The raw variable reluctor signal should look something like the top trace in this image

http://www.molla.org/DIY-CDI/DIY-CDI...ger_Fig05s.jpg

The bottom is a switching signal from a transistor.

It appears that based on your scope traces, that the EGR controller is indeed running the raw signal through some circuitry to convert to a square wave. I'm wondering if the higher signal voltage would be detrimental to the tach over time.

And, I wonder if the appearance of your VR trace as a pseudo-sine wave is due to the closeness of the teeth on the flywhee (trigger wheel), as compared to the 4-tooth trigger wheel normally associated with a transmission for speed signals. (as shown in the following image on the right)

http://www.peachparts.com/shopforum/...edo-drive4.jpg

Interesting stuff....

Is the trace below what you captured on your 85 300D engine?

[QUOTE=mach4;3274321]The raw variable reluctor signal should look something like the top trace in this image

http://www.molla.org/DIY-CDI/DIY-CDI...ger_Fig05s.jpg

The bottom is a switching signal from a transistor.

The square wave is driving the tach. I soldered a wire onto pin 25 of the EGR computer internally, drilled a hole and brought it out of the box and captured that trace while the engine was idling with the tach registering.

That is actually a very pure sine wave from the VR sensor. Not sure why you call it a psuedo sine wave? What you see on right portion of the trace is shifting frequency, i.e the idle is not perfectly stable but varying, that is what you are seeing on right portion of the trace with muti waves. The engine was cold when the measurement was made. After it is fully warmed up, idle may smooth out and the trace may be more stable- but I won't know for sure till I try it.

What is that 4 wheel trigger wheel from? Don't tell me it is what the VR sensor is picking up in the 85 300D. :confused:

I called it a pseudo-sine wave because a VR signal should look like this. (although this is running with reverse polarity from "normal" as it shows a rising edge) There is normally a distinct rising voltage, then a near instantaneous drop to negative. This is what makes it so good for timing as you get a very distinct point where the voltage changes.

http://www.import-car.com/Content/Si...0000019330.gif

As the teeth get closer together (as in the MB flywheel) the falling edge gets less distinct as the following image shows, so much so that if the polarity is reversed, it's difficult to tell what is the rising edge and falling edge. (Note the trace is from a timing wheel, like perhaps an EDIS, where the anomaly is from a missing tooth that marks the beginning of each revolution) Someone could easily mistake the trace as a sine wave, when it is in reality not.

http://img32.picoodle.com/img/img32/...rm_37b2dec.gif

The 4-tooth trigger wheel image is from a 722.3x transmission. It's used as a trigger for the speedometer VR sensor.

I don't own a scope, so everything I've shown is from the internet.

I don't know why you think a VR signal should look like your first pic. Where did you get that pic from? The flywheel, with it's evenly spaced and uniform profile (rising and falling) teeth, should produce a sine wave output from the VR sensor like what I captured.

In your second pic, I have no trouble telling a rising edge from a falling edge.

Depends on the tach circuit. If no one has reported any damage with the mod by now, it is probably ok.

.... So I spliced the the white (RPM sensor signal wire) and grn/bk (to tach) wires from the harness together and guess what? SUCCESS!

I made sure to cover the power lead from the OVP so it doesn't short out on anything. But if you're running sans EGR, there doesn't appear to be any reason to run the EGR module.

YMMV

-Andrew[/QUOTE]
I like this and would like to try it since my tach unexpectedly quit working. The 10amp fuse at the OVP is good but the OVP rattles when shaken. If I were to open the harness going to the EGR black box and splice the two wires you mentioned and presumably the tach works again is the OVP of any use anymore?

It Works
 

This fix works.
Nice job, thanks.
:)

'85 300 TurboD
'78 300D
'70 220D

Just did this to my 85' 300TD, and my tach is now working! The pins from the EGR module were all green and crusty, and I have removed my EGR valve, so one more thing checked off the list of things to do before putting the car back on the road.

Save all EGR related parts that you remove in case some day you may have to re-install them to pass emission. Your car may not require emission test today but that could change with tightening of the noose by the EPA.

Sorry to revive an old tread but I'm in the process of trying to get the EGR controller on my om617 swap working correctly. There isn't a lot of information out about this subject and this was one of the only threads I was able to find that actually discussed the function of the EGR control besides just trying to bypass the thing entirely. Im looking for information on mainly how changing the tooth count on the ring gear might effect the function of the EGR system.

Wow, I just read this entire thread and it is mind boggling.

This reminds me why I love my '81 300SD and its simplicity. I will probably never get rid of it as my almost daily driver.

Good luck to you computer savvy guys; you are admired.

Changing the tooth count will change the perceived RPM seen by the EGR system. If you have a situation where it is critical that it function as designed at various engine RPM, it needs to be assembled as designed with the correct tooth count. There's not really any other way around it.

Thank you Diseasel300 I figured it would change the percieved Rpm I was hoping it wouldn't change it enough to raise eyebrows upon inspection. Kind of at a loss for how to make the system function properly because I am unable to run the factory flywheel in my conversion.

New to me 85 fed 300D turbo had intermittent tach issues. Found this thread and decided to pull the EGR controller and try the jumper wire trick.

I know my EGR valve is bad, assumed failed closed and finally blocked off as of today. I plugged all the EGR vacuum lines last month to see if I could get it running better. I felt it ran worse but never reconnected the lines. Figured I didn't have much to lose by pulling the EGR controller.

The EGR controller and connector had tons of green corrosion. I regret not taking pictures at this point. Cleaned up the connector, installed the jumper wires and got my tach working great, however now it won't shift out of 1st gear, which shocks me.

I cleaned up the EGR controller pins and put it back in. I lost my tach again but it shifts exactly as it did before. I'm not grasping why it refuses to shift without the EGR controller installed.

Since then, I bought 2 EXACT same EGR controllers off e-bay with NO corrosion at all, and it still DOESN'T shift out of first gear, but the tach works...

Of course, still running off the original EGR controller so I can drive the car but have no tach.

Any thoughts?