BUILDING: RELAYS

A thin wire will restrict the flow of electric current. A long wire will do the same thing. The key word here is SHORT, THICK wires.
A relay is a "remote controlled switch" which is placed between the power source (alternator) and the consumer (lights, fans, etc) so the wires can be as short as possible. The long wires from the switch is now only used to operate the relay, and can be as thin as you please. Current-draw in the relay-coil is only 150mA. (0,15A)

Another reason for using relays, is to prevent premature switch wear-out. If a switch has to operate a high-current consumer (ex: the thermoswitch in the radiator that operates the radiator fan), it will not last very long. Each time the switch opens or closes, sparks fly over the contact points, causing a high-temperature-instant-spot-weld-effect, and a soot deposit. Relays are designed to withstand such wear.

There are three ways for the car manufactorers to cope with this.
1. Make the switch very large and heavy duty.
2. Install a relay in the circuit, to take the shunt.
3. Install the ordinary switch (no relay), and make lots of them to have in stock for when the car owners come rushing in to buy replacement switches.

My favourite solution is of course nr. 2.
Sadly, most car manufacturers seem to prefere solution nr. 3. (This is dictated by the bean-counters, not the engineers. Production cost, see?)

THE 99 RELAYs
Saab used bog standard relays in the 99's, exept for two:
-The headlights relay and,
-The headlight wiper relay. (Stateside there were no wipers, though)
The old and new type headlights relays are, as you can see, interchangeable. Only the size and terminal whereabouts differ. Even the 900 headlights relay will fit here. The only thing missing on that one, is the bracket. You'll have to make one yourself.

HIGH VOLTAGE SHOCK FREE RELAYS
Look at the relays above, and you'll see that some of them has a ballast resistor or a diode in parallell with the electro-magnet. "Electro-magnet", is another word for "coil". In the ignition system, we use a coil to induce high voltage for the spark plugs. When voltage is applied to a coil, and then suddenly cut off, it induces a high voltage. Very useful for spark plugs. Less so for the rest of the wiring harness. Especially sensitive electronic equipment, such as radios, injection ecu's and mobile phones.
Anyway, this induced high voltage peaks are sent into the wiring harness each time a relay is shut off. (On my Saab, I could hear a click in the loudspeakers each time the lights were operated. With the stereo switched off!)

AMPERE RATING
Always make sure the relay will handle the load you plan to apply. And leave a little headroom, aswell.
How much is that? Well, lets look at Ohm's law of electrics: If we want to calculate the current draw of the headlights, we use the formula I = P / U.
With 55 w bulbs, and 12 V filled in: 110W / 12V = 9,17A.
A pair of 100 W bulbs will give a current draw of 16,7 Ampère.
As we can see, most relays are rated at 30 or 40 A, which is quite sufficiently.

INSTALLING RELAYs (or how not to)
Some relays have a bracket, so that you can bolt it to the bulkhead, the inner wing, or wherever you're putting it.
I never do that. (The only time I actually use the bracket, is when I put relays into doors, to operate electric windows. Space is limited in there) In an engine bay, relays scattered around, with multicolored insulator sleeved wireshoes, have a very high Brazilian-Rain-Forest factor.

USE RELAY SOCKETS!
This is a marvellous invention which prevents any harness from turning into an amateur lash up. That's why the car manufacturers install these things themselves, in the first place. And you should do it too.
The sockets require special wireshoes with lock tabs. Which are better and CHEAPER than "conventional" color sleeved 'shoes, sold at petrol stations. (Those stations that hasn't yet turned into supermarkeds, that is)
Snap together as many relay sockets as required, and bolt the sockets to the inner wing.

MORE SOCKETS
It's not just Hella that makes usable relay sockets. Bosch has a series of them, but they're actually not as neat as Hella's, so I didn't bother including them here. I only include the stuff I use. There is however another manufacturer on the scene. The sockets are labelled MRO, and there's a wide range to choose from. They all snap nicely together. Problem is, you cant buy those. Unless you manufacture cars. Where did I get a hold of them then? From scrapped cars of course :-) Mostly Saab's and VW's.