Check Out this Crank Position Sensor Wave Form. What do you think?

[Aahj]

Hey guys,

Just picked up my first E30 last week and it seems to intermittently hesitate at idle and under load.

Previous owner apparently did a full tune up including coil. I also sprayed engine down with air intake cleaner to look for possible leaks. All seems to check out.

I was a little slow today and decided to hook up my lab scope to the CPS and check out the wave form.

Here are two videos that I shot and would like your input.

This is a two minute video of the lab scope hooked up with other info I found:

http://www.youtube.com/watch?v=SobqvQhMTW4

This vid is about a minute and shows the harmonic balancer up front:

http://www.youtube.com/watch?v=NHQj5Ie4gtw

Let me know what you think.

Thanks for your input!
AJ

[nrubenstein]

If you've got a doubt, replace it. The CPS is known for causing issues.

[accident]

Hooking up a CPS to an oscilloscope is kinda overkill lol

[jlevie]

Hooking up a CPS to an oscilloscope is kinda overkill lol

Actually it is not overkill. But to really tell if the CPS signal is bad you need to see it while the car is being driven when "events" are occurring.

There are four possibilities w/respect to the CPS. The first, and obvious one is that the CPS can misadjusted or bad. The second can be a bad harmonic balancer (wobbly or shifted). The third can be a bad engine harness, and the last is a bad input section in the DME.

Just because the resistance of the CPS is in spec doesn't mean that the CPS is good. If in doubt, swap in a new OE part. I'll change the DME first if it still looks like a CPS problem and if that doesn't fix it a new harness goes in.

As to the OP's problem, I'd be looking for other causes. The symptoms don't really match a CPS issue. The first thing I'd do would be to properly smoke test the intake/crankcase. Then I'd run the fuel system tests in the Bentley. If neither of those solved the problem I'd try a different AFM and a different DME (swap one, run the car, then swap the other).

[Aahj]

Understood on all of the above...just hate parts swapping. It isn't the best, cheapest diagnosing method. I'm curious if anyone here has any known good wave forms to share?

Thanks!
AJ

[nando]

I can't see the pics

[jlevie]

Understood on all of the above...just hate parts swapping. It isn't the best, cheapest diagnosing method. I'm curious if anyone here has any known good wave forms to share?

I agree, diagnostics are better than throwing parts at a problem and hoping for a favorable outcome. Note that I suggested a smoke test and fuel system tests as the first steps in solving this problem. Those are diagnostics.

The car's symptoms aren't a good match for a CPS issue. A problem with the CPS data will cause the DME to cut spark, fuel, and tach signal. At the least this results in a noticable brief all cylinder miss and at the worse a complete loss of power. When the car is being driven you'll see the tach react to the loss of data.

If the smoke and fuel tests don't reveal a problem the next likely candidates become the AFM and DME. It is possible to test the AFM, but it is easier to swap in a known good unit. It is just about impossible to do diagnostics on the DME and since these are 20'ish year old devices swapping in a known good unit is the best approach.

[Aahj]

Also, reposted the vids on YouTube now:

http://www.youtube.com/watch?v=SobqvQhMTW4

http://www.youtube.com/watch?v=NHQj5Ie4gtw

[CarsSuck]

I never scoped my CPS, but when it went bad in may car, it went intermittently. It was strongly correlated to the ambient temperature too. The colder it got, the worse it behaved.

Like jlevie said, you'll have other indicators. The tach will cutout with the engine, and when that happens, the fuel mileage indicator will swing towards zero (right) as the ECU cuts fuel.

[Aahj]

Update with video:

http://www.youtube.com/watch?v=oZ0dK9t6aPo

I found a couple things interesting. Either the software is
labelled incorrectly, or my car is wired wrong. The Verus
component test feature is telling me that the CKP + signal
wire (Yellow) should be pin #1 and CKP - signal wire should
be pin #2. My car is the opposite. Does it matter?

Thanks for the input!