Réparer son module de climatisation automatique

[Paul]

Le problème

I have a 1996 BMW 328is with about 120K miles on it. A month or so ago, my climate control computer began to turn off intermittently -- but it would leave the AC compressor engaged. The AC light on the AC button would remain lit, as would the air vent button (whichever one had been last selected), but the display would be dark, and hitting the buttons did nothing. So, you would have the AC compressor on, but no fan.

The problem seemed to be temperature-dependent, with high temperatures resulting in more cases of intermittent "off" events. Today, it stopped working altogether. Nothing about this is the Bentley manual. I checked with the folks over at BMW Central, and they all seemed to agree that this was a problem with the Climate Control computer. One of the board contributers pointed me to a couple of other boards where the problem had been discussed and a fix brought up: the The Unofficial BMW Bulletin Board and Bimmerforums.com. Both suggested replacing a capacitor on the board of the climate control computer. Being short on money, but having some spare time and no fear of screwing up (I figured if I had to replace the computer anyway, might as well give this a try first), I decided to give it a try. This is my story.

Le matériel nécessaire

Voici la liste du matériel à avoir pour réparer son module de climatisation automatique...

* Small Philips screwdriver

* A couple of small flathead screwdrivers

* Needle-nose pliers

* Wire Cutters

* Une lame de rasoir

* Radio Shack 30W Soldering Iron

* Desoldering braid

* Solder

* Un condensation : 0,47 Microfarad - 63V non polarisé.

A word about capacitor sizes

When I repaired my climate control computer, I used a 0.47 uF capactitor. Now, since I've posted this page, I've had a number of people contact me and tell me that I was wrong, that it was a 47 uF capacitor instead. So many, in fact, that I managed to convince myself that they were right, and I updated the page to reflect my newfound wisdom. However, I've had other people say, no, 0.47 uF is right. Anyway, I did a little checking, and I did indeed use a 0.47 uF capacitor.

So, if you used a 47 uF instead of a 0.47 uF, does that mean your computer is going to blow up? I don't think so. I am not an electrical engineer, but the capacitor here is being used as a filter -- I would venture a guess that as long as the capacitor is larger than a certain size -- say, 0.47 uF in this case -- the filter shouldn't really care too much if the capcitor is bigger, even if it is a lot bigger. Now, if this were a resonant circuit, clearly that wouldn't be the case, but with my admittedly limited understanding of filtering circuits, even if the right size is 0.47 uF, a 47 uF capacitor should work as well.

Like I said, this is outside of my area of expertise, so if anyone really KNOWS the answer here, by all means let me know.

Why a tantalum capacitor you might ask? Well, the directions on the other boards suggested any type of capacitor of the correct size (0.47 microfarad) and voltage capacity (at least 35 volts) would suffice -- the tantalum was all Fry's had in that size the day I went, except for electrolytics, and you've got to get the polarity correct on an electrolytic or it will blow up, so tantalum it was. Note -- actually it has been brought to my attention that tantalum caps are polar as well, but I've also been told that in this filtering mode, polarity of the tantalum cap doesn't matter. I didn't know Ta caps were polar, so either polarity doesn't matter, or I got lucky and did it right the first time!

On the other hand, if you are dying to use a polar cap (Ta or electroytic) and want to make sure that you get the polarity correct, here's some info from other (more knowledgeable) owners that have fixed their climate control computers:

"I went to Radio Shack and found that the only 47uf caps they had were polarized. I figured that one of the connections was to the ground plane, so I scratched off the negative leg of the other blue cap to the left (Just above your capacitor text in the blow up view of the square cap). I did a quick continuity check and found that the left pin (when viewed in the same orientation of your picture) was in fact the ground pin. In any case all this means is that a polarized capacitor may be used, and should be oriented with the negative pin on the left. Another way to look at it is the capacitor should be oriented 180 degrees from the other blue capacitor."

"Another happy "customer"! Worked like a charm. My only deviation from your instructions was to paint over my work with some clear nail polish to replace the epoxy I scraped away. P.S. I used an electrolytic cap, because that's all that Radio Shack had, but I think I figured out the polarity of the one I was replacing and matched it up. Either that or polarity doesn't matter." "Just wanted to say thanks for putting up the E36 fix. Completed on the GF's 323, she thinks I'm a genius (thanks to you I only saw one issue: tant caps are quite polar. Regardless since the cap is used as a filter: polarity (or non-polarity) doesn't matter"

"p.s. I used a 35v cap instead of a 50 and it seems to do the job."

L'opération

First thing you have to do is get the Climate Control computer out of the car. To do that, first you have to get your Multi-Function Display out of the way. This is very easy; put your hand into your sunglass or whatever slot, put your fingers through the hole in the top of the holder, and push the MFD forward to pop it out :

Oui, je réalise que ma voiture est sale. Slide the MFD forward, pop it out, and let it hang out of the way :

Now, just reach into the MFD's slot and push the Climate Control computer forward with your fingers :

If you flip the computer over, you will need to remove the two wiring harnesses that connect it to the car. The Black one just pulls straight out; the other one, you flip the white lever and the connector pops out. You gotta love these BMW connectors...

Now that it's undone, get it over to your workbench, and take out the four screws on the back. They have a Philips head, but the screws are in tight and are pretty soft, so I ended up using a small flat-head screwdriver to get them out. You mileage may vary :

With the screws out, you need to pop the tab on one side out with a screwdriver, and then squeeze together the tabs on the other side to get the front of the computer off :

Next. pop out the circuit board with the display on it :

Next, remove the fan for the AC control (I think that is what this is, not completely sure). You might want to do this before you get the front of the computer off, because the screws again are small and soft and you may want to have more structural soundness to work with. It worked for me this way, though, so it's up to you. Remove the connector by pulling it straight out :

Next, you need to slide the main circuit board out. This is both easier and harder than it looks. There are two tabs on either side of the housing that hold the board in place:

I got one side popped out easily by pushing the board forward from the back with a screwdrive while applying some pressure to the housing. The other side was tougher -- I had to use one screwdriver to pry away the housing while pushing from the back with the other one :

It wasn't that hard once I figured out what to do. Push the board forward from the back until it slides out easily, and you end up with this :

The next thing you want to do is locate the proper capcitor on the fron of the circuit board -- it's the square blue one closest to the big blue connector :

If you flip the board over, you can locate the two pins for the capacitor in question by first getting into the general area, then looking for a small surface-mount resistor -- the two pins for the capcitor are right next to that surface-mount resistor (the screwdriver is pointing at the resistor) :

I used a razor blade to scrape away some of the laquer coating from the pins, then used my soldering iron and a desolder wick to remove the solder from the pins. Once the solder was removed, I was able to easily pull the capacitor out from the front with my needle-nose pliers (I had to rock it a bit to break away the lacquer, but it came out pretty easily). I chased the holes with a piece of stiff wire (probably an old guitar string) of the right diameter, then inserted the new capacitor. A little solder later, I snipped off the leads, and the new cap was in place :

Reassembly was the opposite of disassembly, as one might expect -- everything went back together very easy. I popped the repaired computer in the car, and it worked perfectly! I'll let you know if it continues to work, but it sure looks good so far.

Total time to repair: About 90 minutes, but now that I've done it once, I could probably do it again in under an hour easy. Total price: 49 cents, plus tax -- I already had everything else. Even if you have to buy everything from scratch. it would still be a LOT cheaper than getting a new or rebuilt computer!

NOTE

If the images don't appear in your browser, I don't know why that is happening -- the pictures are here, and seem to work with most people. I've tested the site with IE and Mozilla, both work fine, but I still have some people saying they can't see the pics. Sorry about that. If you can't see the pics, you can download a ZIP file containing all of the pics here.

Okay, I guess I lied. It really wasn't working with Mozilla. It work with Mozilla now, so you Netscape users shouldn't have any trouble with the pics.

Update

Many moons later (about 25K miles), this fix is still working fine. I've heard from >50 BMW owners from as far away as Russia and Australia, and the fix seems to be working for all of them.

I've also heard from people in the UK that there is some "bloke" (would that be the right English term?) selling a PDF of this webpage on eBay. Now, I'm not about to hire a team of international lawyers to go after this guy for copyright infringement or anything, but what people can do is spread the word that the info in his eBay auction is available FREE here, and at other places that have mirrored this site or copied it wholesale (with my permission) to another site.

Another Update: July 25, 2008

Hard to believe it's been almost 4 years since I last updated this page. Don't really have anything new to report -- still have the car, I just went past 186,000 miles Wednesday -- and the climate control computer is still working flawlessly. Fan stopped working, so had to replace the resistor pack last weekend, and the radio stopped working -- please don't bid against me on eBay, I'm trying to get another stock head unit cheap! -- but the car still runs good. I am still getting emails, over 200 by now from many, many different countries and reqions of the U.S., and people seem to be having success with the repair. I guess as long as E36 BMW's are still out there and being driven, I'll keep the page up!

Happy driving -- I guess I'll update in another couple of years, or sooner if anything new comes up!

Merci à Richard Swope pour cet article écrit en anglais et visible sur son site internet.