Any Automotive Electricians on board?

[Elmer BeFuddled]

SWMBO was returning from work one night last week in her S reg. VW Polo when (I quote) "it was like the battery n everything died, the clock reset and battery light came on". Then she drove home OK.

So I go out a couple of days later, check battery connections, looking good n clean. Yet to find where the earth connects to the body though! (Oh for the good old days when you could stand in the engine compartment and shelter under the bonnet to do stuff!!). Alternator has two small leads (dash indication?) and a larger cable which I suspect is the output/charge cable. That's a bit rusty and has some green corrosion (verdigris) on it.

Has anyone come across anything like this before and has any pointers as to what to look for? Just got a new Haynes as the old one is on a bench beside the A1, so I can delve a bit deeper.

I had something similar happen to me many years ago in an old Beetle when all the power just disappeared from the electrical system. Mind you, that was when a lightning strike hit the ground less than 25yds away! To say I shat me pants would be an understatement!!

TIA for any help.

 

[telectrix]

meter ( ohms ) between batt.- and bodywork. usually the culprit. bad ground.

 

[Elmer BeFuddled]

Cheers, just gotta find the earth point! That'll be tomorrow now, best stay sober!

 

[Robotstar5]

Check the battery to starter motor cable carefully, I had a mini that shorted the exhaust to the battery cable as the engine moved under load, same symptoms, everything died so engine moved back and power restored!

 

[Elmer BeFuddled]

Another helpful tip Robot*5! Thanks.

 

[ajelec10]

meter ( ohms ) between batt.- and bodywork. usually the culprit. bad ground.

This will only tell you if you have continuity and will not check the condition of your earth connection. (1 strand could be making connection but when put under any load it will disconnect) a great way to test faulty earth connection is to use a volt meter set to read 12v dc one probe on batt terminal and the other to the point you want to test. ( so if confirming the engine earth connect to engine) then switch inhibition on and start car this will draw the full cranking amps from the battery and through the earth straps. If the volt meter reads a voltage while doing this there is a potential difference accross the two points indicating a bad connection. Hope this helps it takes a bit to get your head around!

 

[ruston]

Rather than faff on looking for the earth,just make a new one up and link it to body( from batt neg) then engine. Check your main feeds into the fuse box as this is always a problem on older cars. Although not consistent with all your symptoms check the ignition switch too for excessive wear, and the condition of the contacts at the bottom.

 

[chrish_1977]

I wouldnt even bother trying to find the earth, modern vehicle are earthed at various points around the vehicle depending on what is required from the wiring, it sounds more like a loose connection, possibly where the wiring goes from between the engine bay bulkhead and the inside of the vehicle. A loose connection at the fusebox, loose multiplug. or something else, need more information eg what speed, was she turning at the time (which way), braking?, was the battery under full load (headlights on, heating on, HRW on etc). Try to recreate the fault first before you do anything else, that's the first thing I would always ask the customer when I was a mechanic, if you cant recreate the fault, its like looking for a needle in a haystack unless its a common problem to that make and model vehicle.

 

[telectrix]

think the answer lies in the fact that's it's a polo. the problem is the hole.

 

[chrish_1977]

ha ha ha

 

[jon small]

it's most likely a bad earth connection to body or a corroded connection on the starter solenoid / main power lead but also worth checking the multiple earth connector daisy plates (like a washer with about 10 spade connections and a bolt in the middle that goes into a welded nut) usually on inner wings behind headlamps and inside cabin above foot well trims and back from base of dashboard, unbolt daisy plates and sand paint off metal around bolt hole and back of daisies - bolt back on and cover any bare metal with petroleum jelly, most earth connections rely on the bolt rather than the touching faces for a good connection which is okay until bolt corrodes then it's sandpaper time!

that's the usual culprits, if that don't work join rac and let them do the hard work- Jon ( ex auto spark )

 

[ste mac]

According to Autodata there is an "ignition auxiliary circuits relay" which could cause loss of power to certain circuits ie: fuses 4,7,19,22,23,24,25 & 30. For more info I would need the model, engine and year & I might be able to email relevant pages to you. If your alternator has 3 wires 1 will be for the rev counter - red/black, 1 for the battery light - blue & 1 large black cable for alternator output (diesel engine?). There also appears to be a 110A fuse between the battery and alternator.

 

[Elmer BeFuddled]

Thanks for all the hints n tips. Bit of a late feedback but....

Bloody thing wouldn't start today (nice tidy little motor really, 81 on clock), went and did all the clean ups etc. but still no joy. Anyway, to cut a long story short (thanks Spandau Ballet!) bit the bullet and got the new battery that I should've bought before. Hopefully that will be it.

Why on earth did they hide the main earth to body under a plug to the lights that's a pig to remove 'cos the battery tray rips chunks out of your fingers?

On a brighter note it is parked on my locals car park so sustenance was never further than ten paces away!!

So, touch wood that'll sort it, if not, I'll be back!!

 

[Brendan976]

If the battery change does not cure it and it runs flat again check the voltage that the alternator puts into the battery 13.8. - 14.2v or thereabouts is good.
Ben