3.2 Ref/Speed sensors

[IROC]

In trying to troubleshoot my no-start problem, I have narrowed it down to the DME itself or the speed/reference sensors.

I have the correct resistance for both sensors at the big plug that goes into the DME. I attempted a make-shift test of them last night using an analog voltmeter connected to the appropriate pins on the connector and then cranked the engine in hopes of seeing some sort of movement on the scale (indicating some voltage output from the sensors). I didn't see anything. Should I have?

Secondly, is it possible to access these sensors with the engine in the car? Maybe remove the heater blower and lower the engine slightly? If I have to mess with these, I'd rather not pull the engine if I don't have to.

Mike

[ChrisBennet]

The signal is pretty weak (.5 volts?) so you might not see much esp on an analog meter. I haven't tried changing them in the car but I know people do. You would get to them from below the car like you would the starter except on the other side.
-Chris

[IROC]

Yeah, the signal is supposed to be on the order of 2 volts and my voltmeter has a 15V scale, so I thought I would see *something*, but maybe not.

I never thought about getting to them from below. Thanks for the tip....

Mike

[ChrisBennet]

Try setting the voltmeter to the AC scale.

EDIT: Don't expect to get anything from the reference sensor. That sensor only gets a a few pulses (1?) per revolution.
-Chris

[JR Indy]

As Chris said, on the AC scale you'll be able to see if the sensor is sending out something, but I don't think you will be able to determine if it is working correctly.

[IROC]

Well, I'm merely looking for evidence of some sort of output, because my DME is acting like it doesn't know the engine is rotating and I'm trying to figure out if it's the sensors or the DME. Luckily, a friend is going to let me borrow a good DME, but I thought I would try this sensor thing first.

I'll use the AC meter tonight....

Thanks,

Mike

[BGCarrera32]

#1) is your tach fluttering when trying to start? If no, you have a speed/ref sensor issue. I doubt its the DME, but make sure its getting power (DME relay, etc. under seat) Do you have spark?

#2) you need a digital volt meter for sensor checks. Speed sensor, I forget if its BG or DG, will show a stong 2V on the AC setting. These sensors are little AC current generators, they have a magnetic coil inside whose field is excited by the flywheel teeth. The Ref sensor, which generates less current as it only gets tripped once every flywheel rotation is tough to read with a voltmeter.

#3) if you pinpoint the sensors, make sure BG and DG plugs are not reversed at connections to harness

#4) if sensors, jack up back of car. Remove drivers side rear wheel. Remove heater hose from heat exchanger. Sensors located near trans housing. Pull wires through grommet, etc. Observe .8mm gap on sensors (see photo in Bentley manual)

-BG

And in case you were wondering, I've had the same problem before...

[IROC]

Originally posted by BGCarrera32
#1) is your tach fluttering when trying to start? If no, you have a speed/ref sensor issue. I doubt its the DME, but make sure its getting power (DME relay, etc. under seat) Do you have spark?

#2) you need a digital volt meter for sensor checks. Speed sensor, I forget if its BG or DG, will show a stong 2V on the AC setting. These sensors are little AC current generators, they have a magnetic coil inside whose field is excited by the flywheel teeth. The Ref sensor, which generates less current as it only gets tripped once every flywheel rotation is tough to read with a voltmeter.

#3) if you pinpoint the sensors, make sure BG and DG plugs are not reversed at connections to harness

#4) if sensors, jack up back of car. Remove drivers side rear wheel. Remove heater hose from heat exchanger. Sensors located near trans housing. Pull wires through grommet, etc. Observe .8mm gap on sensors (see photo in Bentley manual)

-BG

And in case you were wondering, I've had the same problem before...

Thanks, BG.

No tach fluttering. DME has power. No spark. No ground signal is being sent to the DME relay (85b) from pin 20 on the DME. As for your number 3 comment above. BTDT and I've verified they are correct. The DME is acting like it doesn't know the engine is rotating. Resistance checks of sensors at DME plug check OK.

I will try the AC voltmeter thing this afternoon.

Philosophical question - did your problem end up being both sensors? Only one? Which one? Did the resistance check show OK?

Thanks a million.

Mike

[BGCarrera32]

My problem ended up being two fold, when I did my clutch job and cleaned up the sensors and bracket, I misadjusted the bracket holding the speed sensor upon reinstall. That .8mm gap is critical...the closer you get to the flywheel the stronger the signal. My car started and ran til' it got warm (gap was close enough), and then stumbled and wouldn't restart. The sensors can also appear to function normal when cold, and get funny when hot because they reside over the exhaust. They are some of the most reliable sensors on the car, but occasionally fail from what I hear. The resistance check passed, but that doesn't mean the gap is correct-you really need to check for signal. Cranking over the car will do it. One of the 3 pins on the connector is also a shield in the cable to the sensor, and the resistance check per the Bentley manual doesn't really make any sense, and both sensors must funtion to get the car started which is also contradictory to the DME check in the Bentley. My issue was the speed sensor, the one that generated the most current. If your tach isn't fluttering, I'd really suspect the speed sensor. Again, I don't remember if its BG or DG...

No ground signal is being sent to the DME relay (85b) from pin 20 on the DME

Hmmm...I don't have my wiring diagram in front of me so I'm not sure what to say ???

[IROC]

Originally posted by BGCarrera32




Hmmm...I don't have my wiring diagram in front of me so I'm not sure what to say ???

Oh, this is merely another output of the DME (to the DME relay) that is supposed to indicate engine rotation.

Thanks for your help. I am going to check these again tonight. Heck, maybe I'll leave work now?

Mike

[BGCarrera32]

Heck, maybe I'll leave work now?

Not the worst idea I've ever heard!

[rick-l]

Originally posted by IROC
Thanks, BG.

No ground signal is being sent to the DME relay (85b) from pin 20 on the DME.

Looking at the DME schematic pin 20 is a darlngton transistor that is turned on (20 at ground) when pin 4 is at 12 volts (starter active)

or when the 8051 turns it on.

[IROC]

Originally posted by rick-l
Looking at the DME schematic pin 20 is a darlngton transistor that is turned on (20 at ground) when pin 4 is at 12 volts (starter active)

or when the 8051 turns it on.

Well, I have verified that pin 4 has 12V during cranking. Your statement above seems to indicate a problem internal to the DME?

Where'd you get a schematic for the DME? That sounds like fun.

Mike

[rick-l]

Where'd you get a schematic for the DME? That sounds like fun.

http://frwilk.com/944dme/ should be generally the same as a 911.