Oxygen Sensor Wiring

[Rev Engineer]

If the DME doesn't make the switch to the heater relay, then this would then be the root of the problem wouldn't it?
There must be an input/sensor somewhere to inform the DME to switch it on for a certain time, then switch if off later when the O2 has heated up

tracing back to the engine temp sensor/coolant sensor, since we are running m20 sensors, would this then be the link for the DME to switch the heater relay on and off based on the temp sender from this particular sensor

I'm with tjjartge on this one. For whatever reason, the dme is deciding not to turn on the heater. It's not that the dme is trying to turn it on and there's no voltage/power. I'm not sure where or how this could be a power/gnd/relay wiring issue unless there is another pwr or gnd terminal that needs to be wired to the dme - or the dme is not seeing a sensor signal.

As for the temp sender, I don't think it's there. The cluster and dme temp sensors are different - I'm running the stock s50 engine temp sensor that feeds the dme. I've only swapped out the sensor which feeds the instrument cluster. As far as I know, the dme engine temp sensor is self-contained in the harness.

I'll double check my car for codes, but that last time I checked I got the "no codes" code. Can anyone confirm if they are getting an o2 sensor code?

I just read Jason's comments on the other thread. If I'm understanding this correctly, the e36 fuel pump relay normally powers the e30 green/violet wire which in-turn goes to the fuel pump and possibly some other place which has something to do with the e30 o2 heater? It's this alternate path which must somehow work it's way back to the e36 dme and then doesn't allow the dme to turn on the o2 heater relay? Or, maybe that backfeed voltage winds it way to the o2 heater relay and maybe the dme is trying to switch it's ground signal, but can't!? Jason's fix sounds like it isolates the fuel pump and e30 o2 heater on the green/violet wire - so that the e36 fuel pump relay only feeds voltage to the fuel pump and nothing else? I guess you're looking on e30 schematics to find alternate paths on the green/violet wire.

[Mike B.]

OT this is far reaching but having my o2 relay jumped, could that affect the temp guage?

I also searched around for answers and i came up with jason's old thread.

http://www.r3vlimited.com/board/showthread.php?t=55208&highlight=o2

how would one pull power from the fuse 11 for the green/violet wires? also, if i disconnect the green/voilet wires at the c101 where would i draw power from for fuel pump. I'm a bit confused:( I think you mean x20 on the e36 harness side right?

Nice find! Search is definitely our friend.

If I read Jason's post correctly, in order to get the O2 heater to work properly, we are going to bypass the C101 and fuse 11 to make the connection to E36 harness pin 13 (gn/vi) via a fusable link eliminating the violet/red wiring, correct?

This should be done instead of the pin 13 to pin 13 connection that is typically described for the swap.

Looking at the E30 wiring diagram ('89 325i/is) you can see where you would need to make the changes.

[permit]

I can confirm that I'm getting an O2 sensor code from the CEL.

[tonytony]

If I read Jason's post correctly, in order to get the O2 heater to work properly, we are going to bypass the C101 and fuse 11 to make the connection to E36 harness pin 13 (gn/vi) via a fusable link eliminating the violet/red wiring, correct?

i still dont see how it would work if you bypass the fuse and the c101, wouldn't it be the same thing just more direct? Maybe i'm just brainfarting right now, i'll read it over 20more times. x20 pin 13> fusible link> fuel pump wire

[Mike B.]

I am thinking that there is something in the power distribution box that is backfeeding into the system and elimating this portion of the wiring solves this issue.

It would be interesting to dig into the power distribution box to see what the actual cause of the backfeeding is, and solve the problem instead of bypassing it, but I don't think I have the electrical chops to do something like this.

[Jason89i]

I just read Jason's comments on the other thread. If I'm understanding this correctly, the e36 fuel pump relay normally powers the e30 green/violet wire which in-turn goes to the fuel pump and possibly some other place which has something to do with the e30 o2 heater? It's this alternate path which must somehow work it's way back to XXXXXXXXX Or, maybe that backfeed voltage winds it way to the o2 heater relay and maybe the dme is trying to switch it's ground signal, but can't!? .

CORRECT.

Jason's fix sounds like it isolates the fuel pump and e30 o2 heater on the green/violet wire - so that the e36 fuel pump relay only feeds voltage to the fuel pump and nothing else? I guess you're looking on e30 schematics to find alternate paths on the green/violet wire.

CORRECT. however, the e30 wiring schematics show the fp wiring as isolated. i do not believe that the e30 wiring diagram is correct. i believe the fuelpump circuit is NOT isolated. I DO NOT HAVE ANY PROOF THOUGH. my "fix" is to isolate the fuelpump circuit. now there is no source for the backfeed.

i have trashed all the shells, i dont feel like tearing apart the m3 or my winter beater. so, someone step up to the plate. find out what is connecting the fp circuit to ??o2 circuit?? post c101 and pre fuse.

im not comfortable telling people how to make this "fix" because i have not figured out WHAT is causing the back feed. the internet is already flawed with "hear-say" advice without any knowledge or experience. what the heck, lets figure this out and put it to bed.

cheers, jason

Nice find! Search is definitely our friend.

If I read Jason's post correctly, in order to get the O2 heater to work properly, we are going to bypass the C101 and fuse 11 to make the connection to E36 harness pin 13 (gn/vi) via a fusable link eliminating the violet/red wiring, correct?

This should be done instead of the pin 13 to pin 13 connection that is typically described for the swap.

Looking at the E30 wiring diagram ('89 325i/is) you can see where you would need to make the changes.
]

that is correct. the system is isolated AFTER the fuse. so, i used a blade insert to fuse11, fusable link (15a fuse), to the x20. this bypasses the c101 and all wiring before the fuse. VIOLA. system not backfed. 12v to heater, grounding signal not powered to dme (like its ment to be.)

[Mike B.]

This may be a dumb question, but what is a "fuse insert"? A new baby and lack of sleep has my mind in a fog.

[Rev Engineer]

I was intrigued enough so I dug into my e30 tonight... Here's what I found on my s50->11/87 e30 m3. The violet/red wire goes from c101 directly into the fuse box and then to fuse #11. I opened the fuse box to confirm this - it really appears to be isolated and doesn't connect to anything else. I'm 99% sure of this on my car. The other side of the fuse (green/violet) powers the fuel pump. I happen to have a entire scrapped e30 fuse box assembly and I was able to confirm on this that fuse #11 connects to wires and does not connect to the pcb. All of the wires on this unit we're cut, so I confirmed the violet/red wire routing in my own car. It leaves fuse #11 than heads directly into the wire bundle which goes to c101.

I tested the heater relay in a few ways. First, I jumpered pin #38 at the dme to ground and confirmed that the relay turns on (main relay needs to be jumpered on with dme disconnected). I then turned on the fuel pump relay and confirmed that the heater relay still works. To me, this confirms that the dme has control over the relay and there shouldn't be any back-feed voltages that are preventing the dme o2 drive transistor from switching ground to the relay. For whatever reason, the dme is deciding not to turn the relay on. Or, unlikely, the drive tranistor in the dme is blown.

It remains a mystery... to be continued...

[tonytony]

wow congrats for the work you've done, i'm curious to see what you come up with

[Rev Engineer]

I'm getting further here... I hooked up a 12V lamp to the o2 heater wire and went for a drive. After about 30-60 sec of driving the lamp came on. So, it works! It actually stays on nearly all of the time and only seems to shut off under heavy load. The final assessment for my car is a-ok. I was a little suspicious since I've been getting terrible gas mileage, but I've also been driving with a 4.10 diff. So for me – no codes and the o2 heater works.

[Mike B.]

Good idea with the 12v lamp. I'll have to try that test myself.

So you were not able to see 12v when you checked the wiring at the relay (switched/running), but during driving the DME "flipped the switch" and turned the heater on, correct?

[tonytony]

in my situation my fuel pump relay isn't even turned on it's just working now cause i ground the brown/green wire it out

[permit]

I believe the O2 sensor switches off when you gun it, so that sounds about right.

I just failed smog because of the CEL (due to the heater wire), so hopefully I can get mine to work by following whatever the hell you did (I am the suck at wiring).

[Rev Engineer]

Good idea with the 12v lamp. I'll have to try that test myself.

So you were not able to see 12v when you checked the wiring at the relay (switched/running), but during driving the DME "flipped the switch" and turned the heater on, correct?

Correct. I did not see any voltage on the heater wire with the car just sitting there at idle. Only after driving it did it come on. I also noticed that the heater wire would stay on after the engine was shut off - but I'm not sure for how long. Could be another way to test it. Start the car and take it for a quick spin, stop the engine without turning the car off, pop the hood and measure voltage. If it's like my car, you'll find the relay on.

[Rev Engineer]

in my situation my fuel pump relay isn't even turned on it's just working now cause i ground the brown/green wire it out

Does the main relay turn on? This needs to be on before the fuel pump or o2 heater relay will work. Check the solid red wire to the main relay - should be +12 always. When the main relay turns on, it switches power to the red/white wires which feed the o2 and fuel pump relays. The DME controls everything. If the main relay turns on, and there is power to the red/white wires than it's the dme which might be having trouble turning on the fuel pump relay. Can you ground the dme connection to the fuel pump relay to get it to turn on?

Here's what I did during my testing of the o2 heater relay. You'll need to follow a schematic to determine the dme pin nubmers - be careful the numbering sequence. Remove the dme and short the main relay wire/pin to ground - you should here the main relay click. Next leave the main relay on and short the fuel pump dme pin to ground and you should here the relay click or turn the pump on. Sames goes with the o2 heater relay. This verifies wiring of all relays. If it all works fine than blame the dme.