Classic cars forum & vehicle restoration.

Finch661 · Posted: Sat Mar 07, 2009 11:01 pm Post subject: charging and speedo

hi people, just a few questions about my 76 midget. the fist is the charging circuit. it seems that my battery only last a few days (new battery) so i am guessing that it isnt charging. i had one mot warning saying that my charging light didt go off, but when i drive it, it goes out? very strang? any advice on how i can check to see if the alternator is working or if there is a drain in the system?

my second problem is my speedo. (i dont know if this is supposed to happen on a midget) but it seems that the speedo reading is very jerky, and when i switch off the engine, the needle jumps up then settles to the stop. any ideas

sorry for all the questions, its my first mg! my old austin mini doesnt have any of these problems!!!

thanks Shocked

47Jag · Posted: Sun Mar 08, 2009 1:19 am Post subject:

Finch,

It's possible that your alternator has a blown diode. It will still charge about 10 amps and be enough to put out the ignition light. You should get an output test done by an auto electrician.
The speedo sound like it needs the inner cable lubricated. Detach it from the speedo and pull the inner cable out. Run some 3 in 1 oil down it's length, then wipe it off. Re-insert the inner cable and as it is nearly fully in you may have to turn it slowly while pushing to get it the re-engage the square drive. Re-connect and road test.

Art

Keith D · Posted: Sun Mar 08, 2009 8:51 am Post subject:

Finch,

If a diode has blown as 47Jag has considered possible, then your alternator is trying to put AC into the battery, which the battery cannot accept. The purpose of the diodes is to rectify the current to DC.

My father had an Australian 1972 Marina with a Lucas alternator. The diodes were actually wired into the windings of the alternator. (Only Lucas could come up with anything as crazy as that!!) Therefore not only could they not get rid of their own heat, but they had the alternator's heat to content with as well. Surprise, surprise! The diodes failed whenever the battery went low and needed a high (and hot!!!) charge from the alternator. Under warranty, this rewiring was done several times. The symptoms were an ignition light that ALMOST went out. It could been seen gently glowing at night, but not during daylight.

A local auto electrician fixed the problem permanently by blowing the remaining diodes and mounting a new set of diodes on a large metal heat sink on the inside mudguard. Problem over. Dad had the car for another ten years with no charging problems.

Keith

ukdave2002 · Joined: 23 Nov 2007 Posts: 4105 Location: South Cheshire

An alternator will charge with a single blown diode, but you will get what’s known as "half wave rectification" which essentially means a reduced output, the quick way to check this is to connect a scope to the car. If you have had or given a jump start recently this would be the most lightly cause of a blown diode.

The most common fault with the Lucas ACR is worn or damaged brushes, which cost next to nothing and take 5 mins to change. If its the diode pack a couple of quid and 20 mins. You can buy a rebuilt kit for the Lucas ACR which includes all the electrical and mechanical pars for abut £12, somewhere on this forum I did an ACR rebuild which may be of use to you are not familiar with the alternator.

One other common problem can be an intermittent connection to the warning lamp; the warning lamp its self is a critical part of the alternator working properly, its connected to the alternator by the small blade on the alternator “Euro Plug” when these plugs get removed a number of times over the years the warning lamp connecter is the first to fail, the female blade in the socket gets pushed back, worth a quick check first!

Good luck Dave

Finch661 · Posted: Sun Mar 08, 2009 1:59 pm Post subject:

i have looked on ebay, and there is a full rebuild kit for about £15. is it easy to rebuild? i will check the alternator today to see if it is producing a charge at the battery end.

i have also found a new 65A alternator on ebay for about £50, but being a scotsman, im happy with the rebuild kit!!! Very Happy

ukdave2002 · Joined: 23 Nov 2007 Posts: 4105 Location: South Cheshire

Lucas Alternator rebuild link:

http://www.oldclassiccar.co.uk/forum/phpbb/phpBB2/viewtopic.php?t=4513

Finch661 · Posted: Sun Mar 08, 2009 4:59 pm Post subject:

thanks very much, i will have a look.

a new problem is that my red light (chargin one) no longer lights up at all. it should when the igniton is in position 1. i also checked the voltage at the battery with the car on, it was about 12V, so i am guessing my alternator is packed up!

ukdave2002 · Joined: 23 Nov 2007 Posts: 4105 Location: South Cheshire

If the charging lamp does not light at all, it can only be 1 of 3 things;

1) The bulb has blown (which will stop the alternator charging)

2) A poor connection between the bulb and alternator (which again will stop the alternator charging, or at least intermittently)

3) An open circuit diode in the alternator

Keep us posted Very Happy

Finch661 · Posted: Tue Mar 10, 2009 10:44 am Post subject:

thanks very much for the help, i need as much as i can get

i tested the lamp (by grounding the brown/yellow wire?) and the bulb came on with the ignition in the 'I' position. so i am guessing it is the alternator that is knacked? i have bought a revamp kit and will attempt to rebuild my alt.

will keep you posted

Finch661 · Posted: Wed Mar 11, 2009 10:18 pm Post subject:

evening everybodt. right the story so far, i recieved my revamp kit today, so i fitted it. turned the key (got the red light on) thought thats a good sign then fired her up. i then turned her off. i turned thekey again, but this time the lamp did not light. started the engine and was getting about 12.6V at the battery...

new alternator then?

Finch661 · Posted: Tue Mar 17, 2009 11:16 am Post subject:

fixed at last. i fitted a new alternator and she seems to like it. im getting just over 14V at the battery. even fitted a voltmeter in the cab to let me know what the battery is like

thanks everyone for your input. im now going to take my car for a well deserved drive!!