Autonerdz forums

I'm a member of another group interested in BMWS.

This is a thread from that group including a number of authors with their names and identifying details removed.

I did not participate in the thread.

Also I sought the permission of each author to reproduce the text here.

This is a discussion about scanners and or the need for Oscilloscopes.

It occurred after a fellow had a lot of trouble with diagnosis on a BMW engine where there was an issue with the front crank hub and the crankshaft sensor which works off that hub.



Hi Gents
      I get the the same request from my techs frequently. Let's try a crank sensor. My next question to them is. Did you scope it. Of course they didn't. It could be any component. Maybe even a can signal. It eliminates a lot of unnecessary parts changing.                                                                           
      We have the Snap on Verus (not sure of spelling on that) for scoping and general testing. It has been one of the best scanners we have ever used
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You know I have to admit we have never had a scope in the shop and no tech has ever asked for one.  I suspect all but one of them would even know what to do with a oscilloscope.  This includes myself.   And the one predates modern diagnostic scanners.

I also have to admit that oscilloscope baffle me.  With that said I just ordered one so I will first learn how to use one effectively and then all of my techs will be learning.  We have all(all means everyone one that works on BMWs and calls themselves a BMW professional tech) come to a point where modern scanners have not kept up with the technology in the cars.  In the 90s and most of 2000’s we were fortunate to have a scanner tell us in many cases what the problem was via hard faults.  You know what I mean, example…an M52/54 with cam sensor faults required cam sensors 99% of the time.  Or a map sensor fault meant that the car would be getting a thermostat.  But with many of the E and F series cars our scanners have very few hard faults and mostly soft faults.  Past soft faults often put you in the ball park, but modern soft faults don’t do that.  Honestly you are lucky if they put you in the neighborhood the ball park is in. 

Over the past two years or so I have seen what I consider good techs in my own shop chase their tails on issues and using their gut, waste my money swapping parts to attempt to fix the issue without really knowing if the part would fix the car.  Often it didn’t.  Is that diagnostic?  Sure it is, but it is an expensive way to fix issues and if you are returning the parts in question when they didn’t fix the car you are driving your parts guy nuts and ultimately losing time and money with all these parts coming and going.  And some don’t go and you find them on a shelf or techs cabinet because the parts guy forgot to send back or the tech was embarrassed to admit they were chasing their tail. 

Enough!  It is time to learn how to properly diagnose an issue like a professional and not like someone who is just trying to get buy.  So in comes our first oscilloscope and I am the first to admit that I am a little scared of them.  At the insistence of several of you that work with them regularly I am forcing my techs, my shop and myself into modern professional BMW, MINI and Mercedes diagnostics. 
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Hi XXX.

That explains it all.

In a similar position myself .
You will learn from this but I doubt xxx ever will and he will always be right even when he is obviously wrong !

Difficult and very frustrating at times but as you state its his shop.

The lathe will probably be busy nowfor every low power low revving problem that comes in if xxxx has his way.

XXX

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The wave form(s) were not checked. Here's the story:

At the beginning of this diagnostic issue, two days in, XXX(the owner) decided to take over.

I told him where I was in the diagnostic process, but he has his own way of doing things... did my best to bring him up to speed and made some suggestions on where to go next. At that point I was focused on the "cam sensor" fault, as I wasn't aware of the dynamic of why a bad crank sensor might set a cam code. Sir Ian explained it to me:

"If you lose the crank signal and you are still getting a cam signal, and as there is no monitoring of the crank signal itself it sets a fault for the cam shaft sensor. For if the crank is no longer rotating (transmitting) then if the camshaft sensor is giving a signal…logic, flawed as it is, that the cam shaft signal is faulty. It is true on N42 as well by the way."

So, I can't answer if he tried starting the car after he re-machined the dampener and before he installed the replacement (new Bosch sensor replaced by BMW sensor) crank sensor. I imagine that he probably replaced the dampener and the sensor, tried to start it, and it was successful.

Mike is definitely "old school", so I am positive he didn't bother scoping the sensors (although I mentioned that as a next step when I handed it off to him). He still goes by his "gut" when diagnosing cars, and it drives me NUTS. But, it's his shop and time... I just do my best work, try to be as efficient as I can, and roll with the punches.    ; > P



XXXXXX
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What did we learn ?

Did you check the crank sensor waveform before or after?

Please post so we can all see and learn.

so did the sensor or the damper fix the issue ?

XXX
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Group-

I know few of you are still working on the older BMW's, but we had one that taught us a lesson and wanted to pass it along:

1991 318is  M42
VIN  XXXXXX
XXXX miles

This car began to intermittently (once or twice about every two weeks) lose power, became hard to start and would occasionally backfire thru intake... car would start and idle (smoothly), but at or above 2500 rpm's (even in neutral) the car would misfire (badly), with occasional, mild backfire. When driving engine wouldn't rev beyond 4,000 rpm. All other times the car would run great: smooth, normal power, starts easily, etc. Then it was brought in after it refused to start and run for the 3rd time in a week.

CEL was on (earliest version of diagnostics) and showed only one code for "cam sensor". This is the only code that ever came up/repeated on the CS2000 or GT1.

Replacement BMW cam sensor installed. This made no difference in poor run/ no-start or backfiring.

Plugs were recent and not fouled, no intake leaks found after smoking intake, plug wires and boots passed ohm test and visual, replacement (known good) DME made no difference, ohm check between cam sensor and DME ok, all DME grounds and power ok, visual check of the cam pin ok, re-check cam timing and cam sprocket bolt tightness ok, check crank dampener for rotational looseness = none (applied witness marks to verify lack of motion), visual check of wiring to crank sensor and ohm check ok, re-check of both crank and cam sensor continuity and ohms ok. Do compression check to  assure there are no mechanical issues = passed. Also, pulled O2 sensor to see if cat con was plugged = no difference, car would not start and run. Customer supplied another used DME for testing ( no diff) and another air flow meter (no diff) even though I told him it would not affect the timing.

After checking online for troubleshooting tips and "old fart knowledge" we pulled the crank dampener off and locked it down in the press and re-checked for looseness/lost motion = none, witness marks still aligned. Put the dampener in our lathe and saw slight (approx. 1mm) axial play in outer diameter of hub. Customer ok'd machining outer rim to make it concentric (no desire to overpay BMW for new hub and couldn't find a good used one at the time).

Re-installed machined crank dampener, installed a BMW crank sensor. Engine started and ran immediately!!!

What did we learn?

- Online advice told us that a cam sensor code on this model can also refer to the crank sensor! OH!  Sure -Why not... it's a BMW after all.

- As many know, there were problems with this model's crank dampener coming apart allowing relative movement between the hub and
      outer dampener rim. However, this was not the case in this car. Problem seemed to be the out-of-round/lack of concentricity of unit
      added to the info below.

- People are reporting that Bosch crank and cam sensor quality control problems cause a poor/inconsistent sensor wave form that
      (added to the hub's lack of concentricity) was the reason for the intermittent, low signal strength and  poor waveform that "stacked
       up" to cause this kind of issue.


SO... The car is finally running well and we know why it took us for a ride.


XXXXXXX

I'm from a BMW back ground, but the general consensus out there is the scanner tells you what is wrong, but as we all know that it is far from a reality even though it would make life a lot less stressful if it did.

On board diagnostics have come along way from what they were but they are not perfect.

Some codes do need no further steps but these are normally very rare, like a ECU EEPROM code.

Interesting that a run out issue on the crank pulley will set a cam code and cause a crank no start, but never set a crankshaft sensor code.

Are these a magnetic pickup sensor?

So with the Axial play causes a air gap issue that the sensor doesn't produce enough voltage for the DME to see a crank signal?

Logic wrote on Jan 13^th, 2015 at 2:47am:



I believe this is going to be the biggest thing in next few years with the way vehicles are advancing.
Where vehicles are being pushed towards EV, hydrogen fuel cell or a hybrid.

It really is going to be either sink or swim for some shops out there.

Derreck,

I have no issue with your comments, I do agree that you need the basis as you rightly state.

Do do raise one point that I had not considered however;

Derreck wrote on Jan 13^th, 2015 at 9:41am:

The problem here can be summed up in that first post. As long as people are content with firing parts at a car to test it...there is very little need for a scope.

As long as the public is duped into thinking it's the norm to pay for "We can change this and see if that fixes it," there is very little need for a scope. 

However, if you run a legitimate business that sells fixes, and not guesses, I can think of many, many instances where you need a scope to verify that a part is truly defective.

So, how any hours before you become proficient? Well, for someone who's interested in learning, I think about an hour or two to learn how to do the basics of just proving out whether or not a sensor is working, or not.