Radiator Fan Override Switch (The Easy Way)

[kastein]

winkosmosis, depends on how you want to do it. You could splice a second relay into the heavy current wiring no problem but it's easier to splice into the low current side, requires thinner wire and less care.

I don't presently have an e-fan, because my '98 came without one installed. No CEL because the ECU only monitors whether the relay coil is open/shorted (as I expected) not whether the actual fan is operating. I'm going to my storage unit to pick the fan up and install it tonight, if I can find my set of OEM PDC terminals I'm going to get a head start on this mod.

[winkosmosis]

Originally Posted by kastein

winkosmosis, depends on how you want to do it. You could splice a second relay into the heavy current wiring no problem but it's easier to splice into the low current side, requires thinner wire and less care.

I don't presently have an e-fan, because my '98 came without one installed. No CEL because the ECU only monitors whether the relay coil is open/shorted (as I expected) not whether the actual fan is operating. I'm going to my storage unit to pick the fan up and install it tonight, if I can find my set of OEM PDC terminals I'm going to get a head start on this mod.

But that's what I mean, splice a wire and switch to the wire that switches the relay on and off. According to the OP, the ECU or whatever triggers the relay by grounding it NOT by supplying power to it right? So you'd just need to splice in to that and add a switch with a ground on the other end. At any given time, either the ECU grounds the relay, or your switch grounds the relay, or both ground it.

If the ECU switched the relay by supplying power, then you could have a problem duplicating that with a switch, because the power from the switch would fight the ECU when both were switched on.

[kastein]

You're partially correct. The ECU does switch the relay on by grounding the terminal - but what you're missing is that the ECU will notice if you ground the same terminal by yourself. It'll throw a CEL saying that the fan relay is stuck low for some reason.

What this mod does is turn the relay on without forcing the ECU fan relay control wire to ground, so the ECU never realizes you're doing anything.

[winkosmosis]

Ah OK, makes sense.

How does the ECU know all this?

[kastein]

Hmm... want the electrical engineer nerdy version or the easy version?

Easy version - it watches the output wire and when it isn't doing the same thing it's telling it to, it screams about it.

EE nerdy version - The ECU monitors the ground-side-switched radiator fan relay drive line. When the voltage present on the line indicates it is grounded when the drive transistor is not turned on, or when the voltage present on the line indicates it is not grounded when the drive transistor is turned on, a fault code is registered - most likely P1491, Radiator Fan Control Relay Circuit (An open or shorted condition detected in the radiator fan control relay control circuit. This includes PWM solid state relays.)

[winkosmosis]

The EE version made more sense. It sounds like the ECU is too smart for its own good

[kastein]

I tend to agree. It's fairly shortsighted, all it does is tell the ECU that its own components are working and theoretically connected to something. It's like target shooting by listening for the gunshot instead of seeing if you hit the target. But hey, it's what we have to work with, and I'm glad they set it up that way because it makes it easier to override with a switch without triggering a CEL.

[truckntractorhick]

well kastien if u find the wire colors in your 98 please lemme know! i split my loom and have the relay and splices ready to go!

EDIT: wait.. what harness is it in? the bigger one on the little one?

[MrShaft696]

Ive looked over a few write ups on this issue, and I am just curious where the heck you get a relay, and where exactly do you put it if, just making he fan switched is all you want to do?

[hubs97xj]

waytekwire, delcity, most parts stores...

Truckntractorhick, the wire colors are the same on my 97, so I'd venture a guess that they didn't change between 96 and 99. Same colors as in fyr's schematic, as far as I know.

You can access the blue/pink wire near the connector on the PCM- that's what I did while fooling around with a crude attempt at this. I'll have to check, but I believe it's in the third connector, the rear-most one.

[truckntractorhick]

yea im still tryin 2 find them here in my garage! ive found: dark blue with, orange, gray, black, and white but no pink! unless that gray one is....... ugh i hate all these different s

[kastein]

I did this mod a couple nights ago because I felt like it (I have no need for it yet.) I did it by installing the relay in the PDC, though, so my findings will likely not help that much. It wasn't bad, just cutting and splicing a few wires and I was done.

[truckntractorhick]

pdc= ? fuse block er something?

[Begster]

Power Distribution Center (aka under hood fuse box)

[CharlesS]

Originally Posted by fyrfytr1717

Sorry, I only have a '99 FSM. I'd assume '98 and '99 would have the same colors, but it's entirely possible that they're different. My wires were pretty burried in that loom and required some considerable digging around with a dental pick to pull them out of the bunch.

I did not use a new fuse in my wiring set up. The old relay retains fuse protection via the original circuit and the new relay gets it's fuse protection through the ST-RUN circuit that you tap into.

I believe I used either a 30 or 40 amp fuse, however it really doesn't matter as there is very little load being passed through it. I used the same size and type of relay as many of my other accessory circuits use so that I can carry fewer spares.

As an update for anyone interested in this thread, 4+ months later and this set up is still working perfectly for me. The fan still cycles on and off with the A/C, turns on and off at the appropriate engine temps, turns on and off with a flip of the switch, does not trigger any CEL's, does not blow fuses, no problems at all...

Excellent write up and very clean wiring...
Did this same modification to my '98 almost 8 years ago and much like you it required research with the FSM and probing around with a VOM.