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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.
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Old 07-20-2012, 10:30 AM
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Quote:
Originally Posted by rick-l View Post
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.
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Old 07-20-2012, 02:36 PM
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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.
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Old 07-21-2012, 02:42 PM
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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.
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Old 07-21-2012, 04:29 PM
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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!!
Old 07-21-2012, 04:37 PM
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Quote:
Originally Posted by wnsgc View Post
But we don't have fuel injectors that work..
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
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Last edited by rick-l; 07-21-2012 at 08:08 PM..
Old 07-21-2012, 08:04 PM
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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.
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Old 07-23-2012, 05:34 AM
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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 . . . .
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Old 07-23-2012, 06:57 AM
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Another German car electrical nightmare....

Good to hear!
Old 07-23-2012, 08:36 AM
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holy smokes, way to be patient and persistent. Me, I would've turned it into a pile of smouldering ash by now.
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Old 07-23-2012, 10:55 AM
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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.

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Old 07-23-2012, 11:55 AM
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duplicate
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Old 07-23-2012, 12:00 PM
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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
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Old 07-23-2012, 12:14 PM
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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
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Old 07-23-2012, 12:15 PM
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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?
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Old 07-23-2012, 12:47 PM
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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.
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Old 07-23-2012, 12:56 PM
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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
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Old 07-23-2012, 01:18 PM
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From scarceller's link above New Bosch Fuel Injector 0280150364

$45 apiece or $270 for 6
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Old 07-24-2012, 11:36 AM
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Did you ever fix this or are you ready to sell it to me cheap?
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Old 08-06-2012, 08:06 AM
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(moving this conversation from another thread:
3.2 Voltage at Injectors at key-off position
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
Old 08-13-2012, 01:53 PM
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