TheYellowLandy

Member

• Jul 10, 2022

• #1

Hi all,

Went to move my SIII 109 diesel, and [stupidly] touched the leads onto the battery terminals the wrong way round. Obviously there was a loud bang and some sparks, and I quickly took the leads off. However, since this incident there is now a dead short across the positive and negative leads, with no current and a resistance of 99 Ohms on a multimeter readout across the 2 leads with the battery disconnected, The battery itself is fine, as are all 4 fuses, but there is now a battery-draining short circuit which prevents the vehicle from starting. The ignition was off at the time if that helps.

How much damage have I done? What do I need to check?

 

ExMil109

Active Member

• Jul 10, 2022

• #2

Ouch!
There's not much in a diesel Series 3 that's polarity sensitive, but the alternator is! Likely you've killed the rectifier pack in the alternator. You can confirm by pulling the plug out of the alternator and checking if the short is still there.
You might be able to repair it, but it might be time for a new one...

 

P38 Monkey

Well-Known Member

• Jul 10, 2022

• #3

+1 on likely damage to the rectifier / regulator in the alternator...
You say there is a dead short across the leads, but then you say that the resistance measurement is 99 Ω.
99 Ω would indicate a very low discharge rate (~1.5W), not a dead short?
As you are talking about diodes in the circuit, you might get a different resistance reading if you connect the meter the other way round.
Anyway, first step is to disconnect the alternator as above...

 

N

nickjaxe

Well-Known Member

• Jul 14, 2022

• #4

P38 Monkey said:

+1 on likely damage to the rectifier / regulator in the alternator...
You say there is a dead short across the leads, but then you say that the resistance measurement is 99 Ω.
99 Ω would indicate a very low discharge rate (~1.5W), not a dead short?
As you are talking about diodes in the circuit, you might get a different resistance reading if you connect the meter the other way round.
Anyway, first step is to disconnect the alternator as above...

Click to expand...

Alternator +2.....anything electronic ie radio if switched on or maybe its permanently on.....and other electronic gadgets.

 

TheYellowLandy

Member

• Jul 18, 2022

• #5

Thanks everyone, I have disconnected the alternator, the engine now starts and everything works perfectly. The multimeter shows an open circuit with the battery terminals disconnected. I'll have a look at fixing the alternator, if not then I'll have to lighten my wallet

 

N

nickjaxe

Well-Known Member

• Jul 18, 2022

• #6

Thank your lucky stars you dont have a more modern car with an ECU ect ect.....you could now have a bill exceeding £1000.

Exactly the same will happen if you connect jump lead back to front......£££££££££££££££££££££££££££

 

R

rob1miles

Well-Known Member

• Jul 25, 2022

• #7

Its odds on only the regulator/diode pack and these can be got a kit for under £20 and its straight forward job to replace.