Headlight Relays - The General Case
Many older cars did not have relays to switch power for their headlights. Modern bulbs can draw much higher current than older units and can cause failures in dash or column switches. Fortunately, adding relays to resolve this issue is pretty straight forwards. For extra credit, add inline headlight fuses if your car does not have them.
While the specifics here pertain to most g-body (1974-1989) Porsche 911s, they can be applied to any older vehicle. I installed headlight relays in my old Spitfire the same way so that I could upgrade its headlights to 55W H4 halogen units. The Spitfire has 3 (!) fuses for the entire car and wiring that seems to want to catch fire, so this was high on the to do list.
Nutshell:
The switched power that operates your headlights now will be used to energize a relay instead. The relay contacts will control power to the actual headlight bulbs. That's really all we're doing. This also allows you to run larger wires from the battery and ground which can help the bulbs get their full current.
OK, but what is a relay?
Generally a relay is an electromagnet - a coil of wire around an iron pole - that operates a switch. When you send current into the coil, it creates a magnetic field that pulls the switch contacts towards it. When you cut the power to the coil, a spring returns the switch contacts to their original position. They are simple, reliable and used throughout numerous industries and applications. If your home has central air, there is a relay outside with the heat exchanger which lets the 24V from the thermostat control the 240V service that runs the unit. Same basic idea.
Here is a typical 911 fuse panel. You are looking at the far right section where it ends from across the frunk. Right is towards the front of the car:
Step 1 is ALWAYS disconnect your battery before you do any electrical work. Either terminal is fine.
You will need to run a new lead to the + side of the battery. Use a heavy gauge wire for this. It will run to the relay's common contact.
You will also need to run a new lead to ground - there is a convenient one at the aft end of the fuse panel on the fender. This should also be a heavy gauge wire.
The new + and ground will be the switched power to the headlights, and will be connected to the relay's contacts.
You will interrupt the power to the headlights at the top of the panel, using those wires to energize the relays. The power from the relay contacts will run back to the top of the fuse panel. This means you are not touching your headlight wiring at all. Nothing to worry about. Just go slow.
There will be one relay for the low beams and a second one for the high beams. The only difference in wiring is which headlight circuit they get connected to. 1-2 for your high beams or 3-4 for your low beams.
Relay contacts:
The contact numbers on the relay may seem crazy at first, but they are actually following:
Pin 85: Relay coil (-)
Pin 86: Relay coil (+)
Pin 30: The common contact
Pin 87: normally open contact (off until your energize the relay)
Pin 87a: normally closed contact (on until you energize the relay)
Not all of the Bosch relay coils will have a polarity like this, only the ones with flyback diodes will. There is no harm in wiring them up as if they are all configured this way, plus if you ever need to replace a relay in the future there is less chance you'll have an issue and you'll have 2 different parts you can use interchangeably.
The wiring colors follow DIN-47002 in case you want to match that.
The schematic for the relays are written write on them. You can see that 85 & 86 are the coil that energizes the relay. 30 is the common terminal on the switch, 87a is the normally closed contact (thing is turned on) and 87 is the normally open contact (thing is usually turned off - like headlights!)
The color codes for the sockets I bought online are:
85: White (switched headlight leads from top of fuse panel get re-routed to here)
86: Black. (to ground)
30: Blue (to battery +)
87: Yellow (return to fuse panel to power headlights, 1 for high beams & 1 for low)
Check your sockets to be sure you have the leads you expect, there may be multiple versions of the socket.
Background / Theory (optional)
You need 2 relays because one will switch power for the low beams and the other will switch power for the high beams.
An H4 bulb has 55W (low) and 60W (high) filaments at 12V
Ohms law shows us that
Watts = Volts * Amps
60W = 12 * A
A = 5
This also implies that the filament has a resistance when hot of:
V = I * R
12V = 5A * R
R = 2.4 Ohms
5 Amps of current is a lot, and you will actually have 2 bulbs going at once so that's ~10A of current in operation and still more when it is first turned on (called 'inrush current'). All of that (plus the running lights) has to run through the little contacts in the dash switches normally. No bueno. The little Bosch relay can handle 20A on the normally open contacts, comfortably twice what it will have to handle.
Just to complicate the current ratings of things, the wires are tied up tight in a harness so they can't get rid of heat very well. This means we'll want to use something 2X as large to have a nice safety margin. There are tables of wire gauges to use based on current handling and whether the wire is exposed or in a harness. I used 10 gauge for mine because I had it on the shelf.
The relay coil for the relay we're using only draws 157 milliamps, or 0.157A. This makes your headlight switch very happy. Very, very happy indeed.
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