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'90 4.0 underhood switched power lead?

obaa-xj

NAXJA Forum User
Location
PNW
Hi,

Is there an ignition-switched +12 lead somewhere underhood on a '90 4.0?
I'm wiring in an aux cooling fan underhood, and looking for something I can use to trigger it on only when the ignition is on. I'd like to avoid trying to wire to the interior fuse box as the "empty grommets in the firewall are already stuffed with other wiring, plus it'll be much tidier this way.

Thanks!
 
+12 Key on is Yellow. Looks like the Ign module, O2 Heater realy, FP relay, and even the diagnostic connector have yellow supply wires.
 
The best way to wire it will depend on what your doing with the fan?
 
Thanks!

Is the +12 key plain yellow, or yellow with the black tracer? Generically, any (oem) yellow (or yellow/black?) wire under the hood should be +12 key? (Yes, I'll confirm myself with a meter..... :) I hate cutting through the insulation on a wire in order to tap into it, so I'm going to look for one with an exposed terminal which I can tap into.

As to background, the viscous coupling on my mech fan stopped coupling... I took it off, and my long term mileage records show there is an mpg improvement, and a (psychological, probably) seat of pants improvement. However, in the past couple of weeks, the temps shot up in the PDX area and it started overheating with only the aux AC fan.
I had a spare AC fine, so I wired in a 30A 5pin relay to power it. The current trigger is tapped off the existing AC fan, but that means the new fan doesn't kick in until the engine is already hot.
I want to do to things:
First, wire the trigger to +12 key, so it turns on when the Jeep is running. That should get me by for a while.
Second, I plan on adding a thermostat housing from a '96 that is tapped for a temp sensor switch. I saw a couple different sensors (for idiot light, not gauge).
If the sensor has a single lead, I'm guessing it will just short to gnd when triggered, so I will wire the gnd of the relay trigger circuit to it.
If the sensor has dual leads, I'm guessing it will contain a switch, but I will still wire the trigger gnd through it.
 
The wire is just Yellow, no tracer. You can back probe through the connector to verify key on.
 
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