Manfred

I've been testing my throttle position sensor (TPS) and wide open throttle (WOT) switch by Dwayne's method from this post: https://rennlist.com/forums/928-foru...re-w-pics.html

I'm getting correct resistence reading under all conditions tested other than no matter how far I depress the accelerator I never get a closed circuit with <10ohms. That's the bad news. The good news I can improve my car and make it run better!

So, I hoping it's my ridiculously slack throttle cables that are to blame.



The cable that actually attaches to accelerator appears to have broken off it's plastic adjustment screw:


With slight pressure all the cables can be deflected:







Should all these cables have all the slack adjusted out of them or is some slack normal? Anyone know where I can order the plastic adjustment piece that will allow me to get the slack out of the cable? Or in the interim, can I tighten the cable at the accelerator pedal itself (out to the garage to check on that now--but has anyone else tried this?).

If I can figure out how to tighten the middle cable (that connects to accelerator pedal) my plan would be to tighten this until I get the WOT switch to close at 2/3-3/4 accelerator pedal depression--is that the correct plan? I guess I would also assume that I should get the slack out of the other two cables before I do that step.

By the way--what does the bottom cable do? It doesn't move when I depress the accelerator...

Gretch

I know when I over tightened my cable, the idle switch would not work.......... backed it off a bit and VIOLA.

that is all I can tell you.

Manfred

Good to know! It's probably pretty easy to get into thinking that any slack equals poor responsiveness--the mindset of "the tighter the better"!

Just got back from looking at the attachment of the cable to the accelerator--no easy fix there--no adjustment screw on that end and much harder to get to--lying on your back with your head down by the pedals.

the flyin' scotsman

2 of the cables should be adjusted in concert.............the cable from the go pedal to the mechanism plate and the cable from there to the throttle body.

Take the slack out of the go pedal cable first then the throttle body cable.

IIRC you should see the WFO resistance value at approx 3/4 opening.

Bill Ball

The throttle cable from the pedal is adjusted at the firewall. Is the adjuster broken there? Take as much slack as you can out there, checking that the idle switch still clicks. Then adjust the second half of the cable at the quadrant adjuster if there is still any play left, again being careful that the idle switch still clicks reliably.

If you still do not get the WOT signal, the switch is broken. You can get a new one or cut open the case of the old one by hacksainng at the seam just enough to pry it apart and then resolder the WOT arms where they have a solder joint near the switch plug and glue it back together.

WallyP

There are either three or four cables:
- pedal to quadrant
- quadrant to throttle
- cruise control to quadrant
- quadrant to automatic transmission

The throttle pedal can be adjusted - different attach holes at the base.

The cable from the pedal should be adjusted first - no tension, just a tiny bit of free play.
Next, the cable to the throttle - same adjustment.
Then the transmission cable (automatics only). Some folks like a bit of tension - you can adjust the shift points with this.
You don't often need to adjust the cruise control cable.

It is important that the idle switch and the WOT switch both work properly.
Idle Switch - zero Ohms with your foot off the throttle pedal, infinite Ohms as soon as you move the pedal.
WOT Switch - infinite Ohms until you get to about 2/3rds throttle, then zero Ohms.

BTW - you can check the idle switch while driving on any 928 with the digital dash. Set the dash on instant fuel economy. Drive at highway speed (over 1500 RPM) and lift your foot completely. If the mileage goes to 90 mpg (USA cars) or zero (RoW cars), the idle switch is closed. When you barely open the throttle, the fuel economy must drop slightly to show that the switch is opening.

Manfred

Thanks, I figured there was probably a correct order to follow.

Yeah, I saw that info (I think from you actually) on another post that I came across while searching for info on this stuff. I think I'll use that method for double checking that the idle switch is working.

Is that with meter plugged into pins 8 and 18 of the EZK plug? I thought that was for testing the TPS, or is that the same? Right now, before making any adjustments, I'm getting 0.8Ohms with no throttle, infinite Ohms as soon as the pedal is touched slightly (so at least prior to tensioning the cable it seems to be working correctly.

Ah-ha! I have a 5spd so the cable I couldn't identify was the cruise control cable--thanks.

Didn't see that--this may be a stupid question, but on which side of the firewall? I didn't see any way to adjust if from the footwell area. There was a little white plastic piece that the cable runs through on quadrant that was broken on mine--I touched it to inspect it and another damn piece broke off and fell down into the engine so I couldn't make out what it's function might have been--I assume it was remnants of the cable tensioner but I guess not. It's too dark in garage for me to look on the engine compartment side of the firewall tonight but that's the first thing I'll be doing in the morning after coffee.

How do I do that? Where is the idle switch and when should I be hearing it click? When the gas pedal is just barely touched and then released it should click off and on? Does the battery need to be connected for that? Right now I have the battery disconnected so that I can be plugged into the EZK to monitor the resistance on the WOT.

Thanks for all the good input!
Dan

Dwayne

Hello Dan,
I followed Bill Ball's procedure he mentions above with great success. You asked where the throttle cable adjustment was located "on the engine side of the firewall". Here's a pic - the adjustment nut is pointed at by the green arrow. You will have to remove the air filter box to get at the adjustment nut.


You had also asked about the idle switch adjustment. Before you adjust the throttle cable nut at the firewall, check the idle position switch operation by listening for the "click" when you manually open the throttle from the linkage at the side of the plenum. When you barely move the throttle linkage off the at rest position, you should hear the click of the idle position switch. It should click again when you release the linkage. You can test this without the battery being connected - it's a mechanical switch. Note that the Wide Open Throtle (WOT) switch is electrical - so you won't be able to listen for that one working - you have to check for continuity at the EZK or LH plugs at the computers. Once you're familiar with how the idle switch sounds when operating properly, you can begin adjusting the slack out of the throttle cable.

By unscrewing (counter-clockwise) the nut pointed at by the arrow in the picture, you will take out the slack in the throttle cable. If you continue to adjust it out, you will increase the tension in the throttle cable. This method can help you adjust the tension enough to achieve activation of the Wide Open Throttle (WOT) switch at 2/3 to 3/4 pedal depression. There are two things to watch for when adjusting this cable nut out:

1) don't unscrew the nut all the way out so it's disconnected from the female threads in the firewall. If you do, simply thread it back in.

2) carefully monitor the idle switch operation. It is possible to tighten the throttle cable so much that the idle contact switch no longer makes contact. I would recommend continually checking operation of the throttle linkage and listening for the "click" of the idle contact while you're adjusting the throttle linkage.

By following this method, I was able to adjust the activation of the WOT switch to 3/4 accelerator pedal travel from barely activating at 7/8ths pedal travel before this adjustment. And the idle contact still worked without further adjustment.

As noted by the other posters here, you may need to adjust the cable tension on the cable that goes from the linkage on the side of the plenum down to the throttle body. Do this 2nd after adjusting the accelerator pedal cable tension.

Good luck with the repair and let know how it goes.

Bill Ball

Dwayne's gotcha covered on this adjuster. The end pieces that fractured at the quadrant are not essential and not part of the adjuster.

The throttle switch is buried under the intake manifold. As to the "click" of the idle portion of the throttle switch - yes, it is barely audible just off the idle stop, if adjusted properly. No power needed (it's a mechanical switch). If you tighten the cable too much, you will no longer here the click. You can verify operation electrically with your Ohm meter. The idle portion of the throttle switch is a microswitch. The WOT portion of the throttle switch is is a simple points contact arm on on a cam ramp; hence it makes no click. The idle switch seems to be very reliable, but the WOT switch breaks pretty easily at the solder joints.

Manfred

Thanks Bill and Dwayne--that should be everything I need to know--I'm off to the garage to start taking out the slack...

Dan.

Leon Speed

Hi Dan, I noticed in the first picture the cable to the throttle is bent just behind the adjustment nut, which means the outside is probably broken. As the cable itself is rubbing on the inside it will fray and sooner or later the throttle will stick and the rpm's won't come down. Keep a close eye on it or better yet replace it as a precaution.

Manfred

Well....
The good news: I now can set my cable tension so that the idle switch and the WOT are working.

The bad news: I can only get WOT to kick in at about 9/10ths pedal travel (that's actually generous, it's more like 99 out of 100%). If I adjust any more slack out of either cable (to pedal or to throttle body) then I lose the idle switch.

Anyone have any ideas on how to manipulate the situation differently?

Manfred

I see what you are saying in the picture but I think it's just an optical illusion. I just checked it visually and there's no bend there--the cable is stiff enough that I couldn't make it bend if I wanted to. There is maybe something funny going on there with how the cuff is crimped on (don't know a better way to say this). I'll get a better picture and post back.

the flyin' scotsman

Sounds like the TPS is fried...........add it to next winters projects as you need to remove the intake to get at it.

Manfred

Not sure if this makes any difference if anything but the plastic piece behind the adjustment screws have broken where the adjustment screw crimps onto the throttle cable tubing. The adjustment screw is still wokring as with tightening or loosening it causes me to loose or regain the idle switch function.