Brothers, whatever you do to your car - log it!
The electric cooling fan stopped working. And I have no clue what I did when I installed it 20 years or so ago.
I'm looking under the hood and I have vague memories doing single pass mod and eliminating the stock fan, replacing it with electric one. This electric fan kicked in at certain temperature. I did not connect it in parallel to the existing small fan. So how did I connect it?
Me scratching head ...

 

Find the two wires coming out of the fan and follow then to their sources.

 

Yes, that's the idea. However, where from it is getting the signal? The original wiring has no hardware for temperature triggered power on for electric fan because there wasn't one, by design. Did I add some sensor or what.

 

See post 2 above.

 

OK, fixed. I headed to the garage, armed with printed schematics and diagnostics equipment. Only to find out why I used a stronger fuse 20 years ago. Silly, isn't it. Now, while maintaining and repairing the electrical system I replaced all fuses with correct ones, according to the manual. My supplemental fan blew it ... I even didn't remember there is cooling fan fuse in the headlight fuse box. Darn, I knew so much about this car 20 years ago. All forgotten. But it is coming back to me, sometimes painfully ...

 

Hi Segfault

Have a look at this Photo (from my Parts Car) which is of the Thermostatic Switch, which cuts in or cuts out The Auxiliary Electric Fan, when it reaches a Certain Temperature (don't know what that temperature is) but no doubt someone will

So as Greg said, have a look at your Wiring and get 12 Volts to your Auxiliary Fan (which probably goes through a Relay)

Then all you've really got to do is to put that Thermo Switch in Series, in the Circuit

Then when the Preset Temperature is reached, the Aux Fan should cut in and then it should cut out again after the preset Temperature has dropped

Test the Fan beforehand to make sure it runs freely and that the Blades are not jammed

 

All good advice, but does not apply. I can see now I used existing stock wiring which originally was for little AC fan. Apparently I altered the wiring somewhere else to make it work the way I wanted. The small fan still kicks in with AC. So I must have done something real clever ...

 

It seems to me you are saying two things. In the first post you say that the fan "...kicked in at a certain temperature". In the post immediately above you said "The small fan still kicks in with the AC", and those are two different things. So, depending on your memory of how it worked recently (and not faulting you here, who would remember such things unless you stare at the thing daily)...

1. Maybe you put in a relay, so that when the A/C compressor receives the "on" signal, the relay closes and lets power flow to the fan??? One way to test for this is is to have an assistant sit in the car (unless your have really long arms) while you watch the aux fan. If it goes on when the A/C is turned on, and shuts off when the A/C is turned off, the above might be the case.

Sorry, I'm thinking as I write, the above won't work since the fan doesn't work. But if you put a voltmeter on the fan leads (after testing the fan to make sure it just didn't up and die), you should see system voltage if the above case is true when you turn the AC on.

2. If you think it's temperature or pressure related, maybe you put a trinary switch in the AC system? That would trigger the fan to come on at a certain line pressure, which normally wouldn't correspond exactly with when the AC would turn on and off.

A variation of "1." above. Could you have simply spliced a wire into the compressor "on" signal wire and run that wire to the fan? That would make the fan come on when the compressor would come on without needing a relay. I wouldn't think that wouldn't work very well, but I've made a near-profession out of being wrong.

I think Greg has the right solution. If you trace the fan wires one of them is almost certainly going to go to ground (or earth), and the other has to go to some power source. Find which wire goes to ground, simply by measuring continuity at any of the many ground points. You can forget that wire and just physically trace the other one, and it will lead you to the solution.

Thanks,

John
1987 XJ-S V12

 

Thanks, I managed to restore the original wiring and I'm going to wire the add-on fan differently. And this time I'm going to document it!

 

 

Right now I may wire two fans working together, using an additional relay and fuse. Two fans seem to be a little too much for stock wiring. There are two reasons why it may not be the best solution. First, it may drain the battery when the car is parked in a hot day. And it does get hot here. Second, there may be sufficient airflow at higher speeds and running rather powerful fan is nothing but just putting unnecessary load on alternator and creating noise. There is a paddle switch idea in Kirby's book, but I think I can do one better. There are impulses coming from speedo sensor. Using these impulses I could build a device which switches the fan off when certain speed threshold is exceeded. But right now there are too many other issues with this car, the priority of fan wiring is low. So for now it is two fans working together. Will return to this when I get this car running and registered. Hate paperwork.

 

Other threads have gone over this issue, and the consensus was the electric (large) fan was causing airflow issues at highway speeds. If your cooling system is ok it shouldn't be necessary to have the small Aux fan come on but its nice to have. On my car I've got a very solid clutch fan that locks up quite well when hot, and a toggle-switch for the Aux that I use in stop n' go city traffic, also ignored the thermal switch on the pump as I don't trust it anymore.

I was in too much of a hurry to buy an RPM-activated switch for the fan but it would be nice to have something that kicks on under 2500rpm (or whatever) combined with a 5-minute shutdown timer. I'd consider supplying the parts if someone wanted to do a step-by-step writeup. I can't as my car is my only vehicle at the moment.