1989 V3500 Oxygen Sensor Wire Location?

[Coal creek Chris]

New to the forum. Have a emissions/wiring issue. Wondering if anybody could help me find the general area I should look in the engine bay for a single wire O2 sensor connector. Have an 89 V3500 7.4L that currently has no O2 sensor or cat installed. There is a threaded plug in the stock manifold on drivers side where the O2 sensor should go. Truck runs fine as is with plenty of power, but exhaust smells a bit rich. Planning to reinstall a cat and the O2 sensor but I can't find where to connect the wire to in the engine bay. Any thoughts on where I should be looking or at least what color wire? There is a plastic connector on the driver side beneath the master cylinder that is the right shape and accepts the O2 sensor connector. But it was already connected to something else and when I disconnected it, the check engine line came on immediately. One of the two wires goes to the distributor, the other one goes somewhere else and I did not yet track where it goes. Both are white wires with a black stripe on them.

 

[1987 GMC Jimmy]

Do you have a picture of what you unplugged the connector from?

 

[Rusty Nail]

In my '88 the wire came out of a loom what was on top of the trans bellhousing, driver side up tight at the firewall and I thought that wire was GREEN?

I bet he unplugged the spout connector @1987 GMC Jimmy
You might can fix a three valve Ford that way hahah, but it won't work in your old Chevrolet.

Gray with a BLACK stripe? Yeah, maybe.

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[Charlie]

 

[ajd89]

The o2 wire on my 89 r3500 is purple. And just like Rusty said it comes out behind the distributor on top of the trans bellhousing. The only one near my master cylinder that would plug in to the o2 is tan/white and is the wire you unplug when setting the timing.

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[Coal creek Chris]

Thanks everybody for the helpful tips, much appreciated. In case it matters for my question, this is a manual transmission truck that originally had AC. Truck doesn't have a wire loom that goes over the bell housing/transmission at the firewall, but maybe a previous owner moved wires? Looks like my engine is not original so maybe wire looms got shifted around in the past. There is a wire loom that routes behind the TBI from the driver side, maybe that one can sometimes be down on the bell? It's the same loom as in my first picture below.

Below is a pic of what I unplugged near the master cyl with tan and stripe wires and was what I mentioned in the original post. Looks like the same thing ajdemello had a picture of for timing. This connector fits onto the new O2 sensor.

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I couldn't find a single purple wire looking for a home, but I do have two single green wires that are not connected to anything. One has a spade connector on it and I'm guessing it goes to what looks like a broken off oil sender on the driver side head. My oil gauge doesn't work so maybe that's why.

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The other green wire is on the passenger side, top of engine and has a strange connector end on it that I haven't seen before. Maybe this second one used to have a black plastic part on it that would mate with the O2 sensor? But this one is on the passenger side and I don't think it would reach the O2 sensor location without splicing in more wire and a new plastic connector to mate up with the new sensor.

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Does anybody know where the O2 sensor wire goes to first? Directly to the computer? I could try to follow the green wires back upstream. Wish I had the right wiring diagram, but the one in my manual doesn't seem to be right for this truck.

 

[1987 GMC Jimmy]

Yeah, that’s your EST bypass wire with the corresponding connector. Make sure that’s plugged in the correct place, or your truck is going to be very limited in terms of what it can do about adjusting your timing as you drive. One of those green ones is the coolant temp sender wire. The other is probably the A/C signal wire. The O2 wire plugs into the ECM at D7 (the connectors and pins are all labeled), and it’s a purple wire. You can find the purple wire coming out of the firewall grommet to the left of the distributor. It should be the only solid purple wire on the ECM harness. There is a notation for a possibly purple unspecified sensor ground so I would follow anything you suspect just to be safe.

 

[Coal creek Chris]

Thank you. I will hunt down purple wires coming out of the firewall as the next step. Any thoughts on how important it is to have the O2 sensor if I install a new cat? Could I run it without the O2 sensor like it is now or might I fry the cat?

 

[1987 GMC Jimmy]

Damaging the cat due to a rich fuel mixture is more of a long term thing I’d say, but you need it so the truck can properly trim the mixture, get as good as possible fuel economy, not prematurely waste the spark plugs, not prematurely contaminate the oil, not gunk up the valves anymore than normal, etc. Has it not been giving you a check engine light? You should have had two separate codes, 42 for when the EST was unplugged and 13 for the ECM not seeing the O2 sensor when it reached normal operating parameters.

 

[Coal creek Chris]

Thanks. Check engine light has come on a couple times but it doesn’t stay on. It had code 13 stored when I checked it last. I bought this truck to be used for firewood, log and tree branch hauling but I don’t plan to drive it daily. Since my coolant sensor is unplugged might that prevent it from knowing it is at normal operating temp?

 

[Coal creek Chris]

Ended up finding a cut purple wire tucked back into loom. Spliced it to a new connector and all good. Hardest part was getting out the threaded plug from the manifold to put the sensor in. Not a lot of room to work in there. Thanks again for these tips. Much appreciated.

 

[Coal creek Chris]

Went on my first test drive after a bunch of different repairs done this winter. Re-installed smog pump, properly routed vacuum lines, various wiring repairs, reinstalled new O2 sensor and new Cat. Once the truck is warmed up, it runs fantastic now, much more power and I can accelerate up steep hills in 4th gear which it would not do before. Most people talk about taking off the emissions stuff, but I think with a TBI and a computer it must be necessary to have this stuff installed or the engine gets confused on what to do. Thanks for all the tips.