A place to discuss hardware/software and diagnostic procedures

Scope testing (relative compression test)

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2 years 8 months ago #50192 by Skylocus
I work for a delivery company and work on a mix of gas and diesel vehicles. I own a scope and what to use it more. I been looking at relative compression test. Diesel engine information on you tube is hard to come by. So my question is how do you do relative compression test on a diesel? What if they have 2 batteries does that change the way you preform this test?

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2 years 8 months ago #50194 by Dtnel
Just odd the top of my head if you do or don't have a Picoscope I'd look over at support on the Picoscope website as they have a large collection of training aids available. I'm not one who does stuff all the time so in between times of using the scope for this and that I do other things so when coming back to the scope I have to refresh. Also another resource to check out would be Snap-On as they have training seminars that I think they do weekly that are archived under Snap-On training.

A person who'll give you a little of his time when he isn't busy is Brandon Steckler. Look him up on Facebook, ad him as a friend and message him to set something up to chat with him.

Also if you're in a shop you're eligible to get on DN for Appx $50 per year, that diag.net which has alot of helpful resourceful information. These are people that were over with iATN before things went south from my understanding and there's people like you and I in there daily.

When searching for the Picoscope training search for the training for relative compression on diesel engines so you make sure you get the right training material as we all know diesel is a different breed than gasoline and not having worked with it on the scope I'm sure the settings are going to be different along with some of the equipment because of the pressures due to diesel's compression psi being much higher than gasoline.

Hope some of this helps and I'm sure others will add to this and I'll learn some more as well myself being I stepped away from diesel repair years ago before I went trucking and downgraded to automotive repair. (Less wear and tear on the body)

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2 years 8 months ago #50195 by Dtnel
Also forgot to ask what size diesel engines are you working on as if people see the sizes this will help them be more inclined to reply if they know by reading the post.

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2 years 8 months ago #50207 by juergen.scholl
As long as you measure the starter current it doesn't matter how many batteries a vehicle features, just make sure you put the amp clamp around the thick cable that goes from one battery to the starter.

Another option is to measure the voltage drop from the engine block to B- of the battery where the starter gets it's current from.

To keep the engine from starting you could disconnect the ckp, the injectors or the fuel rail pressure sensor (talking common rail here), depending on accessibility and system.

If you discover a problem you'll have to find a way to sync a specific cylinder in order to detect the faulty one(s).

Once you make the engine a no start to me one of easiest way to get a sync is to pull a glow plug. This way you'll have a clue to guide you thru cylinder identification.

Take special care when you're dealing with piezo injectors: NEVER EVER disconnect one of them with the engine running, the cylinder could get hydrolocked.

An easy way to determine whether you're dealing with a solenoid style or piezo injector is a resistance measurement: a solenoid style will have very low resistance, could be well less than 1ohm. A piezo injector will show very high resistance of at least 100 kO.

An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
The following user(s) said Thank You: Dtnel

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2 years 8 months ago #50213 by Skylocus
Thank you for all that info! I will definitely look into all that.

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2 years 8 months ago - 2 years 8 months ago #50214 by Skylocus
As far as engines we have cummins 6.7L, Chevy 2.8L, sprinter v6 3.0L and sprinter 4 cylinder 2.1L. Also at bunch of gas but the gasoline information is alot easier to find it is just the diesel that I am more focused on for help.
Last edit: 2 years 8 months ago by Skylocus. Reason: Forgot to add something

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2 years 7 months ago #50253 by Andy.MacFadyen
Don't over complicate a diagnostic until you have to.
On common rail diesels disconnecting the the power to the LP pump(s) usually prevents the engine starting  and on the ones I work on disconnecting the cam sensor works.
With relative compression test the first priority is identifying if you have a weak cylinder or not.
Only then to you have to consider identifying the particular cylinder is a priority.
Pulling a glow is a potential can of worms but on a lot of diesel you can identify a bad cylinder from scantool live data by looking at RPM per cylinder at idle.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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