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BMW E36: Repairing the Climate Control Computer

The Problem

I have a 1996 BMW 328is with about 120K miles on it (yeah, I know, a lot of miles, but hey, it's the ultimate driving machine, not the ultimate sit in the driveway machine). 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. 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.

The Tools

Here's a list of the tools that I used in repairing my climate control computer:

• Small Philips screwdriver
• A couple of small flathead screwdrivers
• Needle-nose pliers
• Wire Cutters
• Razor blade
• Radio Shack 30W Soldering Iron
• De soldering braid
• Solder

 

The Parts

Here's a list of the parts that I used in repairing my climate control computer:

• 0.47 microfarad 50 volt tantalum capacitor, about 49 cents at Fry's Electronics

 

A word about capacitor sizes

When I repaired my climate control computer, I used a 0.47 uF capacitor. 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 capacitor 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 electrolytic, 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 electrolytic) 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. If for some reason it blows again, I will follow up with an email. "

The Repair

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:

Yes, I realize my car is dirty. 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 got to 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 screwdriver 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 capacitor on the from 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 capacitor 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 lacquer coating from the pins, then used my soldering iron and a de-solder 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, and 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!