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ChrisBennet's Avatar
 
Join Date: Aug 2001
Location: Nashua, NH USA
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Exclamation 3.2 starts - but not when cranking

Warning: This a high degree of difficulty problem and you will be a god among men if you can figure help me figure this out. :-)

Background: 3.2 Carrera retubbed by my friends for me a while back. Everything was taken from my totaled '86 and transferred to an SC tub. I finished up the wiring but I likely made mistakes along the way. The car runs fine but I've had this problem since the car was re-created.

Problem: When I turn the key to the "start" position, car cranks and fuel pump runs but car does not start - until I release the key.

What I've tried:
1) Tried my DME in a friends car and it starts fine in his car - it isn't the DME.
2) Scoped reference sensor and signal is fine (4V, 8V peak to peak) while cranking.
3) There is no spark and no injector signal while cranking
4) Pin 4 on DME which is connected to yellow starter wire, has ~10v when cranking
5) Power pins on DME (35 and 18) both have power when cranking.
6) Car has brand new ignition switch (the electrical part) and starter.

I'm really stumped here. Any Motronic genius' care to take a stab at this?

thanks in advance,
-Chris
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'07 BMW 328i Coupe
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Last edited by ChrisBennet; 06-08-2010 at 01:59 PM..
Old 06-08-2010, 01:27 PM
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Simple answer: Bad ignition switch (electric part):

Once you turn the ignition ot ON you energize the +15 terminal (DME) As long as you crank it disconnects the +15 terminal from power (this is bad). Once you release the key back to ON the residual rotation of the engine is enough to make it start now that the +15 terminal is energized again.

Think about modern cars. When you crank, the headlights are switched off to make sure more juice is going to the starter. They have more than just a +15 terminal.


You are saying that the DME has power when you crank. Where do you measure?

Ingo
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'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
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How about a NoBadDays DualChip for 964 or '95 993

Last edited by ischmitz; 06-08-2010 at 01:39 PM.. Reason: shoot - should have read the entire thread
Old 06-08-2010, 01:37 PM
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Chris,

Here is what the DME "needs" to start the engine (meaning producing spark and fuel signals):

GND to pin 5
+12V to pin 35
+12V to pin 18
reference and TDC sensor signals

If all these are present the DME will produce spark and fuel signals. Pin 4 is energized from the ignition switch while you crank. It is the same yellow wire that goes to the starter. It makes the DME turn the fuel pump on.

After re-reading your post my best guess is that the +12 volts on one of the two pins are not present while you crank. I have not looked at many schematics and wiring harness configurations but I was under the impression that both pin 18 and pin 35 are tied together in the DME harness. Maybe not - that's where I would start looking.

Ingo
__________________
'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2 - I can help!!
How about a NoBadDays DualChip for 964 or '95 993
Old 06-08-2010, 01:48 PM
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Quote:
Originally Posted by ischmitz View Post
Simple answer: Bad ignition switch (electric part):

Once you turn the ignition ot ON you energize the +15 terminal (DME) As long as you crank it disconnects the +15 terminal from power (this is bad). Once you release the key back to ON the residual rotation of the engine is enough to make it start now that the +15 terminal is energized again.

Think about modern cars. When you crank, the headlights are switched off to make sure more juice is going to the starter. They have more than just a +15 terminal.


You are saying that the DME has power when you crank. Where do you measure?

Ingo
Hi Ingo,
Sorry if I wasn't clear in my original post:

While cranking, the DME connector has power at the pins 35 and 18 (not 15 like I originally thought) in other words, the DME is getting power from the DME relay during cranking.

I replaced the electrical part of the ignition switch with a brand new one.

What is weird is that the fuel pump runs during cranking - that would seem to indicate that the DME "thinks" everything is ready to start i.e. it knows that the engine is turning from the flywheel sensors.

-Chris
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'80 911 Nogaro blue Phoenix!
'07 BMW 328i Coupe
http://members.rennlist.org/messinwith911s/

Last edited by ChrisBennet; 06-08-2010 at 02:00 PM..
Old 06-08-2010, 01:55 PM
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Chris,

the DME has two means to turn the fuel pump on>

1. power to pin4
2. continous signal from the engine speed sensor and at least one pulse from the TDC sensor

If either condition 1. or condition 2. is met the DME relay (stage 2) turns on.

Measure the power pins while cranking. Pin 35 and pin 18. For some reason you keep referring to pin 15.
Ingo
__________________
'74 Targa 3.6 (not stock ) - '01 C4 (almost stock) - '00 ML430 (stock)

I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2 - I can help!!
How about a NoBadDays DualChip for 964 or '95 993
Old 06-08-2010, 02:29 PM
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