Let my computer run the electric fan swap?

I've never seen a guy use the factory wiring / activation to control an electric fan swap.
Sure, your engine would run in a wider temperature range - cooling when the computer sees the need for "extra" cooling instead of "regular" cooling - but that doesn't seem like it would do much harm. I think there is even a guy around somnewhere who can change your fan on / off temps.
Is there anything I'm missing on this one? Really seems like a cleaner / simpler install.

It could work, and I agree it would be a much cleaner install, but there are three gotchas...

1st... the computer controlled factory fan relay does not turn on until like 215 or so, which is ok for an auxillary fan, but not good at all for your primary fan. If you were able to get the ECM reprogrammed to lower that temp to say 195 or 190, it might work. I have no input on whether the reprogramming part is possible.

2nd... the factory supply wiring would need to be upgraded from the battery to the relay, since it was only designed to run one fan and you'd be dropping two or maybe three fans on it.

3rd... you will only be able to turn ALL fans on at once, or have them off, since the factory ECM only has one temp trigger for the fan.... there's no way to set an on point for one fan, and then a higher temp for a second fan, and so forth. This is a problem because turning both or all three fans on at once is going to cause a HUGE spike in amp draw on your alternator and the wiring for the fan. On top of that, having two or three fans running at once is going to make a hell of a racket, and they're going to be turning on and off all the time for just the smallest temp swings.

The third one is the major gotcha for me IMO.

There's a reason why this isn't done. A couple of adjustable thermostatic switches set on different temps is a much better idea, even if the install isn't as clean.

NotMatt said:

It could work, and I agree it would be a much cleaner install, but there are three gotchas...

1st... the computer controlled factory fan relay does not turn on until like 215 or so, which is ok for an auxillary fan, but not good at all for your primary fan. If you were able to get the ECM reprogrammed to lower that temp to say 195 or 190, it might work. I have no input on whether the reprogramming part is possible.

Click to expand...

Since the factory thermostat is only 195º, I'd say turning them on at 215 would be perfect.

Turning them on at 190º or even 195º would have them running almost constantly, and needlessly opening/closing the thermostat a bunch.

NotMatt said:

2nd... the factory supply wiring would need to be upgraded from the battery to the relay, since it was only designed to run one fan and you'd be dropping two or maybe three fans on it.

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Not a big deal. Very simple wiring.

NotMatt said:

3rd... you will only be able to turn ALL fans on at once, or have them off, since the factory ECM only has one temp trigger for the fan.... there's no way to set an on point for one fan, and then a higher temp for a second fan, and so forth. This is a problem because turning both or all three fans on at once is going to cause a HUGE spike in amp draw on your alternator and the wiring for the fan. On top of that, having two or three fans running at once is going to make a hell of a racket, and they're going to be turning on and off all the time for just the smallest temp swings.

Click to expand...

I guess you could call it an inconvenience but it's not a big deal. There's not a HUGE spike draw. Sure there's a decent draw, but it's nothing that the system won't recover from within a second or two.

Yes they make a lot of noise. But I doubt you'll hear it much from inside the XJ. I could barely make out mine.

And there's a thing called hysteresis. Even the most basic of controllers use it. I'd be VERY surprised if the factory didn't use it.

All in all, yes it works. Is it ideal? Nope, but it does work fine.

yossarian19 said:

I've never seen a guy use the factory wiring / activation to control an electric fan swap.
Sure, your engine would run in a wider temperature range - cooling when the computer sees the need for "extra" cooling instead of "regular" cooling - but that doesn't seem like it would do much harm. I think there is even a guy around somnewhere who can change your fan on / off temps.
Is there anything I'm missing on this one? Really seems like a cleaner / simpler install.

Click to expand...

Now being from CA you know it will get hot in the summer months.Just something to think about.Plus how long do you sit in traffic up there?

I use the factory fan wiring as one of 3 triggers for my fans. The A/C and a temp switch are the other 2. I run my fans off of a 75 amp relay to be sure there are no controller issues.

MoparManiac said:

Since the factory thermostat is only 195º, I'd say turning them on at 215 would be perfect.

Turning them on at 190º or even 195º would have them running almost constantly, and needlessly opening/closing the thermostat a bunch.



Not a big deal. Very simple wiring.



I guess you could call it an inconvenience but it's not a big deal. There's not a HUGE spike draw. Sure there's a decent draw, but it's nothing that the system won't recover from within a second or two.

Yes they make a lot of noise. But I doubt you'll hear it much from inside the XJ. I could barely make out mine.

And there's a thing called hysteresis. Even the most basic of controllers use it. I'd be VERY surprised if the factory didn't use it.

All in all, yes it works. Is it ideal? Nope, but it does work fine.

Click to expand...

Some day I might try this, and see what happens. 215 is "too hot" in my opinion (above 210 degrees), based on having owned several 4.0 rigs that always ran below 210 under normal conditions, even in the hottest weather and stop and go traffic. I have rarely had a factory e-fan come on (due to temp, A/C is different) unless there was a problem with the rest of the cooling system that was affecting the cooling ability of the radiator, the pressure was not being held by the bottle or rad cap, or the mechanical fan clutch was bad causing the temp to spike. I guess we have different ideas about what a normal operating temp should be, but in my mind it should not fluctuate that much... and 215 is TOO HOT for the motor to get before something happens like kicking on the efan, whether it's factory or aftermarket, one or many.

Here's the easiest, most foolproof way to do it... wire your fan relay trigger to an ignition on source. This way, the fan(s) always run. This is the way I do it with my Wrangler and one large 16" SPAL fan. With a properly working thermostat, you should see the temp gauge fluctuate very little... a small creep up towards 210 when hot and the t-stat is closed, then a small drop when the t-stat opens and back and forth forever and ever. Let the thermostat do it's job.

I have my 2nd efan wired through a relay to the stock efan. Easy peasy.

You could pretty easily have each fan on it's own relay and thermo switch combo. It would be pretty cool to have "ramped" thermo switchs as well (200, 210, 220F for example) with a 195deg thermostat so it would only use as many fans as needed.

I am doing this small mod right now. The difference is I am going to use a different switch in the radiator. The 2 temp / 2 speed switches out of early water cooled VW's fits in the factory radiator switch location. The switch has a 3 wire connector and the switch temps are 190 Deg and 215 Deg (depending on application). I am going to wire the high temp side into the factory wiring and the low temp into a second electric fan (to replace the factory mechanical fan). Relay the low temp fan side and all should be good. I will let you know how well it works.

yossarian19 said:

I've never seen a guy use the factory wiring / activation to control an electric fan swap.
Sure, your engine would run in a wider temperature range - cooling when the computer sees the need for "extra" cooling instead of "regular" cooling - but that doesn't seem like it would do much harm. I think there is even a guy around somnewhere who can change your fan on / off temps.
Is there anything I'm missing on this one? Really seems like a cleaner / simpler install.

Click to expand...

Put simply .... 218* is way too late for the fans to come on and attempt to cool IMHO.

Vehicles in some locations may work OK with a late control fan activation ... but it will be dependant on ambients.

If you are looking at replacing the mech fan with an electric one ... duplicate the mech fans activation temp.

The sensor coil on the front of the fanclutch is looking for about 170* air temp to engage the clutch fully ... and that would probably translate into about a 180*/185* or thereabouts, coolant temp.

Much better for the thermostat to restrict cooler flow a little ... than have it wide open and hope that fans and rad can handle higher temps.

16vvincent said:

I am doing this small mod right now. The difference is I am going to use a different switch in the radiator. The 2 temp / 2 speed switches out of early water cooled VW's fits in the factory radiator switch location. The switch has a 3 wire connector and the switch temps are 190 Deg and 215 Deg (depending on application). I am going to wire the high temp side into the factory wiring and the low temp into a second electric fan (to replace the factory mechanical fan). Relay the low temp fan side and all should be good. I will let you know how well it works.

Click to expand...

You by chance have a PN for the dub switch? Are we talking OEM only or would one from Carquest, Napa, etc work? I have the DBO triple fan setup, but no controller yet. My plan, so far, is to run a master on/off/on switch for all. Run one on the factory wiring, and at least one always on (unless I manually shut them off ) Maybe 2 if necessary.

I thought about doing this for my dual electric setup, but when the switch is activated the temperatures are going up, and it takes a while for the fans to cool the engine down. You'll probably get up to 230ish often, which is too hot IMO.

Just to clarify, I was planning on replacing both the mechanical and the electrical fans with a single unit. Think "Taurus" fan (this one came from a Volvo)

Thinking about it some more, yes, I'd want the fan to come on earlier than 215. That means either re programming or a separate fan switch. With a separate fan switch controlled by temp, I think I'd want a diode in between the ECM and the relay, just to ensure power isn't being back fed to the computer. Or maybe I'm wrong on that point. Dunno.

Keep your old stuff so you can reinstall it later. It won't work during the summer out here in CA.Do you park your jeep when it gets above 80/90 out?You will with the 1 fan setup.Heck the 3 fan setup won't even work in the desert where i live.It won't even work on mine and mine runs perfect with the AC on when it is 110 out.My jeep hits 195 to 200 in traffic with the STOCK setup.

Johnnie Walker said:

You by chance have a PN for the dub switch? Are we talking OEM only or would one from Carquest, Napa, etc work? I have the DBO triple fan setup, but no controller yet. My plan, so far, is to run a master on/off/on switch for all. Run one on the factory wiring, and at least one always on (unless I manually shut them off ) Maybe 2 if necessary.

Click to expand...

I don't have the part number, but any fan switch from a 85 to 92 golf / jetta would work. That includes any that you would buy to fit that application from the suppliers.

However, going to a wrecking yard and getting one, will also get you the switch connector. Which is kinda important.

I finally got around to finishing this.
Using a Zirgo 14" fan rated for 2125 cfm, I had to:
Trim a little of the upper radiator support
get rid of the mech. fan studs & replace with bolts
weld a few mounting tabs
and run a relay trigger wire to the stock ac fan.
I also wound up getting rid of the old mech fan's shroud entirely, which isn't ideal. I think a custom shroud, or somehow getting the old one to fit, would increase performance.
Still, when both fans kick on (when the computer says so) the temp comes down & they turn off again within 1 or 2 minutes. Thats at 76 degrees ambient air & with the grill partially blocked off. Part of the fan also protrudes down past the radiator.
My install could use some tweaks but as far as I'm concerned, it's a success. Both fans come on, temps go down. I'll see what happens when I hit some hot weather with it.

On my 96 street machine xj, my dad and I fab'd an all steel shroud for a 16" zirgo and ended up running it tied to the stock e fan as I mentioned earlier. It would cool like a sumbitch.

yossarian19 said:

Just to clarify, I was planning on replacing both the mechanical and the electrical fans with a single unit. Think "Taurus" fan (this one came from a Volvo)

Thinking about it some more, yes, I'd want the fan to come on earlier than 215. That means either re programming or a separate fan switch. With a separate fan switch controlled by temp, I think I'd want a diode in between the ECM and the relay, just to ensure power isn't being back fed to the computer. Or maybe I'm wrong on that point. Dunno.

Click to expand...

The relay electrically isolates the PCM from the fan. There's no need to do anything more there. If you're planning on using a two speed fan to replace the whole stock set up I would set it up to have low on all the time, and switch to high when the PCM calls for the AUX fan.