This is how I installed a relay between the AC control unit (AAC) and the aux water pump. This does not apply to 1998-2001.
Why should you consider doing this?
Factory design delivers 12-14V to the aux water pump directly from the AAC unit. By installing a relay, you are protecting the AAC unit in case if seized pump draws too much current or shorts the circuit. This results in burned out circuit board requiring replacement of the AAC unit.
Parts and instructions:
1) Relay with a diode is necessary. You can damage the AAC unit if you install a relay without a diode! I decided on Bosch-Tyco horn relay that has Tyco Part no V23234C1001X008. You can use other relays but make sure diagram looks like the one I posted below. P is the pump. The second terminal 87 is not used on this setup.
2) Relay connector. Aux pump cable has white and brown wires. Cut this cable and solder terminal 85 to ground (brown), 86 to 12v from AAC (white), terminal 87 to battery (+) terminal (fused). Label the wires before installing to the car so that you won't make a mistake. Terminals are marked on the relay.
3) Fuse and fuse holder. Install this between the battery (+) terminal and the terminal 30 of the relay. MB/VW pump draws well below 1 amp. So, 1-3 amp fuse should be sufficient.
4) Extension wire (to the battery), soldering iron, heat shrink wrap. Solder every wire connection and heat shrink wrap the joint.
Why should you consider doing this?
Factory design delivers 12-14V to the aux water pump directly from the AAC unit. By installing a relay, you are protecting the AAC unit in case if seized pump draws too much current or shorts the circuit. This results in burned out circuit board requiring replacement of the AAC unit.
Parts and instructions:
1) Relay with a diode is necessary. You can damage the AAC unit if you install a relay without a diode! I decided on Bosch-Tyco horn relay that has Tyco Part no V23234C1001X008. You can use other relays but make sure diagram looks like the one I posted below. P is the pump. The second terminal 87 is not used on this setup.
2) Relay connector. Aux pump cable has white and brown wires. Cut this cable and solder terminal 85 to ground (brown), 86 to 12v from AAC (white), terminal 87 to battery (+) terminal (fused). Label the wires before installing to the car so that you won't make a mistake. Terminals are marked on the relay.
3) Fuse and fuse holder. Install this between the battery (+) terminal and the terminal 30 of the relay. MB/VW pump draws well below 1 amp. So, 1-3 amp fuse should be sufficient.
4) Extension wire (to the battery), soldering iron, heat shrink wrap. Solder every wire connection and heat shrink wrap the joint.
