Is there a Motronic expert in the house?

B

BonitaCS

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I had some time over the weekend to start working on the Motronic conversion again. Went to the junk yard and pulled a wire harness out of a 735i. I know in the conversion not all of the wires in the harness are used so I started pulling out the ones I know would not be needed and, while I was at it, I ohmed out all of the connectors. My question is what to do with the connections/wires that will not be used; just pull them from the harness or do I need to do something else? To be more specific here are the ones I’m interested (the names are right off the wiring diagram):
Evaporative purge solenoid
Engine speed output (to instrument cluster)
Diagnostic TX & RX (are these RS232? They are not a twisted pair if they are)
Check engine light
Programming voltage
Compressor on input (maybe I should use this one)
ASC & Anti theft, Park Neutral (I could see this one causing problems)
Torque converter
There is a pair for the O2 sensor as well but I haven’t decided yet if I’m going to add an 02 sensor.

As always, any and all help is appreciated!

Scott
 
You most assuredly want to use the O2 sensors; otherwise the car will run pig rich all the time. The O2 sensors tell the Motornic ECU to lean out the mixture, giving you more power and better economy. I had an exhaust shop weld in an O2 sensor bung into my downpipe. The O2 sensor made a huge difference.

What version of Motronic is this, 1.0 or 1.3? (didn't state the 735 year, so can't tell)

The diagnostic ports can be handy. You may also want to put in the Motronic coolant temp and pressure senders, as the ECU takes those inputs into consideration. Ditto for the oil pressure sender at the back of the head.
The anti-theft can cause a problem, but I think for the 1.3 and 1.0 systems, it is a code-based fuel pump cutoff. Since you will have no means of inputting a code, it shouldn't matter.
The CEL can also be helpful, but it is kinda redundant if you still have the diagnostic ports.
If the torque converter is on its way out, it adjusts the signals sent to the trans computer to engage "limp mode", I don't think it changes the engine performance.
The compressor on input is good since I tink that takes over the function of the AC boost relay, which bumped up the idle a bit when the AC was on.
 
Scott
I would appreciate it if you could tell me where you got your wiring diagram from as I am involved in a Motronic conversion on another car at the moment (Audi Quattro UR SWB Rally car)
Thanks
Malc
 
It’s Motronic 1.3 and I didn't see the inputs on the wire diagram for the coolant temperature and pressure which I thought it should have; there was an input for air temp. The wires from the connector for oil sender unit at the back of the head didn't go into the ECU and I didn't see the input on the diagram for it as well.

What is the ECU output signal for the diagnostics? Can you read it on a laptop? I’m sure it’s a bunch of funky code that I won’t be able to understand…

Malc,
I found it somewhere on the web (that's a lot of help!) but I have the PDF so I can email it to you tonight when I get home. I found several on the web it just a matter of finding the right one. This one is for a 91 535i with the Motronic 1.3, it originally it came from a Russian web site (intellectual property rights in Russia?).

Thanks again,

Scott
 
The diagnostics output depends on what diagnostics system you use, the port or the CEL. If you use the CEL, in the Bently E34 manual, it tells you how to read the flashing so you can link the flashes to the right codes, and then what is wrong. The main diagnostics ports interface using whatever standards OBD-I included. To save yourself some hassle, you can snag a code reader for ~100$, which reads out the stored error codes. Some of the nice ones even tell you directly what is wrong, rather than simply giving you a code to look up.

With the CEL, I suppose all you'd need to read it is the code book and an oscilloscope. The diagnostic port, I'm not so sure.
 
Malc,

The PDFs Bert-Ola posted are the ones I was using; they even have the russian web site on them!

Scott
 
The reason I didn't see the input for the coolant temp sensor is the Russian pinout diagram has it labeled "air temperature"...
 
The coolant temp sensor input is different from the air temp input. The air temp input is from the air temp sensor in the AFM (you should be using a matching motronic AFM for this). It allows the ECU to adjust the injector duration to match the air density to maintain the correct air fuel ratio. The coolant temp sensor input allows the ECU to choose the basic engine map based on engine temp (richer for cooler engines).

You should try to use a Motronic temp sensor too. I suspect it wil fit inthe same hole as the orignal coolant temp sensor.

Other than the O2 sensor input, the others can probably just be left open. If you don't want to use an O2 sensor, then use a voltage divider circuit and set the O2 input at 0.5 volts. Some motronics will do this for you if the O2 sensor is not hooked up. TO check install the whole thing, and, once the engine is running, look at the voltage on the O2 line. If it is at 0.5 volts, just leave it open.

HTH

S
 
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