Dead 911 Motronic

[captainq]

Hi All,

My 1984 911 Carrera 3.2 has been laid up in storage for about a year or so. I was working fine before. As is often the case, the battery had gone flat so I charged it up. Tried to start it and... nothing. It just sits there cranking.

I've got a good quality scope so I'd checked all the inputs (which were OK) and the outputs to the ignition and fuel systems (which were non-existant) so concluded that the Motronic unit was faulty. I managed to temporarily blag one off a friend to prove it one way or another and bingo, the car fired up.

Cut a long story short I can't find a used one and the cost of a new one from my friendly Porsche stealer nearly made me fall off my chair. Now I'm fairly converse with electronics (I run a repair facility) but I can't seem to find any information (schematics and the like) on this unit. The ident is as follows:

0 261 200 051
911 618 111 00

Unfortunately (and understandably!) my friend doesn't want me to dis-assemble his Motronic unit to enable me to make some comparisons. So, any schematics would be gratefully recieved, as would any information as to which component(s) may have failed.

Cheers, John

[86 911 Targa]

Originally Posted by captainq

Hi All,

My 1984 911 Carrera 3.2 has been laid up in storage for about a year or so. I was working fine before. As is often the case, the battery had gone flat so I charged it up. Tried to start it and... nothing. It just sits there cranking.

I've got a good quality scope so I'd checked all the inputs (which were OK) and the outputs to the ignition and fuel systems (which were non-existant) so concluded that the Motronic unit was faulty. I managed to temporarily blag one off a friend to prove it one way or another and bingo, the car fired up.

Cut a long story short I can't find a used one and the cost of a new one from my friendly Porsche stealer nearly made me fall off my chair. Now I'm fairly converse with electronics (I run a repair facility) but I can't seem to find any information (schematics and the like) on this unit. The ident is as follows:

0 261 200 051
911 618 111 00

Unfortunately (and understandably!) my friend doesn't want me to dis-assemble his Motronic unit to enable me to make some comparisons. So, any schematics would be gratefully recieved, as would any information as to which component(s) may have failed.

Cheers, John

John,

Try P-Member ischmitz.

Good luck,

Gerry

[rick-l]

This might work

DME - Schematic Download



Lots of other good stuff on that site. There are many on this site familiar with the internals of a DME and willing to share knowledge (except for Loren)

EDIT: Be sure and post your progress and let us know what was wrong and welcome.

[86 911 Targa]

Originally Posted by rick-l

This might work

DME - Schematic Download



Lots of other good stuff on that site. There are many on this site familiar with the internals of a DME and willing to share knowledge (except for Loren)

EDIT: Be sure and post your progress and let us know what was wrong and welcome.

Rick,

Thank you, Thank you, and Thank you for sharing..............

Best,

Gerry

[Joe Bob]

So no spark and no power out from the DME? Have you determined if it has power IN?

Ingo's email is 3.6@cox.net

[xlr8]

(From a previous thread)
If everything else checks out OK then it may possibly be cracked solder joints in the DME computer.



I had to use sunlight and a magnifying glass to see these, looking at the joints from different angles helped me spot the problem.



Once you have the box open, you have to remove the circuit board to access the solder joints.
Carefully disassemble these corner supports by pressing the center pins out of them.







Then squeeze the end of the support to slide it out.





My three cracked joints connected this component to the circuit board.



I used a butane soldering iron, It lets me hold close to the tip and there's no cord in the way.

[86 911 Targa]

Originally Posted by xlr8

(From a previous thread)
If everything else checks out OK then it may possibly be cracked solder joints in the DME computer.



I had to use sunlight and a magnifying glass to see these, looking at the joints from different angles helped me spot the problem.



Once you have the box open, you have to remove the circuit board to access the solder joints.
Carefully disassemble these corner supports by pressing the center pins out of them.







Then squeeze the end of the support to slide it out.





My three cracked joints connected this component to the circuit board.



I used a butane soldering iron, It lets me hold close to the tip and there's no cord in the way.

Well done, excellent tutorial.

Cracked or "cold" solder joints are the most frequent failures on any PC board!

Gerry

[r_towle]

have you replaced just the DME relay and tried your original DME?

Grounds need to be cleaned after sitting for long periods also.

Rich

[dshepp806]

Yes,..MANY threads already on fractured solder joints..(hohum)

I'd check (as well) the junction integrity of that coil driver (TO can).

Inanycase, the schecmatic should take you home if you know your electronics (as you mentioned),..I bet you find it.

There are guys here and Rennlist who do these EVERYDAY, and know the circuits intimately (to include the digital side-not much failure there compared to the other areas). Some will share,..some will provide info for you to start,....various personalities and capabilities.....take it ALL in.

Luckily, you state that you're "electronic knowleadgable",..so you're WAY AHEAD there,..and on a relatively simply box (compared to other complexities noted in this world!), at that!

BEST!

Doyle

[steely]

Maybe check the 5V regulator on the bottom (analog) board (passenger side/ right/rear most of the 3x TO-220's in a row) PN L4705.

[captainq]

Originally Posted by rick-l

This might work

DME - Schematic Download



Lots of other good stuff on that site. There are many on this site familiar with the internals of a DME and willing to share knowledge (except for Loren)

EDIT: Be sure and post your progress and let us know what was wrong and welcome.

Rick, You're an absolute star!! Thanks hugely for that, the schematics are just what I'm after.

Thanks also to everyone else that's posted, it's all useful information. I've got a powerful lit anglepoise magnifyer so all the solder joints have already been checked but as there were no fuel or ignition output signals I think I'll start with the darlingtons... Will keep everyone posted.

[rick-l]

No problem.

No spark and no injector pulses is not the way these typically fail. I'd start with power and see if there is life in the micro controller.

Ingo has a bench that simulates all the inputs, bet you'll wish you had that before too long.

[dshepp806]

Well, if he's running a electronic repair facility (post #1), I would assume he'd have access to function /signal generators galore.....now, to just get the required data as to inputs (which are reasonably simple waveforms.

However, should the digital side of this be the problem,..you've a different a different "fish to fry", as to test gear (he probably has what he needs there, as well......

Curious to hear what you find out.

All equipment aside, there are several here who know this circuit very, very intimately,..not to mention having repaired MANY over MANY years. (THAT surely speeds things up!).

I can tell you from my experience with these (limited, relative to the Pros...BTW), is that I've seen/repaired 2 with solder fractures...in one other case, it was the ignition coil driver circuitry...I watched (and helped) another tech with problematic fuel injection (again relating to the drivers)....as I recall, one would want to make sure your injectors are good to go, insofar as load Z.

Check #1: power (period), all of it, too.

Bright WHITE lights and HIGH MAG (just as you stated!)

The rest have been great reads here at Pelican University of other people's experience.....(great shares!) I DO, however, love it when the discussion dips into the circuits!!!

As rick-l noted, there are (somewhat) "common" failures of this board and his recco' is dead on (M.O., too).

In closing, I think there already exists (somewhere in the cybersea) a list of these inputs (it's not some great "secret"), for the record.

I hope your find is easy (and NOT of heavy digital failure).

BEST!

Doyle

[Lorenfb]

Based on what's also learned here, '87 Surging Idle, both forum members in working together could
be experts in rebuilding the 3.2 DME ECM and benefit all other 911 3.2 owners. Isn't info
found here on Pelican priceless, right? Well, maybe not all.

[masterdave]

I had a faulty Motronic unit on my 924s a few years back. There are places that will rebuild them for a reasonable price ill try and find out where i got mine done i think they were in Florida some where, but i think it was less than or around 300$. I had a spark issue on mine as well, it turned out that the transistor that drove the ground of the coil had fried. When i had it done they said this was a common problem they had seen before.

Best
Dave

[ischmitz]

Let's cut to the chase here. A no-spark, no injector condition is either bad flywheel sensors or a bad DME once you’ve ruled out the “low-hanging fruit” (DME relay, fuses, GND, etc). Since he has confirmed another DME runs the car this leaves the DME.

The “no spark, no fuel” failure is actually a very common one with the 3.2 DME and it is not a cold solder joint. It is caused by a failed custom Bosch IC that decodes the flywheel signals. More specific the IC fails to amplify the reference pulse and thus never triggers INT0 of the 8051 microcontroller. This prevents the DME to go into "run" mode.

{let's see what the Almighty has to say to this one – I am sure I am getting lectured again – so sorry in advance for being such an idiot....... }

[rick-l]

That would be a bad one

I was following you on the DIY-EFI list how you were trying to figure out how it works. Any success?

[Flat Six]

Originally Posted by ischmitz

let's see what the almighty has to say to this one – i am sure i am getting lectured again – so sorry in advance for being such an idiot....... }

rflmao!

[Lorenfb]

"I was following you on the DIY-EFI list how you were trying to figure out how it works. Any success?
__________________
Rick"

Why don't you 'build' a simulator of the DME ECM so members could run 'what ifs'
and possibly determine what would cause the symptoms they're having, as you were
so successful analyzing the Bosch CDI with a software simulator?

[Tippy]

Loren, when you get done with John, can you jump back on my thread? The one about a Carrera and a cliff.........

Oh please, please!

See what you think before I rip out the chip.