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I found this past post from Ari w/a search for 'horn relay.' Assuming I can locate the horn relay behind the fuses, this may be simple enough for even somebody like me to tackle:
The relay you describe is a simple one - just one set of contacts. I assume that when you talk about testing a switch and a relay, you mean the switch that drives the relay.
For example, let's look at the horn. When you hit the horn button, you close a switch in the steering wheel. That switch drives a relay, and that relay drives the horn. Say you want to check out your horn.
I don't have the schematic in front of me, but knowing the way it usually works -
Power from the battery goes through a fuse, then to the horn switch in the steering wheel. The other side of the horn switch goes to 86 on the horn relay, and 85 is tied to ground. Pins 85 and 86 are the relay coil.
Power from the battery also goes through the horn fuse, then to pin 30 on the horn relay. Pin 87 on the relay goes to the horn, and the other wire from the horn is tied to ground.
Let's see how it's supposed to work.
When you press the horn button, power from the battery flows through the horn switch, into pin 86 of the relay, through the relay coil, and out to ground through pin 85. You always need a path to ground, or current won't flow. With current flowing through the coil, it becomes an electromagnet, and that magnetic field closes the contacts - namely, pins 30 and 87 are now tied together. With pins 30 and 87 tied together, current flows from the battery, through the horn fuse, through 30-87, and into the horn. The horn sounds.
If you remove the relay and jumper 30 to 87, then power will flow from the battery, through your jumper and into the horn. The horn will sound without you hitting the horn button, and the neighbors will wonder just what the heck you're doing. This tests is the horn, the horn fuse, the wiring in between, and your battery.
So far, so good. At least you know the horn works. Don't perform the next test you mentioned - namely jumpering 85 to 86. If you do that and press the horn button, a fuse will blow. Why? Look at it this way - power flows through the battery, through the fuse, and right through your jumper to ground. A short circuit. The fuse will blow. The relay coil has a high resistance - not a lot of current flows through it, so the fuse doesn't blow. But if you replace the relay coil with a jumper wire - when you jumpered 85 to 86 - you'll draw a lot of current, and pop goes the weasel (or fuse).
If you want to check the horn switch, pull out the relay and use a VOLTMETER to measure pin 86, with the other side of the meter on ground. When you press the horn button, you should see 12 volts on the meter. That means the switch is working. Say your horn still wasn't working - maybe you've got voltage on pin 86 of the relay, but if there is no path to ground for the relay coil (pins 86/85), it won't work. Set your meter to OHMS, and measure the RESISTANCE of pin 85 to ground. It should be only 2 or 3 ohms at most. If the wire from pin 85 to ground was broken, no current would flow, and the relay wouldn't be working.
If the horn switch works, and the horn works, and the relay coil is getting voltage and has a ground, but you plug the relay in and nothing happens, then you can assume the relay is bad.
This is a pretty basic explaination. There are some twists and turns I haven't mentioned, but they're pretty minor. Get the basics down first.
I hope this helps.
posted by 198.93.76...
, Wed, 12 Nov 2003 08:23:15
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