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Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 21881
Experience:  BEng hons Mech engineering, in auto industry 22 years
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C 220: i have a 1995 mercedes c with a battery drain

Customer Question

i have a 1995 mercedes c with a battery drain
Submitted: 2 years ago.
Category: UK Car
Expert:  Matt replied 2 years ago.
I'd first check the easy things like is the boot light or any other interior light staying on then check that the main battery connections are tight and in good condition also check the earth strap to the engine / gearbox.
Next I'd check the current drain aspect, the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable.
A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.
Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..
Also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.
if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement
And one last thing is – how old is the battery? – As even good quality ones need replacing every 5 years and faulty battery just won’t hold a charge
Customer: replied 2 years ago.
with multimeter conected, ive pulled fuse 10 instument cluster/hazard warning flasher and the battery drain drops. now what
Customer: replied 2 years ago.
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Expert:  Matt replied 2 years ago.
OK the most likley culprit on that circuit is going to be a fused bulb so I'd replace the fuse and see if any of the turn signal lamps are slow to flash or seem dim - then replace the bulb as required and clean up the contacts where the bulb touches
Customer: replied 2 years ago.
had the bulbs out and replaced them, some were dim, cleaned the contacts, replaced the fuse and still the same,
Expert:  Matt replied 2 years ago.
OK in that case there shoudl be a 'flasher unit' that is located in the relay socket , try replacing that as the next step
Customer: replied 2 years ago.
where do i find that
Expert:  Matt replied 2 years ago.
the one is the 13-pin 'Multi-Purpose Relay/Combination Relay'(###) ###-####that control the turn signal
Customer: replied 2 years ago.
the only relays i have is a 12 pin 201 821 00 47 or a 20 pin 202 542 02 32
Expert:  Matt replied 2 years ago.
OK I'd suggest replacing the 12 pin item in that case
Customer: replied 2 years ago.
will be here sometime monday
Expert:  Matt replied 2 years ago.
OK good luck