E36 DME Info Needed for Electrical Gremlins

[seraphiem]

All,

My '97 328iC Auto with 150k miles is having one fun electrical problem. I've checked multiple information sources (Bentley, other online available schematics) and cannot get the complete picture of the Siemens DME operation or sensor operation. I'm hoping someone around here has a bit more experience with this ECU and its sensors then I am and would be willing to offer up a tidbit of information.

The symptoms
*CEL, EGS, and ASC-T warning lights are illuminated on dash.
*Throttle response is very poor and the engine has limited power
*Engine power will cut out (ie tach drops to 1k, MPG needle goes to upper range) for a few seconds and will then resume. Will oscilliate like this.
*OBD-II Code - P0121 - TPS Range/Performance Problem
*DME Fault Code - 12 TPS Range/Performance
*EGS Fault Code - 154 CAN Accellerator Pedal Setting
*TPS Live Data Values Show a MAX of 25% throttle when at WOT (Nowhere close to even 75%)

My Tests to Identify Problem Areas and Rationale
*Due to All the TPS Codes, Check TPS Resistance Across Its Range: Resistance is as expected without any opens or shorts. TPS has been replaced multiple times to verify correct operation.
*Due to successful TPS tests, Check for 5 Volt Reference to TPS Connector: Measuring the voltage between the 5 Volt Reference and Ground on the TPS Connector only indicates 1.5 Volts. Bingo. At least this explains my TPS problem.
*Due to lower then expected voltage at the TPS connector, Check the TPS Wiring from the TPS Connector to the DME Connector to verify continuity: All wires (5 Volt Ref, TPS Signal, Ground) have good continuity with no identified shorts to ground. Performed extreme jiggle tests to try to produce a open/short scenario and nothing found.
*Due to TPS wiring back to DME found to be fine, Check Fuel Tank Pressure Sensor and Wiring (since it shares the 5 Volt Ref from the DME): Disconnected sensor and the TPS was still only receiving 1.5 Volts. Wiring from the Fuel Tank Pressure Sensor to the DME also checks out with no shorts or grounds. Fuel Tank Pressure Sensor is NOT recieivng any voltage between 5 Volt Reference and Ground.
*Due to incorrect 5 Volt Ref and TPS / Fuel Tank Pressure Sensor wiring is fine, Replace DME: DME was swapped out, yet not EWS II aligned. For this testing it is not needed. Still getting the same exact results with a different DME. IE lower then expected 5 Volt Ref to TPS, I'm getting 1.5 Volts.

My Questions

*What other sensors / or other misc devices have the DME feed 5 Volts feed to them? I'm aware that the oxygen sensors and crankshaft position sensor do as well. But the poor Bentley doesn't indicate so. I'd like to determine which devices do use the 5 Volt DME output to isolate each sensor/device and there respective wiring to see if one particular device is shorting and pulling down the source.

*Does the DME bias its signal input lines? With my TPS and Fuel Tank Pressure Sensor disconnected I measure 5 Volts between ground and the sensor signal input line. I'm guessing yes, but would like to verify so as not to think I have a short.

Sorry for the long post. I'm just curious if anybody has any insight to help out.

Thanks Again All.

[berkeleyEE]

You're right, Bentley does not provide details on the internals of the Siemens MS41.1 (or any other) ECU. I guess it's proprietary.

I do know 5V is used to bias the titania oxygen sensor's (not the heaters) but that 5V is through a 30k resistor (a pull-up resistor in the DME), so any short there will not bring down the DME 5 volts. I assume other sensors using 5V are similarly protected, but who knows.

Since the TPS is so easy to access, I decided to check mine.
I found it is 3.5kilOhms between pins 3 and 1.
But pin 2 varied between 885 Ohms and 4.38k as the potentiometer was turned. That tells me there is a fixed 885 Ohms on pin 2 (the arm of the pot).
That just ensures the sensor output can never go all the way to 0 Ohms/0 Volts no matter the position.

With the key in ACC'Y, I found 5.0V on plug pin 3 with respect to pin 1.
And plug pin 2 was floating at 4.9V.

With the plug connected to the receptacle on the TPS, and the car in ACC'Y or running, I still found 5V on pin 3.

And pin 2 had 0.58V at idle, and 0.88V at 2500rpm. Not much of a change.

So you've done the proper checks and substitutions, there must be a reason for the 5V being loaded down to 1.5V at the TPS. It's as if there was an additional 8k resistance in-line with pin 3 at the TPS. Maybe there is a corroded connector or badly frayed wire between the TPS and the DME. Or maybe there is some other bad sensor that shares that particular 5V line.

Good luck,
-Jerry

[seraphiem]

Jerry,

Much thanks for verifying the TPS operation on your car. This information alone is worth its weight in gold for my sanity.

With my low Volt Ref situation I've checked the DME connector to TPS connector multiple times over. The Fuel Tank Pressure Sensor shares the same 5 Volt Ref supply from a single DME pin so this is suspect since the TPS checks out.

The part that throws me and will take some time to diagnose is the complete lack of a 5Volt Ref at the Fuel Tank Pressure Sensor, yet a measured 1.5 Volts at the TPS. Both sensors share the same supply from the same pin on the DME. This heavily suggests a wiring fault.But wiring from the Fuel Tank Pressure Sensor also has been checked out many times over with no breaks or shorts found.

Think I just talked myself into disassembling the DME connector and backing the TPS / Fuel Tank Pressure Sensor 5Volt Ref pin out of the connector to measure it straight out of the DME. If I find a nice 5Volt supply, its looking like I need to physically inspect the wiring to each. If not, I'll continue my hunt elsewhere.

Thanks Again,
Tim

[berkeleyEE]

Just a thought: I only checked my TPS voltages between pins, not between pins and chassis (cuz my TPS is working, so no need).

Maybe your Fuel Tank Pressure Sensor is internally shorted so it measures 0V across all pins, but the FTPS is itself at 1.5V above the chassis. So disconnect the FTPS, maybe that will "free up" the DME 5V. Or maybe the wires from the DME are shorted together and at 1.5V above chassis. Check that out too.
My 2¢

[seraphiem]

Fixed my problem. I'm posting this follow up in case anyone else ever has a similiar problem.

Found an intermittent short between the 5 Volt TPS and FTPS Reference and chassis ground. Repairing the wiring brought the 5 Volt Ref back to working order and more importantly I have a driveable car again.

The FTPS 5 Volt Ref was shorted against chassis ground back in the trunk area on the left side of the car. The large wiring bundle was rubbing against my 'verts soft top lid motor bracket. The bracket had a slight bur and sawed ever so slightly through the insulation. Fixed the wiring and properly sheathed it while filing down that bur.

Thanks All Again,
Tim

[berkeleyEE]

Ah, yes, common source of electrical problems.

read here: Attention: ALL E36 owners beware - WIRING PROBLEM (http://forums.bimmerforums.com/forum/showthread.php?t=79896)