MIL light on after disconecting the battery

[mk1rs]

hi
my MIL light has come on after disconecting the battery to weld a small patch on the exhaust
i reconected the battery and the management light stays on and it wont rev over 2500 rpm
any ideas how to fix this prob ?

thanx steve

[steve_earwig]

Gas or arc?

[mk1rs]

mig

[steve_earwig]

Oh. What you need to do is buy a time machine and one of these, go back to a time before you fried your ecu, don't disconnect the battery this time, put the surge protector across the terminals instead. Better still, take the exhaust off, or at least unbolt is so it's not directly connected to your engine.

You may be lucky but you need to get your fault codes read.

[mjb]

Wouldn't just unplugging the ECU and BSI be adequate? Sure there's other things that could blow, but that'd cover the (most) expensive bits...

Oooooor, pulling the connectors off the battery and connecting them together with a jubilee clip should prevent spikes shouldn't it?

[steve_earwig]

Only in about the same way leaving it connected would. MIG/arc welding is about low voltage, high current, but strikes can generate spikes of many thousands of volts that don't necessarily travel in stright lines. The bulk will go straight to earth of course but some of it finds different paths, like when something's struck by lightening, you wouldn't want to stand too close.

How many ECU's would you need to unplug? BSI, engine ECU, airbag ECU, abs ECU, air con ECU... radio?? Anything with a diode in it??!

[Nicodemus]

Anything with a high input impedence Steve.

It's easy to forget that most modern electrickery stuff is still CMOS based and consequently has a high i/p impedence which will go totally doodleally if subjected to high current pulses even at low voltages just as you say.

I remember years ago (1981'ish) being shown electron micro photos of the blown inputs of some specialised A to D chips from an underwater vehicle project I was working on which needed 3.3kv at 350 to 400 kva 60 Hz at the top end of the co-ax tow cable to start the electric motors for the drybollix pumps on the main vehicle - in early development it blasted those chips (which were mondo expensive) even though they were well on the down side of the telemetry demultiplexer...

It all supports your argument of course - any kind of lekky welding with its' high current is likely to blast the car electronics with induced emf's...

[mjb]

induced emf's...

Duh! I completely forgot about that aspect! I was thinking along the lines of tie the + to the - and prevent any sudden high voltage attached to the body from creating a high voltage over the electronics supplies... I think I deserve a big FAIL for that suggestion ;)

[mk1rs]

right then guys
got the code reader and got
p0380 and p0100

so i took the maf of and look what i found

maf.jpg

so i guess the maff sensor is goosed and a new set og heater plugs needed ?

cheers steve

[mbell666]

you don't need the plugs, for some reason almost all 406 hdi's have the p0380 fault logged, mined did. Its pretty meaningless and can be ignored!