Fixing the DME (ECU) out of a 1983 Porsche 944

Thread Starter

Noahjh17

Joined Jan 5, 2020
5
Hello
I like to mess with/think I can fix things of many different applications, one of which being cars. I own a 1983 Porsche 944 and the other week it quit on me. I'll spare you the automotive details, but I have narrowed it down to the DME which is the main control module in the car. It is a nearly 40 year old computer and quite an interesting piece of electrical design if you ask me. The common problem with these is that over the years, the vibrations in the car will cause one or more solder joints to crack which causes the DME to not work. There is a small amount of information on the rennlist forums, which are a Porsche forum, about how to fix these. The general consensus is, if you can't find the cracked joint, just re-solder em all. I have spent the better part of a day looking for this cracked join with no luck and i'm about to get to soldering but I have a few questions about this process and figured this would be a good place to ask for help. I have some limited experience with soldering and have the necessary tools but some things about this board are puzzling. As you can see, certain joints are bridged with solder. Are these only when two leads share the same pad? I'm assuming the best solution is re-solder the same way I found it, but what is the real answer here? Is it okay to not bridge these joints? In addition, certain leads seem to not be soldered to the board at all. Also, there's a coating on these boards and I've heard someone say you can solder through it but others have said remove it with acetone, which I don't have and don't feel like getting unless I have to. Finally if there's anyone out there who might have ideas on narrowing down exactly what on this board is wrong, that would be very helpful. From what I understand, the lighter side, in the case is all analogue, with caps, resistors etc. while the other board is all digital containing all the IC's that are doing all the maths. I can't find any schematics for the 1983 DME unit although they exist for the later models, but do have some changes. Thanks in advanced for your help, and even if you can't, hopefully people on this forum will at least find this old tech interesting! Pictures should be attached below.
20200105_200650.jpg20200105_200707.jpg20200105_200713.jpg20200105_200738.jpg20200105_200744.jpg20200105_200748.jpg20200105_200855.jpg(This last image is just an example of bridged joints and unsoldered joints)
 

SamR

Joined Mar 19, 2019
2,258
If it was working before... Bridged joints should be correct if it was working. The one unsoldered pin I saw coming out of a hole that had no pad to solder to. The solder joints look pretty good and there is a conformal coat masking the pc board. It could be a bad component, it could be a stressed joint. The old adage "if it ain't broke don't fix it" comes to mind as to resoldering every joint. With good lighting and ~4X magnification examine the joints for damage. Only resolder a damaged joint. As to bad components, without a schematic and a very good knowledge of electronics, you will have to find a replacement board or a very good service/repair tech.
 

Thread Starter

Noahjh17

Joined Jan 5, 2020
5
If it was working before... Bridged joints should be correct if it was working. The one unsoldered pin I saw coming out of a hole that had no pad to solder to. The solder joints look pretty good and there is a conformal coat masking the pc board. It could be a bad component, it could be a stressed joint. The old adage "if it ain't broke don't fix it" comes to mind as to resoldering every joint. With good lighting and ~4X magnification examine the joints for damage. Only resolder a damaged joint. As to bad components, without a schematic and a very good knowledge of electronics, you will have to find a replacement board or a very good service/repair tech.
The only thing I am sure of is that its broke. Being an old automotive part there is almost no information on the electronics. There's companies that refurbish them but they are very expensive. Only success stories are people redoing all the solder joints. Re soldering is kinda my last ditch hail mary before looking for a new one. Thanks for the reply!
 

SamR

Joined Mar 19, 2019
2,258
If you know how to use a multimeter, start with the more likely areas of failure. External connectors to the PC Board. Do they have connectivity? Input diodes. Any opens or shorts? I don't know your level of expertise here and there are guys here FAR more experienced than I am. But I would strongly advise against mass resoldering every joint. To even suggest it tells me you should not attempt it. In particular, I would check closely those flexible multiconductor ribbons.
 

Thread Starter

Noahjh17

Joined Jan 5, 2020
5
I might start going over things with a multi meter that's really not a bad idea. I have some experience with working with electronics, enough to be able to solder THT components on a board but not enough to know how the components work and connect. Isn't the only component that would really fail like that a capacitor? And if it did, wouldn't it be obvious what cap failed?
 

SamR

Joined Mar 19, 2019
2,258
Not always and diodes also possibility. I saw a couple of metal xstrs there wearing heatsinks that I might eventually check. First look for any obvious split/burnt components while closely examining the solder joints. Basic multimeters usually don't measure uF and for some components, you would need to lift a leg or remove them to measure them. But at least check across them for opens to begin with. Next would be to check voltage coming into the unit, if there is any, and go from there. Good Luck and Welcome to AAC!
 

Thread Starter

Noahjh17

Joined Jan 5, 2020
5
Not always and diodes also possibility. I saw a couple of metal xstrs there wearing heatsinks that I might eventually check. First look for any obvious split/burnt components while closely examining the solder joints. Basic multimeters usually don't measure uF and for some components, you would need to lift a leg or remove them to measure them. But at least check across them for opens to begin with. Next would be to check voltage coming into the unit, if there is any, and go from there. Good Luck and Welcome to AAC!
Thanks! For those transistors, can I pull off the metal caps that are held down with the flatheads? I was going to earlier but didn't know if i'd ruin something.
 

SamR

Joined Mar 19, 2019
2,258
The ones I am talking about are small metal can TO-39 transistors standing up off the board with a round halo-like heat sink. Not the large oval TO-03 flat-mounted that may be a xstr or an IC such as a voltage regulator. You can see that the joint compound on top of them is cracked either from normal shrinkage from aging or drying out from heat. Just the fact that they have a heatsink on them says the designer had a concern for their cooling due to heat dissipation. Probably not a problem but might deserve looking at after if the preliminary checks are all OK. I'd leave the flat mounted ones alone as they are soldered underneath. FWIW
 
Top