WOT switch or Dme

[Porsche 935]

I had this same issue last year so thought I would pick people's brains again. I know the Dme works off of on-off type switches. If you bypass the idle valve while setting your idle using the jumper, it also turns off the O2 sensor I believe. Well, when my WOT switch is plugged in I get the same effect. Unplug it and idle speed doesn't change much but air fuel ratio is 14.7. Plug it back in and air fuel ratio is 14.0. Rev to 2500 and air fuel 12.5. Unplug and at 2500 air fuel 14.7. I ohmed the WOT switch and it tests good. If you drive it a few times and plug the WOT switch back in sometimes all goes back to normal again. The wot switch is hard to find, and quite expensive. It can only be a bad switch or a DME issue I believe. Any thoughts? Anyone got a used switch I can use to test this theory?

[E Sully]

It's time to test your full throttle switch. As you note, the full throttle switch and the idle switch are simple on off switches. In any position except wide open the contacts in the full throttle switch make no contact. The DME follows the mapping in the chip along with the O2 sensor to control mixture as soon as the throttle is pressed and the idle switch contacts open.
Only when the throttle plate is fully opened, the full throttle switch contacts should close, bypassing the O2 sensor and running a richer mixture.
The full throttle switch is easy to test. Unplug the connector and put a continuity tester on pins 3 and 18, top and middle pins. In any position other than wide open there should not be any continuity.
If all is adjusted properly, there should be no change in the mixture with the wide open throttle switch plugged in or unplugged with the throttle in any position other than wide open since the contacts are open.
The idle position switch should be closed only when no throttle is applied. This activates the IAC valve, and also decel fuel shut off function when throttle is fully released and rpm drop. There is supposed to be 1mm of play in the throttle linkage, so that the idle position switch opens its contacts just before the throttle plate begins to move. As the switch opens, the IAC valve function ends and the timing advances 12 degrees while rpm should increase by 500 rpm.

[wazzz]

Originally Posted by E Sully

In any position other than wide open there should not be any continuity.

WOT switch actually closes before true wide open. Can't remember exactly, but it should be activated at around 65-75% of full range.

[E Sully]

I set mine according to the Bentley. It recommends setting it at full open.



If needed here is the adjustment for the Idle Switch.

[Porsche 935]

Thanks, but as noted in the original post I have tested both the switches. They both test good. But the only thing I can figure is maybe the WOT switch is somehow corroded and getting a small ground to the DME causing it to switch to the WOT map. If unplugged I have normal map and Oxygen sensor system is working great. But when plugged in the car seems to be in the WOT map and running much richer everywhere with no oxygen sensor control. If car is driven repeatedly, at some point you can plug the WOT switch back in and the car works as it should in regular map and WOT map works as it should when car is floored. The car is an 84 with 218,000 miles.

[wazzz]

Pin 3 of WOT switch is directly linked to prong B of three-prong test plug on left side (the one used to adjust idle).

That means you can test the WOT switch while it is plugged back. Take a measurement between pin B of test plug and ground. This can be either an ohm test or a voltage test:

- Ohm test when engine and ignition are cut off (and maybe battery disconnected to be safe): Between B and C (or ground) you should measure a high resistance (maybe near 47k) while throttle is at rest and on most of its range, then the measured resistance should drop to 0 ohm when reaching the WOT position.

- Voltage test when engine is off but ignition is on: In the same two situations, you should see 5V, then drop down to 0V when reaching WOT position.

[E Sully]

Interesting situation. As you know, the idle and full throttle are simple contacts, and you say they test good and are adjusted properly. There is not much to the switch but you suspect it may have issues with the contacts sticking. wazz has given a way to check at the test port.

Other possible causes? Are the ground wire connections on the intake runner good? You say the O2 sensor is working properly. I know many have had issues with the connector for it being brittle from age.
Another issue I read about with the 3.2 engines is the original single wire head temperature sensor. That is why I replaced it with the newer 2 wire sensor when I rebuilt my '86 engine. I have to wonder if that might be causing you intermittent mixture issues.
Hope you find the problem.

[Porsche 935]

To answer the above posts. #1 The head temp sensor has been replaced with a 2 wire.#2 The idle switch ohms out good and test good to ground. Works well. #3 The wot switch ohms well when at 90% throttle. The one wire on the plug has 5 volts and the other wire tests good to ground. The test port has 5 volts on one pin and the other tests good to ground. Upon starting the car with the wot switch plugged in, everything is normal again. So, after a long time without use, why does the DME revert over to the WOT map? Am I losing a ground or a connection somewhere, maybe a bad connection in or at the DME? Very puzzling. Maybe a spare DME, and when issue presents itself again, a quick swap to rule that as the issue. Where are all the grounds for the various switches terminate to ????? Back to the DME?

[E Sully]

Sounds frustrating.
I am curious, what O2 sensor are you using, and how you are measuring AFR? Have you checked to make sure the O2 wiring or sensor are not causing an issue? Have you opened the DME and checked it out, is the chip in the DME an original or aftermarket.
I know DME's, like the DME relay suffer from bad solder joints, so that could cause an issue. The DME I got with the '86 engine I bought needed to be sent out for repair. I confirmed it had issues by going to a friends Porsche shop and swapping it with a known good one. This option for testing is not frequently available though. I also had to do some light repair to the wiring harness and fix a couple of pins in the connectors.

[cabmandone]

Why are you thinking that it's reverting to WOT maps? 14.0 at idle is actually about right you want warm base idle to be around 14.2 (ish) tip in should (under load) should be in the 13's and WOT will be somewhere in the 12 range.

Have you opened the ECU to see what chip is in it? Your "issue" could be a performance chip was installed.

[mysocal911]

It's possible that the resistor network for the WOT A/D chip input voltage is not correct, or the A/D chip is bad.

[Porsche 935]

The reason I know it is in the wot map is because I was setting the idle parameters one time and forgot to take out the jumper. While in neutral and bringing up the rpm's to 2500, the air fuel mixture was 12.5. After removing the jumper and repeating the process, the air fuel mixture was 14.7 at 2500 in neutral. The o2 sensor is new Bosch unit, and I am using a separate wide band for testing. I never try to diagnose an issue without a quality wideband. The DME was modified when I bought the car years ago and also had an aftermarket chip in it. Couldn't find a stock chip at the time so installed a SW chip. There could definitely be an issue with something in the DME, as occasionally after a WOT engagement the air fuel mixtures in neutral show the same as discovered with the jumper installed. I would definitely like to get a nice 87-89 DME box to see if this issue goes away but funds not there. Car is still great to drive and not having the WOT switch plugged in is not a real problem but it would be nice if everything worked as designed.

[cabmandone]

Now I remember who you are. Did you give up on Sal? I think I offered to send you a stock chip at some point last fall didn't I?

[Porsche 935]

I checked everything as per Sal's instructions plus everyone else's recomendations but the problem came back after sitting for 5 months. I am not familiar with the A/D switch or circuit but there is definately a problem that tends to only return after long periods of non use. Due to the modification you have to do to the circuit board to make the SW chip work, i am not sure if I can install a stock chip back in the DME . That part is above my pay grade.

[cabmandone]

Originally Posted by Porsche 935

I checked everything as per Sal's instructions plus everyone else's recomendations but the problem came back after sitting for 5 months. I am not familiar with the A/D switch or circuit but there is definately a problem that tends to only return after long periods of non use. Due to the modification you have to do to the circuit board to make the SW chip work, i am not sure if I can install a stock chip back in the DME . That part is above my pay grade.

I don't recall, what modification did you need to make?