worf928

I have a questions about the proper operation of the idle/WOT throttle position switch.

The WSM indicates that the full-load circuit (WOT) should be closed after the throttle travels 2/3 of full motion. I tested this at the LH and EZK connectors and got an open circuit through the 2/3s of travel. Thus, I pulled the intake to get to the switch to test directly. What I found was that the circuit was closed when the throttle was at about 90% of travel - not 2/3. (ARG - was the switch working all the time and I just pulled the intake for nothing? Probably not since the #3 pin (that's the WOT pin) was pretty gummed-up.)

It would appear the the circuit is closed by a pin on the linkage coming into contact with a nut on a screw. The picture below shows the bottom of the assembly.

So, my question is: Is the WOT switch supposed to close at 66% or 90%? Is the operation of the switch due to the linkage pin coming in contact with that nut? Are you supposed to adjust that nut to close the circuit at 66%?

And no, I'm not going to pull the intake on the '91 for comparision.

(For the curious - I'm going through the process of running the tests in the LH/EZK troubleshooting manual in an attempt to figure out why the '89GT is missing about 100hp.)

UKKid35

The switch is self contained and can be operated off the throttle body. The manual says that the WOT position is not adjustable, only the idle position.
The idle position was adjusted one mine so that any throttle movement would imediately break the idle circuit and which meant that when 70% was reached the WOT circuit would close.

PS Thanks for your Plenum write up - it's been invaluable!

Jay Wellwood

The throttle switch, as I understand, it provides a signal to the LH unit such that idle speed is transmitted when the pedal is not depressed. This permits the bypass flow thru the idle stabilizer which is just a gated type of 12v controlled gate valve. Once off the 'no throttle position, the LH takes an active signal from the Air Flow meter and begins regulating the injector pulse to ensure that the emissions are correct baed on the feedback from the O2 sensor output. When the the pedal is depressed to the full throttle position it causes the O2 sensor outoput to be bypassed which permits maximum output regardless of emissions. While at idle, the idle switch should be open just after some slight movement so as to trigger the LH unit into compensating more fuel based on the Air Meter reading and making this an active signal loop.

This may not be very clear...my apologies as I have indulged in the vine.


hth-

worf928

Thanks Man. I'm not worried about the idle circuit part of the switch - it works the way it is supposed to and the adjustment is through two phillips screws.

worf928

Yup. That's how I was testing it - buy levering the throttle linkage.

The idle circuit and WOT circuit are independent. When the WOT switch is activate the LH goes into a full-load fuel curve and bypasses the Lambda loop.

But, I'm still not closer to knowing if the setup pictured is in fact the way it is suppossed to be. I know from my tests that the WOT circuit closed only when the linkage pin touched the picture nut. That may be the way it is suppossed to work - but it seems rather non-Porsche like.

I'm glad you found it of use. I'm going to be doing a second edition of that soon - as I've learned a lot of short cuts and better ways to get it off and on.

worf928

Folks, thanks for the prose on what the switch does to the operation of the motor - but I'm past that. I'm not worried about how the switch effects operation of the LH/EZK. I'm just wondering about the actual dynamics/mechanics of the switch's operation on the WOT circuit. I've found a few things on this GT that are obvious quick fixes by previous mechanics - and thus am wondering if there's something missing from the picture.

When the idle circuit closes you can hear a microswitch and the multimeter tells the truth. It's just that having the WOT side of the switch close (the multimeter says so) as a pin on the linkage grinds over a nut on the other side of the throttle body simply does not seem right. What if the nut was turned 30 degrees so that the corner of the nut didn't touch the pin? What happens with the pin travels past the nut and the circuit opens again? Just doesn't seem right to me.

Does the switch have a second microswitch? Or is this nut/pin thing the way the switch is designed to work?

SteveM928

The idle switch and WOT switch are both located inside of the same housing, on the driver side of the throttle body. The nut in your picture is a locking nut for the screw that it's on. The screw is the throttle stop that keeps the throttle plate from closing all the way, and is how you'd adjust your idle speed if it wasn't controlled by the computer and idle stabilizer. Notice what looks like the remnants of the yellow paint on the screw and nut in the picture. That's there as either a thread locker and/or a tampering indicator. The "contact" in your picture is not a contact, and should not be hitting the screw or nut.

UKKid35

If you take the switch off completely you can test both Idle and WOT switches. As you mention you can hear the Idle switch click, but the WOT switch is silent, I don't think there is a second microswitch as such, possibly a circular arm sweeping across a contact instead. I presume this is because the exact point at which the WOT closes is not critical.

The parts in your picture are as Steve says, and a red herring as far as the switch is concerned.

John Speake

Hi David,
The closed throttle switch is a good quanlity microswitch, this is why you can hear it operate. The WOT swoitch is a much cruder thing - just a springy contact rather vaguely operated of a little cam on the throttle spindle.

My experience is that they can operate at anything form 60% to 90%.

If you have any of the L jet or LH jet Bosch books, there is usually a picture of the device in there. If you want a copy, I can scan it for you.

As others have said, the external scrwew you picture has nothing to do with the working odf this switch. I don't know if is possible to take the cover off the switch assembly and adjust the WOT contacts, but it might be worth investigating.

On a GT, I think it would be a good idea to have the WOT switch operating earlier rather than later. With an auto with kick down switch, it really doesn't matter too much if it operates later.

Re your cars power loss - have you checked the knock sensors and the Hall sensor ? They are the usual reason for loss of power.

Regards,

John Veninger

This is simple.
Just make sure the WOT switch operates when you have the pedal to the floor and the idle switch operates when you let go of the pedal. That's the operation when racing, on/off

worf928

Ah. Very good. That is what I would expect. And the answer to my question! Thanks.
As I wrote - it was not my expectation that the pin/nut was designed as part of the WOT circuit. Nevertheless, in this case it is part of the circuit. That fact now becomes the mystery here: why does the WOT circuit close when the pin hits the nut? Probably because the WOT side of the switch is shorted and using the throttle body as part of the circuit? If that is the case then when the pin hits the nut I may be simultaneously closing both the WOT and idle circuits. That however, is idle speculation.

Given your answer above I must conclude that the switch is 1/2 dead as tests of the WOT-side of the switch with it removed from the throttle body showed that the WOT circuit was always open over the full range of rotation. Thus, since the switch is toast I will

- test my hypothesis that the pin/nut is closing both circuits when the switch is attached to the t-body.
- vivisect the switch and take a look at its guts

Very good. I will test the new switch when it arrives to see where in the travel it closes.
You and I have traded paragraphs from those books. I didn't think, though, that one of them would show the inside of the switch. Do you have the Probst book? Or something else?
I agree. We'll see if the new switch closes at 66% as the WSM suggests.
I have replaced the knock sensors - along with almost every other component in the ignition system except the LH harness, EGT sensors, and reference mark sensor. And the Hall sender

Test #4 and #5 - from the Porsche Service Training Center manual - are the Temp II sensor and the throttle switch. In my case both tests failed when performed at the LH & EZK harness connectors, so I dove in. The Temp II sensor is toast - both circuits open. And, as of this morning, I know that the WOT switch is toast. Tests #6 though the end will likely follow.

But, John, a question - the Hall sender is part of the 'protective circuit' according to the STC manual. But, the WSM indicate only that the ignition is retarded in the absence of a Hall signal. Do you know if the Hall sender is part of 'ignition circuit monitoring' system (on 89+)? If so then wouldn't one expect the LEDs on the ICM relay to light due to a faulty Hall sender? That has been my assumption up to this point - as the ICM LEDs are dark.

worf928

[QUOTE]Originally posted by BerNard94
David, the Hall sender is not part of the ignition circuit monitor. /QUOTE]

OK. Well, then, the Hall sender will be the next thing to check (that's test point #6 I think) - after the the intake goes back in with its new throttle switch. Now, I just need to find the black BNC-connector test lead for my 'scope.... I know it's here somewhere...

John Struthers

Re: the nut contact switch activation.
You know it could just be co-incidental.
As a test just turn that nut edge out of the way -no contact possible-
and test again.
This would eliminate the "grounding source" as a trigger for the wot/idle circuits on one hand, and either show a continued closure of the WOT circuit at the same rotational point or it won't.
If you do get the new switch and use it you can always dissect the old switch and look for problems there.
I think all throttle position switches are the same, irregardless of year/model (could be wrong) and do not like the thought of replacing a $90.00+ switch as a diagnostic measure -who does?-.
While the 60 to 90% of rotation to activate the WOT circuit seems to be common lore it doesn't speak well for Porsches' electrical suppliers -specifically-, nor Porsches' electrical design team in general. Shoot the bastards if they even talk to the suspension folks!
The last mechanical thing I saw with a 30% Bell Curve stirred into its operational design specs was the River Queen piloted by the renowned boat mechanic and racer, Humphrey Bogart.
Humor aside, You got me curious as the REAL cause behind the 100HP loss. Keep posting .

worf928

Yup. Now that I have some corroboration that the pin/nut is not part of the design, next time I get a chance I'm going to repeat the tests with the switch both on and off the throttle body. I will probably turn-out the nut and the screw together (whether or not I want to since they are probably bonded together...)
In general I don't. But, even though I'm getting pretty good at R&Ring the intake if there is the smallest indication that the switch is not in perfect health it is outta there - 'cause I do not want to R&R the intake again. All wrenching and no play makes Dave a dull boy...
The 100hp is a butt-o-meter measurement.

I'm am betting heavily on the WOT switch. We shall see.

I'm going through the test procedures slowly but surely. The '89 has good part throttle response but as you transition to into WOT it doesn't go like it should (i.e. like the '91GT) - it almost seems to lose a bit of power at WOT - but that could be my imagination. But, it feels lean (don't know why my butt-o-meter says so) and it's getting 21+ mpg which is too high for a GT - hmm... or too high for the way I drive a GT.

At this point the last items to check are:
- the Hall sender
- the reference mark sensor
- various grounds (some checked, some not)
- fuel pressure (main pump less than three years old)
- LH harness continuity (I've been checking as I go.)

The LH ECU, plugs, wires, coils, caps, rotors, front harness, engine ground, battery ground, various relay, fuses, o2 sensor are new. The injectors are fresh from cleaning and balancing. All new hoses, etc. The '89 runs a LOT better now (ya think?) than when I bought it. But, not 100% yet.

John Struthers

I, myself, was wondering:
How many of us are doing unintended 928 Mechanical/Electrical Master Technician internships.
Hang in there.