Bill Ball

The 928 throttle switch has a Wide Open Throttle cam that is supposed to trip at about 2/3rds throttle, taking the LH off the 02 sensor loop, providing some fuel enrichment. I have found recently a bunch of cars where the idle switch portion trips as it should but the WOT does not. Tightening up the first half of the throttle cable, removing excess slack, got the WOT switch working, but only at 9/10ths+ throttle. Any further tightening of the cable would take the the idle position off the idle stop and screw up the idle switch.

A few months ago, I found this situation in my 89. The throttle cables are good and the second half is new, for that matter. Today I was running Theo's diagnostic program after my car died 3 times immediately during a hot restart attempt yesterday before it ran fine. It NEVER has done that before. Anyway, the LH and EZK tests showed no stored codes. The input check showed everything OK except the WOT switch would not trip. If I manually moved the throttle lever at the quadrant, I could see the WOT trip, but again, at 9/10th's throttle. I took some more slack out of the throttle cable but still could not trip the WOT position with the pedal, and the idle switch became unreliable.

The throttle switch is newish - 6 months old.

Why would the WOT be 9/10ths throttle at best? I found the same on two other cars recently. I'll take 9/10ths, but I couldn't even get that today.

I suppose I could crack open the throttle switch and check out the WOT function, but I'm a bit puzzled why I have this problem.

Jim R.

Bill, I don't know the answer, but I had to pull my intake again to replace a 3 month old defective switch with my fully functional 2.5 year old one. Should never have changed it when I had the intake powdercoated. Needless to say, I was not too happy, but I guess I am pretty much a pro at the intake R&R now.

Jim

AO

Those TPS switches seem to have a shelf life. When DR and I put my SCer on last year, we found the new TPS switch was bad. Luckily it's very accessible with the TS setup.

sharkmeister85

My new TPS switch gave me fits last month, until I discovered the wiring for WOT and idle circuits had been reversed! As it is, I don't think WOT comes in until later than 2/3's as well.
Glenn

Bill Ball

Right. As soon as I put the SC back on, and that will be very soon (I really missed the power in the hills at the open road race this weekend), I will have direct access to the TPS.

John Speake

Hello Bill,
I don't see any possible conenction between the WOT switch non-op and your hot non-start. (not sure if you were suggesting such a connection ?)

I agree that the WOT switch is a lousy design. It is simply a couple of bits of springy material with some contact on their ends.

The picture shows the poor deisgn. By comparison the closed throttle switch is a nice microswitch (with the "click")

Bill Ball

Right, John. The hot start anomally should not be related. I'll be testing temp sensors. The WOT switch finding was serendipitous. It's sure nice to have the Spanner and Theo's diagnostic to find things out you never would otherwise. I suspect half the cars have non-functional WOT switches.

Thank you for the switch diagram. It shows the points-like WOT switch and the cam. It would seem the problem is the cam position more likely than the points, as the cam is not gradual but has a rather definite ramp point. I can get it to trip; it's just very late. I wonder if the cam can be moved. I know it's not intended to be serviceable, but that never stopped me.

John Speake

Do you think the hot start is an electrical problem, rather than mechanical ? I would not suspect temp 2 unless it started in a cloud of black smoke

I don't think the cam is adjustable, but I am sure someone will tear an old one apart.... I don't have one on the shelf at present.

majhopper

Is there a way to test this switch's operation without pulling the intake, like with a VM?

I have had suspicions about mine but was waiting until I did my top end refresh.

Peter F

Bill,
exactly the same happend on my car a week ago.
Car ran great all day, came home and parked outside the house for a couple of minutes.
When I tried to start the car it died on me three or four times, when I got it started with some help from the gas pedal it ran badly.
I have never seen a error code before related to the throttle switch but when running the actuator test it has registered a time out error when trying to activate the full throttle switch.(not always though)
Drove it in to the garage and ran a test with Johns spanner, got two error codes.
One was that the idle switch had a sporadic short to ground and the other showed that the O2 sensor had reported lean mixture.
(never seen any error code related to the idle switch before though)
My throttle switch was old and I had a new one in the garage.
So I decided to pull the intake for the third time in a year .
(I wish I wasn't so cheap when powder coating the intake and tried to save a few bucks not replacing the throttle switch)
Now with the new throttle switch all is fine again.
Still have a drop in fuel pressure when the engine warms up though.
FPR is new and fuel pumps as well, not sure if the dampers can generate a drop if bad?(49 PSI at idle with vacum when cold and a steady needle on the rail meeter, when warm needle flickers back and forth with an average of 43)

Cheers/Peter

Cameron

I tried to service a WOT switch when mine was not giving me a WOT reading 100% of the time under full throttle. With a blower on the car, that is not a good thing. The switch is sealed, and has a small but noticable amount of a thin lubricant inside. I thought that perhaps that the full load contact was causing a problem. When I opened it up, everything looked absolutely brand new, no doubt due to the sealed nature of the device. I put it back together, and it worked like it did before with occassional cruise readings when it should have been telling the LH to WOT! I ended up putting a new one on.

I think that I kept the old one in the garage. Can it be of any help to you Bill?

BrianG

Bill, can you hear the idle switch "click" as it engages at the "throttle closed" position? Can you hear it "click" at the "9/10ths open" position?

As John relates elsewhere, the "idle" switch is a micro switch with a certain "click", while the WOT switch is a "soft contact".

The reason that I ask about where the "click" occurs is that the '85-'86 TPS switch looks identical to the S4 switch but functions in reverse rotation. Each switch will actually mount on either throttle body, but the early TPS unit operates in reverse direction of rotation to the S4 unit. If you have inadvertently got an early TPS switch mounted on the S4 throttle body, you will get a "click" at about 9/10ths open throttle and no click at the idle position.

Since most of us are using the early throttle body on our supercharged S4 cars there is the possibility that you swapped the early TPS back onto the S4 throttle body, and will have a messed-up TPS signal happening.

Hope this helps.

Bill Ball

Pull the LH and EZF plugs an connect an Ohm meter between pin 12 on the LH plug and ground. It should read infinite at idle positon. Push the throttle and at about 2/3rds the resistance should read less than 10 Ohms. (This is for 84-86)

Bill Ball

I have the idle click. This is an S4 switch, but, yes, I am aware of the difference with the early switch as that is the one I use when I have the SC installed (with an 85 throttle body).

I fiddled some more with the throttle cables, and I have both the idle and WOT working, but just barely. Idle switch clicks reliably and WOT comes on just before the pedal is mashed to the floor. I did feel the car was down on power this weekend during the open road race; this may help explain that.

Bill Ball

Cameron:
Thanks for the offer! I will be reinstalling the SC in the next few weeks, so the issue should be resolved then. For the time being, I have WOT tripping again, although right at the end of pedal travel.