Note on 3.2L Motronic Reference Sensor

[carmaneddy]

I recently had a failure of one of my reference sensors with a subsequent no start condition. After researching this site and the BOSCH manual,and Haynes manual. I was not able to find any documentation of the functional role of either sensor. All listed procedures required test equipment which I don't own. Therefore, I acquired a known working sensor from a friendly Bimmer mechanic and replaced first the Ref Mark sensor(BG) and then switched to the RPM senor(DG). Of course in the process I solved my no start condition, but what I discovered was that if the DG is nonfunctional the engine will get spark but no signal to the injectors, and the tach does not flutter with the starter revs. With DG functioning and BG out the engine gets fuel but no spark.

Any other clarifications would be helpful to others like me who try to DIY but lack oscillascope.

BTW I got loads of advice from friendly Porsche friends all over the US ranging from changing the ECU to banging on the injectors while someone turns the key!

[Lorenfb]

The ref sensor determines crank position. The speed sensor provides
timing signals. Both are needed to start. Once started, the ref is no
longer needed (The DME counts the teeth on the ring gear.).

[BGCarrera32]

The sensors are basically small coils that generate an AC voltage that the DME interprets to determine what Loren said above, he is correct. I had luck with an analog volt meter only with the speed sensor getting a reading, as it is much more active than the reference sensor. The ref sensor only generates signal 1 time per engine revolution off of a small socket head capscrew on the flywheel. These sensors can fail from age when the sheathing is melted or degraded, usually by hot exhaust. They are some of the most reliable sensors on the car, but they can fail. On *rare* occasion, the screw for the ref sensor goes missing... Make sure if you have moved the speed sensor in its bracket (like when you were replacing a clutch if you happened to tinker with it) that you observe the 0.8mm gap to the ring gear. If the speed sensor is backed away to far from the ring gear teeth, it will generate no current or a very weak one. I set one of these sensors up on the cross slide of my lathe at work, and backing it away from the spinning chuck just .5 mm at a time resulted in 3V, 2V, 1V...dead...

[Lorenfb]

Spare parts I carry in my 3.2:

1. one ref/speed sensor
2. one DME relay
3. one fuel pump
4. one ignition coil
5. one paper clip to bridge temp sensor connector if temp sensor fails

Those front wheels need more weight!

[carmaneddy]

What do the designations BG amd DG mean?

[Lorenfb]

DG - DREHZAL-GEBER - Speed Generator
BG - BEZUGSMARKEN-GEBER - Ref Mark Generator

[Rot 911]

Loren, when we aren't debating chips you really have some very useful information to pass along!

[3.2 CAB]

I had the same no-start, no-spark problem, back in Nov. Mine had one little quirk though, it was ambient temp. related. If the outside temp was below about 65 degrees, car would crank but not start, if the temp was above that, would fire right up, would not have to turn over twice. I emailed everyone that was supposedly a Pcar guru, ie. Excellence Mag authors, and other authors that we know and love, none had a clue why outside temp would effect the DME system. The car could sit in the garage and the overnight temp go down to 50, car would not start in the morning, but wait till 4:00pm after the car sat in the warm garage cooking all day, about 75 degrees in garage, car would fire right up. I finally got a blow dryer, and when it was cold, started warming up components till I found what the culprit was. It took some time, because, once the car started, it would start fine till it sat long enough till it got below the 65 temp then it would not start again. Finally I worked my way through the components, and with the blow dryer on warm, just a couple of seconds on the spd/ref sensors it would fire right up. I ordered a couple of sensors for $90.00 each with free shipping and fixed the car. Runs fine now, but there was just enough wrong with sensor that it took a little warmth to make it do what it should. One of the sensors the bottom of it was slightly distended, not enough to hit flwwheel, but you could tell it was swollen out some. Just my 2cents.

[carmaneddy]

THe bottom sensor is the DG. And as I discovered its malfunction will result in spark but no injector trigger. Sounds like your BG or top sensor was bad, if you had no spark.----Not to say changing both wasnt a good idea, just trying to document a diagnostic algorithym.

[cab83_750]

So, guys, my '86 930 does not have these sensors as it is CIS and not Motronic, right?

I also do not have the DME relay issue, right?




(I have posted a thread about my quest of building a turbo library collection just a few minutes ago).

[3.2 CAB]

No spark was the problem at cold ambient temps. Everything else worked fine, except only the fuel pump side of the DME relay was functioning, until the sensor warmed up and the sent its signal to the DME relay, wich then closed the DME control unit side of the relay and it would then start. I replaced both sensors because of the condition of the wire harness on them, both had a lot of heat/age deterioration.

[carmaneddy]

3.2 Cab did you replace the head temp sensor at the same time? I have not and am worried I may regret it. Does anyone know if the lifecycle for this sensor is similar to the ref sensors.

[3.2 CAB]

No as it was a recently replaced part. What I have since been led to believe about the temp sensor, it has no effect on spark circuit, just fuel issues on starting.