1996 w140 s500 no spark

[janorman]

Hi,
Ten months ago my s500 went into Limp Home mode. Since then I have had MB diagnose the faults twice with no result, I have replaced the throttle body twice,the second unit having been rebuilt,and I have replaced the Crankshaft position sensor. At each stage the car starts and runs for a few minutes before returning to LH mode. The latest stage, second throttle body, results in the engine not firing at all and,on cranking the motor there are zero revs. recorded on the tacho.I should also mention that I have installed a new fuel pump and filter. Any advice would be welcome. When she runs she's a beautiful beast. I do not want to scrap her.

 

[carat 3.6]

What condition is the engine harness in?

 

[grober]

Do you have an EZL ignition unit?
K6JRF Auto Page

 

[janorman]

What condition is the engine harness in?

Hi Carat 3.6
Among the actions not included in my query is that I removed and checked the engine bay loom thoroughly. All ok.

 

[carat 3.6]

Is it 100% a lack of spark, or it just wont run?

 

[janorman]

Do you have an EZL ignition unit?
K6JRF Auto Page

Hi, Graeme,
Time to demonstrate my ignorance.I've looked up EZL and whilst I think I understand it I could not say if I have such a unit. One of the minor problems I have is that since the car broke I have been waiting for an operation on my back and am relying on my really good, but idiomatically challenged mechanic. He's from Budapest,I'm from London and we live in Spain. Communication is interesting if sometimes frustrating.
I have, in addition to two attempts to diagnose faults with the local MB workshops, acquired various obd1and obd11 testers and obd1 to 11 connectors. My car,built in November 1995 has a 38 pin obd port but only uses 15 of the sockets. Nobody seems able to reccommend a suitable testing kit or to have the relevant fault code lists. sorry,I've gotten carried away. If you can tell me where to check for an EZL unit I will.
Another point I should make is that although built in '95 the engine is a 119980 unit.

 

[janorman]

Is it 100% a lack of spark, or it just wont run?

!00% lack of spark

 

[neilrr]

Your car will have an EZL.

If you looked up 'EZL' you'll know what they look like. Look on the L side of the engine bay. Might be under covers.

 

[grober]

try this section/
K6JRF Auto Page

The EZL ignition units are usually attached to the inner wing somewhere which acts as a heat sink for its electronics. After many years the heatsink compound between unit and wing dries out allowing the the electronics to overheat and eventually fail. First symptoms are the unit is OK on start up followed by partial failure as it heats up [ your symptoms??] Eventually the unit will fail completely.
it will look something like this.

 

[janorman]

try this section/
K6JRF Auto Page

The EZL ignition units are usually attached to the inner wing somewhere which acts as a heat sink for its electronics. After many years the heatsink compound between unit and wing dries out allowing the the electronics to overheat and eventually fail. First symptoms are the unit is OK on start up followed by partial failure as it heats up [ your symptoms??] Eventually the unit will fail completely.
it will look something like this.

Thanks for the info. and pic. Graeme, I will hunt this down although I suspect it will not be easy to test it. Perhaps it will not be beyond the scope of our local Merc. mechanics. I'll let you know how I get on.
regards
John

 

[neilrr]

To test, unplug each ignition lead (at the spark plug end) in turn. Be careful not to pull them apart then stick an old spark in the end & crank the engine.

A spark = good, no spark = problem with your EZL.

You can do this yourself, just don't shock yourself.

 

[grober]

To test, unplug each ignition lead (at the spark plug end) in turn. Be careful not to pull them apart then stick an old spark in the end & crank the engine.

A spark = good, no spark = problem with your EZL.

You can do this yourself, just don't shock yourself.

Make sure the body of the "test " spark plug sitting out in the open is earthed to the engine as it would be if actually screwed into the cylinder head. Often its enough just to rest it on part of the engine casing--- but if not ---wrap a bit of wire with the insulation stripped of round the plug threads and attach the other end to a suitable engine earth point, Remember you want the EZL warm before the test will show the fault.

 

[optimusprime]

I have read this some place .It said it was the rev counter thats not working and the computer thinks it stationary.Just keep it in mind and do all your checks first

 

[grober]

I have read this some place .It said it was the rev counter thats not working and the computer thinks it stationary.Just keep it in mind and do all your checks first

There is a facility on the fuel pump relay circuit which is two stage. Stage one runs at ignition switch on. Stage two requires an engine running signal from the crank sensor or rev counter to signify the engine is running continuously. Without this signal stage two of the relay stops supplying fuel to the engine This is a safety feature to prevent fire. If the fuel pump relay is faulty the engine wont rget fuel and will stop.