I suppose everyone grapples with the dreaded FF44 at one time or another.
Now it’s my turn! I have a 1990 Sovereign with the 4 litre plant. I was getting a strange reading from a recently replaced O2 sensor - +5 volts, coming down to around 3 volts as the car came up to operating temperature. I went so far as to pick up a second replacement sensor but wanted to do one more check before crawling under the car.
I’d been taking my reading by back-probing the signal wire, as-per Haynes instructions. But I disconnected the sensor & harness and found something even stranger. The 5 volts isn’t coming off the sensor…the sensor is actually putting out the millivoltage its supposed to. The 5 volts is already present on the car side of the harness.
Can that be proper?
All thoughts and opinions are welcome at this point!
Roger,
you may be right that every XJ40 owner grapples with the FF44, but we don’t. Our cars don’t have such things.
This forum is for Series I-III produced in 6 cylinder form from 1968-1986. Because the 12 cylinder species of the XJ SIII were carried on till 1992 and the successor to the six cylinder car was identically labelled as “XJ6” there is some confusion. To cut a long story short: people on the forum for “XJ40” (used to be the code name for this product line) will know answers to your questions.
Good luck though
Jochen
75 XJ6L 4.2 auto (UK spec)
Hi Roger, as Jochen has mentioned there is a seperate forum for the XJ40 and I have moved the question over.
And welcome to the forums. Pop into the Pub and have a look.
Roger,
Welcome to Jag-Lovers! I see that you are new here.
Unfortunately you are on the wrong list. This list is for the Series I, II, and III XJ6s and XJ12s. None of our cars have On Board Diagnostics (OBD) and so we never get an dreaded FF44 code. 
Some of us are very happy about that.
The list for your 1990 XJ6 Sovereign is the XJ40 list. I suspect that someone over there will be able to help you out if you posted your question again on the XJ40 list.
Paul
Thanks Paul…thought I had the right forum!
Hi Rodger …
I just completed 12 month of chasing my '89 code 4 (which is basically the same as your '90 FF44} which turned out to be a bad ground and I learned quite a bit about the O2 sensor.
I assume you have a two connector 3 wire O2 sensor. First unplug and check that you’re getting battery voltage on the harness side of the heater connector (the one with the two wires) with the engine running.
Next you want to wait until the O2 sensor reaches 600F when the ECU goes into the closed loop mode of operation …
“CLOSED LOOP … used after the O2 sensor reaches 600F. The ECU then receives info from the O2 sensor, coolant temp sensor, MAF, etc. Using this information It then commands the fuel injectors to lean out the fuel mixture to obtain an optimum 1 to 14.6 fuel to air mixture.”
Now with the engine running and both O2 sensor connectors plugged in put the positive probe of your multimeter on the signal wire (single wire connector) connector and the negative on a good ground.
The reading should be rapidly fluctuating above and below an average of .5v. If it’s higher the engine is running too rich, lower too lean.
Grooveman,
I’ve only come in from the driveway this very second and saw your response.
I’ve just made something of a breakthrough. I found that I had zero grounding at the lug on the head. I cleaned that up and metered it and confirmed it was grounding. Then I started the car and re-metered that O2 sensor wired. There’s still a positive voltage present with engine running and sensor disconnnected but it’s dropped down from nearly 5 volts to just over 0.3 volts. I still don’t know if there should be any voltage at all present here but I’m going to reconnect the sensor and warm it up to see what reading I get.
Thanks for chiming in and I’ll post back if the news is good.
OK…here’s the good news:
Once the car warms up and comes out of closed-loop, the O2 sensor is fluctuating between 0.15 and 0.9 volts - just as good old Haynes tells me it should!
I had an issue with a non-functioning fuel pump about a year ago. Believe it or not, I traced that to a bad ground at a tail light! Current just wants to find a path to ground and it’ll try and take whatever route it finds. In that case, it was making its way through the fuel pump circuit.
Without grounding at the head lug, who knows what was being affected but something was trying to pass current THE WRONG way along that O2 sensor wire.
Many thanks to the forum…it can get mighty lonely out under that bonnet!
Roger …
So no more check engine light … Excellent !
That little tidbit of information only took me 12 months of frustration to figure out.
I think you meant to say when the “car warms up and goes INTO closed-loop”, not comes out of.
Isn’t it closed-loop when the ECU takes control before the O2 sensor begins sending its signal? To be honest, I’m a little too happy with today’s results to worry about my little gaff.
I hope someone else can fast-track to a solution from this, which is the point of the forum, right?
To recap then, the symptom was a mysterious +5 volts on the engine side of the O2 sensor harness. If anyone is getting a strange reading like this, check the grounding on the intake manifold lug!
Cheers everyone!
Groovemen, I’m not suffering from the FF44 issue but thought I’d have a look at the problematic ground just in case. Could you supply a photo or detailed description of where to look for it. Thanks. (1990 XJ6)
Absolutely. Looking into the engine bay from the left side, look just ahead of the #1 injector - the front-most one. There will be a grouping of black ground wires coming into an aluminum ring terminal. It’s held to one of the manifold studs by a 1/2" nut.
Some weeks back, I spent part of the day finding all the ground points and cleaning them all up. Seemed like just a sensible routine maintenance thing to do. But that one somehow escaped me!
Don B provided a nice pictorial review of cleaning all the grounds in his '93 and posted it (in several pieces) in the photo album of the old J-L site: