MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Ignition lamp question

As some of you know I have been fixing some problems in the dash of my '70 GT. I thought I had my Ign warn lamp problem sorted out but. I removed the holder/lamp and changed bad bulb, replaced the alternator. When I turn the ingition to the ON postition the light will only glow when I ground it to the dash?? Obviously this cant be done when it's fitted into it's plastic red lamp holder. Maybe I just missed something when removing/replacing??
Note that the alternator I put in is one of the later with the one large spade and 2 small spades I have Ny to bottom, Lg N to middle and 12 ga N to the top.
Mike
MK Mike

Hi Mike.

I assume that you were grounding the NY side of the lamp, DON'T ground the W side !.

Try disconnecting the NY at the alternator and grounding the wire at that point (ignition on).

The light should come on, if it doesn't I would suspect a broken wire or poor connection on or around that wire.

If it does come on, it sounds like an alternator prob(or connection to the alternator prob, the LG/N wire is a mystery to me, do you mean NG ?, did it have a dynamo originally ?).

HTH.. Don

Don

Don, sorry about that what I meant was LARGE BROWN 10 gague brown wire. The thing is that the bulb holder has 2 wires, the brown/yellow and the White. I don't think this bulb holder needs to be grounded, it should already be at the alternator.
MK Mike

Hi Mike.

You are right, the 'ignition' bulb holder should not have a ground connection.

The 'ignition' lamp circuit works like this :

When the ignition switch is turned on the white wire delivers 12v to one side of the bulb.

Current flows through the bulb (making it glow) and then into the 'ind' tag of the alternator via the N/Y wire.

This current flowing into the alternator sets up the internal magnetic fields so that when the alternator starts turning it is able to start to generate voltage. As this voltage builds up the bulb has 12v on both sides and so goes out.

Don
Don

Thanks, Don. When I ground the NY wire at the alt. the light goes ON. I am guessing it can't be much else but the Alternator? I always thought when the alt went bad the light always glowed. I was told that this Alt was tested =good???
Mike
MK Mike

Hi Mike.

Your ignition light seems to be ok.

The ignition light will show up most alternator faults, but not all.

You either have :

1) a bad alternator.

2) the thin N wire and the N/Y wires on the wrong tags, though I THINK you have them correct.. you do have the heavy N connected to the large tag ?.
What model alternator is it ?.

3) the alternator isn't grounded (unlikely but not impossible).

Don
Don

Don, The alternator I have in now is from the 1500 midget, I believe 43 amp. It has a small tag at the bottom with NY wire attached. it has a large tag in the middle with heavy N wire attached. and another small tag at top which has another N wire, but on my car this wire is like a 12ga wire. I have since found the battery in a VERY low state of charge but don't think that would matter?
MK Mike

Hi again.

I am not familiar with that alternator, if you have the 'ind' and 'sense' wires swapped over (thin N and N/Y) I guess it might give the symptoms you describe.

Hopefully someone out there knows this alternator's connections ?.

If not, you might like to remove it and look for model number and connection markings (eg 'ind', 'sense', '+').

If the battery is very, VERY flat it might prevent the light from coming on.

Don
Don

Mike. I agree with Don. What you probably have is a Lucas 18ACR alternator. This was used on the later MGBs and, I believe, on the Midgets.

The two wires most towards the centerline of the alternator should both be brown. The connector is bridged, internally, and both should be used for alternator output. The outermost connection is for the Brown/yellow wire going to the ignition warning light.

This may solve your problem. If not, please post again.

Les
Les Bengtson

Don, Les. I think you are right, it's the 18ACR. I did switch the NY wire over to the top terminal, the light stayed on even with the ignition off. no luck. I had this alternator around from a Midget as a spare. It was supposed to be good?? My setup as now is thin NY wire to bottom spade. 10ga Brown wore to middle (Large) spade terminal and 12ga Brown wire to uppermost spade terminal. I looked this alt over before i installed it and did not see any markings anywhere!! How can you tell what model it is, I do have another that I originally removed (dead) that I will get rebuilt as that is my next step?
Thanks Mike
MK Mike

Mike. If you have the ignition light on when the ignition switch is off, it means that current is flowing. Unplug the alternator and see what happens. I suspect the light will go out.

Neither of my "spare" 18ACR alternators have any markings except on the plastic piece at the rear. There, it is marked "Lucas" and the proper wiring is described as the "ind" terminal outermost and the terminals for the starter solenoid innermost. My alternators are all for the later model cars and have two large spades and one smaller spade. My daughter's car had an alternator similar to the one you describe with a central large spade and two smaller spaded top and bottom. When I tried to find a replacement, I was told that only the later models were available. I replaced her car's alternator with a late model and purchased the proper connector from Moss. Seems to be working fine. If you are using push on connectors, rather than the original Lucas one piece plug, that, we found, can be part of the problem.

At this stage, I would suggest you contact Paul Hunt through his website, "The Pages of Bee and Vee". Paul has a great deal of knowledge on these subjects and would be your best resource for troubleshooting.

Les
Les Bengtson

On the alternators I have the two identical output spades for the browns are obviously larger than a 'standard' spade, but the IND terminal is a standard spade for the brown/yellow.

68 MGBs had the 16AC Alternator with remote voltage regulator and had 4 terminals - Field, INDicator, + output and - ground wired back to the regulator.

69 to 71 cars had *five* terminal alternators even though they had the internal voltage regulator - two IND terminals linked by a brown-yellow strap in the plug, a +B which was a sense terminal, the + output terminal, and an unused - ground terminal. Both the sense and output wires go back to the battery terminal on the solenoid, and it is the sense wire that controls the voltage regulator. The effect of this is that under high current demand on the alternator, with some volt-drop in the output brown between the alternator and the solenoid, the voltage regulator will maintain the *solenoid* at the correct voltage and the alternator output slightly higher than this, rather than controlling the alternator out to the correct voltage which means the solenoid and hence the battery and the rest of the cars electrics will be slightly lower. It may also have protected the alternator against the engine being run with the plug removed. The later 'two output spade' alternators either didn't bother with maintaining the voltage, or were protected internally against damage if the engie was run with the plug removed.

72 seems to have been a 2-terminal alt with just + output and IND terminals.

73 onwards seems to have been a 3-terminal alt with B+, + output and IND terminals. Whether this B+ was the original sense terminal, and at some time changed to be a second output terminal, or the B+ was a min-nomer and it was always a second output terminal I don't know.

If you have an alternator with the sense terminal I don't know what will happen if you connect the brown/yellow to this instead of the IND terminal, and without a 12v supply on a brown it may not charge at all even with the output and indicator wires connected properly.

With the ignition off the warning light will glow if the brown/yellow is connected to a 12v supply, i.e. a brown, or the 'spare' output spade if only one output wire is being used. It could also glow if connected to the proper IND terminal, but the electronics inside are faulty. Also if the internals are faulty it may *not* glow if connected to the proper terminal with the ignition on.

With a 70 you should have two browns one thick and one thin both showing 12v at all times, and a brown/yellow which when grounded makes the warning light glow with the ignition on. You really need to look inside the alt and see if two of the spades are physically connected together, and if so the thich brown should go to the big spade and the thin brown to the linked smaller spade. The brown/yellow goes to the remaining spade. If the light doesn't work properly then, it looks like an internal fault. Even with the light not working it *may* charge, but may have to be revved to about 3k to start it charging, after which it may charge down to about 600 rpm as normal. OTOH you could have any number of problems inside the alt.
Paul Hunt 2

Hi all.

I have done a little thinking and I believe that :

If the N/Y wire was connected to the second battery connection on a 'machine sensed' alternator there would always be +12V on it so the bulb wouldn't glow when the ignition key was turned on, but might glow when the key was OFF as current flowed from the alternator through the bulb to earth via the 'ignition switched' white wire circuits. (the open / closed status of the points would be significant). I can't see any way in which this would cause damage apart from slowly discharging the battery while the vehicle was standing.

If the N/Y wire was connected to the battery sensing wire on a 'battery sensing' alternator (I have never actually seen one of these):
The battery sense connection goes to a resistor at the top of a potential divider. This would take little current so I wouldn't expect the bulb to glow under any circumstances. Again I wouldn't expect this to cause damage.

Connecting the thin brown wire (which would be carrying permanent +12v) to the 'ind' connection would allow a larger than normal (normal being about 200mA) excitation current to flow through the rotor windings, but they can be exposed to nearly full 12V when the alternator is running under high load, so damage should (I guess) be unlikely apart from slowly discharging the battery while the vehicle was standing.

I don't recommend experimenting with connections, I am only trying to determine what might have happened if incorrect connections were accidentally made.

Don
Don

Hey guys thanks for all the imput. I am thinking it's probably something internal with the Alt. Like Paul and Don said I do have constant power to the 2 brown wires and when I remove the brown/yellow wire the lamp glows. I tried the brown/yellow on the other smaller terminal and the lamp glowed constantly even with key off. Also when I remove the thinner brown wire the bulb glows with ignitioon off. Only other differences are that I don not have the plug in connector and my sense brown wire is not as thin as the brown yellow it's more like a 12ga wire as opposed to the 10ga hot wire to the middle Large terminal. I have the alt I removed that has the 3 correct size spade terminals to fit my application I will have it rebuilt and ask the rebuilder to beef up the internals a bit to at least 43 amps? I will also order the plug. Paul, how high can I go on the alternator output without damaging my electrics/wires with the stock hookup??
Mike
MK Mike

The warning light will glow with the brown/yellow connected to 12v (either from the output wire or the sense wire, which are both conencted to a full 12v back at the solenoid) and the ignition off regardless of the position of the points, the coil is just one of the possible routes to ground.

Mike - Voltage or current? If voltage then blown bulbs are likely to be the first casualty, although a few volts for a long time will more likely cause battery damage due to overcharging. If current then the alternator will only deliver what the load is demanding i.e. a 43 amp load will cause a 43 amp alternator to deliver 43 amps, and also a 1000 amp alternator to deliver 43 amps. The higher outputs from a more powerful alternator are only really brought into play if you increase the electrical load of the system. Only if you are sigificantly increasing the electrical load like from a massive audio system would you need to consider increasing the gauge of the wiring between the solenoid, alternator and the large load. The main benefit of a high output alternator on a standard system is said to be no loss of system voltage and hence charging voltage at maximum load at lower engine speeds, but I have never had a problem on the daily driver including winter V8 with twin electric cooling fans and a heated rear window with the standard 46A alternator. A 70 GT with the HRW may suffer with the 16ACR 34 amp alternator, but the later 18ACR is a 43 amp unit and should be fine.
Paul Hunt 2

This thread was discussed between 21/10/2006 and 24/10/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.