[Mgs] Ignition light always on, low glow

[Paul Hunt]

Depends.  It can indicate a high resistance connection between the 
alternator and the ignition, this should be eliminated as a cause first. 
Measure the voltage to a known good ground on the brown at the alternator, 
the brown/yellow at the alternator, and the white at the fusebox.  Normally 
you would expect these to be within 2 or 3 tenths of a volt.  The more the 
white departs from the brown/yellow the more the warning light will glow.

If the brown and brown/yellow are the same, and higher than the white, then 
it is probably a bad connection in the brown circuit between the alternator 
and the solenoid if the battery voltage is also lower, or if those two are 
the same then between the solenoid and the ignition switch, the switch 
itself, or the white from the switch to the fusebox/ignition relay.

If the brown and brown/yellow are different, and the wires are making good 
connection with the spades, then it is probably an alternator problem.  If 
the voltage difference increases as you switch more electrical loads on then 
*usually* it indicates a diode has failed in the diode pack.  Other symptoms 
can indicate a problem with the voltage regulator, brushes, slip-rings, 
windings etc.  Take care with taking it somewhere to be tested, some place 
are rubbish and can only detect a complete failure, which probably yours 
hasn't (yet).

One thing to watch out for if it *is* a diode, and if it has failed 
short-circuit, is that it will drain the battery.  So as a precaution until 
you have resolved it unplug the alternator while parked, making sure the 
spades can't short to ground anywhere.

PaulH.

----- Original Message ----- 
> During the weekend the Ignition light on my 71 MGB GT has started
> showing a light glow all the time, at all engine speeds.
>
> Is it time for an Alternator rebuild?