Could someone verify my speed and reference sensor measurements?

mskar · 05-09-2002, 05:36 PM

I'm been diagnosing my car in the driveway over the last week... (I posted a question about the idle stabilizer a few days ago, thanks for all of the responses ), now I have a new one.
I measured my speed and reference sensors according to the Bentley manual, each has 3 terminals. Bentley says that they are electronically identical:
1-2 960: ohms +/- 96 ohms
1-3 & 2-3: > 100k ohms

I measured mine (at about 80 degrees), and remeasured them after removing them, I am coming up with:

speed sensor-
1-2: 1083 ohms (outside of tolerance but due to temp?)
1-3: open
2-3: open

reference sensor-
1-2: 1040 ohms (within tolerance)
1-3: open
2-3: open

my question then is, what do they mean by ">"? Infinity is technically ">" but I was kind of expecting a reading. Are there any other tests I can do on these? Thanks for any suggestions!

My car's symptoms: EXCEEDINGLY rough idle cold or warm, horrible hesitation with no load, seems to drive okay at higher RPMS but bucks at lower 2-3k RPMs.

pwd72s · 05-09-2002, 05:42 PM

Sorry...my goof.

Thomas Owen · 05-09-2002, 06:45 PM

What kind of a meter are you using? Check the range when you are looking at higher resistances such as something ">" (Greater Than) 100 thousand ohms. The wrong range will make it appear that a high resistance is 'open' when really it is higher than your scale allows. I would bet your findings are ok.

Tim Polzin · 05-09-2002, 08:45 PM

Off hand, I'd say the sensors are ok. Pins 1 and 2 are the active lines, pin 3 is a shield. Both active lines are individually shielded.

Tim

mskar · 05-10-2002, 10:12 AM

My meter goes all the way up to 20Megaohms, open reading even at highest setting(!)

I'm going to replace the speed sensor as long as I have it off since it is slightly out of spec, really beat looking, and 18 years old, the ref sensor was replaced sometime in the 90's and is in nice physical shape.

Anyone have a better way of gapping this assembly than the "glue a washer onto the old sensor" the Bentley book recommends? I am afraid of dropping the washer down into the flywheel opening if the glue doesn't hold

Thanks 'gain for the responses... hopefully I can return the favor someday though I am far from an authority of the fine inner-workings of Porsches at this point!

ChrisBennet · ChrisBennet's Garage

If the little washer does fall down in there, what is it going to hurt really? I found it difficult to find a washers that was .8mm thick. After measuring all the washer I had on hand I found one in my gravel driveway one day. Save the old sensor and glue the washer to it permanently to make the tool.
-Chris

Tim Polzin · 05-10-2002, 11:57 AM

The insulation on the cable from the sensor to where it goes through the cowl often deteriorates. This is the reason you want to make sure that it is open between the shield (pin 3) and pins 1 and 2. I used some "liquid insulation" on one of mine that had become brittle.

The sensor is just a coil picking up a signal from the flywheel. All that appears on pins 1 and 2 is a series of voltage pulses. In the DME, there is a chip that takes the signals from the two sensors and outputs angular velocity to the microcomputer.

The long and the short of it is that it will either work or it won't. A new sensor will ensure that the shield is not shorting out and will look better, but will probably not help with your current problem.

Good luck...Tim

mskar · 05-10-2002, 12:34 PM

Ya, a "Hall Sensor" right? Remeber them from an analog electronics class I took eons ago. Thanks for putting the mental picture in my head Tim.

Chris- I was worried about it possibly kicking up and damaging or shearing off the reference pin... not a possiblity? Thanks for the responses.