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Hi,

I'm trying to find the source of my error codes in the ECU. One of them indicates incorrect voltage in the Idle Switch but I don't know where to find it. Chilton's even references it when testing the throttle position sensor, it tells you how to test the idle switch but doesn't tell you where it actually is. Is it actually part of the throttel position sensor?

My car is a 1991 4wd Legacy wagon, any help is greatly appreciated.

Thanks,

Mike

Yes, it's part of the TPS. If you do a search you'll find Josh has posted scans from his factory service manual about testing and calibrating the idle switch. You will need some feeler gauges.

search is your friend

http://www.bbs.legacycentral.org/viewtopic.php?t=1298

Yeah, I asked the same question a few months ago

Funny thing is I NEVER was able to adjust the throttle sensor within specs (I have another sensor waiting to go in) and the ecu just "forgot" about the code.

Thanks everyone! I'll search next time before posting.

You should adjust it properly. The idle switch controls other items, if I remember right its even the mechanism that signals WOT to the ECU and turns off the AC at WOT

ciper, I've never heard that before. Do you have a source for that info?

I don't understand how the idle switch would signal WOT...

I thought the ECU just had a certain TPS reading above which it considered the throttle "wide enough open" to warrant shutting off the air conditioning compressor.

Try it out, turn your AC on and go to 85% throttle. The compressor still runs. Then floor it and you'll here the relay turn off no matter the temperature.

It cant just tell that its "open enough" because it isnt based on a fixed voltage sweep. Instead it learns by the difference in voltage over a range. Everyones vehicles are a little different, thats why they specific such large resistance ranges for the TPS.

I understand that the sweep varies from car to car, and I know that the compressor turns off after a certain high percentage of throttle opening, but I don't see what the idle switch has to do with it. I had figured that the compressor shutoff would occur somewhere between 80% and 100% depending on the car, with the variance being due to the variance in TPSes.

Do you remember where you read or heard that the idle switch helped it do this?

From my previous testing here is what I've found out with the TPS and idle switch.

TPS's will vary greatly in their resistance values, so whatever the manuals give you as a range that is good....if you try and adjust the TPS to that range, you may get a CEL. So rather then base everything on TPS's that vary widely in resistance, they take sort of a reference or "snap shot" when you start the car. This snapshot probably is done around the idle switch values and what the current resistance is for the TPS.

Try this. start the car, loosen the screws for the TPS, move the tps around. The idle will get higher and lower, depending on how you turn it. Now leave the TPS in a position where the idle is really high. Turn off the car, and then turn it back on. Idle will be semi-normal again.

I'd have to look more closely at some of the wiring diagrams I have, but I'm pretty sure they show something about a/c compressor cut relay and tps/idle switch.

Well, the A/C compressor cut relay is controlled by a signal from the ECU.

But I guess I see what you guys are saying; it calibrates one end of the sweep by looking at the idle switch.

"it calibrates one end of the sweep by looking at the idle switch"
No, it uses the idle switch at the end of the sweep too, so it knows where the sweep starts AND stops.


In other words even though its labeled as an Idle switch its actually a throttle stop switch. I could be pulling all of this out thin air as well, I should probably find what page I read this on so you guys can check it.

Uh, the factory service manual says that the idle switch is closed only at very very small throttle openings. It even explicitly says to make sure it's open at wide open throttle.

Maybe Im a liar! I could be thinking of another vehicle. Ill try to find what I read.

yes the idle switch is closed, ie conductive from .028" to closed. So somewhere between .028" & .035" the idle switch should go from being conductive to non-conductive.