Oh, The Joys of Modern Cars

Yesterday, we decided to run into town in our '07 BMW 335i, which we had not driven since the “lockdown” in mid-March. It seems to be a bit PO’d about being ignored for so long.

First, when we parked, and sat in the car to eat lunch, the battery warning light came on, which indicates the battery is approaching end of life. Not unreasonable, we bought the car back in about '12, and haven’t touched the battery since. So, went to the parts store, bought a replacement for $160. Not too bad by today’s standards, and it is a big sumbitch. Like all modern BMWs, it mounts in the trunk. When I uncovered it, I was amazed at what I saw - there were about a half dozen cables plugged into or bolted bolted to this gigantic red plastic terminal block fastened to the top of the battery. It covered about 2/3 of the top of the battery. I un-did all that, remove the brace running over the battery, and the clamp holding it to the floor of the battery well. It took almost all I had to lift that big mother out, from a very awkward position. It was even harder getting the new one in. After about 30 minutes, the old battery was out, and the new one was in, a re-connected.

Next surprise? It is no longer enough to simply replace the battery. You have the TELL the computer you’ve replaced the battery, and the kind of battery you’ve put in! Apparently these cars some with either 80, 90, or 100 A-hr batteries, and they are either lead-acid, or AGM (Absorbent Glass Mat) construction. It appears the computer custom tailors the charging profile based on the type of battery, the capacity of the battery, and its age! I had no clue! Luckily, my BMW-specific OBD scanner was able to set the new battery type, and make the computer happy. And, the computer told me the old battery had been installed in '11, probably about a year before we bought the car. So, I just installed it’s third battery, at about 140K miles.

The next, bigger problem is the AC stopped working. No cold whatsoever. And, Holy Moley! Car AC systems have changed BIG-TIME! First thing I noticed when I saw the AC was not working was I heard no AC clutch “click” when I turned the AC on and off. Naturally, I assumed the problem must be a failed clutch, or clutch relay, or a rat chewed through another !@#$%^& wire. But, no such luck! Turns out, there is NO A/C clutch, no A/C clutch relay on this car! Instead, the compressor is a variable-displacement swash-plate type, with the displacement actively controlled by the climate control computer! The compressor is spinning ALL the time, and the displacement is reduced to zero, or near zero, when the system is up to pressure. Displacement, and flow rate, are modulated by the computer, based on load. I had no idea such a change had been made in AC systems, but this is apparently how virtually all car AC systems now function. And, of course, they no longer use R134a. There is a newer, more environmentally friendly refrigerant used - don’t recall what it is. This car just missed that change by about a year, so it is still 134a.

So, what’s the problem with mine? Beats the hell out of me! Fuses are fine, the system is properly charged, but it is developing NO pressure on the high side. The whole system just sits there at 30-35 PSI, on both high and low pressure sides, engine running or not, AC on or off. I’m hoping it’s something simple, like a failed pressure sensor, but I have no clue how to test that, since it’s a solid-state sensor, NOT just a simple pressure switch like on older systems. I fear that, for the first time since we bought this car, I may have to take it to an “expert” who has the equipment and knowledge to debug this problem properly.

Ray L.

Ray, it’s not quite time to despair yet. I suggest you join several forums for your BMW. Like Jag-Lovers, hopefully, they will have sussed out the pattern failures and someone will be able to tell you how to divine what is wrong.

Seesh!, hasn’t anybody told you that BMV is German for money? It’s the vampire effect that Erica alludes to that suck you dry over time. It is, indeed fatal to fall in love with German vampires.

I’ve done that, on the only forum worth beans for this model. So far, not a lot of help.

Ours have not been too bad. The '01 325i is now fast approaching 250K miles, still runs great, uses almost no oil, and is, to me, the better car overall than the '07 335i. It even has 195 PSI compression on all cylinders! I do like the extra 150HP on the 335i! But in every other respect, the '01 is better. The '07 has waaaaaaay too much plastic, and not even good quality plastic - that has been the single biggest annoyance on that car.

What I do like about both is they are generally quite easy to work on. Few jobs are really difficult, and that includes the head rebuild I did on the '01 about 18 months ago. Except for limited access to get the exhaust manifold nuts off, it was a remarkably simple rebuild, and did it all myself.

Ray L.

So… “modern” is a somewhat relative term. Your car is 2007. It’s 13 years old, and by considered by many (not me) to be an OLD car. When this car was new, the original iPhone hadn’t been released yet. (July 2007)

My point is, if you were driving a 2020 car, it would be a couple generations newer still! I’m totally intimidated by the prospect of buying a new car, with a touch screen used to control everything, and systems I can’t begin to comprehend how they work, so I’m totally dependent on someone else to diagnose and repair whatever might come up. God I sound old.

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My family has always owned BMW’s and what I have realized is German cars seem to always be a step ahead in terms of complexity or “over” complexity, at least compared to American and Asian cars. But I wouldn’t worry too much, my 2020 Veloster N while somewhat foreign to me, (I am used to working on classic cars).

I found it very easy to work on, everything is thought out and goes together like “Lego’s”. I added a short shifter, and a stiffer lower engine torque bushing (to reduce wheel hop on fwd cars). What im trying to get at it is, while these cars are very complex, they have also been designed to be easier to diagnose and work on. the short shifter required me to get the shift linkage on the transmission, which require taking the battery, ECU, battery tray, and a few other things out, but it went back together like lego, and the whole job took less than 2 hours.

Plus with modern computers you can get what is called a “piggy back” module and/or tune. This intercepts signals from MAP sensors and wastegate controllers and a few others and tricks to factory ECU into allowing more boost, and the oem ECU adjusts and adds fuel accordingly all on its own. So in modern cars, making another 50-100 wheel horsepower is only a couple hundred dollars away and a couple hours of work. For example on my Veloster it starts out with 275hp (crank) in a 3000 lbs hatchback and with a piggy back tune you are able to get just north of 325-335 crank hp and you can unplug the module and you’re back to stock in no time flat.

The other advantage of modern turbo cars is that the engines are often built much stronger than they need to be from factory, mine is no exception, there is a crazy dude in Korea that is making over 500 crank hp out of the 2.0L I4 and on an engine that has only had stronger con rods, a bigger turbo and a beefier clutch put it. Everything else including the transmission is stock, even the ECU is stock but “jail broken”.

So while new cars a complex, they also offer insane amounts of tuning potential as a result.

Sorry for the wall of text, just wanted to share to awesome world of new cars in the aftermarket world.

Sorry cant find the newest clip but here is the a Veloster making 480 whp: https://www.youtube.com/watch?v=HX-Wv7D8eTQ

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A hood friend has had two: she calls it her, “Velociraptor.”


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Ray, if you need to use an A/C vacuum pump, I have one. I also have a 134-a manifold gauge set.

In our time, political speech and writing are largely the defense of the indefensible.

George Orwell


Thanks. I have both. What I’m lacking is knowledge of this specific system, but I think I’m getting there…

Ray L.

Ray you should not tell me stories like this. My newest car was built in 1975 and have no plans right now for a newer one. Mostly due to the type of things you mention with the newer cars plus most of the styling has become so boring or down right ugly . I only really drive around town since I worked for an airline and get to use airplanes for long trips. I worry about how my plan might have to change though with airlines and how things shake out with the pandemic.

68 E-type FHC

I’ve not the slightest !!! My “modern” car is a 94!!! A USA car!!!

I did have an inkling a decade or so ago as to AC and the PCM !!! The donor car was an elaborate 94 Cadillac Fleetwood Brougham. The AC was managed by the PCM. I connected the jaguar management system to the GM compressor. No longer the big old Harrison more like a Sanden?

My GUESS ! Replacing the battery also requires a reset of the AC management in the BIMMER electronics!!

Ouch! Totally unrelated, but this just popped up on the BMW forum. Why you want to make sure you tighten your caliper mounting bolts after doing a brake job:

Ray L.

This actually happened to one of us on the last Oil Leak near Mt St. Helens. A bit of a thrash for sure.

Hey call mine that too! :laughing:

He wont make THAT mistake, again.

My current daily driver, an Audi, has…65 computer/control modules in it!!! Each door has its own computer for craps sake! Fortunately I have the computer interface and software to access and custom code each and every one of those :crazy_face: 580 HP and Quattro is just toooo much fun to pass up.


“This actually happened to one of us on the last Oil Leak near Mt St. Helens…”
Ahhh, yes…
And a fine job of “kink and limp” creating “sorta brakes” to get back home on, as I recall.

In our time, political speech and writing are largely the defense of the indefensible.

George Orwell

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I think I might have warned you about newish BMW’s a few months ago. There is nothing I can do on my '13 except maybe change a tire and even that is more difficult than it should be.

On my kids '07 we had to take it to the dealer’s computer to tell the car computer to turn the back up lights back on.

I am really impressed with all the info you gathered on your car’s system. Where did you find it?

If I lived in CA, where there is no rust, I would be looking for a good used 1967 auto.

are they the “no reinstall, only replace” type of bolts. ?

Can you tell me again what make and model scanner this is?

Lots of information in the forums, and a few websites that have free on-line access to the BMW factory manuals.

The problem turns out to be nothing unusual - there is a leak, and the refrigerant is low enough the pressure switch does not allow the system to turn on. Next time I go to town, I’ll get some UV dye, find and fix the leak, then re-charge the system. No big deal after all. The clutchless/variable displacement compressor is the only really new thing. And, like I said, they are now pretty much the norm on most cars.

Ray L.