DME/ECU Possible Issues

[VaTechE36]

I'm having some issues with my ECU not holding voltage. My mechanic says its dropping from 9.5V to 3.5V and then cutting the car off. The battery and alternator are working fine and are supply the appropirate 14.4V to the car.

He had his scan tool plugged in and was getting back:
- no speed senor data
- the MAF at idle was reading 269,000kg/hr
- the injectors were out of whack

I'm locating a new ecu to put in, but do these problems sound like that is the issue or should I look elsewhere?

The car is a 93 325i

TIA!

[dannyhsu]

did you take the car to a car wash recently.

[berkeleyEE]

Just to be clear:
Electronic circuits do not "hold voltage" except in the sense of having capacitors on the voltage lines to smooth out variations, filter noise, etc.
The ECU is not a source of voltage, it is a consumer. It is not a battery, it is a load.

If the supply voltage at the ECU is dropping from 10-ish to 3-ish, then no wonder it doesn't work right. Engine sensors that operate on 5VDC, for instance, aren't getting the correct operating voltage.

Now the question is: does the ECU have an internal near-short that is loading down the supply from 10-ish to 3-ish? Or is there a fault in the wiring between battery and ECU (between source and load)? That's what you and your mechanic have to figure out. And good grounding is as important as good sourcing so the current can flow in a loop (compete the circuit). Good luck.

[savage147]

check ECU power supply in the wire harness. I had a similar problem and my power wire was corroded.

Good luck on the fix