It does sound like the regulator is at fault, specifically the cut-out
relay. This is supposed to operate when the generator is producing about
13v, its contacts close and connect generator output to the battery and you
should see charging on your ammeter.
If you manually operate the cut-out relay (it is the one with the 'make'
contact, the other two have 'break' contacts) with the ignition off and the
engine stopped you should see a discharge on the ammeter and the generator
should try to turn, but the fan-belt and the load of the engine will stop it
moving very much! If it does this it implies the contact and its wiring is
OK.
Connect a voltmeter to the D terminal of the generator or regulator
(brown/yellow), start up the engine and watch the voltmeter and the cut-out
relay as you slowly increase the revs. The cut-out relay should operate at
between 12.7v and 13.3v and show a small charge, or at least less discharge,
on the ammeter. Further increase in revs should raise the voltage to about
15v and show a normal charge on the ammeter. At about 15v the voltage
regulator (at the other end from the cut-out) operates, its break contact
introduces a resistance into the field of the generator, which reduces the
output voltage. The relay contact actually opens and closes very rapidly,
you should feel a 'buzzing' if you touch the armature.
The middle relay is the current regulator. If the load (i.e. output
current) is too high this relay operates in a similar manner to the voltage
regulator, but keeping the current through the generator to a safe level,
but the voltage will probably be less than the 'normal' charging voltage of
about 15v.
However if your system is charging correctly for the first five minutes
until you hit the freeway - and presumably higher speeds - then possibly the
commutator and/or brushes are worn and bouncing at higher speeds.
PaulH.
----- Original Message -----
From: Phil <jello@ida.net>
To: <mgs@autox.team.net>
Sent: Wednesday, December 12, 2001 4:47 AM
Subject: control loops
> Does anyone know exactly how the regulator in a 1967 and earlier MGB
> works??
> When I leave work, I keep the low idle of the choke up above 1200 rpm,
> so I can always be generating. No problem for about 5 minutes.
> When I hit the freeway (speed limit 75mph), my car has warmed up, but
> the ammeter is reading negative. The voltmeter gets down to about 11
> volts, and the lights are dim. I keep the heater off for the time
> being.
> After about 10 minutes on the freeway, the regulator starts cycling in a
> high amperage mode. This makes the lights go bright for a couple
> seconds, and then dim for about 10 seconds. This continues for about 5
> minutes. And gets the battery up to about 13 volts.
> Somewhere, the system settles down, and has a net positive all the
> time. I can even run lights, heater, and wipers and keep 14 volts
> (which I understand is the specified high voltage level the system is
> supposed to produce).
///
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