1991 YJ O2 Sensor Wiring - Jeep Wrangler Forum
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Old 11-16-2013, 12:38 AM   #1
brandon.'s Avatar
Join Date: Jun 2013
Posts: 27
1991 YJ O2 Sensor Wiring

Does anyone know what color wires are the signal/ground/heater for a 1991 yj? I've found a bunch of threads and diagrams but none match my jeep. Coming from the harness I have a green wire, a black wire, a brown wire, and a white/grey wire. The previous owner cut the sensor off so I have no idea how it is supposed to go.

1991 Wrangler 4.0, SOA, Dana 44 front 60 rear both locked with 4.10's and 35 inch Pro Comp Mud Terrains
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Old 11-17-2013, 12:54 PM   #2

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Location: North Las Vegas
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Found this looking but doesn't match your colors here is the link to the thread

It was a 94 4.0.

O2 sensor should or should not be connected to ?? - JeepForum.com

Think you're right. i've looked at the wiring again comparing with a 4wire O2 plug layout and found the wire black/light blue comming from the PCM and supplying sensors with voltage so the O2 sensor plug, i think is like this
- Dark green/orange=12Vdc coming from start ignition supplying the heating element in the O2 sensor
- black is ground from the O2 heating element
- black/light blue is voltage from PCM to O2 sensor
- black/drk green is O2 sensor voltage output to PCM, for controlling air/fuel mix.

How far up the harness was it cut off? Can you go back in the wire loom and find the right wires???

89 2.5 TBI, 4" RC lift, 1" Booms, SYE, Adams Driveshafts front and rear, Ford 8.8 with 4:88s, BFG 33X12.50X15, and a ton of other stuff.

AF Jeep Club Tail Number 184
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Old 11-18-2013, 02:22 AM   #3
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Join Date: Jun 2013
Posts: 27
I read that thread before but since my colors are completely different it doesn't really help. I've yet to find a thread where someone has the same wire colors as I do.

The wiring was cut right where it splits from the end of the valve cover to go down to the oxygen sensor. I can trace the wires but I have no idea where they're suppose to go other than the signal wire goes to the ecm and one is grounded. I currently have it wired for grey to ecm, black to ground and both brown and green to the heater. This is based on an autozone wiring diagram but it still smells very rich so I'm not sure if it is correct. I did unhook the battery and drain the capacitors so in theory the ecm is reset.
1991 Wrangler 4.0, SOA, Dana 44 front 60 rear both locked with 4.10's and 35 inch Pro Comp Mud Terrains
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