Motronic - I'm going in! Need Schematics

[OK-944]

I'll be splitting the case of my Motronic unit over the next day or so in addressing my no-start (no spark) issue - having exhausted most other avenues...and will first look at the coil driver soldered contacts to see if any of these need to be re-flowed, plus other possible areas which may need attention.

What I need are schematics/photographs/drawings of the insides of a Motronic unit...to help me identify various components. Thanks!

[djnolan]

There are a ton of writeups with pictures on Pelican and Rennlist.

Search this phrase on google: "944 dme solder joint"

[djnolan]

Here is one I was involved in (grin):

DME repair

[OK-944]

So...that cylindrical part circled red in that thread - identified as "the culprit," would be the coil driver?

You'd mentioned you saw some faint cracking of the solder joints? Sucked out the old solder and resoldered?

Did you use any kind of heat sink while doing this? Or ground yourself to mitigate static discharge? Just wondering!

Kinda looking forward to this! Now...where did I put that desoldering stick and suction bulb?

[djnolan]

I think that is the coil driver. There are two injector drivers on the opposite side that might also go bad. They both already have a heat sync. I touched the iron to the end of the wire and used desoldering braid.

[944 Ecology]

Originally Posted by OK-944

So...that cylindrical part circled red in that thread - identified as "the culprit," would be the coil driver?

You'd mentioned you saw some faint cracking of the solder joints? Sucked out the old solder and resoldered?

Did you use any kind of heat sink while doing this? Or ground yourself to mitigate static discharge? Just wondering!

Kinda looking forward to this! Now...where did I put that desoldering stick and suction bulb?

Check your PMs.

[OK-944]

A bit of brain surgery today, starting with removing the case - finding that all looks more or less pristine:



Opening up the brain…we see the coil driver (my assumed “no start” culprit) in the lower left of this next photo (shiny silver flat cylinder):



Three contact prongs sticking out from the driver looked fine with 10x magnification…so I removed a few more bits - allowing removal of the bottom cover. Under the bottom cover, this is what I found:



After carefully assessing all of the soldered contacts in this unit, I concluded that they all look fine…with the possible exception of one of the three coil driver contacts under the removed bottom cover. So - I cleaned up as much as I could overall, then added just a bit of solder to the possibly offending driver contact. Not much evidence of oxidation, so I did not remove existing solder - but more of a re-flow while adding some new solder. Got everything back together and back into the car.

Replaying the events of the Oct. 2019 stalling event - realizing that it happened after a day of driving through rain followed by a sub-freezing night…then following the wire bundle out of the coil and downward past the window washing reservoir - I see a stack of contacts which I have not addressed, and am now hoping that by cleaning these contacts, combined with the re-soldering of the aforementioned coil driver contact, that I may finally achieve a clean start.

But I do have a question: just how is the wiring from the coil back to the DME/ECU (brain) routed?

[OK-944]

Forgot to mention...that the "stack of contacts" beside the window washing reservoir are grounds. Again...looks like the coil is associated with this stack, and that it also could use a good cleaning - which I will do as soon as I remove the reservoir (cannot reach it otherwise).

My bigger issue is that I need to learn more about using a multimeter. My other issue is that its difficult to find good, specific instructions on exactly how to test. Clarks Garage, for example, in its section on testing incoming voltage to the ECU (Motronic unit) has an illustration of the multipin contact strip - and basically says to "test for voltage." Well...just how do I do this? Where to I attach my multimeter leads to test the incoming voltage?

[djnolan]

So if you are testing for voltage, this is normally 12 Volts, and it is normally between the designated terminal or test point and a ground point such as the engine or chassis ground or negative battery terminal.

So Set the DVM on the 20 volt DC scale, connect or touch the negative black lead to a ground point, and then measure the voltage at the designated test point with the red positive lead to see if it is around 12v.

It may be helpful to have a couple of alligator clamp jumper wires to connect the DVM leads to ground and the other test point.

[OK-944]

OK thanks!

Gotta stay positive about this no-start scenario...because in the end I'll likely know just a bit more - and gain more efficiency, in diagnosing these electrical gremlins!