Hi all, it's been awhile since the last time I was here. My poor old 89 240 has mostly been parked for the last several years. I got it out the other day and noticed that the O2 sensor wasn't working too well. I replaced it with an NTK brand ( I'm not a big fan of Bosch anymore) anyway after replacing it my air control meter stayed at a steady level even after several minutes of warming the engine. If I kept the engine running above an idle it seemed to register correctly. Ok long story short, the O2 sensor is not getting any voltage to the heater side of the sensor. What I need to know is, Where does the O2 sensor get it's heater voltage from and what if any relays are involved? If anyone has a wiring diagram I can refer to or if anyone can just explain the most likely cause for the loss of heater voltage I would greatly appreciate it. Thanks for reading my message. Max. P.S. HI Art, been awhile.

The oxygen sensor heater shares fuse 4 with the fuel pump in the tank. Fuse 4 is only activated by the fuel pump relay, so it is on with the engine running, but not if you merely turn the ignition key on.
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Art Benstein near Baltimore

What engineers say and what they mean by it:
Engineering says: "Preliminary operational tests are inconclusive"
Engineers meant: The darn thing blew up when we threw the switch.

Thanks Art. I knew that it ran off of fuse #4 but didn’t know it was only
active while running. I’m hoping the relay may have a bad connection on one
of the contacts as I get power to #4 while running but still no voltage to
the O2 heater. As long as I’m driving my aftermarket O2 gauge shows it
working property but when at idle it does not vary from .01 to .09 volts as
it should and the idle gets a bit rough. I let the car sit for over a year
and had to pull all the fuses and clean the holders with a brass brush. So
I thinking the relay may be corroded and need replacing. At least that’s my
next step, if it doesn’t work after that I guess it’s time to find the
broken wire. Thanks for your help. Max.

The relay gets involved, yes, but if it were the trouble, you wouldn't have any fuel pumps either.

Sounds like you've made some mods, installing something non-Bosch and something to watch the sensor output. I'd suggest you check over these new connections.

https://web.archive.org/web/20180417040108/http://www.volvowiringdiagrams.com/volvo/240%20Wiring%20Diagrams/Volvo%20240%201989.pdf

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Art Benstein near Baltimore

What engineers say and what they mean by it:
Engineering says: "Modifications are underway to correct certain minor difficulties"
Engineers meant: We threw the whole thing out and are starting from scratch.

Thanks again Art. The O3 gauge I installed has been in there for a little
over 19 years. I disconnected it just in case but my analog volt meter
still shows no voltage change while idling. And yes I replaced the Bosch O2
sensor with an NTK just because of recommendations from other people having
trouble with the Bosch. But I replaced it because the Bosch was the problem
originally. It was acting exactly like the new sensor. So my problem to
begin with was the fact that I can’t get any voltage to the heater side of
the sensor. I had thought at one time I had seen something about the ECM
activating the heater but now I can’t find anything about it. I either
imagined it or I lost the article I was reading. Maybe I’m just getting too
old to do this stuff anymore. I’ll recheck everything but right now I can’t
figure out why there’s no voltage. Thanks again

Is the NTK sensor you put in a zirconia sensor, or is it a titania sensor? What are the lead colors and how are you determining it isn't getting heated? What is the resistance of the heater cold, and what is it hot (if only from exhaust)? How did you adapt the Volvo-specific Bosch harness to the NTK? Many questions, maybe a few pictures would answer, if no other way.

The 89 240 didn't "control" the heater for the oxygen sensor. It just feeds it 12V whenever the motor is running.
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Art Benstein near Baltimore

What engineers say and what they mean by it:
Engineering says: "Essentially complete."
Engineers meant: Half done.

Thanks for being so patient with this problem. It will be a few days before
I can get back under it to answer your questions definitely. It’s raining
now and I hate to get my wherlchair out on the rain plus I have to have my
neighbor help me with the jack stands. But if my memory serves me the
three(3) wires were black, white and green. I spliced the NTK(or if you get
it from their other division it’s calked NGK) into the OEN wiring harness
the same way I did when I replaced the original Bosch sensor about 15 years
ago with the Bosch sensor that is used in the Ford Mustang. This is the
replacement sensor that cross reference s to the Bosch number. . I always
had good results from the Ford sensor. It’s the one that everyone talked
about using to replace the expensive OEM Bosch. All wires are soldered, no
crimps. I determined the heater wasn’t working because after several
minutes of the engine running the sensor never fluctuated, it kept a
reading of around .5 volts. If I drive the car and get the sensor hot it
works just fine. The problem is that no matter what I do there is NO
voltage coming from the two wires that go to the heater from the wiring
harness . I’ll get under it this weekend and verify everything. Thanks
again Art.

Take as much time as you need. Chances are I'm not going anywhere, and neither is the Brickboard anytime soon. If your sensor is switching after a good hearty drive, then I believe you've identified the problem, and its fix is going to be somewhere in that splicing if you've proved fuse 4 is indeed intact and making good contact in the fuse panel.
--
Art Benstein near Baltimore

What engineers say and what they mean by it:
Engineering says: "We predict..."
Engineers meant: We hope to God!

Did your side your 02 sensor in or buy complete with harness?