W124 Crank Sensor change

[dsan]

morning all,

I've been having strange gremlins with the 300TE recently. On cold start, she performs as you'd expect. Drive it for a bit and then try and start her again and it's a no go. Engine will crank but no spark. Leave it for 15 mins (annoying when you are in a tesco car park) and finally she will start with your foot planted deeply into the floor when turning the key on.

After a big trawl through this site, it seems the CPS sensor (L5) is a likely culprit. I've had a look at the various W124 repair manuals but was wondering from the more experience here, how easy a job is it to perform?

Cheers for any help in advance

 

[carat 3.6]

The cps sensor is very easy to change on these, it is located above the flywheel, next to the oil filter. It should come with a long cable, the other end of which plugs into the ignition module.

 

[grober]

If its a petrol engined car ??? is more likely to be the EZL ignition module itself- its mounted on the wing - the heat sink material between it and the wing dries out causing it to overheat. It might ?? just be saved by renewing the heatsink paste - cheap and available from any good electronics shop http://www.maplin.co.uk/high-thermal-conductivity-heatsink-compound-33695 but probably on its way out. They are getting increasingly rare and expensive to replace. One way to test would be to use a bit of non conducting freezer spray on it when the fault occurs http://www.maplin.co.uk/freezer-spray-26296 if it goes away temporarily you have your culprit!
other possibility would be a dodgy ignition coil- again overheating

 

[dsan]

Thanks Graeme,

So it happened again just now. Driving fine until you hit traffic and sit, in which case the temp goes up to 95 and the idle to 1000. When you put your foot down, there's hesitation, almost like it's misfiring. When you eventually do pull off, there is a stutter and then it goes. Once it gets back to normal idle speed, everything is 100%

most random gremlin ever.

Will try the heatsink paste, good idea

 

[dsan]

so driving to one of the suppliers today, the engine temp went up to 100 and the revs crept up. once this happened, the car ran as lumpy as hell

http://hmmm.co.za/merc/IMG_1896.MOV

Give it a minute or so and the revs crept back to what they should be and all is fine in W124 land.

http://hmmm.co.za/merc/IMG_1897.MOV


Headed off to Maplins, car ran fine all the way there. purchased some heatsink and some electrical contact cleaner. proceeded to drive away, alas the cranking like mad but zero spark issue.

sat in the parking lot for 20 mins (people don't have look at you like a perv or something) and then tried again. She started first time.

got home, took off the EZL and whilst there was a good amount of white paste already, i cleaned all the contacts and re-applied the new stuff. Once back, she started up fine.

I'm heading into town now, so will give it a good check. fingers crossed

 

[dsan]

so driving to one of the suppliers today, the engine temp went up to 100 and the revs crept up. once this happened, the car ran as lumpy as hell

http://hmmm.co.za/merc/IMG_1896.MOV

Give it a minute or so and the revs crept back to what they should be and all is fine in W124 land.

http://hmmm.co.za/merc/IMG_1897.MOV


Headed off to Maplins, car ran fine all the way there. purchased some heatsink and some electrical contact cleaner. proceeded to drive away, alas the cranking like mad but zero spark issue.

sat in the parking lot for 20 mins (people don't half look at you like a perv or something) and then tried again. She started first time.

got home, took off the EZL and whilst there was a good amount of white paste already, i cleaned all the contacts and re-applied the new stuff. Once back, she started up fine.

I'm heading into town now, so will give it a good check. fingers crossed

 

[ray_hennig]

so driving to one of the suppliers today, the engine temp went up to 100 and the revs crept up. once this happened, the car ran as lumpy as hell

http://hmmm.co.za/merc/IMG_1896.MOV

Give it a minute or so and the revs crept back to what they should be and all is fine in W124 land.

http://hmmm.co.za/merc/IMG_1897.MOV


Headed off to Maplins, car ran fine all the way there. purchased some heatsink and some electrical contact cleaner. proceeded to drive away, alas the cranking like mad but zero spark issue.

sat in the parking lot for 20 mins (people don't half look at you like a perv or something) and then tried again. She started first time.

got home, took off the EZL and whilst there was a good amount of white paste already, i cleaned all the contacts and re-applied the new stuff. Once back, she started up fine.

I'm heading into town now, so will give it a good check. fingers crossed

I doubt if it's the EZL but, for your sake, I hope you've fixed it.

Look at the distributor cap and, with your symptoms, the rotor. Make sure they are original MB (or Bosch or Beru as second best) and in fine condition.

Basically, make sure your HT is at its best, with the right plugs.

Bon courage.

RayH

 

[dsan]

Thanks Ray.

It happened again, outside the restaurant
I'll pull them both tomorrow, it does feel like it's a spark issue. On the way home (20 miles) she was perfectly smooth without any issue.

the joys of older cars I tell you

 

[paultredgett]

take the crank angle sensor out (10 minutes job) and if it is covered in crap then clean it off the magnet and stick it back in....let me know!

 

[grober]

When I suggested the EZL I wasn't necessarily ruling out the CPS it's just less common on these early cars than the later V6s. Renewing the heatsink compound is a long shot- the damage may be done unfortunately. It's not wasted tho as if its not the EZL failing renewing the heatsink compound will help EZL longevity. Try the freezer spray test on the ezl and the coil when the fault occurs.

 

[dsan]

Paul

apologies for the n00b question, but what's the best way to take the sensor out? from under the car or above?

 

[219]

I was going to mention the dizzy cap , but Ray beat me to it . If it hasn't been changed within the last 30 K , chances are it will be past its best . I have had two M103 cars and one M104 which have all suffered from this ; also make sure spark plugs and ignition leads are in good condition since these can cause problems too .

Another M103 gremlin is a little vacuum breather hose located under the inlet manifold , these sometimes perish and allow air to leak in ( or even split and come off at which point you get no vacuum so engine will not run ) . Cheap to replace and worth doing for peace of mind .

Your problem does sound ignition related , though .

 

[paultredgett]

Paul

apologies for the n00b question, but what's the best way to take the sensor out? from under the car or above?

I have to be honest, I haven't had the privilege of cleaning it on the merc as yet as she seems to be running fine but I have a history of buying Fords with the same problems and it is nearly always the crank sensor to blame due to metal debris sticking to the magnet and sending weak or sporadic pulses back to the ecu.
I brought an 04 Ford mondeo a few months ago...full leather etc etc etc, 85k miles with full mot etc etc, it started fine and revved lovely up to 3000 revs and then always just shut itself down..dead! but would start up again straight away!?
The guy had a new clutch fitted at Ford along with flywheel and starter motor etc (1100 quid!) and it wasnt until a few days later it started playing up, he took it back there and they could not find the fault as hard as they tried.
I gave him £650 for it ,limped home, removed the crank sensor which was covered in old clutch/flywheel debris and hey presto back up for sale.
Renaults are also another good one to watch out for on ebay!

cheers

 

[kth286]

crank sensors usually work or do not work, there is no half way house.

They go out of spec when get hot and lose the signal, and engine stops.

When allowed to cool down again; come back into spec, and car will start.

Do look at the dist cap as condensation can make car splutter. Wipe inside
as a start, and try again.

 

[paultredgett]

crank angle sensors may do that but they are magnetic and tend to get crap from around the flywheel stuck to them which prevents a correct signal being sent to the ecu, the ecu says ....hmmm that is not right, lets try and adjust (hunting and stalling etc) and if it is bad then it just does the limp mode thing.

The cam sensor is the one that gets hot as it is sat on top of the camshaft, it is this one that usually breaks down thru heat damage....the crank angle sensor is not really exposed to much heat as it only just sits above the flywheel etc to pick up a magnetic trace up to tell the ecu the engine is in time with itself.

Paul.

 

[kth286]

crank angle sensors may do that but they are magnetic and tend to get crap from around the flywheel stuck to them which prevents a correct signal being sent to the ecu, the ecu says ....hmmm that is not right, lets try and adjust (hunting and stalling etc) and if it is bad then it just does the limp mode thing.

The cam sensor is the one that gets hot as it is sat on top of the camshaft, it is this one that usually breaks down thru heat damage....the crank angle sensor is not really exposed to much heat as it only just sits above the flywheel etc to pick up a magnetic trace up to tell the ecu the engine is in time with itself.

Paul.

The crank sensor on that car is the number one sensor, it tells the ECU the engine is turning over.

There is NIL adjusting for the ECU to do, as the crank sensor just sends a signal to ECU thats all. If there is no signal the whole system closes down.

There will be no spluttering etc - it just closes down.

So you see it cannot be the crank sensor at fault.