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MG MGB Technical - Red ignition light glowing dully

Hi,

Another question. The red light in the tacho is glowing dully when I drive. What is the significance of this? Engine is turning over beautifuly.

Thanks.
Andrew

Andrew,

Usually this indicates a fault in the charging circuit, possibly a fault in the alternator / regulator / generator.

Glen
Glen Ward

This may also be the water pump about to go. Check the weep hole and try and shake the fan (assuming it has a mechanical one).
SteveM

see the archives...check out blown diode in alternator and check the voltate from the alternator...you may be cooking at 18V

rn
RN Lipow

In my case, dim red light
was due to bad connections. First
check the plastic plug to the alt. and make sure
wires are secure (make sure battery is disconnected --
brown wires are always hot.) Also check ground strap,
fusebox. Check a brown wire connection with a
voltmeter on the
fusebox with engine running, and all accessories
off, you should have 14.5 volts if alt. is working
ok. Diode pack is the culprit probably if all
connections are ok.
Ronald

Also check the connections at the starter solenoid.

They often get dirty and coated with oil, and wet from road splash.
Daniel Wong

FWIW I always suspect the fan belt if everything else checks out OK. They can slip (especially in damp weather and when old) even if apparently tight. Its always a good idea to fit a new one to eliminate it anyway (and carry the old one in the back as a spare)
ian Thomson

I agree with Daniel on the starter solenoid. Mine were just a bit loose here. Test the battery is charging properly - start up use a voltmeter across from the positive to the negative terminal if the battery shows between 13.5v and 14v coming in - thats fine. If not, then there is a bad connection on the brown circuit and eventually you will have a flat battery.

Iwan
iwan

When I parked this evening I noticed the red light glowing after I had switched off the engine but before I had switched off the main lights. When I switched off the main lights the red light went out too. Is it possible that this is due to a bad earth at the tacho itself, causing current to leak into the red light from the tacho illumination?

Just a thought. I have problems with bad earths elsewhere on the car.

Andrew
Andrew Hetherington

The warning light is not supposed to be connected to the tach body (ground) but must be isolated from it. The warning light, unlike the main beam or illumination bulb holders, has two wires and does not rely on its physical connection for a ground. The light should come on as you turn off the engine then go out as the engine comes to a halt. Was the light glowing *after* the engine had come to a complete stop? Was it full brightness or dim? You may have a couple of faults in combination. What is the voltage on the white and on the brown/yellow with the ignition off, engine stopped and the warning light glowing?

PaulH.

Paul Hunt

Paul,

The red light continoues to glow dimly after the engine has come to a complete hault. It glows brightly as the engine stalls, then dimly when the engine has stopped. I havnt checked the vltages becuase I dont have meter. Perhaps it is relevant to point out that the light switch isnt working at the moment, I can switch on the side lights but not the main.

Andrew



Andrew

Paul,

I just fixed the lights and it has not effected the ignition light one way or the other.

Andrew
Andrew

I had a similar problem with my ignition light a while back. The ignition light would glow with the engine stopped, ignition off, but go out once the engine was running. Not surprisingly perhaps the batteries would be flat the next morning. Never did really find the problem except that a replacement alternator cured it. I guess there was some short going on in the alternator itself.
steve
Steve Coulson

Hi Steve

If the warning light is on with the ignition switched off it could be that a diode in the rectifier pack in the alternator has partially failed and letting a small current flow 'backwards'

The alternator produces single phase ac current, and there are 4 diodes forming a rectifier 'bridge' to convert to dc.

I had the same problem on a Delco alternator, and fortunatly there was enough room in on the rectifier board to isolate the faulty one and solder in a single 30A diode. Cost $3.25

Dont know how much room there is in a Lucas alternator, but it could be worthwhile having a look.

Ian F
Ian Fraser

Andrew - as Ian says it sounds like a diode has gone short-circuit in your alternator which will be discharging the battery while parked, confirmed if the light goes out when the plug is removed from the back of the alt. These diode packs (containing nine in a Lucas - 2 for each of three 'phases' of output and three more for field excitation) used to be available as spares some time back. With any diodes faulty there will be some detrimental effects on charging, short-circuit diodes are likely to be causing excessive heat in the alt and hence stressing other components. A complete swap may be safest.

PaulH.
Paul Hunt

Hi All,

Having just found this BBS, I read this thread with interest, as I've had the "slightly glowing" ignition light ever since I got my '66 roadster. Like Andrew, I was concerned, but only at first.

This slightly glowing warning light is not dependent on the engine running or the ignition being on. The only thing which does affect it is having the headlights on (or so I thought). I tried turning on and off everything electrical in the car to see if that affected the light and found that nothing made any difference.

After reading this thread yesterday, I fiddled with the dashboard dimmer switch whilst sitting in traffic. Lo and behold, the glow dimms and brightens with the rest of the dash. This leads me to conclude one of two possibilities: Bleed-thru from the illumination in the dash is making the ignition warning light seem partially on OR there is a leakage current passing through the warning light that is caused by the dashboard illumination circuit. Either way, I would suggest that if your symptoms match what I've written above, then it may be nothing more than an irritant rather than a major issue with the altenator.

Regards,

Chris
Chris Byham

I hate to muscle in on anothers fault with one of my own but perhaps my experience may be relevant(and perhaps not). Recently I have had the ign. light on intermittently. It is difficult to be sure when it happens most often but it seems to occur more after standing a while or when damp (but I may be mistaken). I have a voltmeter fitted and it always shows abt. 14 - 14.5 volts with no discernable change when the fault occurs (though it falls to abt. 13V at night with the lights on making me suspect that perhaps the alt. cannot produce enough current). However, the Alt appears to be an original AC Delco 35A unit so the output is little more than adequate anyway. The strange thing about it is that it gets brighter the faster I go and that when it goes out it does so suddenly and often when on tickover. I have changed the fan belt (twice) and it always starts OK - so it must be charging! I can reach the Batt. cutout when driving and have got the light to go out by disconnecting/reconnecting while driving, but mostly there is no change with the light still on while disconnected. I know this stresses the diodes Paul but I had to try it! Are the diodes easy to get at for testing on these units? Any thoughts would be welcome - and may help the thread originator too.
Ian Thomson

Chris - sounds like your ignition warning light could be in contact with the tach case (which meets both of your possibilities, I think). I would choose to investigate and fix this because the warning light has a connection with the alternator diodes and regulator and I would rather not let inactivity damage that. Just unplug the light from the back of the tach and see what happens then.

Ian - Oooo you bad lad. You would probably have to disconnect the diode pack in order to test the diodes, because they are connected to the low impedance windings of the alt output all terminals would probably test the same i.e. you might conclude they are *all* short-circuit! The warning light acts like a set of balance scales, when the ignition is off and the engine is stopped it has 0v both sides, when the ignition is on, the engine is running and the alt is charging it has battery voltage both sides. At other times it has more voltage one side than the other and so it lights. I would measure the voltage on each side of the light wrt ground when it is glowing to see which side had higher or lower voltage than it should then investigate from there. If you have an unreliable warning light you might not know when your alternator really *does* stop charging ... until it is too late.

PaulH.
Paul Hunt

'lo all...

My dad's '71 B roadster eats alternators. Its engine is a little 'rough' (let's just say that by the time you reach 5000rpm you're really surprised it hasn't burst...) and it shakes a lot. Hence, the leads on the diodes fatige and break. This leads to a brighter-as-you-go-faster lamp that will occasionally get a lot brighter as yet another diode lead breaks. The first couple of breaks didn't stop the car... the third did :o)

Harmonic dampers all round...
--
Oliver Stephenson
Oliver Stephenson

This thread was discussed between 12/11/2001 and 22/11/2001

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