924S no start solved

[Xs924S]

Been chasing a No Start condition since July 20th and finally got it started again today.

What we did to get it started...
BLUF: Resoldered the transformer in the DME

Changed out the sensors multiple times, changed DME relay a few times, bought an oscilliscope, realized sensors were only giving 1 v, replaced starter, replaced new sensor bracket with old bracket, regapped with cool printed tool, got 4 v on sensors. Still no spark. Getting about 10v to coil, coil wires not grounding out when trying to start. took apart the DME, reflowed all the solder at the transformer. Got spark!!!!

[944 Ecology]

Did you mean the DME relay or the DME ECU? Which one, please...

[Monkey Wrench]

I've never opened a Porsche one but in others , most cars of the era have an ECU the capacitors can fail and they are worth close to nothing.

it might have differences with this one that I'm not aware of..
but what Ive done is to open the ECU , look carefully at the circuit board near any electrolytic capacitors. usually small black tin cans.

look at the board near them, if you see the board is discolored it can be because the electrolytic capacitors leaked the fluid they contain. it spills on to the board and discoloration near the component can be evidence.

What I've done is first note which way the capacitor cans are installed and take pics. It's important not to turn them backwards. polarity matters with most electrolytic caps. It does not usually matter with other caps. look on the can and it will give you the capacitance value, something like say 4.3 mfd you may see the micro symbol ( microfarads) it will also state the voltage, so might say 16V , that's the highest voltage the cap is designed for.

look for a line on the can, it indicates which is negative, keep the polarity correct.

go to a electronics supplier and buy them, cheap, about a dollar or less.

take the new cap and wrap the ends of the leads around a pin to make them like a spring coil.

clip the old can with snips, about 1/8" from the board.

remove the bits and you should have 2 short wires coming up from the board.

slip your "spring shaped ends' over the little lead nubs on top of the board. you can squeeze them up and then solder them carefully with a soldering pencil, you want a small tip and properly tin the tip first. once the solder melts and flows stop, try not to heat your board and surrounding parts. your soldering pencil shoudl only ned to touch for a second. You should not eed to hold it here waiting for the heat. If the tip isnt; tinned right it won;t transfer the heat to melt the solder properly..

if you need ot clean the tip put solder on it, wipe it with a damp sponge and put more solder and do this until the tip has a nice silvery coating..
Id use lead solder some may say you shouldn't use lead solder.. ok lead is hazardous..

you can also re melt all the solder connections to try to solve any issues with cold solder joints but it has some chips. Id stay away from tiny traces. leave that fine soldering up to someone up to that task if you need to go there. I wouldnt touch misroscopic parts, my eyes aren't great.

examine the board for any signs of heat. If you were to say see a resistor that's showing bad signs you could replace that. you can compare resistors against their color code. I doubt you will find failed resistors.

The caps age and fail. you can check them with an ESR meter without removal bu they are cheap. most of those other components may last quite well over time.

doing this can result in you messing up something so there is a chance things can go wrong, if so Im sorry for leading you down the path.. but it may also help if the capacitors just got to that age where they fail and they can fail. I;d definitely think this over but if you have a spare to play with you can open it and see if it makes sense to change the caps.

you may find aftermarket companies that make replacement ones , some with different chips or some places will "rebuild" and bench test them , they may have a test bed that can analyze that it's working and sell you it as a rebuilt unit. they may want your old one for the core.

I tried doing this on my van's ECU. it worked fine but I made a little goof up when I clipped one of the capacitor cans I also took a nip out of a transistor. I just wasn't careful enough.

I can probably replace the transistor, it has some very tiny numbers. transistors have 3 wires.
I opted to just order a rebuilt one and it's been ok. i opted not to power it up with a physically damaged transistor , I can still replace that and it should be ok.

you dont want to touch any kind of microchips with a static charge and cause an arc, like when you rub your feet in carpet and touch something and get an electrostatic zap. Often it is advised to wear a static wrist strap that is grounded. you can also take measures to keep yourself and the parts grounded to prevent this chance, that can do damage. dont wear that fresh sweater you just pulled out of the dryer. often I can pet my cat and get a charge that way.

If you would rather and if you know someone that works on electronics then you might have them do this. they will have an appropriate solder station. I sometimes wok on antique radiso and faield caps are a big part of that, the ones I work on are old and with tubes and point to point wiring. I'm not proficient at working on or diagnosing anything with tiny parts. but this you can do without much in depth understanding of how the circuit works.

some boards are made in layers now, and as time moves forward all the parts get smaller to the point where its hard to even see what you are working on. If you dont feel comfortable just stop..

you may find the whole thign is bonded to an aluminum base with heat putty to transfer the heat, and you'd need to do that if you want access to the backside. It may however be possible not top take that apart and to do this from the top of the board.

if anyone has photos of one that is opened up maybe we can discuss if this makes any sense to do on a Porsche ECU , and maybe Porsche doesn't call it an ECU but a DME electronics unit or something similar.

this isn't necessarily going to fix anything but if the caps fail they can be replaced.. its common in lots of electronics to do a recap with a shotgun approach and just see if that fixes it, because they do fail. a bench tech can pull out an ESR tester and perhaps tell if they are bad or just replace one bad cap, but some may want to change them anyway for reliability. Myself I'd use the shotgun approach and change any I felt I had adequate access to. - no diagnosis only, replacing capacitors, then try it, nothing technical.

yes there is an element of risk like how I clipped a transistor things might not go your way so dont attempt this on something you can't afford to loose.

you may find a thin rubbery protective coating you can try a bit of alcohol on a q tip to dissolve enough to get your solder done if that is in the way.

understanding the circuit, that's for someone with a much higher pay grade than myself ;-) . I can't pretend to even start to understand all of that.

[Autobreza]

Fantastic write-up, Monkey Wrench. Love the coiled lead hack.

[Monkey Wrench]

Thanks Autobreza. forgot to mention , sometimes you can find capacitor cans that are "blimped out" the pressure int he can can expand and if you see signs of them puffing up, that's a common sign of failure you may see and be able to recognize.

i work a lot with old paper caps. they are just basically two pieces of foil with basically wax paper separating them. the more area and the more foil used the larger the capacitance. there isn't really a connection through it , unless it fails than it basically shorts out. They do..!

i replace the old paper caps with more modern mylar equivalents or ones that look like jelly beans, They don't fail often. the paper caps or mylar caps are not really sensitive to polarity.

an electrolytic cap is similar but filled with an oily substance. often in old radios these are fairly large aluminum cans. the reason electrolytic caps are used is because they can increase the capacitance but keep the package small. the oil is a smaller gap than wax paper.

modern ones of the same electrical spec are usually quite a bit smaller. in order to keep the original placement and look I will often put the new caps inside the old cans or the old paper tubes , that's just for cosmetics if it's an antique and I care enough but normally these are hidden within the chassis. its a hidden challenge ...no one can see my work unless they take it apart. That's just the love of restoration work..

electrolytic caps are polarity sensitive. If I put those backwards they can explode their guts. motor start capacitors may not be polarity sensitive, they are a bit different.

often the purpose of the larger ones near a power supply is to smooth out the power. a transformer works on AC not DC. they work by induction , basically when you have two coils sharing a iron core the power it transferred and by the number of wraps the voltage can be changed either greater or smaller. after that the power is not DC, its AC power. in order to turn it into DC the power is rectified. If you look at the ripple on a scope it looks like a sine wave with the voltage waving back and forth between positive and negative. stick a diode in there, it stops current flow in one direction, then it would look like a series of broken blips on a scope.

you can then arrange 4 diodes in a way that both sides of the power are made positive, (called a diode bridge) with that it's not pulsating current, its positive ( or negative) but no longer waving between positive and negative. that current is then not clean power. it has ripple. the capacitors in the power supply turn the ripple current into smooth current.

there are other ways to transform power by way of , lets just say magic of modern electronics , this saves metal, the transformer has some iron and copper and that's cost. some modern power supplies won't have a great big iron and copper transformer.. If you look up "digital power supply" it will be explained.

in the old radios I work on they use higher voltages. a common issue is that due to age the caps fail and then there is too much ripple, then the radio hums. often its a typical 60 Hz hum. other caps do other things but I wont go into all that. Its fun to take some non working antique that was stored in a barn and maybe a house for mice and do all these things to slowly revive it. sometimes Ill get so stuck and confused but often it's not much more than parts replacement.


transistor radios , like those powered by a 9 V battery. these dont need great big caps, there is no high voltage. Often Ill pick one up that is from 1970 or so and it will be fine. the components haven't degraded so much. the caps can and still do fail sometimes. the resistors in a transistor set do not usually get hot and fry up, they arent; conducting a lot of amps.

its possible for electronics to be working but to have a higher than normal ripple current due to failed or failing capacitors. This may cause issues.

many circuit boards use a common 5 V power source, most computers do.. on boards sometimes you may see a pin thet says TP 5V or similar, this is a test point so you can check if the voltage there is alright. It might not like 4.5V with some ripple for example. this board might work on 12Vdc and not 5V. I'm not sure. I dont see any other reason for it needing a transformer but I dont know everything.

what I'm getting to here is a bit of a question or a thing that's puzzling me.. the OP said they solved issues by "re-soldering the transformer in the ECU".. so what is this transformer used for? perhaps it runs on 12VDC not 5VDC and this transformer could be something else and not the main supply power, perhaps part of a sensor circuit?

a car runs on DC not AC so to transform the power to different voltages you can't simply use a transformer since the supply is DC they need AC to transform the voltage. to do that you may have other components that are more complex. In the house you have 110 AC so it is easy to transform that since it is already AC.

in a car you can't simply hook 12 V to a transformer because that's not pulsed power or AC. a typical transformer wont operate on DC, it needs a pulse. they may use a digital power supply or a circuit more like that if they need 5 V transformers aren't only used for the purpose of changing voltage , it might be sort of like an amplifier.


some old radios were designed to be used on farms with windmills and lead acid batteries. The way they acquired the higher voltages for tube technology to work back then was they used a component called a vibrator , which was like a buzzer it made the DC into pulsed power, then it can be transformed. Modern electronics will not do it that way. it won't use a vibrator this will be done by more modern electronic methods. old car radios from the 50's would use a vibrator.

by replacing bad caps what you may be doing is helping it by restoring clean power and getting rid of ripple current. you may see improvements in how well it works and quite possibly you won't fully understand the technical details about how the board functions. if you can succeed you may not have to know everything.. the hope is that you made it more reliable and replaced parts that may fail and cause issues down the road.

often if you see a transformer on a board you will also see some of the largest caps near it , these will be the power supply capacitors. these are the ones I would most suspect. In a car where the supply is DC it may not really need to transform that AC supply since the supply is fairly clean DC unliike anything in your house.

other tiny caps are doing things in the circuit that I can't explain. likely they are not seeing a lot of electrical load. they can still fail but I'd target the larger ones near the power supply first.

in an antique radio I will often target any large power supply caps first, then gently power it on I may increase voltage slowly. while monitoring to see if it's drawing excessive amps. often then I will hear them start to work and as i go though and change others I like to test it. this is because if I mess up I have some idea where to look for issues. Ill do basically the same with all the resistors. measure and compare to their original resistance. Often with old radios the power supply is run through the speaker field coil , its basically used as a resistor. i wont normally plug them in without at least changing the power supply capacitors.. what will often happen is they are shorted and then they will explode their guts, fill the room with smoke, maybe wreck the field coil of the speaker too , which is typically hard to find.

modern speakers don't use field coils, magnets got better so they use permanent magnets. thye only have the voice coil. only 2 wires..


that''s all pretty basic. someone that knows more can isolate problems better, some will only target failed components. some will understand the transistors and the microchips better and the digital electronics. diagnosis of the electronics is a career unto it's own. to me most of that is just too complicated to understand. Others know this field much better than me.

replacing capacitors and re-soldering boards to solve cold solder joints is possible and something I would attempt, but I'd go into it with hopes and not expectations of success. I may do this if I for example had issues, replaced the board, it worked , then I know my old one is bad, then I might try to fix that to keep as a spare part.

my 944 is an early 95 version , it seems there were changes mid year.

[Fast Freddy 944]

For some reason my eyes hurt. Congrats on your repair!

[Autobreza]

I do recall the cap construction from my college electronics course. Little electrical miracles ... until they are not. I watch Mr. Carlson's Lab on YouTube, where cap replacements are SOP on all his repairs.

[Xs924S]

Yea, the ECU not the relay.