools for the job.
    I actually do have a set of manifold vacuum gauges. I don't feel the need to waste time and money taking it to a 'pro' to be charged far in excess of what I've spent so far when I'm more than capable of solving the problem myself. Not every AC problem requires a drain and refill as the first thing you do. Incidentally, I have a vacuum pump as well, so if it comes that that I can handle it. However, just to make you happy I hooked it up to both gauges right now, showing 35psi low side, 225 high side; ambient temp 95 degree; this means I probably need to throw in one more can, just as I suspected. However, in sprite of this I still have the fluctuating temp vent temps (which I have even when the AC isn't on,) and I've clearly identified another culprit in the system - a leaking heater valve.

    Let me tell you about the last (and only) time I took my car to a 'Pro', which was a couple of months ago to get it aligned at the local BMW dealership. After being there for 2 hours the service advisor comes out and tells me they broke one of the alignment bolts on the rear suspension, and would have to replace it. I said fine. Then another hour later he comes out again and says they are encountering trouble with the bolts on the other side, and are afraid they will break too. They don't have any more in stock, but will be happy to order them and do the job again in a couple days, with an added $350 for replacing the bolts (they only charged me $30 for the first one they broke.) I told them no thank you, just do the best you can with the other three wheels and I'll fix the bolts myself. I took the car home and got both bolts loose, spinning freely so it could be aligned and not broken in 5 minutes. Took me longer to get it jacked up and the wheel removed for access.

  23. #23
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    Blah blah if you have air in your system it will not work properly. It will blow the seals out of the pump eventually or worse. That’s why you evacuate the system, check for leaks and fill with exact weight and add dye if needed. Sorry if this offends you but your the one asking for help.

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