If you've ever had issues with your blower switch getting hot on any speed setting, this is a very good (but time consuming) solution. In my opinion the factory switch is inadequate for the amount of current it has to pull, and bypassing it with relays will save you from burning up/melting your switch and surrounding plastic.
Needed Items:
Step 1: (no photos)
Remove the dash bezel, Passenger's side knee bar, radio, center console, and everything in between so you can get behind the HVAC panel. Unhook the large wiring harness from the blower switch, and using your paperclips, remove the large solid black wire from the connector. This is the main power supply wire, and it will be hot whenever the ignition is on (verify with multimeter). Strip a section of wire 1-1 1/2" from the end (being careful to not cut the wire through). Wrap and solder a new 12-gauge wire around it, then cover with heat shrink tubing (I covered mine in 3 layers to ensure proper insulation). This will feed power to your relays, and run the fan. Lay this wire to the side and tap into a ground connection (I pulled from the power outlet), pull both wires through the right side under the dash.
Step 2: The painful part - disconnect the wiring harness from the resistor pack, and cut all four of the wires about 4-5" from the connector. Disconnect the black/red wire going to the blower motor and get ready to tuck it away later. You won't need it anymore. Take the end you cut off inside and start preparing your wiring harness.
Step 3: Prepare your relay harness (read-up on how a Bosch-type relay works if you don't already know. It will greatly help your understanding of this process)
Remove all pin 87A wires with your paperclip except for the last one on the right end (whichever end you want to decide is the right end).
Pull all Pin 85 wires to the left side, strap them, and terminate them all to a 10 ga male spade connector. This is the ground connection for all the relays, and will connect to a female spade connector on the ground wire you pulled from under the dash.
Pull all Pin 87 wires to the left side, strap them, and terminate them all to a 10 ga male spade connector. This will terminate to the ignition hot wire, coming from back of blower motor switch.
Relay 1: (from left to right, for speed 1)
Pin 86: terminate to a female spade connector - it will later plug into the Black/White wire coming from the blower switch. Be sure to label it as such.
Pin 30: solder or connect with spade connectors to Black/White wire from resistor pack harness
Relay 2: (speed two)
Pin 86: Black/Blue wire from blower switch
Pin 30: Black/Blue wire from resistor pack
Relay 3: (speed three)
Pin 86: Black/Green wire from blower switch
Pin 30: Black/Green wire from resistor pack
Relay 4: (full speed)
Pin 86: Black/Red wire from blower switch
Pin 87A: Black/Red wire from resistor pack
Pin 30: Terminate to a female spade connector and set it aside - this will plug in to the blower motor where the black/red wire was previously.
Background - Each time the fan switch selects a speed, the current travels through either the white/blue/green wire into the resistor pack, and then out the red wire to the blower motor. When on high speed, the switch connects the red blower motor wire directly to the supply wire. Pin 87A on the fourth relay allows the current to pass through the other three relays and on to the blower motor, until relay 4 is activated, and it connects the blower motor directly to the 12 volt supply wire through pin 87.
Step 4: Fixing your AC clutch switch
For reasons unknown to me (perhaps a smarter member can chime in here) the power supply wire for the AC clutch switch is connected to the black/white wire coming off the blower switch for speed one. Unless you install an SSR here, the AC will only work on speed one. Somehow, changing the circuit by installing relays will keep this from functioning as intended.
With the right knee bar removed, it should be easy to reach behind the HVAC panel and remove the connector going to the AC and Recirculation switches. Remove pin 2 (black/white) with some paperclips (this one is a bear to get out) and attach a new, small diameter wire to it. When 12 volts is applied to this pin, it will engage the AC clutch.
I decided to solder a new wire onto the pin, but you can splice a wire on the end just as easily.
Hook up your solid-state relay to your harness in this order. It will require pulling a few wires out of their terminals to solder them on and do it right.
Input +: Pin 30 on relay four, going to the blower motor + connection
Input -: Any of the Pin 85 wires you have, it just needs to be grounded.
Output +: Any of the Pin 87 wires you have, this supplies 12 volts to the AC switch when activated
Output -: The wire you connected to pin 2 of the AC switch harness
Background: When the fan is running, it is only feeds approximately 3 volts on speed 1, 5 volts on speed 2, 10 volts on speed 3, and 12-14 volts on high speed. If you connect your AC switch wire directly to pin 30 of relay 4, the AC will only switch on using speeds 2-4. Speed 1 does not provide enough voltage to engage it. You can, however, turn your fan to a higher speed, switch on the AC button, then back your fan speed back down to 1 and it will remain engaged. If you connect your AC switch wire directly to an ignition-fed + connection, you will have to manually turn off the AC clutch every time you turn the fan off. Otherwise, it will continue running. The solid-state relay will operate at these low voltages, and we are using it to make sure the AC clutch will only be engaged when the blower fan is running.
Photos:
Here is my completed harness. Right side going to the resistor pack, left side plugging into the blower switch wires.
Other side
Plugged in.
While testing everything I noticed that the ground wire going to the blower motor was getting rather warm, so I quickly made a 10 gauge replacement for it. Doesn't get warm now.
I will probably revise this writeup several times, as I'm sure it's rather difficult to understand at points. Please do not hesitate to make any questions or comments.
Needed Items:
- Various gauges and colors of primary wire
- Wire stripping/crimping tool, wire cutters
- soldering iron (optional)
- Set of 4 Bosch-type 5-pin relays and pigtails (eBay special for $12)
- Male and Female spade connectors, all sizes
- 4-pin Solid State Relay (SSR) for AC switch
- Paperclips for removing terminal wires
- A 12-pack of your favorite brew
Step 1: (no photos)
Remove the dash bezel, Passenger's side knee bar, radio, center console, and everything in between so you can get behind the HVAC panel. Unhook the large wiring harness from the blower switch, and using your paperclips, remove the large solid black wire from the connector. This is the main power supply wire, and it will be hot whenever the ignition is on (verify with multimeter). Strip a section of wire 1-1 1/2" from the end (being careful to not cut the wire through). Wrap and solder a new 12-gauge wire around it, then cover with heat shrink tubing (I covered mine in 3 layers to ensure proper insulation). This will feed power to your relays, and run the fan. Lay this wire to the side and tap into a ground connection (I pulled from the power outlet), pull both wires through the right side under the dash.
Step 2: The painful part - disconnect the wiring harness from the resistor pack, and cut all four of the wires about 4-5" from the connector. Disconnect the black/red wire going to the blower motor and get ready to tuck it away later. You won't need it anymore. Take the end you cut off inside and start preparing your wiring harness.
Step 3: Prepare your relay harness (read-up on how a Bosch-type relay works if you don't already know. It will greatly help your understanding of this process)
Remove all pin 87A wires with your paperclip except for the last one on the right end (whichever end you want to decide is the right end).
Pull all Pin 85 wires to the left side, strap them, and terminate them all to a 10 ga male spade connector. This is the ground connection for all the relays, and will connect to a female spade connector on the ground wire you pulled from under the dash.
Pull all Pin 87 wires to the left side, strap them, and terminate them all to a 10 ga male spade connector. This will terminate to the ignition hot wire, coming from back of blower motor switch.
Relay 1: (from left to right, for speed 1)
Pin 86: terminate to a female spade connector - it will later plug into the Black/White wire coming from the blower switch. Be sure to label it as such.
Pin 30: solder or connect with spade connectors to Black/White wire from resistor pack harness
Relay 2: (speed two)
Pin 86: Black/Blue wire from blower switch
Pin 30: Black/Blue wire from resistor pack
Relay 3: (speed three)
Pin 86: Black/Green wire from blower switch
Pin 30: Black/Green wire from resistor pack
Relay 4: (full speed)
Pin 86: Black/Red wire from blower switch
Pin 87A: Black/Red wire from resistor pack
Pin 30: Terminate to a female spade connector and set it aside - this will plug in to the blower motor where the black/red wire was previously.
Background - Each time the fan switch selects a speed, the current travels through either the white/blue/green wire into the resistor pack, and then out the red wire to the blower motor. When on high speed, the switch connects the red blower motor wire directly to the supply wire. Pin 87A on the fourth relay allows the current to pass through the other three relays and on to the blower motor, until relay 4 is activated, and it connects the blower motor directly to the 12 volt supply wire through pin 87.
Step 4: Fixing your AC clutch switch
For reasons unknown to me (perhaps a smarter member can chime in here) the power supply wire for the AC clutch switch is connected to the black/white wire coming off the blower switch for speed one. Unless you install an SSR here, the AC will only work on speed one. Somehow, changing the circuit by installing relays will keep this from functioning as intended.
With the right knee bar removed, it should be easy to reach behind the HVAC panel and remove the connector going to the AC and Recirculation switches. Remove pin 2 (black/white) with some paperclips (this one is a bear to get out) and attach a new, small diameter wire to it. When 12 volts is applied to this pin, it will engage the AC clutch.
I decided to solder a new wire onto the pin, but you can splice a wire on the end just as easily.
Hook up your solid-state relay to your harness in this order. It will require pulling a few wires out of their terminals to solder them on and do it right.
Input +: Pin 30 on relay four, going to the blower motor + connection
Input -: Any of the Pin 85 wires you have, it just needs to be grounded.
Output +: Any of the Pin 87 wires you have, this supplies 12 volts to the AC switch when activated
Output -: The wire you connected to pin 2 of the AC switch harness
Background: When the fan is running, it is only feeds approximately 3 volts on speed 1, 5 volts on speed 2, 10 volts on speed 3, and 12-14 volts on high speed. If you connect your AC switch wire directly to pin 30 of relay 4, the AC will only switch on using speeds 2-4. Speed 1 does not provide enough voltage to engage it. You can, however, turn your fan to a higher speed, switch on the AC button, then back your fan speed back down to 1 and it will remain engaged. If you connect your AC switch wire directly to an ignition-fed + connection, you will have to manually turn off the AC clutch every time you turn the fan off. Otherwise, it will continue running. The solid-state relay will operate at these low voltages, and we are using it to make sure the AC clutch will only be engaged when the blower fan is running.
Photos:
Here is my completed harness. Right side going to the resistor pack, left side plugging into the blower switch wires.
Other side
Plugged in.
While testing everything I noticed that the ground wire going to the blower motor was getting rather warm, so I quickly made a 10 gauge replacement for it. Doesn't get warm now.
I will probably revise this writeup several times, as I'm sure it's rather difficult to understand at points. Please do not hesitate to make any questions or comments.
How to install blower motor relays in your B4
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