I have a 1989 F-150 5.0L Mazda M5R2 5Spd Tranny, that I've added 19lb injectors, Edelbrock Performer Truck intake, Edelbrock 56mm throttle body, K&N filter, long tube headers, straight dual exhaust with a dual in/out cat functioning as my H pipe. The O2 sensor is installed in the cat. My question is in reference to the 1.5 volts I'm reading off the Signal return line off the sensor. As far as I know, and have ever known, an O2 sensor is supposed to give off a reading of .1 volts (lean) to 1.0 volts (rich) and nothing more. I understand that by adding the modifications to the truck that I have, I shouldn't expect to get the exact same readings as if originally equipped. However, why would an O2 sensor of any kind give off such a high voltage reading? I have an Actron/Sunpro Air/Fuel mixture gauge connected inside the truck to give me an idea of what is happening while driving, but it constantly shows rich except when under heavy load. The sensor is brand new, and although I've had to cut the wires and splice new ones in to make it reach the harness, that should not matter??? It is a 3 wire HEGO and I am getting the correct voltage to the heating element, but for some reason I'm still reading exactly 1 volt over on the return iine than I should. Also, I have a constant "41" code from the computer, which says I'm running lean, contrary to the +1 volt I'm getting on from the sensor.

Any and ALL help would be greatly appreciated.

Luke

 

pull your plugs and read 'em. do they show you running rich or lean? codes and computers are good tools, but the best diagnostic is still reading a plug.

 

The plugs look to be fine, cuz they don't show heavy in either direction. The Haynes manual states that if the computer receives any bad code from the O2 sensor it'll revert to it's default state and not "pay attention" to the signal coming from the sensor cuz it thinks it's a bad signal. The truck is not running lean nor rich, but that's cuz it's running at the open loop default, and although this is a good thing, I want the computer to be reading the O2 sensor correctly as well as get rid of the "check engine" light. I was just playing around with it again, and for some reason I'm getting voltage on the signal return line even without the O2 sensor plugged in!! Now I'm looking for a bad wire somwhere in the hopes that I've got the signal wire contacting another voltage source and it's transfering over to the return line. The voltage is a varying type. It's a high voltage when it's cold, and as the temperature of the engine rises, the voltage drops, until it reaches it's, now common, 1.5 volts. I'm trying right now to trace the signal return wire from the O2 sensor back to the computer w/o physically having to remove the harness and the entire wire loom across the front of the truck. Does anyone know possibly what wire is carrying voltage in the same wire loom that might be transferring?
Any and all ideas would be GREATLY appreciated. I've been trying to figure out what this O2 sensor has been doing for about 6 months now and am coming close to my wits end! hehe
Thanks again.

 

 

I've talked to a few ppl and all have agreed that there should be no voltage from the computer to O2 sensor on the signal line. All wire between sensor and comp is fresh and confirmed of no grounds or bad connects. Is it now possible to believe that my computer is bad??

 

It is strange that you would get a 1 volt stray signal. It sounds like there is a problem in the computer.

With all the wires to the sensor open, do you still get the same voltage? Does it appear exactly with powering on the computer?
And are you referencing true ground?

It really sounds like you are reading a switched output line FROM the computer, instead of an input line.


Have you used a high-impedance analog meter to verify that the one you are using is correct?

Perhaps you could swap in another computer temporarily to test.

After I posted it hit me that you are getting good readings above the 1 volt constant. If there was a short, you would constantly get the 1 volt signal; the o2 sensor is high impedance, and its output would be suppressed by the shorted signal.

So it really sounds like your o2 sensor is not properly grounded.

Try measuring the signal between the o2 sensor ground wire and true ground (with everything connected).

 

The only ground wire is the Heater ground wire, and it's fine. The signal wire is grounded by the actual contact of the housing of the sensor itself to whereever it's installed, in my case the cat. I will check to make sure it is grounded, but the voltage is coming from the computer, so it is slightly confusing...

 

Yes, unfortunately, it does sound like it's a computer fault. I will look at my manual tonight and see what pins are adjacent to that pin. It could be that there is a high-resistance bridge between the o2 sensor input and something else. Could even be moisture, etc. It wouldn't take a very low resistance bridge to cause your symptoms. This could be in the wiring plug or the socket on the computer.

Normally, there is silicone grease in that connector to prevent moisture, but if some dirt or grit got in there with it, it could cause you problems.

I have an 88 F150 5.0, but I also have a 54 F100 with a 92 Mustang engine, EFI harness, computer, etc in it. I got some rainwater into the connector on the computer on a trip once on it and it quit on me. The symptoms were identical to a bad ignition module, but turned out to be water drops in the cable connector.

 

I was thinking the exact same thing in regards to the bridging on the socket. I checked in the Haynes manual and it says the pin next to the HEGO Sig line is the "NDS" which I'm not quite sure what it is supposed to stand for, even knowing that it's the line coming from the clutch safety switch.
Guess I'm gonna go get me a new computer, unless anyone else has figured this one out yet.
Thanks for you help Winfordr. Appreciate the concurrence more than ya know!!!

Luke

 

"NDS" is something like "neutral disable switch", but I'm not sure about the "D"... It's just the neutral/clutch switch for the starting circuit.

 

Cool, just a funky acronym for the switch. It's not something that produces voltage except when clutch is activated correct?

 

Hey Luke,
Because the temp dropped here in N. Texas, I had to scramble up some firewood as soon as I got home yesterday, and I totally forgot to look at the manual on that problem.

I will try again tonight. I am not sure about the clutch starter interface. I would have to look at the diagram. It could be a direct interrupt to the starter switch, or it could be a signal to the computer. If it is an input to the computer, then it could very likely have a small voltage on it, as the switching is normally done to ground for safety reasons. That is to say that the computer sees a "1" until the switch is closed, at which time it reads "0", ones being voltage above about one volt and zeroes being below about .3 volt (approximate values, I don't remember for sure).

Anyway, I apologize, I really intended to look at it, but got too busy.

 

Ok, I just replaced the computer with no apparent change to the problematic voltage on the O2 sig line. Anyone know of a good place to get a complete wiring harness? I thought I saw one somewhere, but wasn't sure if it was Summit or somewhere else. I'm guessing that the problem is right at the connector itself to the computer now, even though I tried coating it with a thick layer of dielectric grease to no avail.

 

I still think you have a ground problem.

Did the braided ground wire from the block to the body get disconnected during the mods?

I hate for you to buy a new harness if you don't need it.

I would remove the harness from the computer and do some resistance checks first. But I still have a gut feeling that there is an open ground somewhere.

 

That is one thing I will definitely check. I believe all my grounds are hooked up but I could very easily be wrong!! Would that be an easy cause for "it" occasionally working? By "it" I mean the gauge showing the right voltage. Almost...almost everytime I'm above 55mph it seems to work fine, and then every now n' then it will work during normal operation. Very inconsistent in when it does work. I'll definitely check on all the grounds. You happen to know where they all might be located, so I don't miss one. I know of the one that mounts mounts block to body(negative battery cable as well), the one that mounts firewall to cowl. Does the Haynes manual list 'em?
I really appreciate the help Winford. Thanks!!!

Luke