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odd electrical fault


Beast5680
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My 300tdi ES auto 98my has a curious electrical fault thats started today when you apply the brakes the rear window demist light on the demist switch comes on, with no brakes on everything works as normal, now i suspect a chaffed wire somewhere feeding the switch but having been out to drop off daughter i stopped at her friends went to switch off the engine and it wouldnt turn off and the check engine light was glowing dimly, reinserting the key and switching it back to on i removed my foot from the brake and switched off as normal, are there any common chafe points i should be looking at?

the fault only seems to occur when the brake pedal is pressed and i cant get it to replicate the not switching the engine off part.

any suggestions welcome

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i would say possibly an earth check all earth connections especially where thay connect on the chassis engine etc take off clean and replace cover with grease to keep water out earth probs can cause many probs chris

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having disconnected the rear screen element and the switch and the relay i,m pretty sure it must be getting a short across from the brake wire that feeds the lights as the rear screen wire(according to the diagram i have) goes from the switch to the element, does anyone know where the loom runs front to back

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Got wiered electrical error in my 200TDi aswell.When i had doors closed the interiorlights was glowing like it was like 7-8 volts and the summer that warns me if i leave the car with lighs on started,when i opened the doors the glowed less like the powere was reduced to half of that.

In my case it was the fuse to the interiorlights that was blown an somewhere in the wiring it was a shared circuit that caused this odd behavior.

So the theory about earth problem mentioned earlier i the topic seems like a possible cause to check in my opinion aswell

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This would be typical of a poor earth connection - had similar problems in the past and the sort of problems caused by poor earths never ceases to amaze me .

Sometimes the connection may appear tight etc but the problem is being caused by ' internal ' corrosion which you can't see . You have to loosen them off and clean them up .

I will try to look up on the cd where there are earth points which could cause your problem .

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The heated rear screen and the RH rear light cluster share an earth .

There is a connector under the panel in the rear of the luggage compartment on the RH side with the black earth wires going to it .

Hope this helps .

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well i,ve found the communal earth inside the back quarter and seperated all the wires out so now nothing is linked together and the rear screen is disconnected from earth and positive and ....it still makes the wire live when the brake is pressed, is there any other communal wiring points? whats the multifunction unit ? i keep see it referred to in the haynes book

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The MFU is behind the RH side of the fascia on the fascia fuse box .

Had a quick look at diagrams and while it appears in the circuit for the heated rear window it doesn't appear in the stop lamp circuit .

It would appear that whatever is supplied by fuses 1 , 12 , 13 and 14 involves the MFU but not sure of this .

Will have a better look when I have time and try to get my head around it .

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well after pulling the car to bits today and tracing the wires i,ve found the fault some weeks ago i replaced both the rear door hinges, unbeknown to me the wires for the rear demist and the high level brake light run right next to the upper door hinge and i didnt see them when fitting the hinge and managed to trap them together between the hinge and the door when i bolted it in although not earthing they had become joined hence the rear demist coming on when the brake was applied,

will learn from that mistake

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