944S Cuts Out on Straight Road - Won't Start - Wait 5 and She Starts

[Lapponia]

Hi Dave!
Yes the old DME-relay is underspecced.......at least one half of it. I´m shure it is the one that closes and break the current supply to DME (computer) and the fuel injection valves. First you get a fixed current from relay to DME (key ON) but then you also have a pulsating curent through the injection valves that adds to the forementioned current when engine is running. The DME (computer) makes and break at pin 14 and 15 to ground thereby activate or deactivate injection valves (all four at he same time). The make time here is the length of time as the injectors are opened and fuel gets to the cylinders and and is dependant of engine temperature, air temperature, rpm, speed and load. So what we can understand from this is that if you are breaking a current through four coils (injectors) and knowing that this current now is inductive as in an ignition coil...this is really baaad for the relay contacts in operation....especially when you stop the engine.....you will have an arc over them like the old fashion breaker points.
Differece here is that you don´t have a capacitor to reduce arc (minimise induction) nor do you have a zener diode between live contact and ground.... and....and... so on. Really bad.
That is why I think a relay with heavier contacts will last longer (greater contact area and cooler) and this is what Í´ve been testing now this summer with good result.
Unfortunately I wasn´t able to use my "new" relay in the same socket as the original one. Instead, I have a 2m long cable with six wires in it. One end is conncted to the relay and the other end finished with those crimping flat lugs that are so common now in electrical appli-cations. Three wires are 2,5 sqmm and three are 0,75 sqmm with lugs 6.3x0.8 mm and the other three are 2.6x0.8 mm I think. I lubricated them with dielectric grease before connecting them to relay socket to avoid any corrosion to come.
One idea is to cannibalise a bad DME-relay and use the bottom with flags to solder the wires to, put the plastic can on and use as a plug into the socket. Relay can be fixed somewhere under the dashboard as so many other relays are.
Car is jacked up for the winter but I will continue to make experiment with my single relay idea during the winter. Neither will this relay fit direct ino relay socket because of the different pin/flag layout. Connection will probably be the same as the forementioned. I will keep you posted. Cheers.
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Lapponia 1984 944 na

[DaveBoettcher]

Hi Lapponia,

Really interesting, I think you are on to something. I have just taken out the contact from my suspected faulty DME relay, the one where I noted the purple wire in the previous picture, and as you can see:



one contact, the 87b contact, is badly burned and pitted. The contact on the other side is perfect, as are both contacts on the other relay. Is this the contact you suspected?

I am sure this burned contact must be the source of my problems. There is a ring of soot around the burned pit, which looks just like the pit burned by a current arc, so I am sure that your conjecture that arcing is tacking place when the engine is switched off, and the relay contacts have to break the current through an inductive load, is correct. (Just like the points in an old style mechanical distributor, as you say.)

This is clearly an inherent fault in the circuit design, because this arcing must inevitably, sooner or later, cause enough damage to the relay contact that it becomes unreliable, and leaves the car stranded, as I now know many 944 owners have been over the years. However, I must say that the DME relay that failed on me is the original one fitted to the car over 20 years ago - I hope the new one lasts as long!

Swapping to a higher current capacity relay won't cure this arcing, surely it needs a zener diode or capacitor (or both) to soak up the voltage spike and prevent the arcing? This would be a nice mod that could probably be built in to the existing housing? Any idea what capacitor or diode spec to use?

Regards - David

[Lapponia]

Hi Dave!
What an exellent picture! A customer would be very pleased to see such a pic in an issue investigation and pay for it gladly. However, I must admit I was wrong here after checking the relay connector 87b on the wiring scheme. 87b is the same as number 4 wich goes to..........the FUEL PUMP and SUPPLEMENTARY AIR VALVE!!! I´m relly surprised!
But if you think of it, the fuel pump has to be a great inductive load too to the relay contacts. Air valve coil my add some load as well (or is it just electrically heathed to open) . I don´t have any figures on the amperage these objects have in action but fuse 2 is 16 Amp!!!!!For not breaking the fuse in the long run, current could perhaps be around 12-14 Amps and that will make a good spark over contact 87b and 30 (2 & 4) when engine is stopped. Well, it is obvious that inductive load is not good for relays and should be dampened in some way. Maybe Ferdinand thought a lifespan of 17 - 20 years for our 944s would be enough, so he wasn´t worried about any relays in the car LOL.
Reminds me, did you push on that relay yoke to see if it could move freely down to the coil before you dismantled it? The two wires (now cut) seem too short in the first picture to allow that. That could also be an issue cause the contact force would be limited and contacts would start arcing when engine is running!
So, to your next question. Not beeing an electronic engineer (just electric) I think a capacitor over the relay pin 87b (4) and ground 85 (1) will do and with values of 0.47 microFarad, 200 Volt....an example taken from an old distributor with breaker.
(I compare the primary coil current in an ignition coil with the fuel pump current here).
A conclusion of all this mumbling of me would be a capacitor from relay pin 87b (4) to chassi ground and while in there why not connect a capacitor from pin 87 (8) and chassi ground as well. If those capacitors could be mounted behind the relay plate, one could use the original relay and live with it forever......perhaps.
I will try this in my experiments as soon as possible and let you know.
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Lapponia 1984 944 na

[DaveBoettcher]

Hi Lapponia,

Yes, I did test that both armatures moved freely when I was trying to figure out what was wrong with the relay before I dismantled it. Everything on both relays seemed to be fine, and the only odd thing I noticed was the purple colour of the armature wire on the upper relay as per my earlier picture. If you look at this picture:



you can see that because of where the fulcrum point is, the end of the armature with the contacts on it can move the 1mm or so to close the contact with only a very tiny movement at the end where the wire is. Sorry the second picture is not so good, but you can see that when I close the contact with the pen, the movement at the wire / spring end of the armature is barely visible in the picture; only a fraction of a millimetre, which the wire does not interfere with at all.

I did think about rebuilding this relay with the capacitor you suggest, but it is not possible to get at the fixed contact of the upper relay, the one that corresponds to the burnt one, to clean it up properly. I may get another relay to carry as a spare, and I would certainly put a capacitor on that one and give it a try - unless you get there first!

[Lapponia]

Great pictures again Dave!! But as I was almost finished with my comments, the power to my house broke down just for a fraction of a second. It all vanished from my PC and I think I need a coffee break to cool down my nerves . I will come back later.
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Lapponia 1984 944 na

[Lapponia]

Now after coffee! So, we can come down to the well known equation; P (watt) = I (current) squared x R resistance. With ageing of the relay contacts every time we cut off the engine we get more and more pitting and soot there. Resistance will increase becase of these issues and
as petrol pump still needs its precise current to operate well, we will have lots of watt here. With time it makes energy which will be transformed into HEATH....glowing heath. That´s not good for the contacts as we can see from your first picture... the purple wire. I would guess there must have been many hundred degrees C to create that color. Strange that the silvery part of the contact isn´t blueish...they usually become that when heated.

I belive we have hit the nail when coming down to the DME-relay issues. It would be a good thing if we could solve this by simply connect a capacitor between ground and pin 87b and pin 87 as well (moving part of contacts). I will go an buy a couple for use in old time distribu-tors if it still possible to get them. Its onother story where to hide them.
Best regards.
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Lapponia 1984 944 na

[Lapponia]

Dave! Can you verify that the moving plate with burnt contact is 87b, please. Relay frame and moving plate in both relays are connected and go to flag 30 in my original DME. Fixed contacts are 87 and 87b (current carrier). Cheers.
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Lapponia 1984 944 na

[smj]

Hope the new DME relay has fixed your problem. Just wanted to add that I had the ignition control module go intermittent in my S some years back, which I'd also heard called the ignition impulse amplifier or somesuch.

In my case the thing would usually cut out while I was in the middle of a turn, though I can remember one or two times it shut down when I stopped at an intersection. engine would stop, tach would smoothly drop to zero and I'd be stranded for an indeterminate time. Fortunately it was mostly happening near home, because once I had ot just leave it for two days before it opted to start again.

My mechanic went through the whole car, redid every ground connection he could find, and logged maybe a 100 miles looking for this one. He'd never seen that module crap out before.

[DaveBoettcher]

Originally Posted by Lapponia

Dave! Can you verify that the moving plate with burnt contact is 87b, please. Relay frame and moving plate in both relays are connected and go to flag 30 in my original DME. Fixed contacts are 87 and 87b (current carrier). Cheers.
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Lapponia 1984 944 na

Hi Lapponia,

Yes you are right, sorry about that. The relay yoke/frame and armature/moving plate carrying the burnt contact are connected to 30, the fixed normally open / closed when relay energised (or when pushed by my pen!) counterpart contact (which is also slightly burnt but nothing like as badly as the contact on the armature) is connected to 87b. The fixed normally closed / open when relay energised contact is not connected to anything.

I only realised last night what you meant by "where to hide" the capacitor - I had forgotten how big the capacitor (or condenser as they were more usually called) across the points of an old style distributor was - item F in the picture below. Of course this wouldn't fit inside the housing of the DME relay. Do we really need a capacitor as large as this? And what about a zener diode, wouldn't that be smaller?

[DaveBoettcher]

Originally Posted by smj

Hope the new DME relay has fixed your problem. Just wanted to add that I had the ignition control module go intermittent in my S some years back, which I'd also heard called the ignition impulse amplifier or somesuch.

Hi Steve,

Thanks for that. Yes the new DME relay does seem to have cured the problem, and from the burnt state of the contact in the old one I am pretty sure that was the problem. However, the ignition control module was mentioned at the start of this thread and, if the problem recurs, that will be the next thing that gets changed. The DME relay being both notorious (as I now know) and cheap was the obvious thing to change first and, fingers crossed, it seems to have done the trick.

[Lapponia]

Hi Dave!
Yes, that´s the thing I suggested and it is big too. My experimental DME-relay lie on the floor behind driver seat so it´s not a big problem to mount them.
A zener diode is by far less voluminous but has to be connected in series to a resistor (not big either) and would fit within the DME relay. However, I feel a bit on black ice now because I don´t know how great a voltage/current this zener and resistor will take when contacts open. Could not find any information of fuel pump in the Porsche workshop manual (current consumption, inductance etc) to be able to make some calculations here. However again, stores of electronic parts sell so called RC-net encapsulated in epoxy and with two legs. It is a capacitor in series with a resistor and could be choosen for a great variety of values and usually are quite cheap and small in volume too . They are in fact used for minimising voltage spikes and arcing in electrical applications But again, I´m not sure to pick the right one yet.
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Lapponia 1984 944 na

[DaveBoettcher]

Hi Lapponia,

Do you have a wiring diagram that shows the connections to the DME relay?

Regards - David

[Lapponia]

Hi Dave!
Sorry I´m not very comfortable copying documents to internet....guess I´m too
old LOL. I found all workshop manuals on www.eucc/dk and electrical wiring is in volume IV page 52 as I remember. All these manuals are good for us 944 owners so I will reccomend you to download them (88 Mb).
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Lapponia 1984 944 na

[DaveBoettcher]

Hi Lapponia,

I couldn't find workshop manuals by following the link you suggested - was there a bit missing?

I have been in touch with the technical department of RS components and they suggested using a varistor in the relay to stop the arcing - see this link

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=289-7575&x=0&y=0

What do you think to that suggestion?

[Lapponia]

Hi Dave! I´m sorry for beeing so stupid. It should be www.eucc.dk of course. I´ve
tried it and it works. Pick and download what you want of it.
Your suggestion of a varistor over the inductive load (fuel pump) is very good and a well known solution in electronic cirquits to dampen voltage peaks. But I don´t know the figures to play with but your suggestion may be good here.
However, I asked a electronic parts shop in this matter and they immediately suggested a "freewheeling diode" which I never heard of before. Like the varistor it has to be connected over the load (fuel pump) inside of relay. They even suggested diode type 1N4007, 1000 Volt, 1 Amp to use here and is faster than a varistor to ground a inductive voltage peak. But as I said before I´m not an electronic so I´m on deep water here. I would try a diode as they are smaller and could perhaps be fitted into the relay and thats what we want, don´t we?
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Lapponia 1984 944 na

[DaveBoettcher]

Hi Lapponia,

Thanks for the link to the manuals, it worked this time.

The first suggestion my guys came up with was tackling the back emf at source by fitting a reverse biased diode across the pump, but when I explained that I really wanted to fit it inside the relay they suggested the Varistor. I think both of these and your electronics shop suggestion of a freewheeling diode are all variations or different names for breakdown diodes like zeners and tranzorbs, and that they will all do the same job as far as we are concerned: after all we are not trying to do anything sophisticated, just kill a couple of hundred DC volts to avoid the arc starting.

Now the next job is to try it out. I think the only way to really test whether our modification is working will be to measure the transient voltage across the relay contacts, both without and then with the protection fitted. This seems to me to need an oscilloscope, which is not something I have access to. Any ideas?

[Lapponia]

Dave! You are absolutely right here. An oscilloscope i needed to messure the transients and would of course be done before and after fitting of the varistor or diode just to make the right decision of parts. I don´t have an oscilloscope either so we just have to test it by trial and error. I´m grounded for the moment by the flu so it is not possible for me to make tests for some weeks ahead but I will purchase some diodes 1N4007 to solder one within the relay can (only one diode will be needed......the one over fuel pump). This could be done over the connection flags 85 (ground) and 87b (fuel pump) at the inside of the relay. Relay parts are soldered onto a printed cirquit board so it may be possible to fit the diode or varistor there. A diode should be soldered with the cathode (white ring end) to 87b, the other end to 85 but one should remember NOT to do this on an old relay with already burnt contacts. A new relay is preferred here........or if one is very keen on fiddeling with very small things, it is possible to rub down a minor contact damage with fine sandpaper and clean them with alcohol. Of course it is essential that the relay is in good shape in other parts.
So, it is a pity that we don´t have an oscilloscope. If this will work we have to wait until our cars are scrapped by age.....17 more years or so, then we can say, it DID work LOL.
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Lapponia 1984 944 na

[Lapponia]

It came to my mind all of the sudden. Car breakdown by unknown issues are more an a electrical matter rather than an mechanical one. Twenty or twentyfive years makes really bad things to all the splicing contacts within the car and there are many. One multiwire in particular is the one over the brake booster. There are nine wires in that connector and they go to so many vital parts, DME/ECU, DME relay and Fuel injectors. The heaviest is 4.0 sqmm and other are only 0.5 sqmm an we can bet that there is corrosion there. So, I would reccomend you all guys to pull that connector, clean male and female contacts and lubricate them with ample amount of dielectric grease. Push the connector halves together and live happily after.
Of course, you should do this to every contact you can possibly reach just to make sure that these contacts will never harm you or the car again.
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Lapponia 1984 944 na

[DaveBoettcher]

Hi Lapponia,

Good thought about cleaning electrical contacts. I cleaned all the contacts in the rear light clusters and applied copper grease to them - no more dodgy brake lights!

Back to the DME relay, as we don't have access to a scope, we could look for the problem we are trying to cure - the arcing across the relay contacts. I imagine that if we tried a standard relay on the car without its can, perhaps in the dark, we might be able to see the flash of the arc when the relay opens. Might need to use a small mirror like a dentists mirror to see the contacts clearly. If we can see this flash, and then we try a modified relay and don't see the flash, then we know the modification is working.

[Lapponia]

Dave!