Here is how the DME controls the fuel injectors.

• Battery voltage through the DME relay is applied to one side of the injectors.
• All six injectors fire at the same time and are wired in parallel. Even though there are two wires going into the DME they are turned on by a single transistor providing a connection to ground.
• These are low impedance injectors ~1.5 ohms 3 milli Henrys if (I remember right). Six in parallel = 0.25 ohms.
• The unidentified chip inside the DME is a peak-hold injector driver. The driver measures current through the injectors.
• To turn on the injector flow the chip applies full battery voltage to the injectors. This minimizes the R/L time constant to develop a magnetic field and open them fast.
• When A predetermined peak current level is reached (I remember someone saying they measured 6 amps) the battery voltage is removed and a freewheeling diode circulates the current through the coils until a minimum is reached and the then the transistor turns on and off again to maintain that current.
• To turn the injectors off the current is routed through a resistor to dissipate the power in the coils and turn them off fast.

If you short one injector all the current will go through that path and reach that peak before the others have a chance to open. The current limiting (not very good without the inductive time constant) could also explain why this is not just a charred cinder.

Dang, Rick -- great explanation! Gonna archive this one -- I learned something new today.

Greetings all,

Update for the day. My mechanic and I spent this afternoon working on my car.

Good news: After all this work we've done, the car is much more spirited than ever before, as it moves quickly thru 1st, 2nd and 3rd.

Bad news: At about 5k RPMs in 3rd, it's like a light switch has been turned off and it still dies....

So, I took the advice of some recent posters.

After the car died, i check each fuel injector, first by touch, to see if any were hot to the touch, and unfortunately, none were even close to hot.
Second, I checked each fuel injector with a meter and all registered at 1.5 ohms.

After it died, I replaced the DME relay and even the DME itself (as I previously stated, my mechanic has many spare parts, some new, some used in good shape, thus he has two known working DME's. One of which is a direct replacement, the other is a European DME). While the car was dead, which lasts for about 15 minutes after it stalls, we checked for spark (again), which we have and we have fuel. But we don't have fuel injectors that work.

Thus we went back to the grounds. As I've stated in the past, the first thing we did when this problem occurred was to check/clean all the grounds. I keep thinking back to the fact the DME relay is always "hot to the touch" when the car dies, which leads us to think there's a grounding issue. But they've all be checked. So we're going to re-re-check them.

Oh, one more thing, one person thought it could be the speedometer and possibly the speedo cable. Thus during our tests today, I disconnected the speedo, and took it for a spin. It died just like it has always died, thus we can rule that out.

Remember, no ideas are bad ideas, please keep them coming!!!
Thanks.

Dude,..I've a fuel pressure test rig that I'll be glad to ship to you. I had one made up for my 89, comes with enough attached hose to reach the driver's area. I've used this only once just to check my system pressures,..but never while driving,..however I had it constructed so I could do so , if needed......I guess you'd just run it in thru the window!!!!!!!

Top shelve gauge,..and professionally built. If you know, for certain, that the rail fitting is the same as an 89, then you've access to it.

BEST!

Doyle


Lemme' know.

I still say you need to use an oscilloscope with a breakout box on a dyno to load the engine to simulate the issue.

"lorenfb's" website called Systems Consulting has the Motronic waveforms to look for when O-scoping.

GL!!

OK maybe feeling for a hot one was not such a good idea, they do represent a pretty big thermal mass. Did you try unplugging them one at a time?


Edit: it is not hard to change out injectors after you do it the first time and they are loose

Greetings all,

So it appears we have found the problem with my stalling 911.

Yesterday afternoon, my mechanic begin re-checking the grounds, which had been checked and cleaned early on in this process/nightmare. He placed some temporary grounds thru-out the car, just to see if there was a "hidden" ground issue. When he took it for a drive, it went further than it had, almost a mile before it stalled out. It was during this stall, he took out his ohm meter and began checking for voltage.

This is where it gets interesting. We have always said we had spark when the car stalled out, and we proved this by hooking up a light meter to the fuel injectors and distributor, etc. and the light did turn on. But this time, he connected the ohm meter to the fuel injectors and discovered he had 5 volts (or amps, I'm not the expert here). Anyway, as the minutes went by, the volts jumped to 6, then 7. He began checking the grounds he had temporarily placed thru out the car. When he got to the fuse panel, he noticed the fuse that handles the DME was the culprit, as it was at 7 volts, then 8, then 9, then 10. At which point, he started the car right up, but then it died. He decided to do some temporary rewiring, and directed the DME to another area of the fuse panel. The car started, and he ran it for a 5 miles without failure.

When he returned to the shop, he did some preliminary testing and it appears there are bad/cracked/cold solders in the back side of the fuse panel. He's going to repair the panel this week, and with fingers crossed, I may get my 911 back by the weekend.

I'll post an update if/when this happens, along with obligatory picture, as I'll be treating this as a new car...

Thanks to all who posted, listened and contributed to my 911 saga. I guess we found yet another area to check on this older 911's, as it never crossed my mind to think about the solders/wiring behind the fuse panel.

Thanks.

Way to be persistent, Scott -- glad you guys figured it out. Sounds like your wrench deserves a six-pack of his favorite brew for going above and beyond . . . .

Another German car electrical nightmare....

Good to hear! :)

holy smokes, way to be patient and persistent. Me, I would've turned it into a pile of smouldering ash by now. ;)

I hate to be the dark cloud but I don't see where the DME power or the injector power goes near the fuse panel.

Fuel pump yes but you said that was not the problem.

http://forums.pelicanparts.com/uploa...1343073107.jpg

duplicate

Rick-I,

I don't have the specifics yet, as my mechanic called me last night, but according to him, the 6th fuse (from the rear), thus 1-3, 1-10, the third fuse in the second cluster is for the fuel pump, thus my mistake when I said the injectors. I'm assuming the bad connection at the fuse that handles the fuel pump, stops sending fuel to the injectors.

Scott

Rick-I,

I don't have the specifics yet, as my mechanic called me last night, but according to him, the 6th fuse (from the rear), thus 1-3, 1-10, the third fuse in the second cluster is for the fuel pump, thus my mistake when I said the injectors. I'm assuming the bad connection at the fuse that handles the fuel pump, stops sending fuel to the injectors.

Scott

Did he measure those voltages at the fuel pump fuse while cranking the engine?

If so could the voltage drop be due to a gazillion amps (small exaggeration) going through the DME relay PCB traces that would be shared with the injector driver?

Another thing I would be concerned about is blindly adding jumpers to provide 12 volts since in a few cases here on this forum the DME relay has served as a fuse.

The injector driver IC in the DME would be impossible to source.

A great Alfred Hitchcock ending!
 

Hi...This thread for myself has hands down contained one of the most comprehensive 'problem solvers- solutions' out there. A real Alfred Hitchcock ending eh!

Like most, I have had many of the same items mentioned in this thread fail on different cars over the years.

I have one more experience that may be of interest in regards to a 'no start' problem that I experienced. This may be the answer to the issue posted by 'kidrock- # 50

Our MB E320 began to be hard to start day by day, and over about a month one morning it just would not start.- In between this time period -(month)- I had it in to a reputable MB shop and they claimed that it was the Oxy sensor and replaced it.

When trying to resolve it at home that day, I could smell gas but could not find anything leaking nor any 'wet' areas on any of the hose's/pipes. I was though curious why one small vacuum hose ran from the regulator to the intake. When I pulled it off it had a slow 'dripping' taking place.

The long and the short of it was that it was the fuel pressure regulator not the Oxy sensor.

It was flooding the engine over night and hence the 'no start'. This hose was meant to dispose of a wee bit of excess fuel when the valves opened and closed. One of the valves in the fuel regulator had failed - A new fuel pressure regulator resolved the problem.

Again...This thread has been -one great mystery read that was resolved - Great Stuff!
michael

From scarceller's link above New Bosch Fuel Injector 0280150364

$45 apiece or $270 for 6

Did you ever fix this or are you ready to sell it to me cheap?

(moving this conversation from another thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/694968-3-2-voltage-injectors-key-off-position.html
back to here)

Rick - Great info. I just read through this thread (something I had not done before) and see that you had posted lots of info over here. My apologies for you having to double post. I've moved this discussion back over here for continuity.

I agree with your assessment that the patch on the fuse panel that appeared to fix the problem didn't make sense. Not that it wouldn't under some circumstances, but Scott and his mechanic have repeatedly confirmed that fuel supply from the fuel pump was not the problem.

The one fact I have to point to here is the 12v I measured at the injectors when the key was off. That's just crazy. So, either the key wasn't off, or there is a problem in the ignition switch or the DME relay is latching on somehow, or there is some other source of power leaking into that circuit. If we can reproduce this symptom this is where I would next look. Not having access to the schematics right now, what is the function of the two relays in the DME relay? One obviously turns on the other and the DME CU turns on the one when the engine starts cranking right? I recall that one of them powers the fuel pump through a red/green wire.

On a different note - one thing we haven't done is replaced all the injectors. I don't see how a bad injector could lead to the symptom of the 12v being present with the key off. But if that really wasn't the problem, I can see from the various explanations how a faulty injector could shut all the injectors down. So, can we run this engine on 5 injectors? I'm thinking we do 6 road tests where for each test we disconnect one injector and floor it down the road. If its a bad injector it should die every time until the bad injector is the one disconnected. Does this make sense? This assumes that we can get the car to reliably start back up after shutting down, something that has not always happened in the past.

If this test does pan out the next prudent test would be to move another injector to that position and see if the problem follows the injector or stays with the wiring harness. Agree?

On the subject of hot DME relays, is it normal for them to be warm or hot? If not, I've got a meter that read up to 10A DC that I can use to check for current coming through the relay on pin 30 or going to the fuel pump; do you know what the current should be? Actually, I don't know how I would isolate pin 30 on the relay for a current flow test - I haven't looked under the seat to see how the thing is mounted.

Finally, I haven't pulled the fuse box apart but it doesn't look like its assembly involves any solder. I see screw terminals with metal lugs going to the fuses. Nothing could be simpler or more reliable, right? There is nothing on the back side of the fuse blocks is there?

Russ

I'm not sure I would want to beat on it with only 5 cylinders operating. When it is warmed up it should start on 5 cylinders if this theory is correct however. If you read through this thread I think 2 or 3 have said injectors solved this problem.

I don't think the DME relay should be getting hot. I don't know what current the DME, injectors and idle air control should draw. I think for periods of time it could also be negative (injectors in hold mode). Does the relay only get hot after it fails?

The 12 volts with the key off is hard to explain if it is not the relay stuck. However the DME is pretty dumb and shouldn't care when it got power.

Again the voltage falling off at the fuel pump (shares a common path in the relay) is interesting.

Rick,

That kind of feedback about running on 5 cylinders is what I was looking for, thanks.

The biggest current draw on the DME relay is the fuel pump and since we put a relay on that it should no longer be an issue. I'll research the other items.

I didn't see the voltage measurement dropping off at the fuel pump, that was Scott's mechanic and since there are several variables there, I don't know what to think of it. Such a voltage drop is a possible cause but when it quit on us on Saturday I confirmed the voltage at the fuel pump (voltage on red/green wire from DME relay) was fine: 11+ volts while cranking the engine that wasn't starting. We got it running again I think by pulling the DME relay - Scott wanted to show me how warm it was. We also found the negative terminal on the battery was loose and tightened that. But as soon as he floored it, it died again and it was during that failure that I noticed the voltage on the injector harness while the key was off. After that weather moved in so we hightailed it home when we got it running again.

Russ

I found this from the other thread: http://forums.pelicanparts.com/porsche-911-technical-forum/694968-3-2-voltage-injectors-key-off-position.html.

Not sure what to make of the 12V on the injectors with key OFF.

My heart goes out to you and your mech. I was really trying to follow this, forgive me if I missed something.

Have you swapped the injectors (all 6 at once)? Consider sending those out?

I tend to agree with Tippy - you should try to instrument the DME, as hard as it sounds, to see what DME pins 14 and 15 are doing before and in failure mode. On the Dyno so you can have the scope in the car.

Pins 14 and 15 provide a gnd pulse when it's working and it is too quick for a DVM. (This is the grey/x wire side of the injector) If you use the scope, it needs to be done in-circuit.

For anyone, with the injectors disconnected, what should the DME pins 14 &15 read to gnd? Some floating value? I am not sure that measurement is of any use when the injectors are disconnected. You want to test your theory that the injector driver stopped, and this can't be done if the 12v from the injectors aren't connected to 14/15 (in circuit)

If you don't want to run with 5 injectors to failure, then drive to failure first and then discon' an injector, one at a time to see if she starts w/o it. (is the 15 minute break reliable?)

other than that, I am leaning toward the DME, but the fact that this happened with 2 DMEs throws us off.

if the FP is really ON (in failure)?, that should elim the speed/ref portion of the DME.

disregard with apologies if you mentioned these before - it's a longish thread.

best of luck to you.

Voltage on lines that shouldn't have voltage on them always brings me back to a bad ground someplace.

I have not seen a conclusive end to this one. So here is my advise:

Once the issue happens you need to measure with a portable oscilloscope across the injector and and across the coil while cranking (no start). Each measurement will tell you whether there is real current going through the devise. For each you expect a flat line with pulses. If interested I can post pictures of typical scope traces you would expect during cranking.

Then while you have the no-start measure each side of the coil and injector with reference to GND (at the battery). You expect to see nice clean +12V permanent on one side of the coil and injector and the GND pulses on the other.

This data will tell you which of the four signals isn't there or is corrupted:

- clean +12V to one side of all injectors
- GND pulses with PWM pattern on other side of injectors
- clean +12V at one side of coil
- charge pulse with spike on other side of coil

Follow the missing signal and you will find the root cause

Off topic but then very similar: I had my ML430 dying randomly on me. It ran weeks and then shut down cold, came back within an hour, ran days, died, etc. Drove me nuts. Turned out to be a bad ignition switch. It was just bad enough so that switched +12V to the coil packs dropped to +4V during dwell time and as result spark was weeeeeak. The rest of the switched +12V goes through a relay that is controlled by the ignition switch and a logic module. So it was only for the millisec dwell time of the coil charging where the T15 dropped. Not enough to affect the rest of the terminal 15.

A test light on the spark plug wire showed spark and it took me days to recognize my error (believing I had spark). The highly intermittent nature didn't make matters easier.

Another thought.... Just for completeness you should unplug the oxygen sensor heater. That could short as it gets hot. Of course this would not be load related as implied before and the injectors would be.

The theory here is the unfused heater draws the voltage in the DME relay distribution land pattern down to the 6 volts observed and a bunch of stuff in the DME stops working.

This sounds like a road map for our next session. Please post the scope trace pictures. I've got a dual trace scope so I can put it on each side of the coil and the injectors and verify that the high side is staying at B+ while under load. The scope is battery powered though I didn't buy the battery for it, so I'll have to sort that out.

Thanks,

Russ

OK, I will take some scope shots tonight. I'll set the RPM to 200 to simulate cranking on the bench tester. For an in-car measurement it actually doesn't matter whether your scope is battery-powered or powered through mains or a charger.

When I work on my bench-tester I have to ensure I don't short out anything with the oscilloscope GND clip. That's why I prefer my battery-powered portable scope for that.

Below is my bad ignition switch in the ML430. Note there the 4 millisecond pulse with a voltage drop of 8 volts. At the end of the dwell time you can see ringing meaning there is a spark event of sorts. But it is so weak it won't run the engine. The measurement is taken between terminal 30 (perm. +12V battery) and terminal 15 (switched +12V from ignition switch, ON and START) during cranking.

http://forums.pelicanparts.com/uploa...1344968424.jpg

If this would be simple to do with your test bench I wouldn't mind seeing that either.

ischmitz - If I understand your scope picture, if your ignition switch had been working correctly, there would be no voltage drop across it so the voltage at the hot side of the coil would not vary as the coil was charged and so would the scope show the ringing or just nothing? My guess is nothing, like the scope probe was connected to B+ and the battery would just sit at some constant voltage. Of course, during cranking, B+ probably varies quite a bit but with the engine running, it should be pretty smooth.

So I am guessing scott's engine will start right up and once it does, idling in the driveway, I'll have the A channel of the scope from coil hot to ground and the B channel from the low side of the coil to ground. The A channel should show around 14v pretty steady, the B channel should show pretty regular spikes and ringing, right?

If I do the same thing across the injectors, picking up the hot side of the injectors on one channel and the DME CPU side of the injectors on the other, again the hot side should be pretty steady and the CPU side should have the wave form that Rick referred to earlier.

If we get that far, we drive the car and watch the scope and see what changes when the engine turns off. We are going to look at injectors first and spark second.

Sound reasonable? I've yet to see the DME unit under the seat but I am assuming that I can get access to the necessary injector wiring somehow from there so that I don't have to try to run scope probes into the engine compartment.

Advice welcome.

Russ

Guys,

Please check my DME relay operation understanding here:

1. Ignition key in start or run position sends B+ out terminal 15 of ignition switch through red/white wire to fuse box fuse 7 which is jumpered to a post between 7 and 8 and to fuse 8.

2. From the hot side of fuse 8, B+ flows across a black wire to DME Relay terminal 86, through the diode, through the resister/coil of the DME power relay, out terminal 85 and through a brown wire to ground at MP XII.

3. This causes the DME power relay to close which conducts power from B+ on terminal 30 of the DME relay to terminal 87 and through a red wire to a junction that provides B+ to:
- the two banks of injectors
- the idle positioner
- pin 18 of the DME CU
- pin 35 of the DME CU
Pins 18 and 35 appear to power the DME CU itself.

4. Under conditions that aren't clear to me (engine RPM over a certain amount, e.g. cranking?) the digital electronics in the DME CU send point ADV7 high which causes the transistors at T480 to conduct to ground which in turn causes pin 20 at the DME CU to conduct to ground which takes the black/white wire going to terminal 85b of the DME relay to ground thereby closing the fuel pump relay and feeding B+ from DME relay terminal 30 to DME relay terminal 87b.

5. B+ flows through the red/green wire from DME relay terminal 87b to fuse 3 and then through the red wire to the fuel pump.

Have I got that right? If the engine dies and the conditions in step 4 that I don't understand are not met, the fuel pump should shut off, correct? For example, if the car is idling and I stall the engine and leave it sitting there with the key on, the B+ on the red/green should go away, right?

But if the engine dies but is still being spun by the momentum of the moving car, I assume that the conditions in step 4 are still being met and pin 20 will still be at ground and the fuel pump relay will still be latched. Right?

If I put a meter or a scope on pin 20 or DME relay 85b, I would expect to see 0v in the key off position rise to B+ in the run position and then drop to near 0v when the engine is cranked. Right?

And DME relay pin 85 should be ground, hard ground. If that isn't right at 0v, that could be a problem. Right?

Remember I saw 12v on the injectors while the key was off? Well that was terminal 87 of the DME relay and if the DME power relay was open, the only way to get 12v to that point is by flowing back through the DME fuel pump relay, e.g. the power was coming from pin 20 on the DME CU itself. Looking at the DME schematic, this means the power was coming from point ADV7 or from pin 4 of the DME CU which appears to be wired back to the start position of the ignition switch. Do I understand that correctly? Just trying to identify things I should test if we see this again.

Thanks,

Russ

I read the majority of the posts, and it recalled a problem a shop once told me:

While chasing a running issue on a 928, many hours into it, they discovered one of the boards the relays plugged into had a bad wire/connection in the back of it. I think it was one of the power legs.

I am not sure of the socket for the relay, (my '88 is 10 years sold) but is there a way to look at the back and verify no cracked power legs/wires loose?
If something was loose, this may cause a heat issue, explain why a relay swap solved the issue in the past...

If you are seeing battery voltage on the injectors with the key off it is probably not coming from the DME.
Earlier in this thread didn't someone add some wiring as a supplemental path?

http://forums.pelicanparts.com/porsche-911-technical-forum/431975-new-dme-relay-did-not-make-my-fuel-pump-work.html#post4198034

How is the scope working?

I removed that wiring (red/green from the DME to the fuel pump fuse 3 had been moved to another fuse - this resulted in the car continuing to run after the key was turned off). I don't think there is any more such wiring. An additional relay has been introduce for the fuel pump - the red/green from the DME relay is now powering this new relay which in turn connects the fuel pump directly to the battery. I have left that in place for now though I need to add a fuse in the new feed wire from the battery. All this was done before I saw the voltage on the injectors with the key off.

Note that the voltage problem is so weird that I am not sure I believe the result - there may have been "pilot error". So I will be looking to reproduce that.

We are going to try the scope tonight weather permitting.

By the way, getting access to all these wires while the car is doing 50 MPH down the road is making me think I should solder a cable onto the inside pins of the DME CU connector so I can access them freely. Scott doesn't think there is any way to get the DME CU and its harness out from under the driver's seat so I can get at it easily. Do you agree?

Russ

I'd agree. I gave up on building a DME Break Out Box. Non-working DMEs/wire harnesses are pretty expensive.

I thought the symptoms here were you could run it till it got hot and then it would quit. Can't you look at the signals when you first start it and them when it quits?

Since I'm getting a number of folks indicating the fuel injectors may be the culprit (something we're going to test later), I've decided to look into replacing them. Can someone point me to instructions on how to remove (and later install) the fuel injectors on an 85 3.2?

Thanks.

Doesn't seem to be heat related. Seems to be load. E.g. when the engine is making maximum horsepower, it shuts off. And note, this is not "stalling", its like you turned the key off. So this really seems to be an electronics issue - something that occurs at maximum load is causing the DME CU to shut the engine down.

We could run it till it quits and then check for weird conditions, we may do that today and then tonight I'll solder a break out cable on to it for future testing.

Russ

I just did this a couple months ago.

On the drivers side remove the heater duct
On the passenger side put the AC compressor on a towel on the fender. Remove the air filter.

Remove the compression nuts from the front and back of each fuel rail. Some gas will spill, let it sit for a while before dong this.
Remove the connectors from the injectors, they just pull off but some you may need to remove the metal clamp wire.
Take the bolts out that hold the fuel rails on and work them off. My injectors all stayed in the manifold.
Work the injectors out of the ports in the manifolds. One of mine was stuck real good.

Put it back together lubricating the new o-rings with gas.

Did I forget anything?

Which is why the injectors are suspect. When the manifold absolute pressure goes up the injectors have to squirt more gas.

Ingo, Is it just pulse width or does the frequency also change?

Here's how I built a break out DME. I got a spare DME, opened it flat and bolted it to a piece of plexiglass. Now all the pins are easily accessible using small hook type alligator clips. With the DME like this it's also easy to make software changes by swapping EEPROMs or using the Moates emulator.

I'm no SW or Iscmitz but I'm not bad at ignition/fuel tuning, plus I've made a few changes for faster starts.

I know what you're thinking, a spare DME isn't exactly cheap (I got lucky and found one cheap - it was broken but I fixed it) but if you're making the decision to work on these cars yourself, then take some of that saved money and buy spare parts and tools (like the o-scope mentioned above) to be able to help in diagnosis.

Chuck.H
'89 TurboLookTarga, 338k miles