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Okay.. an interesting find: DME was beaten to hell by the previous owner, I'm gonna guess. One screw in the corner was completely stripped and the corner of the cover was bent like he decided to just fold the metal back when he couldn't get the screw out... had to work some magic to get it out. Anyway, inside, the only 2 spots that stick out as being fried (I'm assuming, but I don't know-- I only have a very vague idea of how soldering and circuit boards work). The first photo is under the pins of the harness connector. Can't find a diagram of the '84 944 DME harness, but basing it off the Turbo diagram, the dark brown burnt-looking bit falls right on the 5th pin (reference mark sensor ground/shield). Note the hair soldered into the start signal pin next to it lol
In the second photo, it appears to be a broken/bad solder on the pin directly under the ground pin. Another bad solder in the corner at the pin next to the screw, but I'm unsure of what that is actually going to.
Someone, pleaseee tell me this has been the source of my problems. Gonna see if my friend has a soldering iron I can borrow to repair this if someone says it looks like it needs to be, really not too certain of what I'm looking at right here.
If it is sparking, the reference sensors and wiring are good enough. The DME is processing the input and driving the transistor that grounds the coil.
Since you have both weak spark, and low tach bounce, I would suggest looking at the common area.
1) The green wire between DME pin 1 and the splice for the wire that goes to the tachometer.
2) The connection at DME pin 1
3) The splice
4) The entire ground path for the DME (DME connector pins 5, 16, 17, 19,) condition of wires, and ground connection to the frame.
5) And, it could be the DME itself. Specifically, the output driver transistor could be damaged or have a bad connection internal to the DME, like solder cracks.
I would use a digital voltmeter measuring the change in voltage at the green wire connected (negative) on the coil and ground.
The green as stated is connected to the DME which changes from ground to no ground as the engine is cranked or running to buildup and collapse the coil to produce the spark. The voltage should oscillate when starting the engine
I would use a digital voltmeter measuring the change in voltage at the green wire connected (negative) on the coil and ground.
The green as stated is connected to the DME which changes from ground to no ground as the engine is cranked or running to buildup and collapse the coil to produce the spark. The voltage should oscillate when starting the engine
This would best done with a min/max holding DMM, you want the min voltage to be low, like around 1 volt. A regular DMM is crap at catching pulses and reading the value before another sample is taken. Again, an oscilloscope would be ideal. Voltage not going low enough would implicate something on the list. With the right tool your suggestion is an easy way to confirm or reject that hypothesis without having to get under the dash.
I have a half dozen oscilloscopes, but for this kind of work the cheapie DSO150 for about $30 works just fine. If you are not familiar with using one, it will take a bit of learning and they are a bit finicky. But at least the capital outlay is minimal.
The brown stuff that looks nasty and like there is a problem is actually a sealant, The early DME's are coated boards. It can make it hard sometimes to know what's a problem, and what's just a place where too much collected when sealed or has just aged. But you for sure have some joint issues. That one un-soldered pin looks ok in the first pic, the board doesn't seem to have a trace there. (It's not a used pin on the board). Just looking, I'd say the first pic looks bad just due to the sealant. Might be worth cleaning it up some to see what's what. The others are examples of bad joints, however. This board may have been poorly reflowed at some point.
Solder can be a real pain to remove without the right tools or a desolder kit. But reflowing is pretty straightforward. I don't much care for working on tight boards like these. I don't have the eyes or steady hands for it! I don't know if any of this is your problem, but it can't hurt to give it some attention!
Contact Specialized ECU Doctors, located in FL and talked with them. They are excellent.
I believe you have two choices, get this one repaired or find an early good DME to try. ECU Doctors can test DME in real-time using a 944 car as the test bed.
https://www.ecudoctors.com/
Last edited by T&T Racing; 02-07-2023 at 11:08 AM.
Contact Specialized ECU Doctors, located in FL and talked with them. They are excellent.
I believe you have two choices, get this one repaired or find an early good DME to try. ECU Doctors can test DME in real-time using a 944 car as the test bed. https://www.ecudoctors.com/
I have a half dozen oscilloscopes, but for this kind of work the cheapie DSO150 for about $30 works just fine. If you are not familiar with using one, it will take a bit of learning and they are a bit finicky. But at least the capital outlay is minimal.
+1 for DSO150. I am wholly unfamiliar with O-scopes, but I bought one to test my speed and ref sensors and it was very uncomplicated and I figured it out first go. Hooked up up, cranked engine, looked at VPP and my sensors tested good. A $30 tool that ruled out the sensors was to me worth every penny. BTW, I had tried a DMM prior to getting a scope and results were inconclusive with the DMM, at best.
Reflowed the 2 joints that needed it pretty successfully, I think. Unfortunately the car did not magically start. Someone offered to test the DME for me if I ship it to New York. Trying to decide if it’s worth that or if I just bite the bullet and buy a new DME. Don’t know if I want to try getting the current one repaired, just because one of the covers looks like the guy threw the computer down a flight of stairs (which seems like a perfectly reasonable response to this particular car) and I just don’t know if it’s trustworthy!
Examined some wiring under the dash. Fixed some wobbly connections that were being held together with old electrical tape. One of the fuses was blown (at the moment I can’t remember which one, but I kind of doubt it’s a serious one) gotta pick up some spare ones. Was going to swap the DME relay— don’t think I’ve actually done that since the tach stopped bouncing, but had to rush to work last minute.
Charging my battery tonight and gonna climb back down there with a multimeter tomorrow to see what I can find looking for shorts in the wiring through the DME harness. Anything else specific I should check for while I’m in there?
I’ll also look into picking up an oscilloscope. I’m sure it’d be good to have one on hand.
Have I cracked the code??? Today, while taking some readings from the DME harness, I noticed I wasn’t getting any current coming from either the Speed or reference sensor, because of course it would be that after I declared those fine… I immediately jumped to the sensors to test them. Both passed the test (reading about 1020 and 860 ohms), then I tried testing the connector on the harness side… Not sure if I’m supposed to get a reading from those, but I got a reading from one (I believe the speed sensor, the stickers on the harness aren’t still legible, unfortunately), and of course, nothing from the reference sensor… I pull the rubber cover off of the wire to reveal that the yellow and one black were just completely unplugged from their respective pins inside the connector… Now, I feel like an idiot! Was trying to get them back in, but was having a hard time. Pulled the connector completely off, but still can’t get the pins to come out. Is this a part I can pick up at a hardware store? Saw only944.com has a has the whole female connector kit I may try to order, but if I can get these pins out, that’d be nice. Going rig up a connection real quick, just to test if this fixes my problems. Wish me luck.
Good luck sir!
The connectors are molded and the wires are tiny. You of course get new connectors with new sensors, at that side of things. I have no experience trying to replace the connectors on the harness side of things.
Well... that didn't do much of anything. Think I'm getting a stronger spark from the plugs. Not reading anything from the harness on the Ohmmeter, but when I start the car I'm getting 1 volt (which I wasn't getting before), so I guess that was that... Probably gonna call the only Porsche mechanic I can find near me that'll look at it on Monday, even though they're too expensive. I think I'm just going to throw in the towel... My brain has turned to mush and I'm ready for it to be a car again... I don't know what else to do. I swapped tried swapping the relays for fun, didn't do anything. Can't see any visible shorts or detect anything with my multimeter. I just really am out of ideas to try. Maybe it's the DME. I guess I could send it to the one guy in New York that offered to test it for me, but I'm gonna ask the mechanic if they have any spare ones lying around (they're mostly 911s, so I'm not confident).
Not taking it to a mechanic quite yet because your boy doesn't know how to just give up! Bought a new (to me) DME that'll arrive Friday. Praying to Ferdinand Porsche himself that this has been the issue all along!
02-06-2023, 04:57 PM
