OBC Parked ventilation retrofit


Picture soon

Cost: ?


Time: 2-4 hours

Procedure courtesy of Klaus Reichenecker & Bob Knight.

Parked ventilation is a special function of the OBC. It allows you to have the ventilation running while the car is parked and the doors locked. This is handy for these cold winters or hot summers. The drawback is the older E34 are not equipped with this function, even though it is expalined in the owner manual.

So you've read everything you can about parked car ventilation. You've futzed with your OBC timer button in myriad combinations, hoping that just once, once, you'll see "VENT". You've read about other's frustrations and successes. OK. I'll stop.

if you've been lusting after this feature, you'll be happy, too. Because it is NOT rocket science. It takes about $15 worth of parts plus I'd guess it'll take about 2-4 hours of your time depending on how familiar you are with how to remove and replace the: glove box, glove box cover, passenger front (by firewall) foot venting and: associated cover, radio, OBC.
You also, I believe, should have a rudimentary understanding of or facility in DC circuitry, Ohm's law, counting and soldering. The second may not necessarily be a requirement. You should know how to read Bentley wiring diagrams, if only to understand how this fits in.

My disclaimer: I do not warrant that these instructions are correct. I furthermore assume absolutely NO responsibility, fiscal or otherwise, if you decide to do this procedure and end up frying something in your car.


How does it work? (Thanks to Ray)

You call it "fresh air feature". Assuming you are referring to what I know as "parked car ventilation":
1) the feature is not temperature dependent. It will work winter or summer.
2) the feature will not activate unless the ignition key is in position "1". I.e., the ignition must be off, but your ignition key must be turned on suffieiently to unlock the steering wheel.
3) in early E34s, in all early 750iLs, and in all later E32s, the OBC's "timer" function key has a dual function, which was activated by "toggling" the timer button to the left or to the right. Toggling to the left activated an elapsed time function, but is not in effect on E34s post-1993 (except in the 540i), even though it is described in the owners manual. (Don't ask me why.) Removing the elapsed time function allowed the "timer" function to store 2 parked car ventilation start times.
4) when you press the timer button repeatedly, does it scroll 3 messages? The LED should display "OFF Vent", "--.--t 1am", and "--.--t 2am" in succession.
5) there is a way to test whether this feature is working: With the ignition key in position "1", press the timer key to obtain the message "OFF Vent" in the LED display. While continuing to hold the timer button (and the "OFF Vent" message in the display), press the "temp" button. Hold both buttons for 5 seconds, the release both buttons. The LED should now display "1N Vent". Next, press the "S/R" button. A motor should whir (opening the fresh air vent)and the blower should start. Pressing the "S/R" button again will turn off the ventilation. Or, you can now remove the ignition key and either press "S/R" to turn off ventilation, or allow ventilation to continue until is shuts itself off after 15-20 minutes (I forget which).
If the test procedure doesn't work, but your heat/ac system works and the OBC works otherwise, there is probably a broken or corroded connection between the OBC and the heating/ac controller board. It mught correct itself if you remove and reintall the OBC's and the heating/ac board's connectors. If this doesn't cure it, forget about "parked car ventilation"!



  • Phillips screwdriver.
  • hex key to remove radio
  • Needlenose pliers
  • Diagonal cutting pliers
  • Soldering iron
  • Good tin/rosin solder
  • Supply of 14 AWG stranded wire. I'd recommend red, black and green insulation
  • "vampire" wire connectors for connecting a wire to an existing wire without cutting the existing wire
  • Radio Schlock 275-218 DPDT relay (an SPDT will work also)
  • An ohmmeter (DVM, preferably)

Before you start work, REMOVE THE NEGATIVE BATTERY TERMINAL FROM YOUR BATTERY and ensure that your ignition is in the off position (remove the key). If you don't, you may only blow a fuse. But you could possibly blow your OBC. Or something else. Given Murphy's ubiquity and caprice, you'll likely nail something (or things that are) very expensive.

1. Remove the glove box and set it aside (see Bentley's, 513-4)
2. Remove the glove box cover (three Phillips screws: as you look towards firewall, right, left near glovebox latch, left near rear), disconnect the light and the switch. Don't break the plastic tabs at the rear. Slide it towards you. Set it aside.
3. Remove the ventilation duct and cover (see Bentley's 640-12, figure 25). Set the assembly aside.
4. Remove the radio.
5. Remove the OBC by pulling the lever at the left rear of the OBC towards you. The OBC will pull out from its connector (26 pin IDC)

At this point, you have access to the OBC connector and its associated wiring. The wires of interest on the OBC connector are connected to pins 21 (black/white or black/white/yellow, my car was the latter) and 9 (red/yellow, direct from fuse f20). The Bentley calls out OBC pin 14 instead of 21. Do not believe the Bentley. It is wrong. Pin 20 is likely not connected, but if it were, it serves as a timer trigger for heating. But I digress.

My car was built in 7/90. It doesn't have a sword, it has a resistor pack. It also has the newer HVAC control. I only paid attention to wiring diagrams EWD-125 to EWD-127. A cursory glance at the '93 wiring tells me that this procedure MAY work with the 1993.

Now for the relay installation:
6. The OBC connector at the rear of the panel slides to the rightfor removal.
7. Cut the cable tie holding the OBC wire bundle to the green cover for the IDC connector. Take care to not cut any wires.
8. "vampire" a 15" or so wire (I used white) onto the pin 21 wire. Call this wire w7.
9. "vampire" a 15" or so wire (I used red) onto the pin 9 wire. Call this wire w8.
10. Find a power wire that's always on that has sufficient amp capacity (at least 15 amps). There are a number of candidates relatively nearby. Carefully connect this wire (call it w3) to the hot wire and run it to where w7 and w8 are.
11. You will see a 3 pin molex connector bundled with two 4 pin molex connectors mated with only one green wire connected to each. Disconnect the four conductor connectors.
12. One of these green wires goes to the blower switch, position 2. The other goes to the resistor pack. Ring 'em out. The latter should always have resistance on it relative to ground. Call this wire w5. The former will show open if the switch is in position 0. Call this wire w1. I used green wires for w1 and w5.
13. You should now have five wires, labeled (you did that, didn't you?) w1, w3, w5, w7 and w8. Grab the relay and figure what pins
are which. The wire numbers that have been assigned correspond to the connector numbers on the relay. If you wish, sleeve them with
heatshrink tubing of an appropriate diameter before connecting them to the relay. Solder them all into place. Ensure you haven't
inadvertently bridged any connectors. Flow the solder! You don't want any cold solder joints here. Trim excess strands of wire away
from each solder joint. Slide the heatshrink down flush to the relay and shrink it, if you used heatshrink.

You now should have the parked car ventilation feature in place, once things are reconnected.
14. Reassemble the OBC IDC connector and its cover with a new cable tie. The "junction" part of the cable tie should be on the same
side as the socket side of the connector.
15. Slide the OBC connector back into place. Ensure that it "clicks" into place.
16. Ensure that the OBC removal lever is set fully towards the outside of the dash. Slide the OBC into place. If you feel anything but mild resistance, DO NOT FORCE THE OBC INTO PLACE. Check out why its not going into place.

At this point, you can test the installation. Reconnect the negative battery cable. Turn the ignition to position 1. Press the OBC timer
button. You should see "VENT" on the OBC display. If you press the "S/R" button at this point, the two LEDs on the timer button should
flash and the fan should come on. If you press "S/R" again, the fan should come off. The OBC display will change to "VENT on" and "VENT
off" on the button presses. Turn the key to position 2. You should be able to operate the fan normally with the fan switch. Turn the switch
to position 0 and then to position 1. Press the OBC timer button twice. You should see something like "00.00amT1" on the OBC display. Enter a time using the "powers of ten" buttons. "S/R" should set the time and the left timer LED should illuminate. Hitting timer
again will bring up the t2 display, which is programmed similarly. "S/R", after a timer has been programmed, toggles its state (eg. if it's enabled---LED lit---and "S/R" is pressed, the timer is disabled). The timer turn-on time is not affected by the disable. When you're done, disconnect the negative terminal of the battery.

If the above doesn't work as stated, you've got a problem. It is very likely a wiring mistake. Disconnect the negative terminal of the battery, if you have not done so. Go through the instructions again, checking your work. Check fuses, especially f20 and the fuse for the wire you tapped for the other "always-on" 12v. Check that the relay contacts are not touching each other or something else. Be meticulous.
Be patient. If you get frustrated, walk away from it and do something else. Have someone else check your work. You'll eventually find out
what's wrong.

So you've tested things and you're satisfied it works. Time to button things up:

17. Replace the radio.
18. Replace the ventilation duct.
19. Replace the glove box top cover. Ensure that you reconnect the switch and the light.
20. Replace the glovebox.


Notes from Janis S:
I would like to add some comments about this article that can make life
easier for another people trying to repeat the procedure described above.

1. I used standard automotive 12V 30A relay, which has different pin numbers. So in my case they were:
w1 - 87a
w3 - 87
w5 - 30
w7 - 85
w8 - 86
2. There is no need to remove radio and OBC, you can use the original BMW park heating/ventilation relay box connector to connect wires w7 and w8.
This is an unused 8-pin connector (shown in the picture) hidden somewhere near to OBC. Connect red/yellow wire to w8 and black/white/yellow wire to w7.
3. "Always on" power w3 can also be taken from the same red/yellow wire, it is protected by 30A fuse that is more than enough for blower, so wires w8 and w3 can be connected together.

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