Discharging battery

[pandjack]

Apologies for duplication, but, whilst the below is the latest update in my previous thread (R400 K series misfire), I've narrowed it down somewhat :

 

New alternator fitted & the battery indicator on the Stack is showing 12.5v with the engine idling on a re-charged battery (not the usual 14-14.5v), this then gradually drops over a 20-30 min blat to 11.9v, &....return of misfire on sudden throttle inputs. Thinking the previously spikey alternator outputs have fried the battery I then invest in a brand new battery & the same thing happens - after a 20-30 min blat the voltage drops off & the misfire returns!!!

 

I've checked the integrity of the thick cable from alternator to starter solenoid & all OK, also fitted an extra earth cable from the battery negative to the chassis & still no improvement.

 

Scraped through Harewood at the weekend (actually managed a PB!!) by charging/swapping batteries between runs but I really need to get to the bottom of what's stopping the alternator from charging the battery at the normal 14-14.5v.

 

Any further pointers (or recommendations of a good auto-electrician in the York area in lieu of my numpty understanding of the black art!!) would be VERY much appreciated........

[tbird]

what sort of resistance have you got from Chassis so -ve battery (with new lead fitted) to Engine

 

and have you checked the Alternator earth is clean and shiny

follow current path all the way from Alternator to battery cleaning each connection

follow earth path back from Alternator to battery cleaning each connection

 

If new alternator and new battery the only thing left is leads/connections.

 

I wouldn't rely on stack, what voltage have you got at the Alternator output,

then what voltage have you got at the battery.

 

Tim

[pandjack]

Thanks for the quick reply Tim - I concur, the only things left are cables & connections but I've checked/cleaned all the obvious ones when replacing the alternator.

 

I've not checked resistances etc yet & although I have a multimeter I'm not electro-savvy so therefore haven't got a clue what the numbers should be anyway....??

[tbird]

for voltage checks

 

with battery connected and engine running, meter set up to a range that can read 12vdc

If you check voltage between alternator output, +ve lead and battery earth -ve lead , might need two people for safety one, on each lead (mech safety more than electrical)

make a note

now check voltage directly across battery terminals the readings should be within .2 volts or so

just after start I would expect IRO 14.3v to 14.8v

 

 

in terms of resistance I would be looking a lowest Ohms range on meter and resistances under 1 ohm between alternator connections and battery connector. as you say you are inexperienced I would make resistance measurements with battery

disconnected

if resistance look ok reconnect, run engine until fault appears then disconnect and re check resistances see whats changed

 

BTW

Do not spin alternator with battery disconnected or stop engine by lifting battery terminals, may kill alternator

 

 

 

Tim

 

 

 

Edited by - tbird on 8 Jul 2013 20:52:12

[Paul Deslandes]

Just realised you've switched threads

 

Try the following:

 

That's really bad news and sorry to hear that you've laid out on a new alternator and battery, only to have the same symptoms.sad smiley

 

It would appear that the alternator is not charging the battery and the engine is running on the battery until the voltage drops below an acceptable level. So, why?

 

You've checked the main cablesthumbs up

Do you have an isolator switch, if so are all the connections tight? Is this a new switch and has the problem just occurred since it was fitted?

How many individual wires go to the alternator and to how many terminals, 3 or 4?

Are there any alternator terminals without wires connected, especially a single terminal, separated from the others?

Is there a brown/yellow wire connected to the alternator or floating nearby with no home to go to?

The reason for the above questions is that OE alternators have a separate terminal that senses the battery voltage and tells the alternator how many volts to put out. If this is disconnected, it won't give anything out and the battery will go flat. This wire should go between the alternator and the battery but if a cut out switch has been fitted its easy for it to be left unconnected.

 

I can't help with an auto electrician in York but I suspect you really need a 7 specialist. See how we go with the above and I'm sure we'll get there.

 

BTW, congrats on the PB at Harewood.

 

Paul

 

 

[7TOM]

Hi.

Do you have an issue with the excitation of the alternator ? If you put a test light (not meter) between a chassis earth and the small lead on the alternator with engine running does it start to charge then? If so then that's your issue. Not sure on your set up I.e does your 7 have a charging/ ignition lamp for exciting the alternator? Kind regards Tom

[pandjack]

Tim : Thanks - I'll work my way through these checks.

 

Paul : Factory fitted isolator from new but will check connections; 2 wires to alternator - thick one from starter solenoid already checked/OK, & thin one disappears off into engine loom - no terminals without connections. The thing is, I haven't made any changes & the car was previously running perfectly. The only thing you've now got me thinking about is I removed the isolator key a couple of times recently when parked (for security) which I'd never done previously - I wonder....??

 

Tom : Thanks -will check this also.

[Paul Deslandes]

is the alternator body definitely connected to the engine/chassis? You can check with a a meter or try connecting the body to the block using a reasonably thick wire. I suggest you make a semi permanent test connection with the engine switched off to avoid any sparks or surges, rather than dabbing a wire on whilst things are running.

 

Edited to add, what colour is the thin wire from the alternator

 

Edited by - Paul Deslandes on 8 Jul 2013 22:04:23

[Paul Deslandes]

Failure of the isolator switch usually results in no cranking (big switch terminals) or no ignition (small terminals) so I doubt its that causing it not to charge.

[pandjack]

SORTED - AT LAST!!!! Tom was on the right track as it was indeed a lack of excitation current path. Still not sure why , but rather than stripping back the loom into which the thin brown/yellow cable from the alternator (now known to be the exciter cable) disappears, I've wired a separate excitation circuit from this terminal via a warning lamp to a new live feed. With the ignition switched on the lamp lights as the circuit's completed then goes out when the engine is fired up as the alternator kicks in (ie the system most 'ordinary' road cars work on). I now have a steady 14.4/14.5v battery charge & no more mis-fires