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Wayne 962 Wayne 962 is offline
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959 Project: A/C & Heater Troubleshooting... (10-25-07)

Hi all. I was able to do some more work on the 959 today. Since yesterday's drive was unbearable due to the heat on full blast, I decided to tackle the problem today. The factory manual states that if any of the 3 temperature sensors are disconnected, then the heater will go to full blast with no method to turn it off (I guess that's a warning). If one of the sensors shorts out, then it goes to full cold. So, I took the thing apart today, and tested all of the temp sensors, and they all tested fine. Then I went to the fuse box and tested the heater valve coil located in the engine compartment - that tested fine too, although it wasn't getting power like it should be, which indicates that the control unit wasn't sending the right signals. So, my conclusion was that the control unit was faulty.

Usually with older electronics like these, the big problem is what is called a cold solder joint. I talk about this in my DME repair article: The solution is to reflow the solder. Well, on this particular unit, that's very difficult, and it looks like someone already tried to repair the unit that way anyways (which is always a bad sign). Basically the insides of this thing look awful (see pics below). So, what I'm going to try to do instead is adapt a 944 AC control unit to work in this car (if an original 959 is not available from the factory). The control units look exactly the same on the front and back - I think the main difference is the outer case of the unit. I would be surprised if Hella made a special unit just for the 959 - seems unlikely. Also, all of the sensors seem to carry over from the 928 & 944 cars. Makes sense that it would be the same unit - they seemed to have used it on the 944 up through the 968 cars, and including the 928. So, there's hope in case an original one is not available.

Here's the photos from today...

First shot is a daily 959 shot. Looks nice huh?

Here I am using four small screwdrivers in lieu of the radio removal tool (coming tomorrow). When I inserted the screwdrivers and pushed on the rear of the unit with my hand, it simply popped out.

Here I am pulling the unit out. It says in big letters "code protected", and I certainly don't have the code, and don't want to have to chase down a code for a 20-year old stereo, so I left it plugged in. Weird though, as I have disconnected the battery for several days, and it didn't ask me for a code. Still, I'm not going to temp fate - maybe there is a switch built into the connector that activates the "code required" feature. Actually, that would make a lot of sense, as disconnecting the battery for service is pretty common. Although as a theif, I guess you would just cut the harness. Oh well, who knows...

Front of the radio. This was standard equipment on the 959. Lots of buttons on the front, I'm not sure what many of them mean (ARI, CPS?, M, U?). I would love to get a manual for this, anyone know of where someone might have one? Maybe downloadable on the net? I'll have to check...

Here's the side of the radio, that looks like a 911 part number there.

The top with a bunch of information, most of it in German, of course...

In the center of this photo is the screw you need to get to in order to remove the AC control unit. I think the outer housing is what makes this unit unique to the 959. Hopefully a plain vanilla 944 control unit will work.

Prying off the plate. I'd hate to snap this - I used a lot of caution, as I always do tugging on things on this car...

Old 10-25-2007, 10:33 PM
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