Help- Jump leads wrong way! audi a4


Registered User
Feel like a tool and i fear the worst.

The last few days i have been having battery problems.

every morning ive had to jump it.

jumped it this morning with new leads i bought.

these new leads have a red handle on all prongs so when i was connecting negative to my audi i saw the red handle and assumed stupidly connected it to negative instead.

my friend started donor battery and i was in my car and started mine after a 10 seconds of leads connected.
i started to see smoke and immediately shouted for him to turn off engine.

the leads have melted.

tried it again with leads connected right way and nothing!!!

when i try and start my engine now one dash light (traction) comes on and there is a faint ticking noise.

i have a 2003 audi a4 1.9 td.

i am scared ive fried ecu or more!!!!

can anyone tell me if there is a safety fuse on my ecu on this model or what sort of damage i can expect?
also where i can look to see extent of damage?

getting new battery tomorrow and hoping it will work.



Registered User
just checked fuse box. dont think i have one for ecu unless its somewhere else.
all fuses i have are intact tho.
i checked the fuse that runs from the battery and it is still ok as well. maybe good news?

when i turn ignition there are now more lights, however some flicker on and off then eventually most go off.


shifting to Hyperspace... .
VCDS Map User
Fuses generally do not blow if you get the battery polarity the wrong way round as per jump lead mis-connection.

Get a new battery, if it starts then you half way there.
Get a VCDS scan. clear the DTC errors - drive a mile or so to clear ESP light etc. and then check with a re-scan.
Anything blown may show up - there are a lot of black boxes besides the Engine ECU on an A4/B6.


Registered User
check the big fuse (60amp or sumurt) under the engine ecu box cover on the o/s next to the servo/brake master. this is the main supply fuse to the fuse box.


Registered User
do i need to unscrew the ecu cover? or is it located beneath.
i shall go have a look now.

if this fuse is intact does that mean there will be no damage to ecu and other electronics?

new battery wont be here til the morning now.


shifting to Hyperspace... .
VCDS Map User
First check the 150A main supply fuse on the battery positive terminal - shown below highlighted red.......

This 150A fuse ,ay be blown if you connected the negative jump lead to the load side of the fuse (nut connection top left of picture) - easy to visually check.
If you connected the negative jump lead straight to the positive terminal then probably not blown.

With regards to fuses in Plenum chamber (for Diesel engine) which houses the engine ECU, this lot is mounted below the ECU but accessed from the top under the cover, (you will need to unclip the ECU to see below it)..........


[A] S282 "Engine Electronics Fuse" is rated at 15 Amp.
The large fuse in the plenum chamber, S39 "Glow Plugs" is rated at 80 Amp.
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Registered User
Thanks a lot mate, great help.

The 150 Amp main fuse is intact.

i started taking off the ecu cover, although need to wait til i get a socket wrench to remove window wiper bolt so i can remove cover.

do i need to remove ecu case from the ecu to see fuse box or should it be apparent when i remove cover?

I i tried to jump the car again.
once i turn ignition on with donar car jumping me, all of the dash lights came on as normal, as soon as i turn the engine though it dies out and after i try ignition again, only some dash lights appear(battery symbol and esp flashing).

if ecu was fried then would these lights work?
will get tools and new battery tomorrow and see what the craic is.



Well-Known Member
VCDS Map User
id wait till you get the new batt
and if it dont start then start to worry about damage it could have caused
you may be lucky


shifting to Hyperspace... .
VCDS Map User
It's obvious when you take the plenum chamber cover off - the ECU is in a sealed metal enclosure but "clipped in" with a metal strap. Release the strap and you can get below to see. Assuming you are Manual gearbox? (If Auto then transmission ECU also in this location under engine ECU.)

Engine ECU will communicate with Instrument cluster to put up relevant warning lights on/off.

In my 3.0 V6 I have around 12 main ECU's (5 sub ECU's) for the relevant systems in the car which communicate with each other,
  • [01] Engine
  • [02] Auto Trans (Tiptronic Gearbox)
  • [03] ABS Brakes
  • [08] Auto HVAC (Airconditioning)
  • [09] Central Electronics
  • [15] Airbags
  • [16] Steering Wheel
  • [17] Instruments
  • [45] Interior Monitor (US Alarm sensor)
  • [46] Central Convenience Module + 4 door modules + Alarm Horn [H12]
  • [56] Radio
  • [77] Bluetooth
So there is a lot of potential for damage - but I am sure Audi have put module protection into these units as it is a pretty easy mistake to make connecting jump leads up the wrong way. Using a simple Diode costing a few pence in the supply feed of the module would protect it for incorrect polarity connection.

Lets take a fresh look at what you have with a new battery.


Registered User

Managed to get a new battery in her!

good news and bad news.

good news = She starts fine!

pretty much everything works perfect.

Bad news = The cd player doesn't work at all and the Brake lights are constantly lit, even when key is out of the ignition.

I checked the fuses on the side of the dash although everything still seems intact.

Im hoping the cd player will be due to perhaps the 'radio' relay being blown in the ecu.

But no idea about the brake lights?

i had to take out brake light fuse to ensure my battery doesn't drain overnight.

Going to take a look at relays in ecu tomorrow.


Well-Known Member
VCDS Map User
check the brake swich above the brake pedal 10 min job

im sure theres a fuse at the rear of the radio


shifting to Hyperspace... .
VCDS Map User
Look at the obvious stuff like fuses but do not waste your time trying to diagnose the fault without first getting a VCDS scan - if anything obvious is blown it will show instantly on the scan - including brake light switch!


Registered User
There is a fuse on the back of the radio next to the ISO socket.


Registered User
as said, theres a 10 amp protection fuse on the back side of most stereos.

dont know why the brake lights would be constantly on tho. hmmmmmm.


Registered User
Dropped the car off in a friends garage to get checked.

Id imagine they will do a scan and find the faults and hopefully it wont be too much damage.

Will keep you's updated on progress and solution.

to be honest just happy i didn't fry her too bad!

easy going Si

Registered User
Richie have you also checked your alternator putting 24 volts across a 12 volt system might not have fried the ecu but I am pretty sure the diode pack/rectifier or regulator at the back of the alternator may of blown get your mechanic to check the alternator to check the charging current sorry to add to your woes


Registered User
wasnt this thread over 3 years ago? lol surely fixed by now

easy going Si

Registered User
lol lads see what you mean was trying to find if anyone had altered the battery lead on an A4 and stumbled across this thread I didn't even think to look at the date I am sure it wasn't such a happy ending at least the back end of the alternator would of been fried and possible the ECU at least a few hundred squids for parts alone


Fire up the Quattro!
VCDS Map User
He wouldn't of put 24v across the system if he put the jump leads on the wrong way round. The battery would have been in parallel (albeit with incorrect polarity) increasing the Ah. not in series format, thus increasing the voltage.

I'm guessing it was not a great outcome, or cheap at least.

easy going Si

Registered User
S4_Dan can't of been cheap lol and your right it is 12 volts I humbly apologise

would that much current have fried any of cells in the battery or made the jump leads either glow red or possible catch fire

electrical experiments by Mr Photovolteric on you tube brick in the washing machine is a good one