Speed sensor signal: Question for Experts

[ChrisBennet]

I used my oscilloscope to look at the speed and reference signals for my 3.2 911 and a 944. The reference signals had the spikey look I was expecting but the speed signals looked like a sine wave - not like the sawtooth the manuals show.
Could someone who has looked at these signal before comment on what they should look like?
thanks,
Chris

[Early_S_Man]

Chris,

Sounds like the speed sensor needs to be adjusted to the proper air gap. Try adjusting the gap and take a look to see if the waveform doesn't change to the expected shape ...

[ChrisBennet]

Originally posted by Early_S_Man
Chris,

Sounds like the speed sensor needs to be adjusted to the proper air gap. Try adjusting the gap and take a look to see if the waveform doesn't change to the expected shape ...

If that was the case, would both car's signals look the same (944 and 911)? I'm thinking it might be something to do with my measurement technique.
-Chris

[BGCarrera32]

If that was the case, would both car's signals look the same

Not necessarily. The ref signal might but I recall reading somewhere that the 944 flywheel has more teeth on it than the 911's, even though the DME is similar and the sender is the same... (You may know more on that than I do)

-BG

[ChrisBennet]

Perhaps a picture will help.


I did notice that my speed signal wasn't very strong. Less than .5 volts I think. Car runs fine though which leads me back to my measurement technique being wrong theory.

-Chris

[BGCarrera32]

Can you change the range and sample rate of your scope? If the upper peak to lower peak range on your display is decreased to say 2.0V max, and the sample rate display is narrowed, it may look more like the book. What does the book list for values against the graph that you show from the book? I'd be suspect of the book anyway...

If you think you have weak signal, what are you getting for a VAC value from the speed sensor?

[BGCarrera32]

I did notice that my speed signal wasn't very strong. Less than .5 volts

O.k., duh, I just reread your post. I get like 2 VAC at the proper .8mm gap. Every tiny distance you move that sensor face away from the flywheel, the AC current generated by the sensor drops off exponentially. I tried this once by mounting the speed sensor to the toolpost holder on my lathe, and moving the face closer and closer to the chuck face. Got up to like 3.5V with the big chuck teeth and almost no gap.

What I was getting at in my previous post was that even though your graph looks different, it could be just displayed different (duh) even though peak to peak measurements are really the same, so we need to see values...

[ChrisBennet]

Originally posted by BGCarrera32
Can you change the range and sample rate of your scope? If the upper peak to lower peak range on your display is decreased to say 2.0V max, and the sample rate display is narrowed, it may look more like the book. What does the book list for values against the graph that you show from the book? I'd be suspect of the book anyway...

If you think you have weak signal, what are you getting for a VAC value from the speed sensor?

Good idea(s) BG. I'm going to make up a test plug and try again, maybe this weekend. I'll let you know what I find. That plug is right near the coil and spark plug wires. I've have some shielded wire, I just need a plug from the local Metric hardware place...
-Chris

[Early_S_Man]

Chris,

It is very possible that the bandwidth of the scope you were using is responsiblr for the sine wave display, rather than the sharp waveform you expected. If the scope is a relatively inexpensive model with bandwidth under 1 MHz, then that is the problem. Many scopes these days do not have the bandwidth to accurately display complex waveforms -- any 'sharp' waveform components require bandwidth many times higher than the fundamental frequency of the basic 'rep' rate.

Even a 500 MHz scope displays a sine wave when fed the sharp waveform from a time mark generator at the 2 ns setting!

[ChrisBennet]

Warren,
The scope is a Fluke 192 (60Mhz). That should be OK shouldn't it? I was using it in "auto" mode. I'll try some manual settings and see if that helps.
-Chris

[Early_S_Man]

Chris,

Yes, 60 MHz should be adequate, but, were you using simple wire & clip leads into a banana plug ... or a 'proper' 10:1 compensated & shielded scope probe?

[ChrisBennet]

Originally posted by Early_S_Man
Chris,

Yes, 60 MHz should be adequate, but, were you using simple wire & clip leads into a banana plug ... or a 'proper' 10:1 compensated & shielded scope probe?

I was using the sheild probe that came with the scope. I'm not sure if it is 10:1.
-Chris