replacing a failed relay with a new one.

Hi... new to the forum. Here comes post #1.

I've got an external wiring harness in my car that provides power to my retrofitted HID ballasts. The harness was purchased pre-made and has given me good service for almost two years now.

The design is simple:

Two harnesses combined into one unit within a common wiring loom. Two SPST relays (one for each headlight) two inline 15A fuses, all the necessary connectors, and lots of 12 gauge wire.

The problem:

Recently, one of the relays failed and with that both headlights (strangely enough) were affected (maybe due to the ground being common). I have now purchased a replacement relay however I am faced with a slight problem. The new relay is SPDT (Bosch) and thus I have an extra 87a terminal. Not a problem. However, the old relay (Song Chuan... made in China) has absolutely no markings on it whatsoever in regards to which terminal is what.

Questions:

1. How do I figure out which terminal is what? The only way I can see it possible is to go by the wires coming into the relay.

2. When the new relay is in place, 87a will not be hooked up to anything. Is this a problem?

TIA.

If you have a DMM, measure the terminals for a pair that shows some resistance value, about 100 to 200 ohms for example. That should be the coil input for the relay.
If you have a diode check function on the meter, it's usually a good idea to make sure your relay doesn't have a suppression diode installed, by checking the reading with the leads connected both directions. The reading should be the same regardless of how you connect the leads.

Using your meter, check for continuity (less than 1/2 ohm) from the 87A to one of the other terminals. The one you see continuity on will be the term 30 Common terminal.
If you apply 12V to the coil inputs, the relay should click on and you should measure continuity from term 30 to the remaining terminal, which should be 87.

Since your old relay failed, you may not be able to check for this, but you can measure the voltage in the socket, to see which terminal pair goes to 12V when you turn on the HID switch. It could be either ground switched or +V switched, so check between the two terminals you think should correspond to the coil inputs.
The other two terminals should show 12V across them whether you have the switch for the HIDs on or off; they should be the ones going to term 30 and 87.

It won't matter in your application if the 87A is not connected, so that's no problem.