compression test with engine out of car, starter issues - PeachParts Mercedes-Benz Forum

petecooke · #1 10-06-2011, 01:24 PM

I have a 83 240d motor in my garage and I have the starter connected to the engine. The tranny is off the motor. I want to test the compression on the motor. I have a battery with plenty of juice.

There are 3 electrical connections to the starter. 2 are large and 1 small. There is 1 large connection closer to the top of the engine and another large connection that is toward the bottom with a metal strap going into the starter.

I want to attach jumper cables to the starter and then touch the cable to the battery to have the engine turn over.

I connected the negative to the larger terminal closer to the bottom with the strap going into the starter. I connected the positive to the small connection that has a screw going into the starter.

I didn't touch the large connection closer to the top because nothing happened when I did.

With this configuration, the starter engages but does not rotate the engine. The engine is not seized.

Can anyone help with where the negative and positive should be connected?

Thx.

pete

leathermang · #2 10-06-2011, 02:15 PM

If we just told you where they were we would be doing you a disservice ...

Jumper cables do not have enough surface area to do this right..

You really need a good connection... like as when in the car to get the rpms to get a good test...

Do you have cables and such you could connect up with...
and connect solidly to the battery...... and use a solinoid controlled by you to do this... there are reasons the system is made as it is...

petecooke · #3 10-06-2011, 02:26 PM

Thx for the info. I can easily pull the red positive cable from the car and connect it to the battery and the starter. The negative cable in the car will be a little more difficult.

Is the top connection on the starter the positive and the bottom negative?

I assume the little screw contact just makes the connection(completes the circuit) so the starter engages.

Is this correct?

Thanks greatly for your input.

pete

barry123400 · #4 10-06-2011, 02:56 PM

I cannot remember everything. There is no purpose ground connection on the starter . It is grounded through the starter casing to the engine block. So attach the ground to somewhere on the engine. Or use a starter mounting bolt to the housing for the ground if practical. You want the positive cable to the upper side large terminal on the starter in most cases. Not on the large terminal that has a strap or wire entering the starter motor itself. That terminal is just to process the voltage to the motor once the internal switch in the solinoid is activated.

The solinoid is then energised by touching a screwdriver between the small terminal and the large powered terminal once this is done. The solinoid closes the motor power switch and moves the starter drive forward into the ring gear as well.. In effect you have completed the circuit the key switch normally does to activate the starter.

If you put the positive cable on the top large terminal and the negative on the lower large terminal. Then activate the system you may no longer have a good solinoid. It would probably burn out the switch contacts in my opinion. Or weld the switch contacts together in the solinoid. Anyways forget putting the negative cable on either terminal. Take it to the block.

If you use the wrong large terminal for the positive connection the starter motor will just spin continiously without the starter gear engaged with the ring gear.

Now if the compression tests work well I would remove the lower oil pan cover and plastigauge the first rod bearing. This is a simple and cheap method of checking the clearance. Can be done after the purchase if the engine checks out well compression wise.

If the 616 engine is run for a long time with low injection pump base pressure. For example never changing the secondary fuel filter until it obstructs. The number one cylinder bears a heavier loading than the other cylinders so excess wear can be present on the bearing. This in my unproven opinion is the reason for the larger incidence of the number one rod failures on the 616 engine. So if that bearing is worn more than it should be. Change it before installing the motor.

There is also another great contributor to this issue on the 616 engines as well. This can be left for another time after the engine is installed to check.

Other than this these engines are pretty well bulletproof if the oil was changed as it should be and total accumulated miles are not too great. Make sure you have valve lash clearances on any cylinder that checks low before condeming the engine..

Many owners where not even aware the valve lash is not automatically dealt with. As it is in cars with hydralic lifters. Valve lash was recommended to be at least manually checked every fifteen thousand miles. So some of these engines developed basically no lash with no maintenance and the valves do not close well enough to get a proper compression reading.

petecooke · #5 10-06-2011, 07:00 PM

Barry,

Thanks for the info.
I will test this tomorrow.
I don't think I damaged anything so far because the starter gear engages but since I used jumper cables, it wasn't getting enough juice to turn the engine.

I have regular metal feeler gauges that I've used to adjust the valves. Hopefully Advanced Auto sells a plastigauge.

Do you know what the clearance should be?

Thx again.

pete

tangofox007 · #6 10-06-2011, 08:42 PM

Quote:

Originally Posted by petecooke

I didn't touch the large connection closer to the top because nothing happened when I did.

Now there's a description that would make W. J. Clinton proud!!!

leathermang · #7 10-06-2011, 09:24 PM

LOL

barry123400 · #8 10-07-2011, 12:05 AM

The clearance check of the first rod bearing can wait until just prior to installation. It is just precautionary in nature. You do not have to actually set the valves for the compression check. Just make sure there is some clearance still there on any lobes for a given cylinder if the compression check is lower than normal.

vstech · #9 10-07-2011, 02:02 PM

Quote:

Originally Posted by petecooke

Thx for the info. I can easily pull the red positive cable from the car and connect it to the battery and the starter. The negative cable in the car will be a little more difficult.

Is the top connection on the starter the positive and the bottom negative?

I assume the little screw contact just makes the connection(completes the circuit) so the starter engages.

Is this correct?

Thanks greatly for your input.

pete

ok, the bolt closest to the starter motor has a wire on it going to the starter itself.
the other large bolt gets positive power.
the negative needs to attach to one of the starter mounting bolts or the block itself. you need to use at LEAST #2 copper wire for this test, as the starter will be turning a LOT of mass.
the little screw energizes the motor, and engages the bendix onto the flywheel to actually turn the engine over.

leathermang · #10 10-07-2011, 03:52 PM

I would suggest that it is proper to set the valves before doing a compression check....
' Virtual setting ' not allowed....

47dodge · #11 10-07-2011, 05:04 PM

virtual setting is fine to just see if there is reasonable compression. why waste time just to find a bad cylinder. Of course once ready to use the engine a proper setting should be done. I find that on an engine that the valves have not been adjusted regularly that the first setting will change in a few thousand miles. Then another adjustment should be done, then normal intervals after that.

leathermang · #12 10-07-2011, 05:15 PM

47dodge, I can go with that....
but I have a couple of questions....
Please describe ' virtual setting' and tell me if that is another word for ' halfassed setting ' ?
How much longer does it take to do a regular valve lash setting as compared to the 'one you hope will be good enough' to determine if the cylinders have enough compression to mess with fixing the engine ?

vstech · #13 10-07-2011, 08:04 PM

Quote:

Originally Posted by 47dodge

virtual setting is fine to just see if there is reasonable compression. why waste time just to find a bad cylinder.

what is a "virtual setting" of the valves?

leathermang · #14 10-07-2011, 08:20 PM

John, I think it is ' skip the setting and hope it will be ok enough to show which cylinder is so far off that I can make an uninformed decision'...LOL

47dodge · #15 10-07-2011, 09:14 PM

As Leathermag coined the term "virtual adjustment" I have just stuck with it.

it's just make sure there is clearance. can be done buy just checking to see if the rocker arms can wiggle. This is not a replacement for proper adjustment just a quick way to check for basic clearance. Then do a compression check to see if the engine has any hope before taking a lot more time. Now if the engine is a known engine that was good when pulled, sure take the time to adjust the valves first. Of course if it were known to be good then a compression check would not be needed.