Ignition light on (and off)

My ignition light is staying on most of the time, but can be made to go off...

The light behaves as it should to start with: on with the ignition, then on starting, light stays on briefly but then goes out (when alternator exceeds its minimum speed)....
Trouble is that light comes on again after a few seconds (and can be seen flickering slightly).

Had a closer watch on it this afternoon, noticed curious effect.
Tried giving it some revs. At high revs, the light can be made to go off, and it will stay off until the revs drop a long way back again (almost to idle).

I fitted a new battery 2 years ago (after I'd run the old one flat by mistake, using the work lights for too long, and it wouldn't re-charge) then later discovered that the alternator must have been failing too, as that needed replacing a few weeks later.

I must admit I haven't tested the charge voltage yet.

Could this symptom be caused by a slipping belt, and if so, how can I tell if it's slipping? (it isn't making horrible screeching noises)

OK, fired it up this morning (8am -- the neighbours will love that!) to check the voltages:

• Ignition OFF -- 12.4V (just powering the clock)
• Ignition ON -- 12.2V
• Started -- dropped to around 10V (!) then stabilised at about 12.2V (ignition warning light still on)
• Higher Revs -- up to 13.3V when warning light off, back to 12.5V after revs had dropped, when light came on again

(The battery was out and charged in the garage, but not left on trickle charge. It's also been used for the test starts and runs yesterday. It would benefit from some normal usage again to bring it up to scratch, but I know it is more than adequate for the job.)

We need the truck for holiday next Saturday, so I need to know soon if there's a simple fix to try, or if I have to order another alternator quickly.

Any ideas, anyone? (Please?)

It's worth checking the alternator connections for dirt and corrosion including the earth connection via the alternator mounting. If the belt is tight and clean and not perished (running surface goes hard and shiny) then it probably isn't slipping - you can probably see if it's slipping by watching the pulley as you rev the engine.
If that doesn't sort it then it looks like the regulator has gone and it's new alternator time.

Oxides, thank you for the suggestions...I'll have a look at the belt tonight.

The alternator was bought new in Sept 2008, and has barely travelled 6000 miles, so that side of the connections should be fine; however it uses a 3-pin plug connector (which is now 17 years old!) on the loom so that might be more suspect (the previous owner used to take it wading).
Hadn't thought about the earth connection - good pointer.



The light came on quite suddenly while we were out driving back from previous holiday - no hints of problems prior to that. There was no significant electrical load (only the stereo on) as it was a dry morning, although the truck was heavily loaded.

My gut feeling is that as a sudden failure it's more likely the regulator than a dirty connection (although I will certainly check for the latter).
The voltage regulator is separately replaceable.
Can anyone tell me the part number?
The alternator is a Lucas 127 (70A) with the 3-spade connections in a plug.

ALTERNATOR REGULATOR LUCAS A127 FORD AUSTIN ROVER BUSH on eBay (end time 25-Aug-10 17:34:01 BST)

Bought one off this guy a while back with no troubles at all - just undo the small bolts ( need a very thin wall socket - had to grind the edges of one of mine down ) . Angle the pack out slightly and remove the wire .

just be carefull with the bushes - dont just slap it in as they'll brake ...
Apart from that its very easy to do , If it still dont work then its the alty itself fooked .

I had a similar problem earlier this year. In haste I rushed out and bought a new battery. Once that had gone flat I checked the alternator - the problem lay in the fact that the carbon bushes were clogged up. Cleaned them up and now it works sound.

Something else to try, good!

Needed some diesel so had to take it for a short drive. Ignition light went off and stayed off for most of journey (flickered on a couple of times). Still off when I got home -- revving and letting it drop back to idle didn't make it go on again. So tried with it in gear against the handbrake and lifting the clutch -- eventually, once the engine was juddering, it went on and stayed on.

Next, check for slipping belt:
Sitting on the wing giving it some revs and watching the alternator pulley/fan with a torch. No sign of the fan stalling or anything else untoward...
...except that the extra revs had made the light go off again. :doh:


So, tomorrow I'll pull out the air filter etc and check the connections and brushes (too dark now).

Don't you just love intermittent faults?

It could be a fault inside the ignition barrel - spray some WD or similar in the key slot? good luck

Y'know I still don't think of using WD40 to fix electrical faults...

The truck was in for some welding recently, and I asked them to look at the horn, which had stopped working (I was on holiday without a multimeter, or time to investigate). They fixed that with WD40 too (the mind boggles!)

As I don't have the circuit diagram to hand (I'm at work) what part of the circuit is routed through the ignition switch, and how might it cause this effect?

I use the 200tdi one in the Haynes manual - the ignition powers the glow plugs and hence the warning light.