Look at my weired OEM gnd layout

From DIY contraptions to sophisticated FreeEMS-specific designs! Plus general hardware development!
Post Reply
User avatar
HotCat
LQFP112 - Up with the play
Posts: 110
Joined: Wed Apr 13, 2011 2:35 am

Look at my weired OEM gnd layout

Post by HotCat »

I spend the whole day with my friend to study the real harness of my car. We remove the OEM ECU and explore the connector pins against engine block using a multimeter. Then I draw this diagram.
7.JPG
I think the center of the gnd start is not the engine block but left side of body frame, there is quit a distance between F and ABC. About D and E, I think it is the return path of oil pressure switch and a CHT sensor(not the one for ECU). I think it is very wrong, but it works most of the time. I start to suspect that the car was so sensitive to the whether condition had something to do with noisy sensor?

Another strange things can happen when I connect a bunch of large capacitors between + and -of battery, when deaccelerate, the car can go more distance than with out it, when change to high gear, the RPM takes more time to drop to specific range. That's means large capacitor can interfere the MAP sensor? How could that happend?

Also my car has a huge fuel consumption when drive in the city and thick gas smell when open engine oil filler cap but the engine doesn't burn oil. The OEM ECU injects more fuel than engine can digest. This is my primary objective to run my own EMS, hope FreeEMS can answer all my questions
User avatar
Fred
Moderator
Posts: 15431
Joined: Tue Jan 15, 2008 2:31 pm
Location: Home sweet home!
Contact:

Re: Look at my weired OEM gnd layout

Post by Fred »

Nice diagram! :-)

D and E also supply the negative connection to your starter motor, however you're right, usually the temperature gauge has a dirty "we don't care" ground in this fashion too.

The F ABC distance shouldn't be an issue, the body is likely a fairly low resistance conductor for your purposes. The main thing is what you do with those three wires. See other thread for my thoughts/questions on that.

Connecting caps could potentially raise your battery voltage as spikes could charge them and the current drawn out not be sufficient to discharge them before the next spike :-) RC filter! :-) That will cause you ECU to do funny things, but who knows exactly what, that's highly dependent on their code and algorithms etc. If it were FreeEMS sensing a high voltage it would cause shorter pulsewidths and shorter dwells so bad ignition and lean mixture. Perhaps your alternator and/or regulator are failing? If the battery voltage is noisy then the ECU won't like that either, neither will FreeEMS. Have you scoped the battery voltage while driving? What voltage level does it usually sit at? At idle? At 3k steady?

Big capacitors are the norm for serious car audio at 1F or more, and I've never seen an effect on the engine from one. If your regulator was spiking high, though, that could cause the effect that I described.

FreeEMS can't fix your physical issues, but it can help you diagnose them effectively (with good logging) and tune the engine how you like it, rather than what the French thought was best.

The OEM ground wiring looks OK to me! If you make sure that the three wires from the ECU to the chassis are wisely used and correctly connected you'll be fine :-)

Fred.
DIYEFI.org - where Open Source means Open Source, and Free means Freedom
FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!
Post Reply