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91 560sel - it cranks but no start

9K views 30 replies 7 participants last post by  banyan 
#1 ·
I narrowed down the possible cause on the ignition circuit. There is no voltage at the coil when the ignition key is on start and engine is cranking. Before pointing to the ignition switch as the cause, I wanted to know if there is a fuse that might be in beetween? The schematics are not clear that I can see. Any advise would be appreciated. This is a daily car. It was working one day. Parked it and tried to start it the following day and it would not start.
 
#2 ·
PM me your number and I'll walk you through it. I have the same car. 2 actually.
 
#3 ·
The coil receives the impulses from the EZL, is not connected to the ignition switch. Using the diagram check if the EZL receives voltage. If yes, check the crankshaft position sensor (CPS is connected to the EZL and the sensor is located behind the engine, driver side). If the EZL does not see the signal from the CPS, it will not fire the coil. There have been sensors that we know failed after so many years, I would not be surprised to hear the sensor is actually the cause of your problems.
 
#5 ·
Removed cable from EZL and measured the resistance of the crankshaft position sensor (see pic in red arrow) - assumed this is the CPS as the cable leads towards the back of the engine. It measured 0.830K Ohms. Is this an indication of a bad Ignition control module (EZL)? or do I need to measure voltage to confirm?
 

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#6 · (Edited)
Yes, that is the one. It appears your sensor measures okay. Have a look here: http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/2686426-crankshaft-position-sensor.html

You can measure its voltage as you crank the engine (w/o cranking the engine you should not see any voltage). I don't remember the exact voltage, but I think it was above 1volt.

If that sensor supplies voltage to its round plug, then I would connect it and then I would go downstream:
1) I would make sure that the coil receives voltage at it primary coil terminals (under the black plastic cover) when the engine is cranked. If yes, the EZL functions. I would go to next step.
2) I would disconnect the high voltage wire from the distributor, connect a spark plug to it and make sure that the plug is also grounded (if is not, the EZL may fry up). Are sparks showing up while the engine is cranking? If yes, then the coil is good and I would go to next step.
3) Reconnect the high voltage wire back to the distributor. Disconnect one of the spark plugs from any cylinder. Connect your spark plug to that wire and make sure the plug is grounded. Crank the engine. Are sparks showing up? If yes, you are receiving high voltage to that cylinder. I am assuming that all plugs are receiving high voltage, so your engine should fire.

It could be either the coil, or the distributor (the rotator arm or the spring loaded "brush"). I do not remember the coil's resistance values, but they can go bad too. You should also measure the coil resistance (primary and secondary coils). Before you do that disconnect the wires at one primary terminal of the coil so you get correct readings.
 
#8 ·
Glad to hear you got it running. However, I would suggest still check the coil and the conductivity throughout the rest of the circuit downstream of the EZL and the grounding because an interrupted conductivity can cause an EZL to go bad. In other words make sure that you don't have something wrong that caused your EZL to go bad and that "something" is still there.
 
#10 ·
liviu165,
The has been working fine until a couple of days ago when it just stalled. I had to have it towed. Today I had the chance to work on it. It started right up and stayed at idle for about 5 minutes then dies and would not restart. I measured the CPS resistance to be 930 ohms which I think is good. I then measured the voltage accross the primary of the coil while cranking and it was zero. It appears the EZL has failed again. The coil's primary resistance is .7 ohms with the EZL connected. Assuming the coil is good, what other areas would be suspect to cause the EZL to fail again? Grounding is good as well.
 
#13 ·
Assuming the coil is good, what other areas would be suspect to cause the EZL to fail again? Grounding is good as well.
From others on this forum I recall that a danger to the EZL could be incorrect discharge of the high voltage. For example incorrect plug gap + resistor plugs + resistor wires + worn out distributor rotor.

Other causes could be additional work, like the example with the radio installation. Correct heat dissipation is also important.
 
#11 ·
This happened to my old car 2-3 times. A high end stereo was installed improperly, causing the stereo to short out to chassis. You could feel a slight zap when touching things such as the door ends. The EZL got zapped as a result.

Also the EZL depends on cooling and the proper application of heatsink paste, so if you installed with the old paste, that will not keep the unit cool.
 
#12 ·
I would go bare metal where the EZL sits and a nice even coat (no air bubbles) of a quality paste to help with heat dissipation. Our cars/parts are old and the layer of paint at that point is counterproductive since the inner part of the wheel well is the heatsink.
 
#15 ·
To be honest, I don't think in his case the problem is heat dissipation. In fact the EZLs had heat sink paste applied directly on their back, and then a plastic film. Therefore when installed over the inner fender the paste was still not coming into direct contact with the paint because of the film applied. That is why after taking the EZL off my car there is no paste on the car's paint. My car did not have an EZL problem and as you can see the paste never touched the paint directly. However, now is reinstalled with paste directly touching the paint.

I am pretty confident he is having a different problem, some kind of electrical discharge on the EZL. Better heat sink will not hurt, but I don't think will solve the problem either. The electrical diagram should offer some clues.
 

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#16 ·
Interesting.

When I pulled the EZL off my 88 420SEL, there was a nice thick smudge of the stuff on the fender. I've also pulled a couple in yards and both of them left residue on the fender.

That's why I recommended the computer processor paste. It's what I used when I put it back on.
 
#17 ·
I build aquarium lights for friends and family with multiple 50W, 100W and 150W high power LEDs using either 6" or 8" aluminum sinks and fans. Each bulb takes about 1-2 grams of paste on metal to metal application. I'm guessing an EZL it would be easily 8-10 grams with no film or paint barrier in between.

I'm not saying this is the OP's problem, just commenting that today's electronic heat dissipation has advanced from what was understood in the 80s.
 
#19 ·
I tested the suspect EZL into anot her 560 with running game EZL and it w I uldated not start so I am concluding that the EZL has gone south.

My next steps then is to determine which offending components might be causing the EZL to fail. These are the ones downstream that I can think of. I think I should replace the EZL along with the components that might be causing the EZL to fail
1 coil
2 spark plug wires
3. Rotor
4. Dist cap
5. Spark plugs

Any other possible parts? These are tuneup parts that have not been replaced since 2008 - about 80000 miles ago.

At this point I am ruling out the CPS as possible cause.

Thoughts?
 
#21 ·
Two more things to keep in mind (not saying that they are the problem), but you never know:
- Under the ABS unit there is one more ground connection. I can't remember what is for, could be the ABS, but you may want to check that too,
- At the coil terminals you probably have 4 wires (two on each terminal). Second picture shows a bunch of wires where the coil should have been. Also please notice one of them is a ground. Two of those wires are coming from the EZL. Where are the other two going? You may want to study the diagram, those could be the RPM indicator?

Very good catch GreenT! I do remember that ground connection caused problems before and is not hard to get loose and dirty.
 
#30 ·
- At the coil terminals you probably have 4 wires (two on each terminal). Second picture shows a bunch of wires where the coil should have been. Also please notice one of them is a ground. Two of those wires are coming from the EZL. Where are the other two going? You may want to study the diagram, those could be the RPM indicator?
The other 2 wires are going to X11 Diagnostic socket, according to the MBZ diagram
 
#23 ·
replaced EZL, spark plugs, rotor, cap and ignition coil. checked all grounds. car is back on the road. there is one thing that did not go away. It still idles rough for a few seconds during start up. no other issues so far after about a week. let me know if there are other checks that I need to do to make sure the EZL does not fail due to other components. Is there a chance that the CPS might be dirty or about to fail, resulting into the rough idle at start up? Resistance of CPS is within specs, cold or warm. How about the camshaft position sensor? Is this also a component in the ignition circuit?
 
#26 ·
I had the same problem with a 380sl I was working on. I guess what you are calling the ezl modual is what I was calling the ignition modual. A small sq box located on the left fender of the engine bay. Anyway couldn't find test procedures for the modual itself but I did come across something pretty hilarious. It said to test all related components and if they were good then replace the modual lol. I really got a kick out of that but I guess that's alldata for you.
 
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