Dme computer

[Porsche 935]

After hunting down running issues for quite some time I am down to a possible dme issue. I know people here buy parts to have "just in case ". Anyone have a good dme they are willing to sell or loan out for a quick test? And if the dme is my issue, anyone suggest a good refurbisher? I think sw chips has that service, but if anyone has had success somewhere else that info. would be appreciated.

[John W.]

I made a note that John Walker recommends loren @ systems consulting for ecu repair. Good luck.

[DavidMn]

I purchased a refurbished DME from Vertex two years ago. Think it was $700 with exchange. Solved a no start problem. Fortunately, I had a local mechanic switch it with a known working unit to verify that it was the problem.

[ischmitz]

What sort of running issues are you dealing with exactly. There are really only very few failure modes of the 3.2 DME that cause poor running. The one common issue that comes to mind is when the solder joints at the ignition final driver are about to fail. This leads to hard cutting out and/or intermittent no-starts.

In my time after fixing hundreds of these boxes I only came across one DME where the actual A/D converter that evaluates the sensors was bad leading to wrong mixture. Most everything else are pretty clear-cut cases: If the DME was damaged there was a very obvious no-start or non-existing idle or similar symptom.

And just beware there are some out here that postulate such nonsense as "found an out-of-spec transistor". Don't fall for that nonsense.

Cheers,
Ingo

[stomachmonkey]

Originally Posted by ischmitz

...The one common issue that comes to mind is when the solder joints at the ignition final driver are about to fail....

Can you post a pic with the location of those points on the DME?

[ischmitz]

It's been posted many times - just do a search or see here for some examples:

DME Bad Solder Joint Repair

DME repair

Make sure the work is done properly. I have seen some very horrible burn marks where people confused electronic soldering with copper pipe plumbing.

[brighton911]

Put your faith in Ingo's work, you won't be disappointed.

[mepstein]

Originally Posted by brighton911

Put your faith in Ingo's work, you won't be disappointed.

I agree. He’s been my go-to for a couple dme’s.

[tperazzo]

Originally Posted by Porsche 935

After hunting down running issues for quite some time I am down to a possible dme issue. I know people here buy parts to have "just in case ". Anyone have a good dme they are willing to sell or loan out for a quick test? And if the dme is my issue, anyone suggest a good refurbisher? I think sw chips has that service, but if anyone has had success somewhere else that info. would be appreciated.

I have a running 3.2 engine on a test stand right now. If you want me to try your DME I'd be happy to do that.

Its always better to try a DME on a known good car versus the other way around. For example, I will not send you my good DME to test.

[stomachmonkey]

Originally Posted by ischmitz

It's been posted many times - just do a search or see here for some examples:

DME Bad Solder Joint Repair

DME repair

Make sure the work is done properly. I have seen some very horrible burn marks where people confused electronic soldering with copper pipe plumbing.

Thanks, the shear number of DME related threads makes it hard to find to find when doing a search.

[Porsche 935]

I purchased this 84 911 a number of years ago and it had a lot of issues. I started with the basics - fuel filter, plugs, wires, cap , rotor and installed a wideband in the cat bypass pipe. First major issue, car would not leave a traffic light without giving the car a lot of throttle. Finally found a air leak at the hose between the two metal pipes going to the brake booster. Every time the engine torqued, the crack in the hose opened up to a bad vacuum leak. Next I tested the fuel system. The fuel pressure regulator was bad, replaced it. the fuel pump was weak, especially after driving for awhile. Replaced it. Sal sent me information on testing my air flow meter. Everything in spec. But I didn't drive car for a few years with the covid and some work lay offs. I did start it and bring up to temperature every couple months and add fresh gas. Oh, I found some type of after market chip in the computer, couldn't find a stock one so put in a Steve Wong chip based on several recomendations from members. The injectors were giving me issue's 37 years old and 218,000 miles so replaced them with an aftermarket unit based on another recomendation here on the board. This is what may have bit me in the ass. Car kept reading lean based on wideband, after much searching found bad exhaust leak at crossover pipe on right bank. Just fixed that and wideband showed rich everwhere. I had moved the toothed dial in the air flow meter to compensate for the many lean condition, pump, fpr, etc, Just put everything back to stock. Tested wot switch, ohm readings were all over the place. Was going to replace but no new ones available. Took mine apart, cleaned contacts and adjusted. Ohms perfect now. So here is my current issues. Car starts, warms up, cruises and revs in neutral all with perfect air fuel ratios. While setting idle speed with jumper in, I forgot about it and reved car in neutral and air fuel was 12.0. Removed jumper, back to 14.7. But the next day it was back to 12.0 like it was stuck in the wot map. (i was told the jumper wire puts the car in wot map so you can set idle speed without the idle valve doing anything) The next weekend I restarted car and all was normal again. I drove car, but when trying to engage wot switch car felt like it fell on it's face and fuel mixtures went wacky again. After a few minutes car seemed ok and I unplugged the wot switch. This is why I think there is some type of problem with the dme. My second issue is with moderate throttle car is too rich, most likely the after market injectors. Steve Wong said they could do a custom map to get it where it should be but I spent $550 for the first chip and he wants to only sell me a new one for $875. Funds to do that definately not available right now. So, car drives really well right now, but need to fix wot map issue sticking and the rich acceleration. Thoughts? Maybe Sal can chime in?

[Porsche 935]

When I had a generic running issue, I received a lot of responses. But now that I have isolated at least one of them down to the wot issue, no replys. No ideas why the dme would get stuck in the wot map or not come out of it even though the switch is working?

[mysocal911]

Originally Posted by Porsche 935

But the next day it was back to 12.0 like it was stuck in the wot map.

How do you know that the DME ECM was actually in the WOT map? Just because the AFR ratio indicates 12, one can't conclude it's in the WOT map.
Remember, others factors can effect the AFRs, e.g. fuel pressure, AFM, the temp sensor.

Originally Posted by Porsche 935

(i was told the jumper wire puts the car in wot map so you can set idle speed without the idle valve doing anything)

That's not correct! The switch just eliminates the effect of the ICV during an idle adjustment.

Originally Posted by Porsche 935

My second issue is with moderate throttle car is too rich, most likely the after market injectors.

If your injectors aren't the low impedance type (2-3 ohms each), the AFRs will be too rich and cause problems for the DME ECM in closed-loop.

Originally Posted by Porsche 935

Steve Wong said they could do a custom map to get it where it should be but I spent $550 for the first chip and he wants to only sell me a new one for $875.

Sounds like a real bargain!

[ischmitz]

The WOT and IDLE switch are both evaluated by channel 7 of the 8-channel A/D converter in the DME. The input of the A/D converter be at the following values and that allows the code of the DME to enter one of four modes:



The WOT switch and the IDLE switch cannot be closed at the same time under normal driving conditions but the WOT input is also present at the 3-pin diagnostics port. When you short it while the idle switch is closed the DME centers the ICV valve.

[Porsche 935]

As for the comment from mysocal911. That information came from Steve Wong at SWChips. I would hope he is not misleading people when it comes to diagnostics. And the injectors are low impedance. As posted many times, there is no o2 sensor hooked up during any tests or driving conditions. Could there be an issue with those channel 7&8 in the converter and if so can that be tested and repaired at dme repair center? Could there be a reason when it swiches to that circuit it does not switch back? The wot switch has had a bad connection probably for a long time, could it have damaged anything? Does the wot switch supply a voltage signal or a ground?

[mysocal911]

Originally Posted by Porsche 935

Could there be an issue with those channel 7&8 in the converter and if so can that be tested and repaired at dme repair center?

Yes, there could be a problem with the A/D converter chip, and yes the chip can be easily tested for any problems.
A complete testing of the A/D chip functions should done any time the DME ECM is bench tested.
The A/D chip typically becomes damaged when a spark plug wire is left unconnected near the AFM or the Idle/WOT switches,
and a high voltage spark enters the DME ECM.

Originally Posted by Porsche 935

Could there be a reason when it swiches to that circuit it does not switch back? The wot switch has had a bad connection probably for a long time, could it have damaged anything? Does the wot switch supply a voltage signal or a ground?

The WOT & Idle switches just provide ground signals to the DME ECM.

[Porsche 935]

That is some great information. Looks like my next step is sending dme in for testing and or repair.

[Porsche 935]

Sending mysocal911 message.

[mysocal911]

Originally Posted by Porsche 935

Sending mysocal911 message.

Check your messages.

[Porsche 935]

Sorry, at work. Thanks for the info!