JP Rodkey

Based on what others have seen, and what I now know, I wouldn't hesitate to do a resolder as a first step, followed by a rigorous test for continuity.

This is a control circuit, so the contacts are not carrying any current, which is why they will not experience pitting or other problems associated with contacts that switch power to relay coils, wiper motors, window lift motors, etc. Just a slight cleaning should be adequate.

ew928

BB,
With your longer than average bouts of WOT on them Nevada flyways, I would think it might be interesting to wire in parallel another switch for the WOT circuit inside the cabin.
Maybe some dash toggle with bright LED so you don't forget to switch it off.
Those NOS boys must have some cool heavy duty toggle you can use.

PorKen

I think an adjustable vacuum switch would be a good replacement.

Some euro L-Jet equipped cars (not 928) used a vacuum switch in place of the WOT switch.

RyanPerrella

I was reading the 968 throttle position switches can effect idle, is that true of the 928's Throttle position sensor. I thought that the switch ONLY is there to detect 100% throttle and make adjustments to the fuel and ignition maps. Does it have any other effect on the car, specifically with idle?

Louie928

There are two switches within the 928 throttle switch. One is closed at >~70% throttle and is the one this thread mostly deals with. Activating that switch has the LH add the WOT enrichment map values to the base map fueling values. It also switches to an entirely different map in the EZ-K for WOT timing. The idle position switch is closed when at less than a very few degrees of throttle opening, possibly 3 degrees or thereabouts IIRC. Basically, when your foot is off of the accelerator. When the idle swich closes it activates the idle speed control valve and if RPMs are above approx 1700, it shuts off the injectors. As the RPMs come down to below approx 1700, the injectors are brought back on incrementally so the engine settles into a stable idle. The 1700 RPM injector shut off isn't firm as other things affect that point. There should be no interaction between the idle switch and the WOT switch. They do completely different things.

Bill Ball

Ernest & PorKen - good thinking!

PorKen

I have been meaning to buy some of these to experiment, but I keep getting distracted:


Omega

derporsche928

Hello John,
This is the SAME switch I posted a pic of first, and also the one Bill circled-up.... Don't know why the pics look like they were photo-shopped golden / rust....
The actual TPS doesn't look like it's been in water.......
FYI, the old ones have tabs which can be pried-on to open them up easily.....
The newer ones are different and definitely sealed better, but can be opened up too..... just takes a little more patience.....
Just so you know..... I'm not an expert on this...... but as John replied , these should not be susceptible to a lot of pitting...... All of the three I've simply re-soldered are still functioning fine..... The contacts I've seen had very little, if any, pitting.....
Although I'm sure the components can fail over time, it's unlikely...... I'm around a lot of this type thing where I work, and failure of limit-switches or non-current carrying contacts is usually the last place to look, and invariably, if your not receiving a signal back from one of them, the problem is most always connection or wiring related......
As far as the switch itself, it is a very simple device, and they should NOT be prone to failure in-and-of themselves...... Micro switches (idle) rarely fail, and the WOT portion can ONLY fail if there is pitting or poor contact at the points, (unlikely) so that only leaves the solder joints, which are not that great from Mr. Bosch..... Of the ones I've fixed, three have had WOT solder connection issues, and one had an idle switch solder issue...... All are still functioning after re-solder.....
FYI, the idle switch malfunction manifested itself as a poor return to idle and also stumbling / cutting-out when back on the gas..... So far, I've not seen one with a failed idle switch or WOT switch contacts themselves,......
Not saying the components themselves couldn't fail, especially very old and / or water entrained ones, but I'd be curious to know if someone can succinctly show us one that has.....
(Still leaning toward poor solder joints......)
(and a little disappointed that someone didn't say I wasn't (dumb) )

Best,
Greg

Louie928

I'm not sure about that Ken. You can have low, or no, vacuum at low/moderate RPMs and only partially open throttle. That's going to switch in the WOT enrichment and the EZ-* less advanced WOT timing map when that may not be what you really wanted to do. You could have an external throttle activated reliable switch controlled by an arm, or cam, on the throttle linkage. Pretty easy to do and the activation point could be adjustable.

PorKen

I'm concerned that a vacuum switch might be too slow to react. The other option I consider is an arm under the dash, pushed by the pedal, which would trigger a switch or switches.

How/when?

Are you able to bridge the Kármán line with that 600HP?

Louie928

Hi Ken,
Nope I haven't got there yet. We are at about 1/4 mile up from sea level so only 61 3/4 miles to go. Actually, I'm not driving it much and still tuning. I find there is a huge difference between getting the engine to run fairly well and having it right. The intake air temp as well as engine coolant temp compensations are driving me nuts. They have to be right before the general maps can be judged to be correct. Sure, you can get a map to be close at some temperature, but to have that same map be close at 50 deg F hotter is something else again. Or to have it be close when the engine temp is either 60F or 200 F is another puzzle. The standard air density formula should work to set fuel compensation for differing IAT and it works pretty well below about 100F. However its not even close for the IAT on a hot start at 160F. Each morning is spent starting the engine from overnight cooling at around 55F, and logging temperatures and mixture as both engine temp comes up and intake air temp comes up while holding a constant RPM. Then analyze results, make compensation map changes and do it again the next day. It's getting better, but I'm not getting much else done either. I find that I have to keep some enrichening in until the engine temp comes up to around 155F. I find that I can mask all this fussing around and correction tuning by simply having my base map rich and depending on the closed loop mixture control to take care of it. I want it to be good enough to not depend on the closed loop control so I have to do this.

Regarding low intakevacuum/high intake pressure with part throttle do this. Have the engine running steady RPM at 2000 in say 2nd or 3rd gear. The throttle opening is quite small and there will be high vacuum. Quickly open the throttle to 1/2 throttle. That'll be more than 1/2 pedal movement because the relationship between pedal movement and throttle movement is not linear. You should see manifold vacuum drop to very low level until the RPMs come up, then manifold vacuum will increase again. When you open the throttle to near full open and leave it, the vacuum will initially drop, but it won't recover as much.

PorKen

Your experiences make me appreciate my MAF.

I dunno, stabbing the pedal sounds like a full load situation. I thought maybe you were going sub-orbital to see no/low vacuum at part throttle.

I wonder if it would be of any benefit to have the EZ and LH be triggered at different throttle positions with separate switches?

Louie928

All I mean is that for some reason Bosch decided the LH/EZ would go to the WOT map at X throttle opening regardless of load. Whether that's good or bad, I don't know. The manifold pressure is usually an indication of the engine load. When you are at low to mid RPMs you can reach full engine load (high manifold pressure) at a lot less than the approx 70% throttle opening where the 928 WOT operates. Maybe switching at some value of manifold pressure (load) would be of some advantage, maybe not. The WOT maps are not load related, only RPM related, so maybe they are expected to only be used at near full load. Time to experiment.

heinrich

Louie it sounds like you have been doing really tedious work. I'm glad I don't have to ... when will you get to just drive your GT? We've been waiting for what -- 5 years now? Dangit. I oughtta just come down there and see you or I may never see you again. Are you coming up for Park Place later this year? I trust you and yours are well :thumpsup:
Heinrich

Louie928

Hi H,
Sure, come on down anytime. I find that 928 ownership is a lot less expensive if you don't drive them. Just work on 'em. I had an 82k mile GT 5 yrs ago and it was higher than average miles. Now I have an 85k mile almost low mile GT. Funny how that works. This tuning has been a real education and eye opening to how easy it is to get an engine running mostly ok. Good enough to get by on, and how difficult it is to get it right. Howcum you won't see me again? Are you going somewhere? Gotta say something about WOT switches or this will be a thread hijack. Did you know that you have a WOT switch in every 928 you own? Some are probably broke too, but you can fix them.