Using an o'scope for DME\engine testing

[ianc]

Hi folks,

I've been working my way through the DME test plan, and it seems that to get good results for testing some of the components, an o'scope is required.

I haven't read much here about using one of these for tests. The test plan mentions two types: a Bosch Engine Tester MOT 300/400, and a SUN Engine Tester 1010/1080/1019.

I have not priced these, but they are probably not available anymore and\or magnificently expensive.

Are there low-priced alternatives one could use to make these types of tests? Has anyone here used one of these for this type of work before?

Just curious...

ianc

[ianc]

I'm moved to post here again and surprised by the lack of response to this. Wouldn't it be relatively inexpensive to obtain a scope of some suitable type? For a primarily mechanical sort of guy, could it yield some useful diagnostic information? Has no one here tried this?

Thanks again,

ianc

[Bill in OKC]

I have not read the test plan... but I am relatively familiar with the DME after tracking down two no-start conditions. So with that, I've seen inexpensive Oscilloscopes for sale on ebay. I am pretty sure that just about any scope made in the last 10 years would have the features needed to measure any signal a Porsche can generate. From the posts I've read, it looks like you would be needing a scope to look at things like (near) square waves from the speed and position sensors - fairly slow and 12V max - well unless you want to look at the coil secondary or something. HP made good equipment until bought out by Agilent - and Agilent is good stuff. A good ebay buy on a used working Tektronics o-scope would work fine. Maybe look for a battery powered/portable scope for ease of use & storage purposes. Lots of higher-end equipment if you want to spend the money but my car is 24 years old and the best scopes would be tremendous overkill.

[Wayne 962]

I have a scope I bought for $40 at the Pomona swapmeet. Works well, but it's difficult to use if you don't use it every day. I just recently used it to check the sweep pattern on the flywheel sensor for the 959. Lots of interference in the signal, it was good to see that it was working, but it would have been difficult to tell if it was intermitant or not...

-Wayne

[Porsche_monkey]

I bought a scope based engine tester off Craigslist for $200. I see them there all the time.

[dshepp806]

I've worked with 'em all of my life, in many different technologies,...and can say that this particular app (DME) would be the simplest environment. There are several specs that differentiate one from another, with bandwidth being the most significant. One would do just fine with a 10MHz BW in this application (ie: a 500 MHz BW would be overkill (and higher in cost) in this app,..although that's what I have and would easily be up to any DME tasks). Sensitivity would be another spec that you would pay for (no need for extreme sensitivity),...# of channels, yet another spec (single channel would do fine I would imagine).
Another: digital vs. analog,..analog would be fine... Any of these specs would differentiate one scope from another,...the more the merrier!!!

Now if one were to get into looking at the processor signaling, you'd want to be aware of clock frequencies (BW),......yet still this is quite primitive circuitry, although I'm sure the people that repair these (Loren, etc.) could chime in with additional comments as to the digital side of things. Also, one could easily see any noise being generated, to and fro. These signals really are quite simple insofar as "goes into, goes outa".

NOW, when one delves into the circuits (processing of these signals internally), this WILL require a degree of electronic knowledge in order to troubleshoot/repair,..regardless of whatever test equipment you may have in your hands.

Handheld units are quite nice in that one wouldn't have to drag a bench device out to the car but would be a preference,..price could be outside of one's budget.....what can you afford?

Used scopes can be had quite cheaply and would be a good bet for this app (ins/outs). One would want to know the device is calibrated properly but extreme precision wouldn't necc. be a factor here with the DME,,,,,,accuracy: yes.

Maybe we can start to lay out some sources for this piece (used or new)..

http://oscilloscope-tutorials.com/

http://science.howstuffworks.com/question8.htm

http://www.williamson-labs.com/scope-main.htm

http://www.apogeekits.com/oscilloscope_hps40.htm

http://www.electronickits.com/gold/hps10.htm

my 2 cents....

Best,

[911quest]

This one from Fluke looks promising

http://us.fluke.com/virtualdemos/28X_demo.asp

The problem with the Sun machines Bear etc. is they are really hard to get parts for.....that being said you can find them for a couple hundred bucks but the early Sun Machines are not small.

http://www.allstates.com/Emissions.html

This is what the 80's had to offer in scope gas analyzer technology

[Porsche_monkey]

In hindsight I wouldn't buy an automotive scope again, unless it had the CO analyser.

[hcoles]

I'm starting to put a scope on various DME pins. My first try today will be to see what is on the 3 pin "test" connection. What is the signal supposed to be on pin 12 on the DME? It just says Terminal A test connector.

I'm asking about the three pin connection mounted on the throttle body with a black rubber cover over it. This seems to be the easist thing to get to to give my scope a try.

[911quest]

It is the TDC test connector I believe that was used for RPM and timing that being said you will probably get a square wave pattern. The old bosch machine plugged into that as it's reference.

[scarceller]

I have a Techtronix scope from the 80s works great and you can find these cheap on ebay from time to time. These where top notch scopes back then and are way more than you will need. see listing #260208297748 on eBay for a good example of one. This one is very much like what I have. Here's another on ebay 220152729815

Then if you have a loptop you have new bread of scopes like this on ebay 300196854456, never tried one but sure seems cheap and you view the image on the laptop. Plus easy to record stuff. I'm tempted to get one of these myself.

As for Gas anylizer that in my oppionion is old school. I bought a Wide Band O2 system for about $200.00 and it's amazing what you can do with this thing! Beats the Gas Anylizer any day for what we need. I much rather know Air Fuel Ratio. Here is what I bought:
http://wbo2.com/2j/default.htm about $175 shipped

Then I just bought the WBo2 sensor from Autozone Bosch #17104 for $50.00

The software is free.

See this thread for the project details:
Wide Band O2 + data logger for 84-89 Carrera

[hcoles]

Tony, thanks....we'll see what it looks like. I should be able to post a scope image.

[LM3929]

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