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Discussion Starter #1
I'm moving from a dual cat system to a single cat system on a cali truck and I'm trying to deal with the dual downstream sensors. I would like to just use one sensor and feed that single signal to both downstream channels of the PCM. Note: Upstream O2's are unaffected....I'm essentially moving to a 3 O2 system.

Looking at the wiring diagram, I think this is possible. Attachment 1 is the diagram for the current setup, modified to show the complete loop from the PDC back to the PCM for the downstream 02 signal circuit


You can see that the sensors are in parallel, so voltage wise, it shouldn't matter if I were to do something like attachment 2.

Can any of you wiring gurus tell me why this would not work?
 

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Turbo Dakota Junkie
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What year is your truck?

My '99 has this system and the PCM is looking for different voltage return between O2 sensor 1 and O2 sensor 2.
 

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You could wire it that way but it will throw a cel sooner or later when the pcm starts checking cats, o2 heater resistance and o2 response time. You are better off trying to use an 02 simulator or you could weld an extra 02 sensor bung into the exhaust and mount the other sensor there, problem with this though is that the rear o2 sensors do affect fuel trim, it is minimal but it does affect it, now when you get both 02 sensors reading the same thing for left and right side of the engine it will affect fuel trims accordingly.
 

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Discussion Starter #4
it's a 2001. The PCM compares the downstream against it's matching upstream to make sure there is lag in the signal. I figured one sensor after the combined cat would produce enough lag to make both upstreams happy.

How much does the PCM use the downstream to adjust fuel trim? I though it only tweaked the LT trim to try and maximize converter efficiency. How do O2 sims not screw with it? Is it because they read 'perfect' air?

Does anyone know what the thread size on the O2 sensor is?
 

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Discussion Starter #5
Hey l1tech, are you positive about the PCM checking the heater resistance on the downstreams?

Reading my wiring diagram in the FSM, the heater circuit for the downstream 02's shares a common power lead and a common ground. Power is supplied straight from the PDC (through a relay), and the ground is just the common earth ground....there is no input anywhere to the PCM.

The actual sensor shares a common power lead from the PDC, and each grounds in the PCM (which is the signal).

The upstream sensors are different, in that they do ground the heater circuit through the PCM. However, if this wiring diagram is correct, the PCM should just see signal and not be able to diagnose which sensor it is coming from (unless it cross compares downstream O2's).
 

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Blown and Squirted
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Just put two bungs after the one cat. They are like two dollars. The PCM will be dumb and happy.

Sims are bad news on (*some) of these trucks.

I did the opposite, and went from one cat to two in my duals. Didn't need to wire an extra o2 though.... :jester:
 

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Turbo Dakota Junkie
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Just put two bungs after the one cat. They are like two dollars. The PCM will be dumb and happy.

Sims are bad news on (*some) of these trucks.

I did the opposite, and went from one cat to two in my duals. Didn't need to wire any extra o2 though.... :jester:
Hahahaha.... maybe his 2001 would be.

I have both O2s in my crossover pipe and the PCM thinks one of them should be showing double the voltage.... so it's all pissed off. I showed it. I just unplugged one of them and made the PCM stay in open loop - and then pretend like I don't see the CEL. LOL
 

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Discussion Starter #9
I'm trying to avoid the sims. I really don't want to spend $120 for something that may or may not work.

I also don't want to just zip tie them to the frame, because that seems like a hack job, it screams bad news in the winter and supposedly doesn't work on the 2001's (although from the wiring diagram, I'm not sure why).

I'm also avoiding running 2 sensors post cat because:

1. The exhaust past the cat is stock, and I'm not sure I can find a shop that would just add an extra bung. (One bung is built into the cat).

2. Having 2 sensors inline, monitoring the same thing seems like another hack job install.

I have thought about finding a mff pipe T fitting and doing something similar to the spark plug anti fouler trick, except double sided, but I don't know the dimensions or threading of the sensor
 

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Turbo Dakota Junkie
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Considering the fact that having an O2 sensor for each bank was a hack from Chrysler to get around the unbalanced fuel delivery because of the returnless fuel system.... adding an O2 bung doesn't sound too bad actually.
 

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we add o2 bungs all the time to custom exhausts its not a hack at all, way better then cutting into your wiring
 

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Discussion Starter #12
Cut it? Hell no. I was just going to make an extension harness that y'd the two leads into one.

I still have to extend the wires. What gauge do you guys use and do you use any special loom?
 

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Hey l1tech, are you positive about the PCM checking the heater resistance on the downstreams?

Reading my wiring diagram in the FSM, the heater circuit for the downstream 02's shares a common power lead and a common ground. Power is supplied straight from the PDC (through a relay), and the ground is just the common earth ground....there is no input anywhere to the PCM.

The actual sensor shares a common power lead from the PDC, and each grounds in the PCM (which is the signal).

The upstream sensors are different, in that they do ground the heater circuit through the PCM. However, if this wiring diagram is correct, the PCM should just see signal and not be able to diagnose which sensor it is coming from (unless it cross compares downstream O2's).
While it may not exactly check the heater resistance what it does is turn on the heater and then check the signal wire to make sure that the voltage is acting accordingly as the sensor heats. If I am not mistaken the 02's start off at 5 volts and as they heat up the voltage comes down to about .450 volts and then normal operation takes place from there. When the pcm turns on the heater relay it expects to see the voltage change adn being that you have wired both sensors together it more than likely is going to freak the living shit out of the pcm.
 

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Considering the fact that having an O2 sensor for each bank was a hack from Chrysler to get around the unbalanced fuel delivery because of the returnless fuel system.... adding an O2 bung doesn't sound too bad actually.
The reason they added O2 sensors on each bank was to be able to better control fuel delivery...you can't honestly think that each cylinder will require the exact same amount of fuel to perform to it's fullest capability. there are systems in the works that have an 02 sensor on each cylinder...that's gonna be fun
 

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Discussion Starter #15
Yea, but the pcm can't controll the heaters individually ( at least the downstream). It throws the downstream heater relay and all downstream heaters get power. In this case the one sensor would get power and report back a signal that matches that. The pcm will just see 2 heater warming up as usual; it would have to cross compare the two signals to notice anything fishy.

The one caveat I did think of involved how the pcm measures the voltage in the line. If that measurement is current sensitive, then the split in the harness would screw it up.

I kind of want to try this just to see if it would work. I just need to find 2 bad O2 sensors (for their connectors) and a connector off a factory harness.
 

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Turbo Dakota Junkie
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The reason they added O2 sensors on each bank was to be able to better control fuel delivery...you can't honestly think that each cylinder will require the exact same amount of fuel to perform to it's fullest capability. there are systems in the works that have an 02 sensor on each cylinder...that's gonna be fun
I did some testing on my 4.7.....
There was a 15 rwhp gain just from balancing the temperatures at the injectors on the fuel rail. The reason these engines have always turned sluggish when they got was in a large part because one side of the engine was rich while the other side was lean. The returnless fuel rail was the reason.
 

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Discussion Starter #18
That's interesting. If you were to balance the rails but leave it returnless, would you pick up that power?

I think I can scrounge the connectors off of 02's easy enough, but does anyone know where I can find the connector like the one attached the factory harness (the one the O2 plugs into) without buying a full extension kit?
 

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Turbo Dakota Junkie
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That's interesting. If you were to balance the rails but leave it returnless, would you pick up that power?

I think I can scrounge the connectors off of 02's easy enough, but does anyone know where I can find the connector like the one attached the factory harness (the one the O2 plugs into) without buying a full extension kit?
I left the rails returnless - but installed a cooling system on the top of the rails to get the temps to each injector to be much closer to the same. The only real pickup in power is actually that it stays much closer to the same power as it is when the engine is still cooler and the fuel rails haven't heated up yet. Balancing the rail temps doesn't actually gain any power - but more like it keeps it from losing some power.
 

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You need to take a few basic electronics courses and then go back to the drawing board. Hooking two O2s into one will definitely throw a code in the PCM as the voltage(the signal the PCM is looking for) will not be the same.
 
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