Electrical Gurus - Does this work?

[Tom Schenk]

Hello,

This is the wiring diagram I'm going to use for the 2 speed fan in the TJ. I want the low speed to come on with the A/C clutch, and the high speed to be activated by the thermostat. I DO NOT want the two to be on at the same time! With this set-up will both high and low be on, when say, I have the A/C on and the water temp hits 200 deg? If so, where can I put another relay to prevent this? :confused:

Thanks,
http://www.virtualjeepclub.com/attachment.php?attachmentid=588&stc=1

[danno]

Tom,

Are you sure that the fan will allow the high side and the low side to be on at the same time?

If you really need to only turn one side on at a time, there may be some kind of a SPDT relay that you can put on the low side to disconnect the low side when the high side is on... Alternatively, you could put a relay in front of the low side that is the reverse of normal... when the juice goes to it, it is open, but when no juice, it is closed. (Kinda like the Foglight relay on TJs.) You would wire this relay to energise when the high side is on thus killing the low side.

Sorry, I am trying to type this fast, my wife is trying to get us to leave for the friggin airport 4 hours in advance.

[JamesT]

This should fix it:

http://www.virtualjeepclub.com/attachment.php?attachmentid=589

There Probably should be a fuse on the clutch side too, maybe 5 amp.

[Tom Schenk]

:cool: Thanks guys, I think that it all makes sense now. I'll post the results after its in (this of next weekend).

[Brad]

Here's how I would do it.

In your picture the relay's look wrong, as if you have the fan connected to the normally closed position. I am assuming that is not the case and that the top contact that you have the fan connected to is the normally open contact.

As you can see I eliminated all the diodes as they are not needed with this setup. When the thermostat is off and the top (high speed) relay not energized current can flow through the normally closed contacts to supply power to pin 30 of the bottom (low speed) relay so that the fan can run on low speed contolled by the fan clutch supply. When the thermostat energizes the high speed relay the normally closed contacts will open, cutting power to the low speed relay while at the same time supplying power to the high speed side of the fan instead.

HTH and Happy Thanksgiving
Brad

[JamesT]

That would work too. The only problem I see with it is, running the power through the top relay all the time, instead of the way I did it, running the power through one relay for high and one for low. There is nothing wrong with the way you did it, it just makes trouble shooting a bit harder and if you blow the top relay you loose both high and low.


I have no idea what the heck those diodes are for. I assumed they were some factory-feed-back thing. I remember seeing them on some wiring diagram someplace, but their direction really doesn't make sense to me.

[Tom Schenk]

Ok heres another revised version. The relay off of the A/C clutch should disable the low speed when the thermostat kicks on the high speed. Right? Also, the two diodes on the fan are to keep the fan from sending current back into the system when it is spinning freely. The diode off of the A/C clutch is to keep the clutch from being activated when the system is energized. By the way, thanks for all of the input on this. I really don't know stink about any of this. I appreciate the feedback!


http://www.virtualjeepclub.com/attachment.php?attachmentid=593&stc=1

[Brad]

The diodes you have drawn around the fan are going directly from the hot side to ground which will create a short. When both relays are open (fan off) there is nowhere for the current to go back into the system when the fan freewheels. If you give the current a place to go the fan will not be able to easily freewheel (fan motor acting like a generator under load) and will block the airflow. Even if there were a way for the current to feed back into the system it would be such a small current and low voltage that it's nothing to worry about, far lower than the 12 to 14 volts on the other side of the relays.

Brad

[Brad]

One more thought... If you intended to have the two diodes connected to the hot side of the fan and not to ground you will need some really high current diodes to carry the full load of the fan. Those probably won't come cheap and will need heat sinks.

Have a good one
Brad

[JamesT]

Tom,

That will work too. I guess my thoughts are the same as Danno's, I don't think the fan will operate at high and low both. My feeling is that the high will over ride the low, so there is no reason to shut it off. My thoughts on this come from knowing a bit about motors, what they probably do is use more windings in the motor for high than low. In other words, low probably uses the same windings as high just not as many of them.

Brad, IIRC those diodes from the factory are huge and do have heat sinks on them. In addition, when you get the fan they are mounted to the fan, so there is no need to buy them seperately, they are just seperate on the schematic to show them being there.

Good luck whatever you do Tom.