On my 89 XJS I have two micro switches on the throttle. I can see one of them is a full throttle switch, but the other is a mystery to me. What is it?

 

On an '89 XJS, the switch on the bracket attached to the throttle cable and adjacent to the throttle pedal is for the kickdown solenoid in the transmission.

The microswitch mounted on the bracket at the outer end of the left throttle operating rod is for the digitally controlled ignition timing. The switch is open when the throttle is closed to retard timing. I never measured the amount of advance provided when the throttle opens, as the timining does not require adjustment on a Marelli system. It's probably like 10-15 degrees.

It's all part of the Marelli Industrial Complexity. (I Like Ike too)

 

Do you know the best place to buy the timing micro switch? Mine has desintegrated. Although it doesnt seem to affect performance much, unless it's been bad since I purchased the car. Oh well, it needs fixed so I'm fixing it if I can find a new one.
Thanx,
Jarrod

 

Any of the usual Jag suppliers...like MotorCars Ltd...can fix you up.

Its probably a generic micro switch....most of them are....so if you Google "Burgess Snap Action Switch" or "micro switch" you'll have lots of hits. It'll take some time to find the right one as there are dozens of varieties. Might be easier to just buy one from a Jag supplier.

Or visit your neighborhood electronics shop.

Cheers
DD

 

Hi, just taken a look at ours; it doesn't look very generic- the switch is also riveted on to the bracket and it's one part in the Jag parts list. Assuming you have Marelli ignition, the part number is DAC 6049 (idler switch). I'd go for the Jaguar OEM part to save hassle, as the adjustment is quite sensitive when you com to set it.
Basically, it tells the ECU that the engine is at idle or over-run condition, and you should make sure that the switch lever is depressed when the throttle is closed.
Best wishes, Michael.

 

Ooops. Don't mind me. I was confused

Yeah, just get one from Jaguar

Cheers
DD

 

Michael,

Please explain what an engine in "over-run" condition means, and how the ECU prevents whatever that is.

 

HI,
Actually, I was quoting from the manual and I should have written "overrun", but, as I understand it, it just means your foot is off the gas and the engine is being turned by the drive train as the car slows down. As an aside, I think the fuel injection system also cuts the fuel completely on overrun on later cars, until the engine rpm fals to a certain value, around 1000 rpm I think. Must look it up. Our car is a '90 model V12 convertible - I think the more sophisticated fuelling comes with post face-lift cars. Interestingly, on our Marelli car, the ignition system doesn't talk to the injection system at all, except for one wire which links the two across the top of the engine and passes the engine speed signal to the injection ECU. Michael.

 

That's nonsense, the idle switch does what I described in post number two.

From XJS Model Year Update 1989 Supplement, Publication number S-60/SUPP
1988 Jaguar Cars Inc., page 6:

"Closed throttle running is controlled by the idle switch and is programmed by engine speed and coolant temperature. When the idle switch is closed, the ECU does not recognize the engine load (manifold absolute pressure sensor) input and uses the idle portion of the ignition strategy for ignition timing".

Which is exactly why, in practice, the engine idle speed will be in excess of 1200 RPM when the throttle switch is not adjusted correctly to close, or when the switch is faulty. If either of those two conditions exist:

"Ignition timing during open throttle running is programmed by primary inputs from engine speed and manifold pressure. and correction inputs from coolant temperature and intake air temterature."

The idle switch breaks the circuit through the Strategy Selection Link, which means that "switch closed" in the manual description actually means "circuit open" so that the idle timing strategy is enabled. That is why removing the strategy selection link for use with low octane fuel retards the ignition timing to reduce the onset of spark knock: you are disabling the open throttle ignition timing strategy by opening the same circuit. When I wrote "the switch is open when the throttle is closed", I was stating that the circuit was open at idle: my description is a little more clear than the manual's. "Closed switch" means that the microswitch is depressed, which means that the circuit is open.

None of this has anything to do with overrun, or with injector shutoff on deceleration as controlled by the EFI ECU. And you can twist and tape any electrical circuits on your car that you wish, so long as correct and reliable operation is not your concern. Buy the right switch or rig up whatever whatever you want.

 

Thank you for the "nonsense" comment. You have described in great detail the operation. I was simply quoting the basics from page 15 of the Supplement section of the Repair Operation Manual (admittedly for the '92 year on, but it's the same).
"Throttle idle switch. The switch enables the ignition ECU to sense when the throttle is in the closed position, the engine is idling or in an overrun condition. It is therefore important that the contacts of this switch are 'CLOSED' when the throttle is in the closed position." End of quote. Jaguar's words, not mine. What is there to argue about?
Michael.

 

Ummmmm, I'm just going to buy a new Jaguar replacement switch and let it figure out what it is and isn't supposed to do. I hope it can take the pressure.hee hee.

 

You're right to giggle, Jarrod! And there I was just trying to be helpful..............M

 

I answered the original poster's question accurately and as completely as the question required. Any time a nut twiddler wants to ignore or rewrite my post, he had best be accurate or I will call him on it. Best get busy, you have another thousand or so to proofread.

 

Heh, I dropped the ball on this one, I forgot to mention that my cat has a LUCAS ignition.

 

In that case, if the switch you are talking about is the one I identified on the end of the throttle linkage, it does nothing at all. Which means you can take it off your car and send it to Jarrod, who will have even more to grin about.

 

OK, in that case, we're back to genreic micro switches. One is for the trans kickdown and one is for enrichment at about 3/4 throttle

Cheers
DD

 

I can't find the idler switch anywhere. Everyone has the kickdown switch which they explicitly state is not the one attached to the throttle cable. Anyone have any ideas? I tried motorcars ltd., coventry west, jag graveyard, jag bits, and jag parts.
thanx fellas,
jarrod
P.S. the switches look similar any chance that they're interchangeable?

 

That's pretty amusing. PM me if you want one.

 

Yes, you can interchange the switch. Who here at Motorcars told you we didn't have one?

 

Sorry Ken I just looked at the website and saw a switch with 3 contacts. Mine only has 2. no biggy I'll just duct tape a toggle switch to my console and flip it when I floor the gas pedal!