R129 SL500 2000 cranks but no start

[rocketfixer]

My R129 has been running well but has begun failing to start on several occasions. Now won’t start.
Starter cranks engine well, and I can hear pump running. Jumped fuel pump relay and good fuel supply at rail test port.
Im assuming I have an ignition issue. Immobiliser / alarm seems to work correctly, althogh boot/trunk can be slow to unlock.
Any help and ideas would be helpful. I have scanned the internet and owners forums , but can’t nail down how the ignition side works ??
HELP please

 

[Wearsafoxhat]

Hi, perhaps a camshaft sensor or maybe the crank position sensor though the later usually breaks down when warm..

however far better to get some diagnostics on it and just replace the defective item, rather than guess and change and spend more than necessary..

Sent from my iPad using Tapatalk

 

[alexanderfoti]

Does it still have the main dealer fitted imobiliser? (with the square fob in addition to the flip out key)

 

[rocketfixer]

Thanks very much for the info.
I have removed a spark plug and confirmed no spark. I have reseated many engine sensor plugs that I could get access to. Also gained access to what I believe is the engine manage unit... all looks clean and no contamination.
Is it possible to do a continuity check on cam sensor .?
My fob has flip out key and 3 buttons.. no additional square fob, if that’s what is being asked. The alarm sets and upsets. Doors locks work ok but a very slightly slow.

 

[rocketfixer]

I have an OBD code reader , the rectangular type. Is it possible to use a circular adapter on the SL to get codes. Never needed it befor.
Is there any way to confirm if I have an immobiliser fitted ?
many many thanks in advance.

 

[Wearsafoxhat]

Hi, I don’t think the OBD reader, even with an adaptor will work on an R129.
The OBD is only 16 pin so would expect the lead to loose 50% of the info from the 32 pin, and think that the technology is different too.
Not sure about expected reading of a good or bad cam sensor either, sorry not much help to you with this..

 

[gassed up]

If you can't access diagnostic info via the diagnostic port, I've got star and I struggle, then you need to know whether you have an injector signal. The sequence is: do you have a spark, yes/no? Do you have an injector signal, yes/no? Are you getting reference voltage at other sensors e.g. water temp? No voltage in the system means an ECU problem. No injector signal suggests no CPS signal. The crux is if you have an injector signal and no spark then the issue is in the ignition.

 

[LostKiwi]

As this is an M113 engine it's almost certainly a crank position sensor but diagnostics will confirm.
Symptoms are cranks, no spark and fuel pump will only run briefly to prime the system.

Ignition turned on will prime the fuel system. The pump will then stop.
Cranking will then provide the signal from the CPS to tell the ECU the engine is turning which will then tell the ECU when to inject fuel, when to provide spark and that it's safe to turn on the fuel pump. No crank position sensor signal and none of these things happen. No injection, no spark, no fuel pump. Only use genuine (Bosch) sensors. Probably the most reliable of a bad bunch but still a common problem.

On the 38 pin diagnostic port each pin is a different ECU or control module and an external multiplexer between diagnostic and car determines which module to query.
OBD2 works using a comms line that talks to an onboard multiplexer and tells it which module to query.

Totally different systems.

 

[rocketfixer]

Hi, I don’t think the OBD reader, even with an adaptor will work on an R129.
The OBD is only 16 pin so would expect the lead to loose 50% of the info from the 32 pin, and think that the technology is different too.
Not sure about expected reading of a good or bad cam sensor either, sorry not much help to you with this..

Thanks for coming back with that info. It’s confirmed my thoughts. I wonder if there are any reasonable priced bits of kit out there that can read info from these cars ??

 

[rocketfixer]

If you can't access diagnostic info via the diagnostic port, I've got star and I struggle, then you need to know whether you have an injector signal. The sequence is: do you have a spark, yes/no? Do you have an injector signal, yes/no? Are you getting reference voltage at other sensors e.g. water temp? No voltage in the system means an ECU problem. No injector signal suggests no CPS signal. The crux is if you have an injector signal and no spark then the issue is in the ignition.

Cheers very much for replying. I’m pretty good at troubleshooting, but it’s tricky without diagrams and basic info.
Would you have any idea of rough voltages/ current these various sensors and injectors work with. I have jumped the pump relay so I know I’m getting continuous fuel supply. I would like to put a meter across an injector plug to check for signal. I’m definitely not getting a spark...but don’t know the logic.. if no spark, does ecu signal ‘no fuel inject trigger’..?

 

[rocketfixer]

As this is an M113 engine it's almost certainly a crank position sensor but diagnostics will confirm.
Symptoms are cranks, no spark and fuel pump will only run briefly to prime the system.

Ignition turned on will prime the fuel system. The pump will then stop.
Cranking will then provide the signal from the CPS to tell the ECU the engine is turning which will then tell the ECU when to inject fuel, when to provide spark and that it's safe to turn on the fuel pump. No crank position sensor signal and none of these things happen. No injection, no spark, no fuel pump. Only use genuine (Bosch) sensors. Probably the most reliable of a bad bunch but still a common problem.

On the 38 pin diagnostic port each pin is a different ECU or control module and an external multiplexer between diagnostic and car determines which module to query.
OBD2 works using a comms line that talks to an onboard multiplexer and tells it which module to query.

Totally different systems.

Thanks Lost Kiwi, really helpful.
Does this engine have a cam position sensor, and Crank position sensor. I can see the cam sensor. Have you an Idea of the different functions they each trigger ? I checked the 3 pins on cam sensor and have continuity between top two pins (which would connect to pink center pin and brown/red middle pin).

What do you think about this - I don’t drive the car much, lay it up in winter. But it always would start ok. It was about a year ago it just spun , but wouldnt start. Then started after a few weeks. Then would be ok for months. Happened a couple more times over the months, but would start, maybe next day. This time it started and I moved the car,, went back and wouldn’t start... just cranking.?. What would cause this intermittent problem... ie hasn’t been a Hard failure. My thought is a loose or dirty pin,, or dry solder joint. I actually don’t want it to start so that I can do a permanent fix.
Im going to manually turn the engine a few degrees... flat spot on crank sensor .?
thanks in advance.

 

[LostKiwi]

Crank sensor sits on the bell housing just behind the passenger side bank. It's held in with a Torx bolt (either E8 or E10 from memory). Undo the bolt and if you're lucky it will pull straight out. They can stick and be a barsteward too though...
It picks up from the flywheel so forget flat spots etc. Normally they fail by working when cold then fail when hot and work again cold etc but that's not a given.

They're about £50 if you hunt round.

 

[rocketfixer]

That’s really helpful info about crank sensor,, and sounds like a good place to start, especially as I have an intermittent fault. Is it possible to do ops check on it ?
Cheers

 

[LostKiwi]

Code reader is about the only way.

 

[brandwooddixon]

I've had similar symptoms in the past.
On my old E430, it turned out to be a fob battery (car unlocked but wouldn't start only turn over).
On my W215 it was a camshaft sensor.
I did have a crankshaft sensor issue once on the E430, but it would start when cold.

 

[alexanderfoti]

BTW the 38pin to OBD adapter will allow the engine to be read by a standard code reader. I have had to take some apart and correct their pinouts in the past but they do work.

 

[LostKiwi]

BTW the 38pin to OBD adapter will allow the engine to be read by a standard code reader. I have had to take some apart and correct their pinouts in the past but they do work.

Only if the adaptor is connected to pin 4 (usual one) and if the reader talks the correct protocol. It's a gamble as many adapter/generic reader combinations don't work.

 

[alexanderfoti]

Yes lots of the adapters are wrong but any elm327 reader will usually talk to them.

 

[rocketfixer]

Gents. These posts are really giving me a lot of positive vibes.
Im going to investigate sourcing another code reader.. or any other bit of kit to decipher the problem. I don’t fully understand, but sounds like pin 4 needs to be utilised in the adapter/ code reader. The company I work for maintain there own fleet of Mercedes trucks.. and has Star. I can not get the car there, but perhaps I could get access to wiring diagrams ?? Not sure though.
Going to take a good look at the cam, crank sensors.
Can anyone give me a clue of ascertaining if I have an immobiliser.. it’s a mystery .
Thanks

 

[rocketfixer]

I've had similar symptoms in the past.
On my old E430, it turned out to be a fob battery (car unlocked but wouldn't start only turn over).
On my W215 it was a camshaft sensor.
I did have a crankshaft sensor issue once on the E430, but it would start when cold.

Great info. Sounds like the sensors can be a problem.. they operate in a harsh environment. Do you know if they can be checked ?
I will also check fob batteries.. but I have had this problem when fob batts were new ?