Strange solution to a problem

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- Baker9
  
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posted
18 years ago

Mon, Apr 10, 2006 7:31 PM

My 02 Vaux Astra 1.7 diesel put on the engine warning light on friday. The light was flashing on and off but with no real pattern. The engine was running ok, power was as usual and I didn't have time to get it looked at. On saturday I went to my local one man and his son garage, met the son as I was walking into the garage and explained the problem to him. He has served his time in a main dealer ship on Renaults and Nissans and is now an RAC patrol man. The first thing he said to me was are your brake lights working, so I said what the @"** has this to to do with the brake lights. We checked, both the low level lights were out but the high level was on. We put two bulbs in the brake lights and the warning light went out. I asked him why the light came on and his reply was the majority of people would not be bothered about a brake light not working, but if the engine warning light comes on they will look about it. VWs also do this. The idea is now if engine warning light comes on, check the brake lights are working before you go to the garage and avoid looking like a prat

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posted
18 years ago

Mon, Apr 10, 2006 7:36 PM

very common fault, its the first thing we check when a customer comes in with a warning light fault.

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- www.fuelsaving.info
  
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posted
18 years ago

Wed, Apr 12, 2006 5:12 PM

AFAIK, the real reason is completely different (though that's an imaginative answer!) Modern diesels are all "fly by wire" with no mechanical connection between the accelerator pedal and the engine. So if something goes wrong with the electrics or the ECU, you can potentially (under exceptionally unlikely circumstances) end up with the engine delivering full power while the poor driver stands on the brakes in an attempt to stop the car.

To prevent this, the brake switch is also wired to the ECU - if the brake is pressed, then the maximum engine output is severely limited. (It also disables cruise control, on cars where this is fitted.) But what if the brake switch is also faulty? Easy, you have two independent switches, typically one "normally on" and other "normally off". Both feed into the ECU; one also works the brake lights.

Crucially, if the two switches don't agree then something must be wrong, and so the engine warning light comes on. My 1998 Golf diesel did exactly this. And the OP is absolutely correct, the brake lights are a very sensible (if non-obvious!) first thing to check if you have an engine warning light.

Tony