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Tue Jul 29, 2008 9:32 pm

i am about to replace my entire exhaust system with an S&S system, including a new performance high flow cat and o2 sensor. the PO installed a full custom midas exhaust bypassing the o2 and no cat. now i'm putting a car and 02 back in it and i need to figure out the wiring. luckily for me the old o2 wiring harness was left plugged in in the engine bay. now i just need to know which wire goes to which wire. it has one black a two white tires. which white wire goes to the same plug with the ground black and which one goes to the single lead on it's own plug?

Wed Jul 30, 2008 4:19 am

Two of the wires would be the wiring for the "heat up" mode of the O2. My guess is that the white wire that is with the black, assuming they are seperated in this way are for the O2 "heat up" function, and the single white wire is the O2 lead, which will connect to the ECU. My 84 is a single wire setup; the one wire of the O2 connects to the 1 sheilded wire of the vanagons wire harness (which connects to the ECU). At some point, the Vanagons used a "heated" O2 sensor, meaning it was quickly heated up to temperature to become accurate quickly; a big advantage over the one wire, non heated O2's.

Wed Jul 30, 2008 1:00 pm

Mookman62 wrote: i am about to replace my entire exhaust system with an S&S system, including a new performance high flow cat and o2 sensor. the PO installed a full custom midas exhaust bypassing the o2 and no cat. now i'm putting a car and 02 back in it and i need to figure out the wiring. luckily for me the old o2 wiring harness was left plugged in in the engine bay. now i just need to know which wire goes to which wire. it has one black a two white tires. which white wire goes to the same plug with the ground black and which one goes to the single lead on it's own plug?

Hi
I have a problem very similar. I was failing emission test and the local wizard decided the O2 sensor had to be replaced> So he did with a Bosch "Universal" Part No. F 00H100 083 8951631 12V.
When time came, this local genius cried Uncle and declared that he could not hook it up as he "did not do electrical". I am now stuck with an unwired O2 sensor.
It has three wires, two White and one Black.
The fact that I am running a Subaru 2.2 KEP conversion in the crate might be relevant.
Anyone with affirmed advice and knowledge who could tell me which wire goes where ?

It would be most appreciated.

Wed Jul 30, 2008 1:31 pm

There is a simple way to test this, beside using your ohm meter. I cannot find the exact resistance of the heating element, which is one of the black and one of the white. But if you install it and run your engine and let it warm up a bit, like five minutes, only one white black combination will produce voltage in the .5-1.0 volt range. The other white is to the heating element.

Thu Jun 03, 2010 6:29 am

I'm about to replace my o2 sensor and I'm still confused, clearly I'm missing something. My Bosch o2 sensor (11027 / 0258001027009 from vancafe) has one black wire.

Am I correct in assuming the stock wiring is
black = ground
white = heater
white = sensor
?

If so then I'm a little confused by this statement and could do with some clarification: Dogpilot wrote: let it warm up a bit, like five minutes, only one white black combination will produce voltage in the .5-1.0 volt range. The other white is to the heating element.

Is the black/white .5-1v pair the ground/sensor pair?
In that case why is it not necessary to have two wires going to the new 'single wire' sensor? Grounding through the exhaust system seems likely to be problematic to me.

I'd really appreciate someone spelling this out.

Thanks /pauric

Thu Jun 03, 2010 8:16 am

If you have a one-wire sensor the single black wire is the signal, you connect that to the green coax signal input wire. The sensor grounds thru the metal of the exhaust tract.

The common misperception many have about the sensor is that it somehow receives power and modifies it like other sensors do. A reasonable assumption, but incorrect. An O2 sensor is based on a cell, a cell produces a voltage based on a chemical differential, in this case the differential between the concentrations of oxygen on the interior vs. exterior parts of the sensor. Compare it to the car battery, which is a series of 2V cells; the battery grounds at one end, like the sensor cell, and produces a higher electron potential (voltage) at the opposite end. The O2 sensor works a lot like that, it just makes a very weak 0-1V signal that varies in proportion to the changes in oxygen differential.

On three-wire sensors there is usually a single black wire, which is the signal, and two white wires for the heater element. Since the heater is just a resistive element there is no polarity, the two white wires can be connected to the heater power circuit the vehicle has. Actually, any older car with a single wire sensor can easily have a heater circuit added, it just has to have switched 12V power and a ground. This delivers faster sensor warmup and more consistent signal when the exhaust is cooler, such as at idle.

Three-wire sensors do not ground the sensor cell thru the heater circuit, they still ground the sensor cell thru the sensor body screwed into the exhaust. There are 4-wire sensors that have a dedicated cell ground. Once you move to 5-wire sensors, you're looking at a wideband sensor which can't be run in a vehicle without a special controller circuit.

Thu Jun 03, 2010 8:26 am

RadioRental wrote: I'm about to replace my o2 sensor and I'm still confused, clearly I'm missing something. My Bosch o2 sensor (11027 / 0258001027009 from vancafe) has one black wire.

Am I correct in assuming the stock wiring is
black = ground
white = heater
white = sensor
?

If so then I'm a little confused by this statement and could do with some clarification: Dogpilot wrote: let it warm up a bit, like five minutes, only one white black combination will produce voltage in the .5-1.0 volt range. The other white is to the heating element.

Is the black/white .5-1v pair the ground/sensor pair?
In that case why is it not necessary to have two wires going to the new 'single wire' sensor? Grounding through the exhaust system seems likely to be problematic to me.

I'd really appreciate someone spelling this out.

Thanks /pauric

I could be wrong, so those in-the-know please correct me if that's the case, but I believe all Digi-Jet and Digi-Fant systems use a 1-wire O2 sensor. The other wires are there to heat up the sensor, because the sensors don't give reliable data until they are really hot. And the quicker the system can rely on the sensor data, the quicker the ECU goes into closed-loop control, which is more efficient and results in lower tail-pipe emissions.

SO.... There is no "stock" wiring on the Digi's for anything other than a 1-wire sensor. But you're in luck, because you can figure out which of the 3 wires the ECU wants, by doing what DP suggested. Namely, run your van until it's well into the operating temperature range (10 minutes or so), which ensures your O2 sensor is giving reliable data (0.5V-1.0V ish), and measure the voltage on all 3 wires. Use anything handy as a ground (like the left cylinder head grounding point), remember that the O2 sensor is grounded to the exhaust pipe which is (eventually) grounded to the chassis, so the voltage difference is between 1 of those wires and chassis ground.

When the sensor is hot, only 1 wire should show a voltage, the other 2 will be floating around zero.

Once you find that wire, label it, and hook that up to your ECU, and you're done!

Thu Jun 03, 2010 8:36 am

Thanks guys! reading between the lines I will be doing the following to R&R the sensor

First - I have a 3 wire connector, connected to a 3 wire sensor. I am replacing that sensor with a single wire sensor. I do not have any shielded coax or green wire near the connector.

1) run engine till hot
2) measure voltages out of the sensor. 1 wire will have some reading, the other two will not. Note which wire it is. This wire is most likely the black wire.
3) remove old sensor.
4) insert new sensor and splice it's single wire in to the wire noted in step 2

sound correct?

thanks!

Thu Jun 03, 2010 8:51 am

Quote: but I believe all Digi-Jet and Digi-Fant systems use a 1-wire O2 sensor.

David, Digifant got a sensor heater circuit, a 3-wire was standard then. It was to improve the overall emissions profile for the US market. That's the same reason the oil/water heat exchanger was added, too, faster warmup means cleaner burning sooner.

Quote: I'm about to replace my o2 sensor and I'm still confused, clearly I'm missing something. My Bosch o2 sensor (11027 / 0258001027009 from vancafe) has one black wire.

The black wire is the signal output.This is true on a one-wire (like, duh?) as well as a 3-wire sensor.

Quote: First - I have a 3 wire connector, connected to a 3 wire sensor. I am replacing that sensor with a single wire sensor. I do not have any shielded coax or green wire near the connector.

You said you have an '87 2.1. There is no factory three-wire connector involved in the O2 sensor wiring, there is a fat green coaxial wire for signal, and a black two-pin Bosch-style connector for the heater. These are both hanging around along the left side of the engine bay. If it differs from this then someone has modified your wiring and in that case you're on your own.

Thu Jun 03, 2010 8:58 am

tencentlife wrote: Quote: but I believe all Digi-Jet and Digi-Fant systems use a 1-wire O2 sensor.

David, Digifant got a sensor heater circuit, a 3-wire was standard then. It was to improve the overall emissions profile for the US market. That's the same reason the oil/water heat exchanger was added, too, faster warmup means cleaner burning sooner.

I should have known. I'm happily stuck in the Digi-Jet world, and sometimes I assume everybody else is too. Digi-Jet is just SO simple, like me! :D Thanks!

Thu Jun 03, 2010 11:02 am

tencentlife wrote: You said you have an '87 2.1. There is no factory three-wire connector involved in the O2 sensor wiring, there is a fat green coaxial wire for signal, and a black two-pin Bosch-style connector for the heater. These are both hanging around along the left side of the engine bay. If it differs from this then someone has modified your wiring and in that case you're on your own.

I stand corrected. I didn't follow the wires back up to the green wire.

I'm not sure about the efficacy of shielding half a wire and only connecting one end of the shield to ground... but I'm sure there's probably a great reason for it.

Thanks again, now I only have to evict the old sensor, which it's reluctant to do.

Thu Jun 03, 2010 12:23 pm

Quote: I'm not sure about the efficacy of shielding half a wire and only connecting one end of the shield to ground... but I'm sure there's probably a great reason for it.

After all the back and forth about the shielding over the years, I've settled on the need being due to the O2 signal wires being bundled in with the injector triggering wires, which have what amounts to a pretty good ac current going thru them. Ac produces RF, and RF induction is worst when conductors run close and parallel to each other. There are other strong RF sources in the engine bay, but the injector wires are closest to the O2 signal wire. That's why the shielded coax only continues until the signal wire has left the wiring bundle, then stops.

Shielding is only ever grounded at one end. Grounding it in more than one location negates the protection it provides.

Thu Jun 03, 2010 1:25 pm

[quote="tencentlife"] Quote: Shielding is only ever grounded at one end. Grounding it in more than one location negates the protection it provides.

I beg to differ. Look at the coax for your analogue tv, cb radio antenna etc etc. Shields are usually tied off to the outside of the connector and then off to ground.

http://en.wikipedia.org/wiki/Coaxial_cable#Signal_leakage
"An ideal shield would be a perfect conductor with no holes, gaps or bumps connected to a perfect ground." the idea that shields should only be grounded at one end runs contrary to this principle

Clearly the design, as vw implemented, works but if I read the advice to check the grounding on the shield to reduce problems with the o2 sensor it just confirms the idea that either emi is getting in (a poorly ground shield only connected at one end is in effect, an antenna) or by expecting sensor signal ground to make the return path through the engine you induce a ground loop
http://en.wikipedia.org/wiki/Ground_loop_(electricity)

It's a small point but best practice, generally speaking, is to tie both ends of a shield off.

Thu Jun 03, 2010 1:47 pm

If the shield is grounded at each end, you could have two different potentials at each ground causing current flow. This will make the signal noisy. If there are grounds at multiple points on the shield, they must be common.

Thu Jun 03, 2010 2:42 pm

Keane wrote: If the shield is grounded at each end, you could have two different potentials at each ground causing current flow. This will make the signal noisy. If there are grounds at multiple points on the shield, they must be common.

And if the shield is grounded at one end and floating at the other? My understanding is that you are open to emi.

If you have potential difference between the ecu ground and sensor ground then something tells my you'll have issues with the single from the sensor as it relies on the path through the exhaust/block/bulkhead to return through.

"If there are grounds at multiple points on the shield, they must be common."
I'm not sure I understand what you mean here. Are you saying the exhaust, block & bulkhead are not common? That there's significant pd between them?