Alternator light on

[flying farmer]

I have been asked a question by Janet Taylor who owns 1 5TAG which has me baffled.
The alternator light has come on, but the voltmeter is still showing a good charge even with the lights on.
Has anyone come across this so I can give her some advice, as she is off to Yorkshire in the Stag next week with the Stoke TR group
Thanks, Neil

[european voyager]

I would try it with a proper multimeter in case the volt meter on the car is wrong , Ive had modern alternators charging but still bringing the light on

[4x4]

I have been asked a question by Janet Taylor who owns 1 5TAG which has me baffled.
The alternator light has come on, but the voltmeter is still showing a good charge even with the lights on.
Has anyone come across this so I can give her some advice, as she is off to Yorkshire in the Stag next week with the Stoke TR group
Thanks, Neil

It probably is still charging - the fault is just the warning light circuit.
Had the same problem myself in the past.
Problem at the end of the day - do you want to leave it that way?
If not - it's replace the alternator time......................

Good luck.

Adrian.

[wilf]

Good multimeter needed to see what there actually is at the battery terminals when "charging". 14.3V is the target.

My bet is the alternator has a dead diode or two.

[Motorcycle mad]

Hi Niel

I would go along with Wilf’s suggestion, a faulty diode.

I am assuming the car in question has a ACR type alternator fitted as opposed to the earlier 11AC type, in which case the ACR alternators are a three phase machine producing an AC currant then rectified to DC via the 9 in-built diodes. What normally happens is that one of the diodes fail, knocking out one of the phases which reduces output by about 50% (amps), output voltage will still be about the same as normal, but could be slightly down.
Another check you can try is watch the charge warning light, run the engine and very slowly increase rev’s, closely watch the charge warning light (on the Stag this is very difficult to do because it is in the cluster) you should notice it dim and go out, and then come back on glowing, but less bright than normally, this is caused by an in-balance of voltage on one side of the warning light caused generally by a faulty diode on one phase of the alternator.

Long term you need to change the alternator or rectifier bank, short term you should be able to run the engine, but don’t use lights otherwise demand is going to be greater that supply, result flat battery and unable to start.

Incidentally the 11AC alternators only have 6 diodes and work in a slightly different way, the above would not apply.

Ken

[V Mad]

I agree with Ken, vey well explained.

I have a similar fault on my Spartan and had it for months now. Sometimes the red light does not even light wth ignition on, but when it does, it needs the engine to be revved quite high before it goes out. Then it stays out until nect time the fault appear. But it never runs the battery down. I think in this case there is a bad connection tot he warning light.

I have a spare alternator in the boot waiting to be fitted, when it really needs it.

[tonypy1]

I had a similar issue were the alternator was actually charging but I had a dead short in the warning light circuit. I can't remember whether it was one of the power steering pipes or something similar was pinching the wire. A search on alternator problems should flag up the diagnostics that identified the problem.

[jleyton]

My alternator was overcharging and I was all set to replace it however, luckily, I opened it up first and was able to change the voltage regulator (doddle of a job) - equally it would have been easy to change one or more diodes and far cheaper than complete alternator.

Cheers

Julian

[GrantE]

I seem to have a similar problem. Ignition light even stays on when key removed and voltmeter showing only about 11 volts. Any one with ant suggestions?? Thank you

[peterl]

Dear all,
My two bob's worth. This is the order that I would chase this problem.
1) If the charge light is on intermittently and becomes worse the more occaisions that you drive the car, chances are it will be the carbon brushes are worn. replace the brushes. This is the cheapest and easiest outcome. I usually check this with a DC tong ammeter and turn on the headlights.
2) If the charge light is on all of the time then likely to be the regualtor no longer working, and or the brushes are that far gone that they are no longer making contact with the slip rings.
3). Check the diodes, possibly at least one is short circuit.

I have had all of these problems in the past with a variety of vehicles.
Regards to all,

Peter

[Motorcycle mad]

Hi GrantE
I think you should open a new thread with your problem, as I see it forum members are reading the initial problem and then not always reading of your fault, they may have an answer for you.

Regarding your fault, my personal view is that this fault is not a known condition, it could be any one of three or four problems causing your trouble, and therefore not able to say go look at so and so as in the case of the alternator problem in the original question.

I have the view that you have some wiring issues shorting out causing a feed back via some other wiring or component on your car, why I say this is you are not getting a full 12 volt reading suggesting something else is in the circuit feeding the circuit(s).

1. Have you carried out any wiring mods recently, or fitted something, if so disconnect it, it may be causing feedback.
2. Pull the fuses out one by one on your fuse box to see if fault goes away, if it does you need to check that circuit.
3. Have you or previous owner had any welding work carried out on the vehicle, may be wires have fused together.
4. Any wiring rubbing and worn through on a moving part, such as steering.
5. Check multiplug connectors for corrosion shorting/conducting, pay attention to the white, or green wires

Good luck

Ken

[wilf]

Grant - for your ignition warning light to stay on with the ignition switched off, you have something very wrong with that circuit.

I would be looking for a misbehaving ignition switch to begin with - i.e. checking to see if volts remain on outputs that shouldn't have any with the key "off". Then I would check for any shorts to earth in the wiring from the bulb to the alternator.

Also, your alternantor is not charging it would appear, so you need to ensure that voltage (through the ignition warning lamp) appears at the relevant alternator connection. At 11V, your battery is well flat, and needs charging pdq to avoid it dying completely.

You may have more than one fault going on here.

[GrantE]

Thank yOu Peter. Does your oPtion two include the ignition light stayin on even with the key out??

[GrantE]

Thank you so much for your kind responses. Today I also spoke with a most helpful chap at Wards. He thought it worth checking the connectors under the washer bottle. I had a look and found a loose connectore. After cleaning it up and checking for a good connection, i restarted the engine and it now charges at 0ver 13 volts, which is good but ignition light still stays on with key out! At least it is now charging. He also suggested i might look at the connectors at the back of the alternator for a short, as you described above. It's too late to look now but will have a butchers on Saturday. As you suggested, it appears to be two problems. Thank you again! Grant

[GrantE]

BTW, I did plug a solar charging panel into the cigar lighter recently - could have done anything possibly?