I forgot to mention the engine size in my last thread, so I am reposting for any additional hints. I have checked out the ignition with a volt meter and ohm meter. Everything appears to check out okay. I just tried a few of the hints with no success. This is really getting frustrating becuase I am not sure which direction to go next. If anyone can help please let me know.

Thanks
Dana Desko

so what is not happening?
no fuel or no spark?

george

Turn the key on, do you hear the fuel pump for a second, try it again. With the key on and the engine off move the throttle, does a pair of injectors click twenty times. Any amount indicates the fuel system is functioning. These two tests without tools will give you a basic idea if both items are functioning. You really need to check fuel pressure, but if the above tests seem OK then you should go for ignition. A test light placed on the coil negative should flash if the engine is cranked as the coil is loaded and fired.

Points were a common problem... they don't wear out they just get fouled with the oil vapors slightly frying with the small current used to trigger the transistor switching box. Drag a dollar bill through the points while holding them closed.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

280se-man

i had a similar problem this past summer.[1972 350sl] what i would suggest before looking for the big bad problem is that you check the basics.

i would look at the ignition spark at the plug. ground the plug then have someone turn the engine over. you would be looking for a brite well lit blue spark. if that looks a bit weak start tracking the primary ignition components. resistors, primary wire, to coil and after coil. start connecting block pins #50, 16 etc: make sure all the wire is clean and with good connection. also the wire to the pos side of your battery. if any of the plastic insulation is missing replace that wire. here's a picture of my conn block . the phillips head screws would not come out. the loose connections you see is how they were when i started looking for what caused my 4.5 to quit running. it's really a wonder it had run at all. look at the coil to dist connection. that wire gets heat over the engine block. check the dist. points to see if they are clean and that they open when the dist is rotated. my 350sl had a switching box under the fender. if you have ignition the box is working.

the fuel is relatively easy, turn the key on and listen for the pump. it will pump shortly then go off. if you put a pressure gage on the rail you will only get pressure for a short time. it seems the pump stops unless it senses the engine running. when it runs you should see i think like 28 to maybe 32 pounds constant. if you have the cd i think it gives you a method for testing the injectors for firing. wet plugs are a sign of no electrical juice to fire the plugs.

after this then you can start looking for a bigger problem.

good luck. i am curious what you will find as your problem.

Attached Thumbnails

 

__________________
Thanks Much!
Craig

1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
See My Cars at:http://mysite.verizon.net/res0aytj/index.html

Pound it to fit then Paint it to match!

There is only First Place and Varying degrees of last!

Old age and deceit will overcome Youth and Enthusiasm every time!

Putting the square peg in the round hole is not hard... IF you do it fast enough!

Old enough to know better but stupid enough to do it anyway!

I pulled the coil lead from the distributor and held it close to the engine block while someone cranked the engine, no spark. I purchased a new coil last week when I didn't get a spark from the old one. I do know the new one is funtional because I had it tested before paying for it. Other than the switch gear, what else should I look for. I checked all the ground wires and connections and could find no broken wires or loose connections. In my previous posting I indicated that I parked the car and went out of town for a few weeks. When I got back, I started the engine and it ran rough with absolutely no power when shifted into gear. It was then I decided to replace the districutor cap, rotor and points. I replaced the plugs and wires earlier and the car ran fine. I even reinstalled the old components to see if any of the new ones were defective. No luck.

Thanks for the feed back so far.
Dana Desko
1972 280SE 4.5

Steve, I really like your dollar bill trick and of course you are one of the several exulted heroes of this fourm ,but wouln't a hundred dollar bill be better for a classic like his 72 280 SE? ..................
William Rogers..............

so, does the new coil work ?
mr brotherton gave you good advice. have you tried
anything yet?
got compression? how about the power wire at
the battery for the computer. thats the seperate wire
at the posative clamp.

george

Getting fuel through the system...The fuel pump and injectors activate when the ignition is turned on and the throttle is moved.

Didn't try the dollar bill trick after checking to see that no spark cam from the new coil after grounding the coil wire from the distributor. I will try the test light to see how or if the coil is firing. It is a new coil that was tested before purchase.

The wire for the computer is properly attached to the battery terminal...couldn't visually see any problems..but will test with ohm meter to see if its okay.

I orginally bought the car in Florida from a car collector. My wife named the car Mabel. It was love at first site...the car that is. I have always been a Mercedes fan since high school.

Attached Thumbnails

 

280se-man

not sure what effects it has but your original coil type is for external resistors. did you get that type of coil? if you didn't then you have twice the resistance than you should.

what i was talking about previous was any and i mean any primary wire that has anything to do with engine starting having signs of corroding insulation falling off should be replaced. testing just a primary wire tells nothing because you don't have any scales to measure resistance by. but... take several wires and connections adding the sum of their resistance can lead to a weak ignition.

Now the ignition switching box. you said you grounded out the hi voltage coil wire and had no spark. is that also true if you pull a plug leaving the plug wire on and then grounding the plug. spark??? no spark would mean your switch Ignition control unit may not be doing the job. but before going there re-evaluate your primary side and the new coil and the procedure used on the coil's secondary wire going to the dist. did you let it arc or jump the gap. obviously if you got a spark on that test go to a plug wire thru a plug and see if there is any spark. the rotor has a 5k ohm resistor imbedded so you really need a good primary and secondary system.

if you have done all the above and feel that the ignition componets are operating ok then you got to look at the ICU box. but that is like 500 bucks. i'd be sure it wasn't something simple first.

post back so we can see how you progress
tks
craig

__________________
Thanks Much!
Craig

1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
See My Cars at:http://mysite.verizon.net/res0aytj/index.html

Pound it to fit then Paint it to match!

There is only First Place and Varying degrees of last!

Old age and deceit will overcome Youth and Enthusiasm every time!

Putting the square peg in the round hole is not hard... IF you do it fast enough!

Old enough to know better but stupid enough to do it anyway!

Gorgeous car!

You probably need to check to make sure the points are opening and are conducting, first -- connect your ohm meter to the green wire going to the distributor and check for continuity sitting and cranking. Must show a connection to ground when points are closed and on/off pulses when rotating. If not, points either aren't opening or are dirty enough to fail to conduct. Cost about $8.

From here, I'm going to give you the factory training manual tests:

Voltage test:

block points open with something non-conductive. Check voltage to ground at both coil connections and both resistors. Should have battery voltage at all points. If you have voltage on one side but not the other of any component, it's broken, replace. If you have something other than battery voltage, unplug the transistor unit and recheck -- if you now have battery voltage, switching unit is bad (in which case, a Crane or Pertronix system is both better and cheaper than fixing the factory unit!)

Points closed testing:

Attach red lead of voltmeter to terminal 7 of junction block (green wire). Attach black lead to distributor housing. Ignition on. Less than 0.1V, points good. More than 0.1V, clean or replace points.

Attach black lead to ground, check voltate at terminals 15 (resistor side) and 1 (switching unit side) on coil. Voltage must be 8.2 to 9.0 on termnial 15, 4.8 to 6.3 on terminal 1. If not as specified, check resistance of coil and both resistors. Coil should be .38 to .43 ohm, 0.4 ohm resistor .35 to .43 ohm, 0.6 ohm resistor .55 to .65 ohm.

Voltage drop of switching unit:

Attach black lead to ground, red lead to 0.6 ohm resistor connection going to swtiching unit. Voltage must be less than 2.5 V. If higher, switching unit bad.

It is possible to wire the points directly to the coil for troubleshooting. unplug the swtiching unit, then connect a wire from terminal 7 to the side of the 0.6 ohm resistor that goes to the coil with a 1 ohm resistor in series. This replaces both resistors with a single 1 ohm resistance and uses the points to switch the coil.

If it runs with the points connected directly, you must either replace the switching unint (if you can find one), repair it -- it's usually the big switching transistor that's gone, or get a breakerless unit (Crane and Pertronix both sell one that fits).

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks for the assistance. I will go through all these checks this weekend and get back with what I found.

Dana Desko

Just finally had a chance to check everything. Here are the results:

I tried grounding the spark plug and coil wire...no spark. I check each wire for resistance to see if maybe there was a short. No broken wires.

stevebfl suggested a test light on the coil negative. The light did come on with the ignition. However, the light did not pulse when turning over the engine. Even tried the dollar bill trick, no luck.

afmcorp suggested verifying the new coil. I matched the part numbers for both the new and old coil, identical.

psfred gace me a list of items to check.
1. I was able to measure battery voltage at both coil and resistor connections. I did this with the switching unit connected. Do I need to unhook the switching unit and check this again?

2. When performing the points closing test, all voltages checked find. The only exception is that I got battery voltage reading when checking between terminal 7 (switching unit) and the distributor housing. I did the test with two different sets of points and got the same result.

3. When checking for voltage drops from ground to the 0.6 ohm resistor, the voltage was less than 2.5V. I am assuming the switch unit checked okay.

Is it possible for the engine to be out of time and not spark? I have never seen this in other cars I worked on that were out of time. I was always able to get a spark.

Any further advise would be helpful. I can't quit scratching my head on this. It still won't start...no spark.

Thanks so far
Dana Desko

Dana:

If you don't have flickering at the coil 12V connection, the points aren't working or the switching unit is shot.

Check to make sure the points go to ground when closed, and that the condenser is disconnected -- it can sometimes fail and cause the point circuit to "bounce" so badly the transistor unit doesn't work.

If any of the voltages specified for checking the switchbox are out of spec, it's shot.

did you try hooking up the points "simple" and testing?

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks for the quick response Peter

I used a regular test light, not a timing light if that is what I should have used and had no pulsation from the coil. I also closed the points and checked the voltage. The reading was equal to the battery voltage. I am assuming this is what you mean by the points going to ground? I already had the condenser disconnected.

With the points closed and the ignition on, I checked the voltage at coil terminals 15 and 1. The readings were within the ranges you posted previously. Also both resistors checked out okay as well for both continuity and voltage.

I also checked the voltage drop as you described by placing the black lead to ground and the red lead to the .6 ohm resistor connection to the switching unit. With the ignition on, the reading was less than 2.5V (about 1.7V). I did notice the resistor getting hot.

I was not able to hook up the points "simple" and test. I have to get a 1 ohm resistor.

I hope this clarifies things,
Dana