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Discussion Starter #1
my bronco o2 plug has 4 leads in it...and the o2 sensor that "was" in it was a 4 wire bosche.

HOWEVER, I had extra o2 sensors laying around (sadly all bosche)...that were three wire, though they had the exact same pig tail that fit right into my bronco...

I tested voltage, and the one I used seems to be bad...stuck around .515 volts or so, whether I introduce a vacuum leak or not....

I go to parts stores, and they have both three and 4 wire for the same car.

Research tells me the 4th wire is only how it grounds...whether it grounds through the harness...a 4 wire, or whether it grounds through the o2 chassis into the exhaust (3 wire)....

So anyone have any more input on this?? I'm going to buy a new o2 sensor, since I was told to stay away from bosche, and the ones I have don't seem to be operating very well...I was looking at Denso...or ngk, any opinions there??

Just want to make sure the sensor I buy will work properly.
 

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Charlie don't surf..
'92 Ford Bronco XLT
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Pretty sure up to a certain year, the EEC to ground for the O2 sensors went somewhere on the chassis. If you have a 4 wire harness going to it, I'd stick with a 4 wire O2 sensor. You don't want to be without ground.

Also, for what it's worth, I've never had any trouble with Bosch O2 sensors..
 

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Some vehicles us a two ground system and some use the same ground for both the 02 and the heater, if you have a 4 wire comming from the chasis side of the plug then you need a 4 wire o2 or you will be missing a ground, more than likely the one for the heater circut. So no you can not use the 3 wire where a 4 wire is present. as for 02's we use alot of the Bosch and have good luck with them except in Jeeps. I personally use factory parts in my rigs whenever possable. As for the best aftermarket o2 , mabey NGK.
 

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On both a 3-wire and 4-wire HEGO, there is always a separate 12v PWR/GND circuit for the heater. This is key-on between the fuse panel and chasis ground. The difference between the 3 and 4-wire is in the SIG RTN (GND) to the PCM.

On a 3-wire HEGO, the HEGO SIG is from PCM Pin #29 and the HEGO GND is through the body of the sensor. I have read many issues with the 3-wire sensor threads developing corrosion that reduces ground contact which impact the HEGO SIG from the PCM. This is why a 4-wire HEGO is recommended as a dedicated HEGO SIG RTN (GND) is sent directly to PCM Pin #49. Also, I believe the 3-wire and 4-wire harness connectors should be different, at least on the FFI web-site they are.

In other words, stay with the more OEM 4-wire HEGO that your PCM is looking for or expect potential problems down the road.




1990 Bronco Speed Density HEGO



1995 Bronco Speed Density HEGO



1995 Bronco Right MAF HEGO (note HEGO SIG and SIG RTN on PCM PIns #44 & #46)



1995 Bronco Left MAF HEGO (note HEGO SIG and SIG RTN on PCM PIns #43 & #46)
 

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Not to hijack the thread, but I have a question related to this. I just did a MAF conversion using the RJM harness on a 1990 model with a 302. My original harness (now my passenger side O2 sensor) has four wires. My conversion harness uses a 3 wire. Should I run a 4-wire sensor in the one and a 3-wire sensor in the other?

Thanks,
Greg
 

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I just did a MAF conversion using the RJM harness on a 1990 model with a 302. My original harness (now my passenger side O2 sensor) has four wires. My conversion harness uses a 3 wire. Should I run a 4-wire sensor in the one and a 3-wire sensor in the other?

Thanks,
Greg
Great question, Greg. Caused me to do some additional research (Thanks... :goodfinge).


The answer is... It really depends upon what your harness is wired. As an example, my Master 1990 Ford Truck Diagrams book shows the Bronco/Econoline/F150/F250/F350/SuperDuty as all having a single 4-wire HEGO. The 4.0L Aerostar also has a 4-wire, but the 3.0L Aerostar/Ranger/Bronco II have a 3-wire HEGO. Again, this is for 1990 Ford EFI Trucks.

Additionally, we know that EEC-IV MAF Trucks (49 States) use 1-HEGO, EEC-IV MAF Bronco (California) use 2-HEGOs, EEC-IV MAF Mustang use 2-HEGOs and EEC-V MAF Bronco uses 3-HEGOs. In general I believe this to be true, although there are probably variables.

Anyway, whether you need 3-wire or 4-wire is dependent upon the wiring harness used for your MAF Upgrade and the number of HEGOs is dependent upon the EEC used for your MAF upgrade.
______________________________________

In summary, the Ford Fuel Injection MAF Upgrade Harness was set up to use the common A9L/A9P with stereo 3-wire HEGOs by adding a new Left HEGO Harness Connector.
  • If you are using a 3-wire stereo HEGO A9L/A9P EEC, the new Left HEGO goes to Pin #43 and the old (now Right) HEGO stays on Pin #29 as before. Your 3-wire HEGOs will ground the signal through the HEGO body and the not used HEGO GND wire will be blank in the old 3-wire HEGO Connector.
  • If you are using a 4-wire single HEGO WAY1/VEX1/AKCO EEC, move your existing HEGO from Pin #29 to Pin #44 and HEGO GND from Pin #49 to Pin #46 (SIG RTN).
  • If you are using a 4-wire stereo HEGO BI0 EEC, the new Left HEGO goes on Pin #43 and the old Right HEGO goes from Pin #29 to Pin #44, and the old HEGO GND goes from Pin #49 to Pin #46 (SIG RTN). You will simply have to run a new HEGO GND wire from the new LEFT HEGO connector also to Pin #46 (SIG RTN).
  • And don't even get me started on a four HEGO EEC-V PCM or 5-wire Wideband Zirconia UEGO Sensor...

Remember, in all of these scenarios we are not talking about the HEGO HEATER PWR which comes from the 12v EEC Relay to CHASIS GND. We are talking about the 0.1v-0.9v HEGO SIGNAL and its separate HEGO GND or SIG RTN. Using the SIG RTN in the EEC creates a much cleaner ground than using the body of the HEGO. This provides more accuracy, which equates to a more efficient closed loop air/fuel ratio.




Is that clear as mud? :crazy I'm sure Ryan will straighten me out if I have my head in the toilet... :itsatrap
 

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I don't know about V8only, but that helps me a ton. I did the conversion and fired it up for the first time this week. I only have one code (canister purge valve code 85 - I don't have a purge valve nor did my original harness), but it's not related to this. I'll post another thread about that. However, I used a computer from a 1990 mustang. It turned my exhaust cherry red during cam break in, but I believe that was due to extremely retarded timing. I was doing this alone and couldn't adjust the timing once I fired it up and brought it up in RPMs. I will stick with my 3-wire HEGOs. Thanks!
 

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Discussion Starter #8
some absolutely fantastic info here, and thanks all for posting....

Since I had a box of spare o2 sensors from late 80's mustangs that share the same connector (but are all three wire, and not a 4 wire) I decided to go out just for giggles and see what would happen.

I put in my 4 wire o2 that I suspected was bad based on the extreme richness, and indeed after motor completely warmed up, engine was rich as hell.

I went through my box of three wires, and decided to try one, as it'll ground through the chassis, and based on all my readings, should still work (but perhaps not as accurate)....

My results were night and day...I was able to watch the truck go from rich start up while cold, all the way through warm up, and the tail pipe sniff test changed 100% and now smells like what I would suspect it to smell like for a late model efi truck.

I'm friends with the smog tech, so b4 I plump down $50 for a new 4 wire o2..he'll do a free idle test of my emissions...I just want physical proof that my used o2 brought my idle hc's down, which I know for a fact it has, but I'd like to see how well it's working...I'll report back when I do.
 

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Hey sorry to bring a thread up from the past, but im currently hitting the same issue (sort of) Im running an a9l through a BRONCOII harness into a Tbird engine harness which has a 4 pin o2 sensor. Looking at the all the schematics i can get my hands on, the 4 pin hego seems to have the Sig return on pin 46 (as mentioned) However, the on the a9l which is (again mentioned) a 3 pin native HEGO sensor computer, but with the schematics I found, it seems that the 4 pin sensors just tie into the the sig return signal pin 46 which is native on both the A9l and any other ecu that is both 3/4 pin native. So if i tied into the sig return circuit on the a9l computer, theoretically, the A9L will be able to comprehend the 4 pin HEGO sensor.. correct? or am i way out the realm of possibility?
 

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This should work. The reason you cannot use a 4-wire HEGO with a 3-wire harness connector is there is no pin/wire from the connector going to SIGRTN. If you provide and confirm a HEGO Harness Connector SIGRTN wire to A9L EEC Pin #46, I don't see why this would be an issue using a 4-wire HEGO - except to confuse someone at a later time. Remember this should be the same for both stereo HEGOs. The easy way is to just use 3-wire HEGOS with either 3 or 4 wire connectors.


MAF Upgrade Notes:
  • You can use a 3-wire HEGO in a 4-wire Harness Connector.
  • You cannot use a 4-wire HEGO in a 3-wire Harness Connector.
  • When using the A9L/A9P Mustang PCM, use stereo 3-wire HEGOs.
  • When using the WAY1/VEX1/AKCO PCM, use a single 4-wire HEGO.
  • When using the BIO0 PCM, use stereo 4-wire HEGOs.


HEGO 4-wire Sensor Connector




HEGO 3-wire Harness Connector




HEGO 4-wire Harness Connector
 

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Good info.

thanks
tim
 

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i just found this thead...i've been having problems w/my mass conversion and this just might be it!!
i was and am using 4wire HEGO's on my 95 bko since the mass conversion install. would this have burned up pin46 trace inside the A9L eec? if i go today and buy 3wire HEGO's for the right&left should everything be ok??
 

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i was and am using 4wire HEGO's on my 95 bko since the mass conversion install. would this have burned up pin46 trace inside the A9L eec?.
Shouldn't, we are talking +-1v.

i if i go today and buy 3wire HEGO's for the right&left should everything be ok??
Should, as this is correct for the A9L.



See these links and note Pin #46 & Pin #49 differences between EECs:

SD to MAF Computer Pin-Out Reference

Oxygen Sensor Wiring Reference
 

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Seattle fsb,
do i need to clip the pin49 wire going to the A9l after i change to the correct 3wire 02 sensors or just leave it as there is nothing there eec wise on the A9L?

btw, THANK YOU for all the info i've been able to get from your post's!!!!!

AND...would this have anything to do with my CEL not illuminating when i first start the bko nor being able to pull any sort of codes???
 

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Seattle fsb,
do i need to clip the pin49 wire going to the A9l after i change to the correct 3wire 02 sensors or just leave it as there is nothing there eec wise on the A9L
Leave it. Should be blank on the A9L and nothing will be connected to it anyway.

AND...would this have anything to do with my CEL not illuminating when i first start the bko nor being able to pull any sort of codes???
Doubt it. Try using a paper clip to check for codes. Then read Ryan's myriad of no code causes below:

No Codes?
 

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Hey, first post guy here with a clarification question. On my MAF conversion I am using the AKC0 ECU for the E4OD and think I have a good idea of everything that needs to be re-pinned, but on the comparison chart Seattle FSB linked a few posts back it shows SIG RTN on #46 and HEGO GND on #49, then on the swap SIG RTN stays on #46 (unmoved) but the HEGO GND needs to be removed for the TOT on #49, so my question is, where does the HEGO GND go for the E4OD application? Does it just splice into the SIG RTN or does it just get pulled and capped or should it be moved to another pin? Thanks for all the info, there is so much on here I thought I would never get my first post in. :thumbup
 

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The reason you cannot use a 4-wire HEGO with a 3-wire harness connector is there is no pin/wire from the connector going to SIGRTN.
New guy on here but not new to Fords. Found this thread, which seems to discuss what I'm dealing with. Specifically, I've got a factory 3-wire HEGO vehicle that I want to use a 4-wire HEGO on (I must have at least 5 of them in the drawer). My plan is as follows:
1) Leave the 3-wire HEGO connected and snip the 3-wires near the sensor.
2) Snip the connector off the 4-wire HEGO.
3) Butt-splice the appropriate 3 wires on the 4-wire HEGO.
4) Put a ring terminal on the 4th wire and find a good Gnd for it on the body.

Should work, yes?
 
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