mj1pate

OK...after replacing the WOT throttle switch in my 86.5 with a new one (and employing no small amount of labor), I discovered the brand new switch failed to send a WOT throttle signal, with the throttle nearly wide open. Keep in mind, both the old and new WOT throttle switches were capable of sending a WOT ground signal, but the contact closure was outside the range of the actual throttle rotation. I'm not replacing the &%$!# thing again.

So.....here's a possible alternative. How about a WOT vacuum actuated switch? The Design Flex vacuum switch is a possible example:
http://www.designflexswitches.com/switches/psf102.php
This one has a field adjustable set point - from 0.1" H2O to 25 psi. Does anyone know if that is that sufficient range? The idea is not to replace the WOT switch, as the idle sensing portion of the switch needs to be retained. The lead from the WOT switch would be extended to the vacuum switch and the vacuum switch could provide a WOT contact based on engine vacuum.

I would be happy to try this out if someone could identify a vacuum profile based on engine RPM. I know this is not a perfect replacement for the WOT switch, but the factory WOT switch was far less than perfect. My experience is that any RPMs over ~3700 (in an automatic) is likely a product of WOT, therefore the basis for a vacuum switch operation. Thoughts?

Mike Frye

Mike,

I'm not sure about the setup you're considering or even if you want to incorporate vacuum into one of the few systems that doesn't already rely on it.

I'd do a couple of things before giving up on the stock setup:

1. See if you can get the WOT switch to trigger by just pulling on the rod at the front of the engine. If so, you may be able to get it to work.
2. If you are able to get it to trigger by pulling on the rod, get all of the slack out of the throttle cable and see if you can get the WOT to trigger with the pedal. You can meter this all the way back to the LH by reading the pins on the cable.

One reason I wouldn't try an external WOT switch is that the stock system is set up to trigger the WOT fuel map when the throttle plate is all the way open. If the plate isn't opened up all the way then I don't think you're going to get the right mixture and you're not going to be running right.

PorKen

That looks like a good switch, AFAIK.

The trouble I have with a vacuum switch is that vacuum is always a tiny bit behind the throttle opening, so you may feel a little 'lag' if you snap open the throttle.

The factory WOT switch is supposed to close around 2/3 throttle open.

Cosmo Kramer

That would work well in boosted application to get the LH into open loop at the onset of boost. I see they send out one free switch to businesses, wonder if I can get one. Hmm.....

mj1pate

Mike; no...i have all the slack out of the cable to the point where I'm just above disabling the idle switch. I get no WOT switch closure.

Good point, but the WOT closure operation/timing looks (examining the innards of the original switch) to be less than exact. Judging from when I am at 3700 RPMs with an automatic, I am generally near WOT. Fudging it up higher ~ 4k RPMs, I am definitely on the metal.

One important question would be how necessary is WOT to performance any where near legal speed limits.

Landseer

Something sounds not right. I run and 85 and 86 automatic and can't relate to your RPM comment. I hit the gas on either car and the biotch is pushing 6000 rpm in a heartbeat. Maybe I misunderstood.

Mike Frye once reported on a problem with his gas pedal pivot linkage bearing inserts causing a surprising lack in throttle actuation, you might check for that.

I disassembled the switch and bent the contacts to actuate earlier on one car, after taking all slack up on throttle cable at the firewall adjustment plastic screw.

Access, I guess you know, can be achieved without pulling the intake runners. The center runner only can be pulled out, the throttle body plate split in half with four bolts and the unit canted to access the switch for removal, manipulation, and remounting.

Pcplod

I have never been sure that my WOT switch is working properly either. Is there an easy way to test it to ensure that it is sending the right signal??

danglerb

Aren't there 3 or so versions of the TPS?
Maybe you have the wrong switch?
Or a bad wire/connector?
Or a bad switch?

Landseer

There are test procedures using the plug-in port on the front of the early 32V engines.

That's also where the porken blink'r tool is fitted. It lights up to make detection of switch function easy.

There is history of broken contacts inside the switch, occluded from view by the case, but the OP says the contacts work, he just isn't getting rotation. I think something on his throttle linkage is screwed up.

mj1pate

Could be....but there is no slack in either the linkage under the hood, or in the pedal linkage (having sorted that out already).

I mean, thinking about it: The linkage has to allow the idle switch to engage at no throttle (achieved). And with with no slack in the linkage and the pedal set for maximum range of motion, a WOT contact closure should be possible within that range of throttle motion, and unfortunately is not.

Now....as I understand, the WOT contact is supposed to be made with throttle at 3/4 open, not as late as full open throttle. If that's the case, that was the basis for considering a vacuum switch with variable set point for activating WOT closure. In my auto, I'm rarely in the higher RPM range unless I'm seriously on the metal (like 3/4 throttle). The situation could be very different for a 928 with a manual. With variable set points, I could make the WOT at 4700 - 5k RPMs. Its just something in the brain storm stage right now.

BTW: I replaced the orig WOT switch in my 86.5 by way of removing the center tube and doing the necessary actions under the intake tubes. This is not a task for those with repainted or powdercoated intake tubes. Try as I might, I managed to scratch my newly painted tubes and it was (being careful) quite a time taker. All this only compounded frustration dealing with the switch module.

Mrmerlin

I think you should make sure you have the throttle rod on the correct position on the throttle pivot quadrant and the throttle cable connected correctly as well this would account for the throttle opening but not enough .
i dont have a throttle quadrant picture

mj1pate

Hmm.....THAT would be a surprise if they weren't connected properly. Not sure if anything would work right, the rods were misconnected. But that should be verified. Merlin or Landseer; can one of you upload a digital picture of your pivot quadrant, if yours is an "S" type? Thanks.....

Mrmerlin

here you go , from my early 86 5 speed.
The top cable is the throttle the lower is for the CC.
the missing hole is for the bowden cable , this car is a 5 speed

dprantl

So it only costs ~$19:

http://www.newark.com/jsp/search/pro...-OP&CMP=AFC-OP

I think this is something similar to what Dave Roberts uses for boosted 928 applications to activate WOT when a certain positive pressure value is detected in the intake. I would expect WOT on an NA car to be a pressure number very close to 14.7psi, if not exactly that. This switch definitely has the proper range for this application. You could set it to something like 10 - 12psi for it to activate at slightly lower than full throttle.

Dan
'91 928GT S/C 475hp/460lb.ft

mj1pate

Thanks merlin, for the pictures....I'll look things over in the morning. Thanks Dan for the info.......it would be interesting to experiment.