Open v closed loop at idle

@gregma Is the neutral safety switch, which is closed in P and N on the PRNDL, what tells the ECU to operate in open vs closed loop in P and N? ( I don’t remember whether it’s open or closed in P and N.)

When I did the trans conversion, to get the car to start, I connected the neutral safety wires together, permanently. I’m wondering if that is causing my ECU to stay only in open or closed loop. (Again I can’t recall which is which in neutral.)

The car runs great, but I’m just sitting here wondering if I in advertently caused myself a potential problem. It’s easy enough to wire in to a true neutral safety, the transmission has a neutral safety switch built-in, so maybe I should just do that?

Not sure if the trans switch is open or closed when in P vs D. But I do know in P and N, the ECU is in open loop and the O2 sensors are not used. When in D or R, the ECU is in closed loop and the O2 sensors are used.

The “Neutral” switch that I wired closed must be closed in P and N. Otherwise the car won’t crank.
I am not sure how the ECU would know if the car was in D or R (the “Neutral” switch open) except for that switch. I’ll look at the wiring diagram. I just want to make sure I did not inadvertently wire my car to be always in open loop…

Yes, the park/neutral switch triggers a relay to inhibit closed loop. I think you have to rethink; maybe a switch on the clutch pedal that only engages fully down (so that it doesn’t activate at each shift). Following the open loop map will definitely hurt fuel economy…

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Maybe just a momentary button that I push and hold to crank…poor man’s security system. I need to go read the Book and remember why I ever want open loop in P and N…

So the ECU keeps it in open loop, even in D or R, for about the first 5 minutes, when engine is cold.

I tried forcing mine to closed loop even when in P and N, but I did not like the idle bouncing back and forth. The idle is more steady in open loop. I don’t really notice the bouncing back and forth in D or R, could be the torque converter smoothing it out or just being a lower idle.

This is what the Roger Bywater has to say on the O2 sensors being inactive in P and N:
“To avoid an unstable idling condition the system used the transmission inhibit switch to disable feedback when idling in N or P. The condition is aggravated by the low exhaust temperature at idle allowing the Lambda sensors to fall too close to, or even below, their minimum operating temperature. The disable function could itself be disabled by removal of a link so that idle adjustment could be safely made in N or P. Some people claim that the disable link should be removed when the car is “smog tested” but that is not what it is there for at all and a knowledgeable tester might be inclined to fail the car for having illegal tampering with its emission system. In practice that isn’t likely and removing the link will probably help a marginal car to pass but it is still a bodge that shouldn’t be necessary for a healthy, properly adjusted car.”

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I don’t remember if that Lambda disable function in P or N was deleted when the 3-wire sensors were introduced. Anyone know? Presumably the sensor heater would prevent the idle instability.

I di the same and had the same thoughts.
Nicer smoother idle on open loop.

Don’t know when the 3-wire O² sensors where introduced but the 16CU retained that function still.

Mine has 3 wire sensors and still has the inhibit relay and jumper. MY’88, VIN 146xxx

My wiring diagram shows the disable function and I have 3 wire sensors.

To summarize:
In addition to disabling 02 feedback and thus forcing the ECU to open loop, the P N switch also grounds the starter relay so the engine can crank.
I currently have the P N switch wires jumpered (so that I can start the car) so presumably I am in open loop all the time. Which is not O.K.

Options as I see them:

  1. Bywater mentions a “disable the disable” link. Pulling this link sounds like the best approach. Does anyone know where it is??

  2. Replace my P N jumper with a momentary switch that I push to make the connection and allow cranking. Poor man’s security system and a little inconvenient.

  3. Un-jumper the P N leads and add a new direct ground to the correct lug on the starter relay.


I wonder what the factory manual trans XJS-S’s did? I’m sure someone in the UK must have one?

The jumper is by the ECU in the trunk, next to the diagnostic plug that you use to set the base fueling map. Red connector if I recall. Adding the direct ground and removing your jumper will also work. ECU decides on closed loop by coolant temp and the P/N switch status, so could be your hot start idle problem.

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I am admittedly unfamiliar with this area, where would the diagnostic plug be?

they are usually tucked away behind ECU back inside rear fender. Should be a long wire with two plugs. One larger one for testing, and the small one with a cap that disables Open Loop when you remove it.

yours looks very different than mine. I don’t think that’s it. Here’s some crude photos from the Jaguar Manual. Top one is the testing plug, and bottom one is the open loop disable plug. Of course, I have no idea if the Marelli cars changed this or not?

Thank you Greg. The testing plug looks about right, but instead of a two wire jumper plug, mine just has one wire. This thing I’m looking at goes right into the ECU harness. I will dig a bit more.

OK, here’s a diagram for cars up to Vin 153559. That is before my car. The red pen is pointing to the jumper, it’s marked BG, it goes to the ECU pin 25, O2 sensing disable.

The black pointer is pointing to the Feedback inhibit relay, which is what the jumper enables or disables.

My car is 159803. Here is the diagram. No feedback inhibit relay, no jumper. I’m going to study things a bit more, because the neutral switch also affects the idle relay, and I need to refresh my brain on how that works. My plan is just to ground the starter relay, I just need to think about any unintended consequences.