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MG MGB Technical - Ignition warning light...

...But probably not what you're thinking!

Went out for run yesterday, came back and left the car outside the garage. I'd noticed a 'wizzy' noise on the way back and thought it sounded like alternator on it's way out. Came out later to put the car away and noticed the ignition light glowing - without the key in the ignition!

I'm thinking I've got a short somewhere, probably in the alternator but haven't had a look yet. Wondered if it might also be the ignition switch. I'd had a hell of a job to get the old steering wheel off and wondered if all the bashing about might have hastened its failure.

Are there any other likely candidates for causing this?
Dave Smith GT

The wiring for the ignition light is very simple. It gets 12v from the ignition live (white) circuit, hence, in theory, only with key "ON". Also the alternator "IGN' out. The idea is that as the alternator senses power drain it pushes out more current and this "balances" out and the bulb goes out. Technically without ignition the only place the light can get 12v is through the alternator as that is permanently connected to battery pos. Hence probably is an internal short on the control circuit. Disconnect the ign out (brown /yellow, from the alt. The build should go out. If it doesn't there might be a problem with the ignition switch.
Allan Reeling

Does sound like the alternator, probably one of the diodes.

However disconnecting the brown/yellow i.e. unplugging the alt will make the light go out regardless of whether the fault is in the ignition switch or the alternator. If it stays lit then you have two faults, one on a white wire or the ignition switch, and one on the brown/yellow wire, one to 12v and one to earth, which would be pretty unlikely. If it were the switch other ignition stuff would continue to work, and I doubt you would be able to stop the engine with the ignition switch anyway.

Really you need to unscrew the ignition warning bulb, alternator harness plugged back in, then see if you have 12v on the white wires or the brown/yellow wire, and I'm guessing it will be the latter.
Paul Hunt

Thanks Allan. I had a quick look at the circuit diagram last night and was wondering where the warning light was getting a ground.
Looking at the diagram again now I see that the warning light is connected to the '2' position of the ign switch, as is the fuel pump, so it might be grounding though the coil there, but with insufficient current to energise it. I'll have a play with it tonight, it's certainly a new one on me!
Dave Smith GT

Thanks Paul - yes, I think the alternator is most likely. Not done a fraction of the miles that its predecessor had covered, but then there are many posts on that subject too!
Dave Smith GT

"so it might be grounding though the coil there"

Indeed, that's what causes the ignition light to glow (in most cases) when you turn off a running engine. The alternator is still generating a voltage while it is spinning, and current can flow via the lamp to earths at the coil, fuel pump, instruments etc. However the lamp keeps the current too low for the coil to continue to produce sparks, so the engine does stop when you turn the key. When the engine stops the alternator stops generating, so the lamp goes out - normally, but with various faults in the alternator current can flow from the battery through the lamp to keep it glowing. Remember that when it is glowing it is discharging the battery, and other alt faults could develop to create a catastrophic short-circuit, so when parked the alt plug should be pulled out until you have fixed the problem.

"However the lamp keeps the current too low for the coil to continue to produce sparks, so the engine does stop when you turn the key."

North American spec cars with the ignition relay had a design fault where on switch-off the ignition warning lamp earths through the ignition relay, and in that case the current is enough to keep the relay operated, and it is the relay that connects 12v to the coil, so the engine keeps running - at least it does do if there is a fault pretty-well anywhere in the emissions plumbing or anti-runon systems. On those cars it is the anti-runon system that stops the engine, by sucking the fuel out of the carb jets. That fault crops up from time to time on North American cars, and causes some head-scratching. It was discovered immediately on UK cars, so the wiring was altered, but the mod was never applied to North American cars.
Paul Hunt

I had a look at this last night and sure enough the current flow through the bulb is from the alternator side. I pulled the alternator out and got it on the bench to remove the rear cover and get a look at the diode pack.

Now I'm unsure as to what I have here - this is a replacement from a few years back but does not have any markings on it to tell me if it's a 17- or 18ACR. I thought it was the latter, but the diode pack fitted looks like that of the 15/16/17ACR, at least according to JCR Supplies website, (see photo). So maybe that's what I have after all.

Both are described on the JCR site as 12V 45A alternators, so what's the difference between the 17 and 18ACR units?




Dave Smith GT

The 18ACR has the higher output I think. 45 amps.
17ACR is 36amps.
Allan Reeling

Opinions vary according to where you look. 16ACR is said to be 34 amps (WSM), the 17ACR either 34 or 43 amps, and the 18ACR 43 (Haynes) or 45 amps.

How many wires and what size spade connectors do you have at the moment?

The one you picture has two large output spades, originally used with 2-wires from the harness one large terminal being unused, but later on with two thick brown wires on the output spades for better current carrying capacity.

At other times three wires - the additional wire being a standard gauge brown - were used with three different shaped spades on the diode pack, as shown here: http://www.jcrsupplies.co.uk/products/ALTERNATOR_REBUILD_PARTS/LUCAS_ALTERNATOR_RECTIFIER_DIODE_PACKS/LUCAS_ACR_ALTERNATOR_RECTIFIER_3949_see_notes_in_advert.html
Paul Hunt

Currently (!) I have two wires, small one (brown/yellow with 1/4" spade connector) to ignition warning light and a larger one (brown with a 3/8" spade) to starter & battery.
I took the diode pack out the other night and had a closer look. The diode that has short circuited is not seated correctly (see photo), so I suspect that it has been unable to dissipate heat so well as the others, run hotter and given up the ghost early.
If this had been an original Lucas alternator I was fixing it would be easy, but the one I have here is a recon and has nothing on it to say what it is. It has a wide stator ring, so it's either a 17 or 18ACR. I've read that stator windings on a 17 were varnished or painted orange, and the 18's windings were red, but who knows with a recon?
Having spoken to the chap at JCR I've gone with the diode pack for the 17ACR, as that's what was fitted before. Electrically, I don't think there would have been any harm in fitting the later rectifier, but I don't know if it would have fitted under the cover with the extra right-angled spade connector.


Dave Smith GT

Those diodes are much beefier than the ones in 70s, which were a little blob smaller than the solder blobs on the ends of those diodes, as in the attached. The semiconductor element sits in a recess in the fin and has a blob of silicone sealant on the top. The one nearest the camera has been pulled away from the fin.

The larger size now is all heat-sink to give better transference to the cooling fins above and below. It looks like the tab it is soldered to above is in the wrong position and probably was from new, which means the body has never been properly in contact with the fins. The 15/16/17ACR one from JCR looks to be the one to use. Check all the diodes are seated correctly!
Paul Hunt

This thread was discussed between 19/10/2015 and 26/10/2015

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