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MG MGB Technical - charging
just bought a 72 bgt running 2 6v batteries. its charging at 13.6 volts. not much difference between tickover and revving. is that ok.
|It's showing that the alternator is doing its job, even at idle.|
|H J Adler|
|According to the WSM it should be 14.3 to 14.7v at 5000 rpm, so if are getting that with nothing other than the engine switched on, then either the batteries are still taking a lot of charge or the output is a little down. The more current you take from an alternator the lower its voltage will go.|
But it depends on where you measured it, as weak connections between alternator and solenoid, and solenoid and to various points in the circuit can also cause the voltage to be lower than it should be.
Normally one would measure on the brown at the fusebox, as you can expect the white and green to be slightly lower. But to eliminate the other points you need to measure the brown at the alternator. If that shows low as well then the alternator is perhaps suspect, if that is higher then there are one or more weak connections between there and where you measured your 13.6v.
Another test is to monitor the voltage while gradually turning more and more circuits on. The original alternator on a 72 had a capacity of about 34 amps. Parking lights take just over 4 amps. A pair if 45 watt dipped beams add 7 amps. The headlamp flasher and 60 watt main beams adds 10 amps. A pair of brake lights adds 3.5 amps, as does a pair of reversing lamps. That totals a theoretical 28 amps, which should be enough to check the alternator. If it is still outputting more than 12.8v at, say, 2000 rpm, it is fine.
|thanks all. one of the 6 volt batteries was naff. put a new 12v on and all is good. one other problem iv noticed is overdrive works in 4th but not 3rd. any ideas.|
|If it is an ex US car then they only had it in 4th - if it is a UK car then maybe it needs a washer taken out from under the inhibitor switch - try pulling the gearlever towards you when in third and the OD switched on.|
|Chris at Octarine Services|
|Sounds like marginal switch adjustment. Try pulling the gear lever towards you in 3rd, and see if switches in then.|
If so, it is marginal. There were originally two fibre washers under the switch to adjust the depth of engagement, removing one is normally enough, or maybe the switch is just loose. However the 4-synch overdrive switch is extremely difficult to get at without removing the engine and gearbox! You can just about get to the wiring and terminals with the tunnel carpet pulled back and small access panel removed, but that's about it. If you do do that see if you can move the switch with a long screwdriver, if so you may be able to tighten it. A pal had to cut an additional access panel in the side of the tunnel to deal with his, replacement, I think.
If pulling the lever towards you makes no difference then it probably still is marginal adjustment. Late-model North American spec cars only had overdrive on 4th using a different plunger in the gearbox, but I doubt it's anything to do with that. To get at that means gearbox removal, or again maybe another big hole cut this time on top of the tunnel.
You may just have to live with it ... or rather without it :o)
Some say OD 3rd is the same as 4th but it isn't. It's lower, although it is more than half way from 3rd to 4th if you see what I mean. I find it a useful gear for spirited twisty bits or certain level inclines, but don't use it very often.
|thanks lads will try that when next out. can't say iv ever used it in 3rd on any cars iv had, just really used it as a 5th gear but it bugs me when something isn't working right.|
|See here for how to get at OD inhibitor switch|
|Chris at Octarine Services|
|clickable link, hopefully|
|Dave O'Neill 2|
|well done dave, looks like you have too much time on your hands . :):):)|
|well done lads, pulled back the gearstick and o/d engaged. thanks, good advice as usual.|
|if i leave it as it is will it stay like that or could it lead to anything else getting worse, like getting stuck in o/d or losing it altogether.|
|If it is loose and gets looser, or worn and wears more, then you will eventually lose OD altogether. Unless something else is wrong it won't cause OD to be engaged when it shouldn't be.|
This thread was discussed between 04/05/2017 and 15/05/2017
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