CPS issue- 2003 W220 500L- FRUSTRATED

davidbownes

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Hi all,
Hoping someone may know something to help.
Car showing signs of CPS failure so had this replaced. No change so checked it again and had replaced again. Car seems slightly worse, revs dip while braking towards traffic lights like car is stalling and the cranking takes as long.
Dealer telling me that wire must be broken and need to replace the wiring harness at £££.
Is it possible to replace the CPS cable in insolation without needing do the whole harness? I checked and didn't see anyone say these fail...
Also, could it be something else I am missing- secondary air system failure or something?
Any help or thoughts appreciated...
Thanks,
DB
 

Frontstep

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Sounds like an easy job for an auto electrican.
Or any competent mechanic for that matter.
 

WG M-B

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Have you have the codes read? Are there any codes?
 
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davidbownes

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Yes there is the code for CPS failure (forget the number). Garage says that the entire engine wiring loom needs replaced- is the CPS cable a simple case of unplugging one and plugging in another?
 

WG M-B

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It's like this......
The cps will not cause a communication problem, ie the engine fail to crank and fan to run full speed. This is a communication problem related to the engine ecu.
Is it cranking now all the time? Is is logging a permanent fault code with the cps???
 
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davidbownes

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It cranks ok, usually as normal but sometimes with a delay, but the fault code seems permanent. The occassional slow crank when warm doesn't worry me too much, it is the fall in revs when I brake which almost causes a stall.

What do you think the best course of action is?
 

Philedge

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I'm no expert on this but CPS failure or partial failure shouldn't cause slow cranking.

As I understand it when the CPS starts to fail the symptoms are engine cutting out when hot and failure to start when hot. If the engine is left to cool it restarts until such time as the CPS has fully packed up.

If you've got a wiring fault on the CPS so the signal from the CPS to the ECU is lost or intermittent then the symptoms would be the same but not affected by engine temp in the same way.

I don't see how a faulty CPS or CPS wiring will affect cranking speed??
 

Philedge

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Just tried to edit the post below but failed dismally!!

It may be that the slow cranking doesn't allow the CPS to pick up a good enough signal for the ECU to register the engine as turning. In that case its a cranking issue rather than a CPS issue.

If the engine cranks slowly for no reason I'd get that looked into as it could be something that is about to fail- if you cant turn the engine over any CPS issues wont be of much concern to you!!!
 

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