obd1 o2 sensor wiring?

Hi guys,

I'm replacing my o2 sensor and I want to check the wiring on the engine harness to make sure everything is functioning properly.

It is a 92, so it's got the OBD1 wiring.

On the engine harness, I have no idea which wire is which...I don't have the best lighting and they all seem black and identical except for one which is larger and light blue. Three of the four have some continuity to ground, and with the key on I can't find any that have 12 volts against the negative battery terminal; the large one has .17 volts according to my multimeter. I'm thinking something is wrong with the heater circuit.

If anyone knows, can someone tell me what each of the four wires on the plug is for, and also the location of any fuses or relays for the heater circuit? I've searched a fair amount and I see lots of references to Renix or OBD2 Jeeps, but not for early 90s models.

(The haynes manual engine control schematic for "91 and later" models shows what I assume is the main power lead for the heater circuit running through a "ballast resistor relay" which I have been unable to find. This schematic contains two fuel pump relays and two ballast resistor relays...doesn't make sense to me unless this is some kind of one-size-fits-all schematic)

Hopefully someone can shed some light so I can pass emissions by Wednesday!

Thanks- Rob

Hallo. The O2 sensor is for a '92 a Zirconium type and has 4 wires.
2 wires are white and are for the heater inside. I can't find a fuse for the O2 heater.
The black one is the signal for the ECU and the grey one is signal ground.

The Ballast Resistor Bypass Relay is near and outside the A/C pump. (There is only one and one Fuel pump relay)

'92 XJ .:kissyou:

hey, thanks!

I figured there aren't two fuel pump relays; I was more or less complaining about the Haynes diagram.

Since I can't find 12volts at any of the connectors, I'm trying to trace this circuit for the o2 heater. I'll check out this relay by the fuel pump. Does it make sense to you that the ballast resistor relay would switch power to the o2 heater?

I appreciate your help- Rob

Hallo. I don't think so. I believe the time is too short.
The O2 sensor heater is switching by the ECU (warming up mode).
The BRBR is only giving the fuelpump the full 12 Volts when starting.
After that the ballast resistor takes over and gives only 11 Volts to the pump for the sound reduction.
What I think is that the ECU gets a signal from the BRBR to use that for the O2 sensor. But I am not sure about that. :huh:

'92 XJ :smootch:

ok, I just did a more accurate resistance test, and I think there must be a short on the heater power lead.

On the engine harness side of the o2 plug, there are four connections. Assuming the connector clip is the "top" of the plug, then according to your earlier post the two lower connections should be for the heater, so one should be ground (bottom right) and one should be a switched 12 volt supply (bottom left, judging by the larger diameter wire that feeds into that spot). The top two connections should be an ecu ground and an ecu signal (top right and left, respectively, correlating with the O2 sensor wires).

These are the resistance values (using the negative battery terminal) with the ignition off:

Top left (ecu ground): 3 ohms
Top right (ecu signal): infinite
Bottom left (heater power): 5 ohms
Bottom right (heater ground): 3 ohms

3 ohms seems to be the inherent resistance in the chassis electrical system, judged by grounding to various things in the engine bay.

With the ignition off, I would expect the heater power lead to have infinite resistance to ground; to be connected to some relay or switch that is not switched on.

So I'll try and trace this circuit further. I don't see any evidence of wires melted by the exhaust manifold or anything.

Appreciate your help- Rob

okay, so maybe I'm just thinking too much here.

I just tested voltage between the lower two connectors (heater power & ground). I put the multimeter where I could see it as I was turning the key. For a second or two I got voltage, around 10 volts I think (hard to tell on a digital meter). Basically I got voltage when the fuel pump ran, which is only for a second or two. If there was a short in the circuit I don't see how it could read a voltage at all. So I'm not as worried now about frying the new o2 sensor; I'll install it and then test the plug voltage again with the engine (and fuel pump running).

Oh, and I was looking near the A/C compressor for the ballast resistor bypass relay you mentioned. The closest relay I see is more like behind the battery, bolted to the bracket for the coolant overflow tank. Is that what you meant? I'm curious about this relay b/c my fuel pump is quite noisy, but seems to operate fine in terms of providing adequate fuel pressure. I understand the ballast resistor is meant to quiet down the pump...so if it is being bypassed by this relay, then maybe that could cause a noisy pump. Something that could be ruled out before replacing the pump, at least.

Thanks for your help, -Rob

just tested it with the engine running...14.xx volts through the heater circuit. Not sure what I was doing yesterday. So, much ado about nothing, but thanks hoppenbrouwers for your replies.


-Rob