Should all motronic sensors receive approx. 5 VDC?

[Robert K]

I was just checking the water temp sensor, idle control valve, air flow meter, etc. per the Bentley manual. Usually when checking for supply voltage to sensor, it says to check for approx. 5 VDC to ground. My idle control valve had 5 VDC going to it. The water temp sensor and AFM appeared to have about 2 VDC. Does this sound right? Also, when I do the stomp test, I get a short light, long pause, long light, long pause, then the process repeats. I do not appear to be getting any codes. I disconnected the battery overnight on Friday to reset things and still get this reading. Do I have a DME problem? Are these two symptoms related?

This all goes back to the fact that I have a slight surging at cruise speeds of 50-70 mph. I've adjusted the valves, put in new plugs and plug wires, cleaned the cap and rotor, replaced a few bad vacuum hoses and cleaned the throttle body. The idle control valve appears to be working fine. Everything appears to be in great working condition. I just don't seem to be able to find the cause of this.

I would like to check the fuel system next. I just need to come up with a high pressure fuel guage. Any suggestions on any of this would be appreciated.

Grace and peace,

Robert K
slightly ailing 1991 535i

[Jeff N.]

Interesting on the codes. Not sure what that means. And, I'm no help on the voltage to the sensors. I do know the DME sends a 5v DC signal to the AFM...not sure about the rest of the sensors.

Slight surge? Hmmm...how old's the O2 sensor? Just for grins, you might try running the car in open loop (O2 sensor disconneced) and see what that does.

For about $20, you can buy a high pressure fuel gauge from Summitt Racing. Go to a local hotrod shop and for another $10 or so you can get a "T" and tap it into your fuel like under the intake manifold. The problem will likely be trying to observe the gauge at your 50-70 MPH speed.

Curious issue...

[bill g]

Bentley manual states there should 5V at AFM and coolant temp sensor and that is what I found when I tested mine. Most sensors operate with a varying resistance, except for example the throttle position switch which is just an on-off-on switch. The reason sensors are supplied with 5V instead of full battery voltage is to prevent incorrect sensor readings due to variations in available battery voltage, which can drop a few volts at times, such as when starting the engine. It seems to me if you only have 2V at AFM etc. these sensors will send incorrect information back to the ECU.
If the ECU is supplying incorrect voltage and odd or no codes it sounds like it could be faulty. Some voltage drop can occur at really poor (high resistance) connections, but this great a drop does not seem right.
I also have a question - I have found about 11 volts supply at the throttle position switch on my car, but the BMW troubleshooting manual on CD indicates it should be 5V. Does anyone know what this voltage should be? I suspect the voltage here does not really matter, as the car seems to run OK including at full throttle.

Regards
Bill G

[Hector]

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[Robert K]

I've sprayed some carb cleaner in it and made certain the rotary valve rotated freely. Also, the other day when I just turned ths key switch to run without starting the engine, I got a buzzing sound like Bentley says I should. I still think I need to do the full fuel pressure diagnosis routine as stated in Bentley. I have new plugs, new plug wires and have cleaned the cap and rotor. The electrical system should be fine. I've replaced nearly every vacuum hose and made certain all joints are tight, so it shouldn't be a vacuum problem. That leaves fuel and sensors. I'll recheck the supply voltage today to the various sensors and see if I'm still not getting 5 VDC. With the new parts and tuneup, my car should be running perfectly. This is really annoying.

P.S. There are absolutely no leaks under my car!!!! Hurray and thanks for the help with the oil pan gasket replacement.

Grace and peace,

Robert K
1991 535i

[George M]

remove the vacuum hose from the FPR and see if any gas is present, i.e. the diaphram inside the regulator is perforated which will cause your intake to ingest raw fuel and your idle to hunt...before you move on to check fuel pressure. My personal opinion is it is all a matter of perspective. I personally wouldn't be that plussed by the whole thing as I have been there many times. An enthusiast will view your issue as a challenge and have faith that the answer is near after you have exhausted all the contributing factors.