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MG MGB Technical - Voltage Stabilizer ???

Today on what was planned to be my last ride of the season my '72 MGB ran into a little problem. After jump starting it (battery was low, hadn't run it in 3 weeks), I noticed none of the gauges and directional lights and radio were working. I drove it 2 miles, stopped, and had to jump it again. Got it back home, put a volt meter and noticed it wasn't charging (about 8.8V were indicated). The alternator and battery are only 2 years old, and I haven't had a problem all summer. My guess is the voltage stabilizer is not working, I last replaced that about 12 years ago when the gauges stopped working. All exterior lights, work fine. Can a faulty voltage stabilizer account for the lack of charging as well?
Joe Dufresne

"Can a faulty voltage stabilizer account for the lack of charging as well"

No, the voltage stabilizer only purpose is to provide 10 volts to the instruments. I suspect that the regulator in your alternator has gone south (unless you have an external regulator, in which case, it is the external regulator that is FUBAR). Cheers - Dave
David DuBois

Neither voltage stabiliser nor alternator not charging would account for *none* of the gauges or the radio working, but the latter could well account for the indicators lighting but not flashing. If the indicators weren't even lighting then they and the tach (and possibly the radio depending on how that has been wired) depend on the green-circuit fuse. That also supplies the voltage stabiliser and from there the fuel and temp gauges. The oil gauge should be mechanical and so should have worked regardless
Paul Hunt

I charged the battery overnight, it started up but only 12.2V across the battery, not charging. What is also strange is the red light indicating no charge was not working, and that has always worked in the past when it wasn't charging (if idle was too low, etc.). I am ready to change out the alternator to fix the charging problem, but feel that doesn't correct my lack of temp. and fuel gauges + directional lights + radio, checked all 5 fuses and none are blown. Not certain what has happened, as the car ran well all summer, did change the ignition back in May, but had 5 months of pleasurable driving. Any suggestions beyond a bad alternator (or built in voltage regulator)?
Joe Dufresne

If the warning light circuit has failed that affects the alternator charging, as the current through the warning light circuit acts as a 'pump primer' to start the alternator charging. In my experience alternators will charge without this, but have to be revved to 3k or so before they will start. After that they will charge normally down to 600rpm or so before the charge stops, and the engine has to be revved again. This is used alts, new ones may not do this, and some manufacturers may not either.

Unplug the alt and with the ignition on connect an earth to the brown/yellow wire in the plug. Be careful not to connect an earth to the brown wire(s)!

If the light doesn't glow either the bulb has failed (very rare) or there is some other break in the circuit. You will have to pull the bulb from the holder and see if you have 12v on the white wire in the holder with the ignition on. If not the break is back towards the ignition switch, if so either the bulb has failed (try the high beam warning light bulb if that works) or the break is towards the alternator.

As far as the other problems go with the ignition on check you have 12v on the white and green sides of the second fuse up in the fusebox.

If not on the white there is a problem back towards the ignition switch, probably the 4-way connector in the mass by the fusebox.

If there is 12v on the white but not on the green the problem is in the fusebox itself.

If there is 12v on the green at the fusebox there are a lot of branches and tees in that circuit so it's possible a bullet has pulled out of a 4-way somewhere behind the dash. By listing what does and doesn't work of the fused ignition circuits i.e. including the brake lights, reversing lights, washers and wipers in addition to those above it should be possible to work out where the break is.
Paul Hunt

a few of quick thoughts

check fuses for electric continuity and not just visually, I've had fuses that look fine but are not

try replacing or checking that the existing ignition/charging bulb is working

for charging, check the wires and connections in the plug that goes into the socket of the alternator (have battery disconnected)

your alternator could be repaired or reconditioned at less than the cost for new

have a look at Paul's excellent web site for info and wiring diagram for your model to see what common points there are for the items not working - mgb-stuff - http://www.mgb-stuff.org.uk/hammertext.htm

ETA: from Paul's site - http://www.advanceautowire.com/mgb.pdf

Nigel Atkins

sorry I was still typing my post as Paul put his post up

for the voltage stabiliser check the connections to it are clean, secure and protected and that the voltage stabiliser is well secured (earthed) and is level (horizontal I think but check)
Nigel Atkins

yes horizontal (in this example at least) - http://www.mgb-stuff.org.uk/ivs.htm#1

voltage stabiliser - http://www.mgb-stuff.org.uk/electricstext1.htm#stabiliser

all from Paul's web site in case you missed them
Nigel Atkins

Another thing, when checking electrics with the ignition turn on disconnect the coil, or you could wind up burning it out.

Herb
Herb Adler

Herb,
from me being an idiot I can tell you a coil in good condition will take a lot of being left on (next day, possibly even 24hrs, can't remember) and another time something being wired wrong and the coil getting hot (can't remember details now but I thought I knew the ignition switch wiring then realised I didn't and had forgot how they were before I messed with them)

better to follow your example than mine thought - and note what goes where before disconnecting(!)
Nigel Atkins

Nigel,

I believe how long the coil will last with the ignition on isn't dependent on the condition of the coil. Rather it is dependent on whether the points are open or closed at the time the ignition is left on.

Charley
C R Huff

Charley is correct, but switching off a running engine 'usually' stops it in one of two positions and in those positions the points are normally closed. After-market electronic triggers often have protection against this and cut the current through the coil if left on for more than a few seconds.

I'm not sure about burning a coil out, but they will certainly get much hotter than normal if left powered. I had occasion to change my coil after a dozen years of never having left the ignition on, then shortly after I left it on not once but twice, for an hour or to. It was very hot and the engine wouldn't start, needless to say I was extremely annoyed with myself. However after cooling it down with a wet cloth I breathed a sigh of relief when the car started and there have been no problems since. 12v coils (chrome bumper cars) will get a lot hotter than most of the 6v coils used on rubber bumper cars. However the later North American spec with the 45DM4 electronic ignition have special low-resistance coils, and if the ignition is changed to points or some other electronic systems they will get much hotter than even 12v coils.
Paul Hunt

Paul,

I left the ignition on in a Volvo Amazon I used to have. By the time I noticed it, it would not start even though the battery was still hot. I opened the hood to find the top of the coil missing and black tar like stuff sprayed all over the engine bay. Looked like it was a pretty good explosion.

Charley
C R Huff

good point Charley I'd forgot that

plus I don't have those nasty CB points (shudder)

when I was trying to find the correct wiring for the ignition switch I got the coil very hot but it was fine, it was a Lucas sports coil but whether that makes much difference I'm not sure (but as they only a few pounds more I use them)

I once gave away a sports coil to a mate because I thought it might have been weakened/damaged/injured and as far as I know he's still using it - over the years my tolerance to car that wont start or run well has decreased and it was never very high to start with, I expect my classics to be able to sit unattended outside in the coldest of winter for weeks on end and still start easily and fully operate and run well and to be reliable (getting them reliable takes effort, time money but mostly preventative maintenance and year round regular use)
Nigel Atkins

This thread was discussed between 27/10/2013 and 30/10/2013

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