i was just curious why the 86 terminal on a bosch relay goes to ignition power. Will i burn out the relay if i just run this constant power. I'm thinking i might want to run the fan with the car off sometime. One less long power wire too.

If you want it hot all the time, you don't need to use a relay at all. You might prefer to switch your relay with a thermostat rather than the ignition, but the purpose of a relay is switching high current draw devices with a low current switch.

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The 86 terminal turns the relay on. It sounds like you are using it for your cooling fan. If you connect the 86 to the ignition switch, the fan will be on with the key.

You would have to drop the power from the 86 wire to turn the fan off. That is why the ignition key is used. Otherwise, If you just used a switch, you may leave the fan running and kill the battery. Connecting it live all the time would not let the fan shut off.

As far as the power wire length, I would mount the relay on the rad support with the power coming straight from a fused wire to the battery (or horn relay). Then, the wire to the ignition is only a control wire and the current draw on it is minimal.

The best way to do it would be to use a thermostat to run the fan.

Peter

According to the diagram i found in the tech archives, the 85 is the terminal that runs to the switch. That's why i didn't understand why there are actually two switches (ign-86 and thermostat or manual-85). I am using this for a cooling fan, and i do plan to mount it right next to the battery. I'm going to follow the diagram and do it right. Thanks for the tips guys.

I've confirmed with a couple of sources that the terminals are as follows;
30 is +12 input
85 is Ground
86 is Signal or Trigger from Switch
87 is +12 Output

You connect a live all the time, 12V fused from the battery to terminal 30. You connect your fans 12V wires to 87. Ground the other wire or frame of your fans. Connect terminal 85 to ground too. Then, when you apply 12V to 86 your fans will run. Because 86 is the coil of the relay, it only requires a small guage wire and draws little current.

You could probably interchange 85 and 86 with no ill effect, but 86 is usually the 12V wire, not 85.

Peter

The reason that I really like the Bosch relays is that they put a little diagram (molded) on the side of it!

Terminals 86 and 85 are the ones that energize (or pick) the relay. You need 12 volts on one and a ground on the other in order for it to work.

I just read the prior posts real quick and I'm not sure what the installation is in, but here are a couple thoughts;

I wouldn't worry about the relay being energized for a long time if it's not too close to direct engine heat. (You could use one for an electrical fuel pump)

What you should think about is how much current is going through the contacts. As an example, if you are running a couple large radiator fans, you may want to run two relays in parallel.

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Wes. Vann
Technical Reference section
Gold Member #5

I'd recommend keeping "86" the positive side or trigger switch (+), and "85" ground.

The relay may work if you get the ground and source to the coil switched, but remember the coil is an electromagnet, and polarity is important. The manufacturer designed those so they would be most stable and reliable holding the inner contact with "86" positive and "85" ground so the current flows in one direction.

Oh, and it doesn't matter what you "switch" either the (+) or ground side of the coil as long as you practice good safety and what's convenient for you.

For example, you could have the (+) side of the coil hot all of the time, and have the ground switched--connecting it to a thermostat that grounds itself completely the electrical path. Most people (yours truly included) usually "source" a relay. That means have the ground connected to the car ground all of the time, and switch a hot (+) wire to the coil to complete the circuit.

Wes has a good point on using relays in parallel, as this creates a "current divider" circuit, where as long as equal size wires split up the main power wire feeding the fan, fuel pump, whatever and go into the respective relays, each relay has less current flowing thru it than if it has all of the current flowing thru it.
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Joe

<BLOCKQUOTE>quote:</font><HR>Originally posted by malibufil:
According to the diagram i found in the tech archives, the 85 is the terminal that runs to the switch. That's why i didn't understand why there are actually two switches (ign-86 and thermostat or manual-85). I am using this for a cooling fan, and i do plan to mount it right next to the battery. I'm going to follow the diagram and do it right. Thanks for the tips guys.<HR></BLOCKQUOTE>


The reason for the two "switches" is so the that fan(s) will only run when both the engine is warm (thermal switch closed) and the ignition is on. If only one or the other is true, the fan will be off:
water hot/igntion on = fan on
water cold/igntion on = fan off
water hot or cold/igntion off = fan off

Most thermal switches are ground-seeking, so that they will have only one terminal, and when activated, that terminal is connected to whatever the switch is mounted to(normally a conductive part of the engine, which is connected to ground). I'm sure there are isolated thermal switches (which would have to have at least two terminals) if you really wanted to have it in the positive side of the relay coil, but it really doesn't matter, as it is a simple series circuit, and the current it would be switching is the same regardless of where the switch is electrically located.

An Afterthought:
Things can get a bit more complicated if you wish to have the thermal switch and have A/C, because the fan should be running if the A/C is "on", regardless of whether the water is hot or not. I think what I would do in that situation is run an additional (smaller) relay off of the compressor clutch power source that would basically put a short across the thermal switch when the clutch is engaged.


[This message has been edited by Gene McGill (edited 07-10-2000).]

Thanks for all the input guys. What i finally realized was that the thermostatic switch would actually act as a ground. Hence, my assumption that there were TWO switches involved. I had a guy take a broken relay apart for me yesterday and now see how simple they are to wire up. Thanks again.

You can also get really fancy and wire up a three-position toggle switch (SPDT) so you have full control of the fan's operation. One position on the switch causes the fan to always be on, one always off, and the other lets the thermal switch control it (as well as the ignition switch if it's wired that way). You just have to make sure everthing is wired in on one side of the relay's coil circuit, or it won't work properly. Either one side should always be grounded, or the other should always be hot. Let me know if you want a wiring diagram for this.

Turbojet's idea of adding that switch is a cool one! But, why would you ever want it turned off?? I always try to approach this sort of thing with the idea of possible "F-ups" and I'd hate to bump the switch to the off setting and overheat the engine.

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Wes. Vann
Technical Reference section
Gold Member #5