Fan belt stuck on

[Deleted User]

Hi,
I have a Mercedes C saloon 2015 and it has been brilliant up to Tuesday morning when the battery died.
Mercedes roadside technician fitted a replacement battery but since then the car's fan belt has been kicking in inappropriately, most often when the car is locked and stationary.   
Is this a known "thing" to any of you?  

I'm not happy with the dealership insisting the malfunction was not triggered by their roadside technician but also asking for more chargeable time to diagnose the fault, as the potential is there for costs to escalate very rapidly. I have already paid £200 for a new battery and £170 for their observations at the garage; now they are asking for another £340 to cover the cost of a second report "to start with."

Any advice gratefully received,
ZPZ

AdrianC

Not the "fan belt"... that just goes round when the engine does, and can't possibly otherwise.
I presume you mean the electric radiator cooling fan?

One thing's for sure, it's very unlikely that changing the battery caused the fault. It's far more likely that you simply didn't notice the fan kicking in before... until the battery died...

Modern cooling fans are controlled by the engine's ECU. It might just be a temperature sensor fault, but it might be something within the ECU. Given that it's an intermittent fault, it's a lot harder to diagnose. Yes, of course you're going to have to pay for their time... I presume this is a Mercedes main dealer? So very, VERY premium labour rates. £170 could easily be just one hour's labour - ~£140/hr + VAT wouldn't be at all unusual for a premium-brand dealership.

If t'were me? I'd be finding a good local Merc specialist or a generic auto-electrician.

Lomast

A fan belt can only move with the engine running as it is driven by a pulley on the engine. What I suspect you mean is the electric fan is kicking in, these can run with the engine switched off (normal after a long run when the engine is hot).
I suspect an electrical gremlin somewhere  shichnis going to cost some money to get diagnosed  but I would be looking for an independent auto electrican rather than a main dealer unless it is still under warranty 

Lomast

Just too add its more likely the fault happened first and killed your battery rather than the other way round

[Deleted User]

That does make sense, thank you both.
Yes, you surmise correctly; out of warranty, main dealer charges, full service history. I have used a good local Merc specialist too.

[Deleted User]

Car is still at the dealership with fan cutting in intermittently.
Workshop asking for a minimum of a further eight hours diagnostic to check the communication of the linbus system. "To start."
As a complete non-technician, I supposed diagnostic examination would identify the fault. The workshop have had the car since last Tuesday. They put in a new power train control unit but not solved the phantom running fan problem. I'm not going to authorise until after I speak to the independent Merc specialist on Monday morning - (part of the problem has been the tardiness of the dealership emails, and my guy was rushing away for the week-end) - but eight hours strikes me as super excessive. 

A quandary for me. Feeling burned.

Lomast

Unfortunately as cars get more complicated finding electrical gremlins can be both long winded and expensive

AdrianC

ZingPowZing said:

Car is still at the dealership with fan cutting in intermittently.
Workshop asking for a minimum of a further eight hours diagnostic to check the communication of the linbus system. "To start."
As a complete non-technician, I supposed diagnostic examination would identify the fault. The workshop have had the car since last Tuesday. They put in a new power train control unit but not solved the phantom running fan problem. I'm not going to authorise until after I speak to the independent Merc specialist on Monday morning - (part of the problem has been the tardiness of the dealership emails, and my guy was rushing away for the week-end) - but eight hours strikes me as super excessive. 

CanBus, not "linbus"
Basically, it's a computer network that runs all the electronics in your car. The "powertrain control unit" is the computer that runs the engine and ancillaries.

If there's a problem with the computer network, then perhaps the diagnostics that are generated by the computer network aren't always going to be accurate...? B'sides, they'll say where the ECU thinks the issue is, but not what the cause is. Damaged wiring? Corroded connections? Sensors that aren't saying what they should? Water ingress somewhere?

SteveJW

Remember the good old days, just wire in a manual switch
Guess if you tried this nowadays it would throw up all sort of faults and go into limp mode

[Deleted User]

Two weeks on, no further forward with the garage who sold this car to me 5 years ago.

Raising a complaint, must be first to Mercedes but where do I go after Mercedes HO back up their dealer?
I don't want to go their preferred route, which is the Motor Ombudsman so, is Trading Standards better than the Small Claims Court, please?

The dealership have had the car 3weeks and tell me it has been in a semi-stripped state with exposed wiring - parked in the open - for at least a week.  Should I pay RAC or AA to report on my car where it is, iyo?