WHERE IS THE FUSE FOR THE HEATING CIRCUIT FOR THE O2 SENSOR?

I've read that the 02 sensor's heater circuit has an in-line fuse somewhere,
and I suspect that my wires shorted out and blew that fuse, but I can't find it, I don't know where to look, and don't know what it looks like.

I also saw a reference that the MY1993 XJS has it's own relay to power the heater for the O2 sensor. Can anyone confirm if this is the case, and also tell me where to find that relay? I didn't see it listed on any of the electrical diagrams that I looked at. I've read that the MY1992 used the fuel pump relay to power the 02 sensor's heater circuit.

 

The O2 heater comes directly from pin 87 of the fuel pump relay. If it's in the same position as the 89 then it's above the battery on the RHS of the boot.

http://www.jagrepair.com/images/Elec...Electrical.pdf

 

Thank you warrjon. You even gave me a link to the electrical diagram
for my model and year! That's what I call helpful.

While I had the 1993 MY Updated document, I somehow overlooked the diagram you linked to in your post.

Thank you again... I'm now reading it and trying to digest everything.

Sincerely,
-Philip

 

So, it seems that the 02 heater circuit DOESN'T HAVE A FUSE.
It's powered by the output of the fuel pump relay, and as best I could tell, there is no fuse.

I tested the output under the hood, and although it read +12V, when I plugged it into the 02 sensor it dropped to 0 volts. I think that the output from ECU is weak. It only takes 3 amps to drive the circuit, but still, I think that was too much.

My solution:

I tapped into the output of the fuse for the A/C fan relay which is inside the cabin on the passenger's side. I upgraded the fuse from 3 amps to 10 amps, and tapped into the output wire, ran the new wire through the firewall grommet that the main harness uses, and into the engine bay. Then I crimped it to the blue with red wire on the harness leading to the 02 sensor.

For the black wire on the 02 sensor harness, I cut it off, crimped on a wire with an 0 ring, and attached that to the ground post on the fire wall. Lastly, I carefully cleaned up all of the grounds in the engine bay, and then (finally) I cut off the shielded blue signal wire from the 02 sensor, removed some of the shielding, and crimped on a 1/4 male spade with heat shrink tubing.

The result... all new connectors, with heat shrink tubing, and a new +12V for the heater on the 02 sensor and a new ground.

It worked!!!! My car doesn't through a FF44 code anymore, and I passed the emissions inspection !!!

Next todo.... I'll need to go to the ECU, and check that that power feed to it is good, and remove, clean and reattach any ground wires, assuming that there are some by the ECU. Maybe the ECU has a poor ground, and that's what was causing it to be anemic in terms of supply the 3amps of +12V to the 02 sensor heater circuit.

I hope this fix helps someone else.

P.S. -- The wire that I patched into IS small, but only has to power one relay fo the A/C fan plus 3 amps for the 02 sensor... in other words, I'm fine... it's not overloaded.

-Phil

 

If the ground to the ECU was that bad the ECU would not be able to drive the injectors. I would be looking for a dirty connector first.

You can use a 4ohm 100watt resistor as a substitute for the O2 heater to fault find. This will simulate the 3amp draw from the ECU. If a connector is at fault then it will get warm when power is applied to the circuit.