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Has your cars battery been dying overnight?

You may have a parasitic battery drain! Recently at The Automotive Clinic we had David’s pride and joy come for a visit, His 1999 MGF convertible. This little car has been immaculately looked after over its life. Despite the perfect maintenance history the vehicle had one fault for us to get to the bottom off, a battery that would die overnight ! This vehicle had (as usual) been to a few other shops before hand, the car has had the radio replaced with an aftermarket unit, a new cigarette lighter and the last workshop was looking at a replacement engine control module.

Before we dive into the problem with this vehicle lets first explain what a parasitic battery drain is.

As you turn your vehicle off and close all the doors, all electrical components are supposed to turn off (go to sleep) and the current getting drawn from the battery should go down to an acceptable amount. Most older vehicles should draw around 20 milliamps when the vehicle is completely asleep, newer modern vehicles with an abundance of control modules will draw up to 50 milliamps. This sleep procedure that should happen when the vehicle turns off could take anywhere from 10-45 minutes, this is why parasitic battery drain testing can be such a long procedure.

Back to our MGF

Our initial step was to open all doors, roof, boot and bonnet and latch all locks, we then proceeded to leave the vehicle for 30 minutes to allow everything to fall asleep, once this procedure was completed we then came back to the vehicle and checked our battery draw with our Picoscope. Our battery draw while the vehicle was sleeping was at around 140milliamps, way too high !

After confirming that we definitely have a issue with the vehicle, we then proceded to volt drop test all fuses in the vehicle, while this is time consuming this is the only reliable way to check for current flow in a modern vehicle, pulling the fuses in newer vehicles DOES NOT work as it will cause you to wake modules up inadvertently. After voltage drop testing, all fuses the only fuse we had any current flow on was the 100A battery fuse in the under hood fuse box.

Our Findings

When tracing this wire back this wire goes straight t back to our starter motor Battery positive post, from there that post branches off to multiple places, relay blocks, alternator, fuse box in the rear etc. fortunately we were able to confirm via use of our amp clamp that the current draw was going to the vehicle’s alternator.

All this testing while it may sound like a long process has now eliminated absolutely all electrical components in the vehicle that are not linked to the alternator battery positive wire.  This dis proves all previous repairs and assumptions.

Now as we got up to the vehicles alternator we were able to voltage drop test from the cable lug to the stud of the alternator, while the connection did look ok we had a large voltage drop over the small amount of space ! There is our issue! After removing this connection and cleaning and re installing miraculously our battery drain had gone down to a perfect 28 milliamps.

Test don’t Guess

This case study proves why logical, non intrusive, well informed diagnostics always gets to the underlying issue and provides us with the ability to provide our customers with a reliable first time fix !

Have your cars fault correctly diagnosed at The Automotive Clinic

The Automotive Clinic are Melbourne’s fault diagnostic specialists, if you are wanting to have your vehicles fault diagnosed by our specialised team, book an appointment at The Automotive Clinic.

Battery drain test
Extended capture for testing battery drains over a long period
MGF diagnosed at The Automotive Clinic