D

Alright I'm coming up to this part of my build and figured this is the cleanest thread to wrap my findings up in. My car is still on the lift but is getting closer to the first fire up and I'm determined to have the AC system working as the Porsche gods intended before it comes off the lift. Firstly - summing up my understanding of the AC Relay located behind the rear seats on Relay Support 2 (in a dumbed down version so that even I can understand it): A relay is just a physical switch controlled by an electromagnet (coil). On the AC relay the coil is controlled by the DME. Pin 86 should see 12v+ when the key is on - this is the 12v+ side of the coil. Pin 85 is the DME side and will see this same voltage potential between it and ground since it's just the other side of the coil. Pin 85 goes to the DME at pin 62 where the DME grounds it when it wants to call for the compressor to activate. This closes the coil circuit and activates the physical switch in the relay. Pin 30 is the 12v+ side of the physical switch and will show 12v+ when the ac pressure switch (located in the front of the car under the passenger side cowl) is happy and fuse D6 is happy. Pin 87 is the compressor side of the physical switch. When the switch is closed (DME calling) the 12v+ is provided to the compressor and in a OEM porsche setup is grounded through the compressor housing to the engine activating the compressor and giving us cold luscious air. The Relay can be tested by shorting pin 85 to ground which activates the coil and closes the switch. Now... I have a buddy who works for Porsche and before he was promoted to his current position he worked as a tech back in the 996/997 days. I asked him if he could provide some insight on what parameters the DME needs to see to call for the compressor. I'll sum up our text conversation below: ME: Long time no chat. Searching for some info and this might be a long shot but... I'm wondering exactly what inputs the 996 DME needs to see to decide to power the AC compressor. I've got all the wiring diagrams and understand that bit but we're (I'm in a group who are looking for the same info) not seeing the DME provide a ground to pin 62 (comp on) which signals the ac relay to send 12v to the compressor. Does the DME need more than rpm? Throttle position, coolant temp, phase of the Moon...? Trying to have the Porsche oem side run the gm ac compressor independent of the gm ecu HIM: Hey man! It has been a while! Looking at the diagram now, it looks like there’s a signal line from the DME to the climatatronic that is PWM. And information from the Instrument cluster to the climatronic cu for vehicle speed, engine speed and immobilizer. ME: So if the cluster is showing outside air temp, coolant temp, rpm, and speed correctly it will report these values to the Climatronic via a PWM signal. Then as long as the intake temp (is this the iat or the ambient via the sensor on the front of the car do you think?) is over 3.5 the DME will call for the compressor to activate provided the Climatronic is asking the DME for it? HIM: Yeah, so as long as the instrument cluster is happy with all of those, the data will be sent on the k line to the climatronic. It’s been a long time, but I think the k line is a LIN bus. Input to the climatronic on A1 marked k lead in the diagram. Intake air sensor I think is on the intake pipe to the engine, it should go from there? I think. Haha. Little bit of a puzzle to figure out how to trick it! ME: Any mention of the DME not calling the compressor because of the throttle position? I.e. No ac at wide open throttle? HIM: No mention in the SIT book, but I know 997 and later cut the ac compressor under full throttle. __________________________________________________________________________________________________ So - if the relay is working correctly and has 12v+ where and when it needs it, and the cluster is showing speed, rpm, coolant temp, and outside ambient temp, and the DME is seeing the (assumed) Intake Air Temp, and this intake air temp is above 3.5C, and the climatronic is calling for AC (snowflake on), the DME should ground pin 62 which closes the coil on the relay which provides 12v+ to the compressor turning it on and blessing us with cold air out the vents on to our hot, sweaty face. Ohhh that's nice. I am not certain that TPS reporting correctly to the DME is required for the system to function. My car is a few weeks out from being able to test any of this but I thought I'd post here so someone else can sanity check my logic or demonstrate that the above does or does not work. Hope this helps someone! Cheers!