Stable idle speed mod for manual conversion

Converted my 87 XJS to a Getrag 290 manual tranny. One annoyance is once the O2 sensors start to work, the idle varies up and down in concert with the O2 sensor voltage varying. I’ve watched it on a scope, and the RPM follows the O2 fluctuation.Its pretty annoying when sitting at a stoplight, or just letting the car idle. The Lucas ignition cars had a microswitch that was activated in neutral and park gear lever positions to go to open loop to stabilize the idle. In gear, the auto tranny torque converter stabilized things. Even with the A/C on, there isn’t enough load to stabilize the idle. After considering options of how to switch the system to open loop at idle, I’m thinking about adding a RPM activated switch to replace the microswitch. Most of the available RPM switches I’ve found have a minimum activation of 2000 RPM. They can be set to correspond to the number of cylinders you have. If i set it to 6 cylinder, I should get activation at 1000 RPM with my V12. I think that should be low enough. I’m at 750 RPM at hot idle. The switches are about $65.

If anyone has a better idea, or easier or cheaper implementation, I’d be glad to hear it.


Yep, on the flip side, i thought I’d force my idle to closed loop in Park/Neutral. Roger Bywater even suggested it to me.

It was terrible, hearing the engine go up/down when parked. Exactly what you’re experiencing I guess. Amazing how TqC can stabilize that when in gear.

How about a signal from the brake light switch? Use a relay activated by the brake lights to switch the vacated Park/Neutral input to the ECU…

Hmmmm, I’ll have to think about that. It would operate at times when I’m not idling. That might work. I’d probably hook it up to the handbrake also. Might be worth trying out.
Thanks for the input.


How about a microswitch at the throttle pedal or the throttle pedestal?
Activated when throttle is depressed, i.e. on idle, and disactivated when you press on the throttle.
Coasting would not be a problem but don’t know if there would be a transition issue the instant you press on the throttle (in theory, if your base fuel map is set correctly it should not).
I think it’s easy to implement and test.

You could use a diode in line with the handbrake warning switch…to not interfere with the warning light function. As Aristide said, should not interfere with coastdown if base fuel setting is correct. But it might inhibit fuel cutoff (which could be a good thing if your ECU hasn’t been modified).
Be sure that you check your TPS is correctly set at idle, and that it provides a good signal; when mine failed the idle became (more) erratic and the fuel cutoff didn’t function. Plenty of info about using other brand TPS.

I wondered about that. But wouldn’t it also activate every time you lift the throttle to shift?

Is there any way to get the switch to activate when transmission is in neutral? That would be ideal.

Except that every time you pass through neutral when shifting, the switch will activate. Not sure what impact that would have.

True. Didn’t all XJS V12 factory manuals have no cats, so no closed loop? And L6 manuals with cats didn’t have the same issue as the V12?

Sounds like an engineering mod that Jaguar never had to come up with.

You could remove O2 sensors and redo ECU for no closed loop, but thats a lot of work and money.

The Marelli ignition cars got rid of the switch, so apparently they modified the ECU, or more likely, the Marelli ignition unit, to stabilize the idle. But you are correct, the HE’s were never equipped with manuals, and the 6 cylinders used an entirely different engine management system.


There are some places (somewhere in Idaho is one I know) that do manual conversions for HEs. May be worth phoning and see if they came up with a good solution?

Wonder if you could find a three way switch to fit where the Sport’Normal switch resides…and wire it so you can “activate” open loop when you know you will be idling a few moments and then switch to S or N as you drive sway?

The ski slope was changed to a manual tranny type, and an 87 didn’t have that switch anyway.

I’ll fit a switch to the throttle pedestal, and try that out.


It could, but there is a solution for this, a delay relay that would delay the signal for a couple seconds.
Also easy and redily available.

It’s the only way to find out.

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This is what I up with for an idle switch addition to the throttle pedastal. Things are very tight, but everything clears throughout the throttle movement. I ran the wires over to the inhibit relay which has two green/black wires coming out of the same pin in the relay socket. One of them goes to a diode and on the the a/c spplementary air valve relays. The other goes to the ECU. I figured out which was which, and cut the one going to the ECU. That wire now goes to the switch, and the other wire on the switch goes to a ground point by the inhibit relay.

I haven’t driven it yet, but i verified that the switch grounds the O2 sensor defeat wire at the ECU at idle and is open off idle. When i drive it later today, I’ll report back.


So far, so good on driving it with the mod. Idle seeems more stable. Didn’t go very far; will evaluate over next several days.


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Jon are you running a stock ECU or has the fuel cutoff been modified (like by Bywater) for a manual? Should have no impact on the above issue, I’m just curious.

Stock ECU. I talked to Simply Performance, where I got the manual conversion. They said it wasn’t necessary to do the mod, and might create issues. So far, I’ve seen no reason to do the mod.