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Wayne 962's Avatar
959 A/C Control Unit Electrical Puzzle - Need some help (10-26)

Okay, so Porsche-O-Phile (Jeff Novak) was kind enough to donate to me a used 944/951/968 air conditioning / heater control unit that I will be picking up from him in Long Beach sometime tomorrow. So, I decided to check out the electrical diagrams from the 944/951/968 books that I have and compare them. Seems the units are very similar, but the wiring harnesses seem to be wired slightly differently. The good news is that I doubt that plugging the 944 unit into the 959 will cause major electrical damage because all of the lines seem to match up. Some of the sensor plugs are reversed, but those are non-polarized anyways, so it shouldn't matter.

Here's a spreadsheet that I did comparing the two electrical diagrams:



The items in light blue are where the signal wires are similar but not exact. Like pin A5 connects to pin 2 of the footwell flap sensor on the 959, and pin 1 of the footwell flap sensor on the 944. The polarity of the motors is reversed, so I think that this is correct for the 959. The 959 uses a unique heater assembly that may be wired backwards from the 944 one. Seems that way from the electrical diagrams.

Items in yellow are a direct match. Items in green are not used on the 959 but appear to be used on the 944.

Basically the main difference seems to be that the 944 has some type of third flap and sensor that the 959 doesn't have. This is called the "temp mixed flap" on the electrical diagrams, and I don't know exactly what this does. I'm thinking that it's a flap that meters fresh / cold air and hot air and cycles back and forth depending upon how much is needed. Perhaps 944 experts could chime in on that.

All the other sensors seem to match up. The two items in red are the oddballs. The 959 has a water cycle valve that is ON when there is no power to it (a safety mechanism so that the car fails to full heat when the system fails - better to sweat than to freeze to death in the winter?). This valve is connected to pin A11 on the 959. Pin A11 on the 944 connects to the motor on the mixed temp flap. The 959 has a vent sensor that connects to pin A1. On the 944, pin A1 connects to the mixed chamber temp sensor, so those seem to match, sortof.

The big question is with pin A11 - will the 944 work the way that the 959 one does? That is a good question, one that I'm not sure of. The 944 diagram seems to indicate that +12V would be applied to this motor terminal at one time, and then possibly switch to the other terminal (A10 which is not used by the 959), to reverse the direction of the flap. If this effect is equivalent to the cycling effect of the heater valve then it might work. Then again, pin A4 of the 944 unit seems to do what I would think that pin A11 would do. I'm not sure. Any thoughts or suggestions are welcome.

Here are the links to the files:

http://forums.pelicanparts.com/uploads11/944-heating_elec_diagram.JPG

http://forums.pelicanparts.com/uploads11/959-heating_elec_diagram.JPG

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series 101 Projects for Your Porsche 911 How to Rebuild & Modify Porsche 911 Engines 101 Projects for Your Porsche Boxster & Cayman 101 Projects for Your Porsche 996 / 997
Coming in 2014:
101 Projects for Your MINI Cooper
Old 10-26-2007, 11:20 PM
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Another weird thing is that there are some wires that seem to have been added to this unit after the fact. Perhaps the factory hard-wired some of the 944 units to make them compatible with the 959 unit. I will have to open up the 944 unit tomorrow and compare firsthand the differences between the two...

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series 101 Projects for Your Porsche 911 How to Rebuild & Modify Porsche 911 Engines 101 Projects for Your Porsche Boxster & Cayman 101 Projects for Your Porsche 996 / 997
Coming in 2014:
101 Projects for Your MINI Cooper
Old 10-26-2007, 11:22 PM
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Got the unit from Jeff this morning. Two out of three circuit boards are identical, the third is different. Not looking good for a direct replacement. I will post pics shortly.

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series 101 Projects for Your Porsche 911 How to Rebuild & Modify Porsche 911 Engines 101 Projects for Your Porsche Boxster & Cayman 101 Projects for Your Porsche 996 / 997
Coming in 2014:
101 Projects for Your MINI Cooper
Old 10-27-2007, 11:59 AM
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Can you swap the two identical boards with known good ones? Then you could at least narrow down the problem to the "unique" board if it doesn't go away.
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Old 10-27-2007, 01:58 PM
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Quote:
Originally Posted by Gogar View Post
Can you swap the two identical boards with known good ones? Then you could at least narrow down the problem to the "unique" board if it doesn't go away.
Yes, I would like to try that, but they are hard-soldered together. That would be difficult to do smoothly.

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series 101 Projects for Your Porsche 911 How to Rebuild & Modify Porsche 911 Engines 101 Projects for Your Porsche Boxster & Cayman 101 Projects for Your Porsche 996 / 997
Coming in 2014:
101 Projects for Your MINI Cooper
Old 10-27-2007, 05:09 PM
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