What do you need to know?

 

For all the OM 615/16/17 engines normal compression is 24 to 30 bar or 348 to 435 psi.

 

For future reference, this information is incorrect.
According to the factory service manual:
OM615, OM616 and OM617 non turbo (00-115 Checking compression pressure)
Compression pressure normal (22 - 24 bar) (319 PSI - 348 PSI)
Minimum compression pressure (15 bar)
Max permissible difference between any two cylinders (max. 3 bar)

For OM617 turbo:
(01-010 Checking compression pressure)
Compression pressure normal (24 - 30 bar) (348 PSI - 435 PSI)
Minimum compression pressure (15 bar)
Max permissible difference between any two cylinders (max. 3 bar)

Currently the manual is available for free on Mercedes StartekInfo website.

 

What need to know is how and where to connect the compression test and what are acceptable reading?

 

You need a special compression tester for diesel engines; standard gasoline engine testers don't go high enough. The proper tool should thread in place of the fuel injection nozzles and Jdc already stated the proper readings. However, any cylinder that reads more than 30-40 psi different from the rest has a problem. If you have low compression in any or all cylinders, you'll want to perform a leakdown test to determine the source of lost compression. What is the reason you want to check the compression: just for good measure, or do you suspect a problem?

 

Harbor Freight sell diesel compression checkers for $20.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93644

Don't know anything about their accuracy or reliability, though.

Jamie

 

I was just looking at the compression testers when I was at the store this weekend, but I didn't see any for diesels. It's good to know they have one that's affordable.

 

I'd like to test my car before I start puttting a lot of money into it, plus I have my eye on another one. I did see the copression testers at the store and hoped I could figure out how to use it.

 

You can also test through the glow plug hole with the proper size adapter.

 

Mercedes Source in Bellingham, WA has a compression tester with a bunch of fittings and a booklet on how to do a compression test for $72.00. www.mercedessource.com is their web url. I bought 1 but have not had the time to do the job yet. I should also have purchased their injection hose but I forgot to order it.

 

Have you done a google for compression test tutorial? You're bound to find something. The job is pretty straightforward, you connect the gauge, after removing all the injector pipes, in place of the injector or the glow plug depending on the fitting you have available and then crank the engine while you watch the gauge, repeated for each cylinder, any more than 10% variance between cylinders shows a loss of compression in the cylinders with lower compression. If you inject oil in the cylinders with low (this is a leakdown test) compression and the compression comes up, you have worn rings, if it stays the same, you have worn valve seats.