Albert Escalante
Yes its normal to have the diode on the 1977 1978. I think it was to
deenergize the coil windings and let the power drain off the alternater.
Sometimes you wil hear stories of different owners cars not shuting off when
they turn the car off and take the key out of the ignition. This was the
cure to keep the
small amount of feedback out of the tickler wire.
I copied this part of a thread from the mg enthuseast green page archive.
The other problem is a basic design flaw of the ignition circuit on the
later cars. On later cars ??, the ignition switch relay will not turn off
when the ignition is turned off. This is dependent on the variation of the
relay hold in current, and the resistance of the ignition light bulb. Of
course this doesn't matter if the running on valve works. There is a simple
fix for this. Some cars have this fix by the dealer. Possibly by the
factory, but I don't know. To fix this, place a diode (like the ones found
under the glove compartment) in the brown/yellow wire coming from the
alternator. The diode should point to the alternator. The brown/yellow wire
is the wire for the ignition light. This is detailed in the Jan '92 issue of
the Octagon, of the AMGBA.
> This week I replaced the alternator on my 1978 MGB roadster. I had
suffered
> an electrical meltdown that burned the wiring from about 6" away down to
and
> through the connector that goes onto the back of the alternator to attach
the
> wiring. After seemingly completing the job and having bolted and wired
> everything back together, I went to start the car. I noticed that the
panel light that
> says "IGNITION" on the dash in between the two big gauges (the Tach & the
> Speedo) stayed lit no matter if the ignition was on or not. A first I
thought that
> it stayed on due to a short caused by two wires that had melted together
when
> the alternator went ka-blew-ie!! (technical terminology, heh, heh!) I then
> carefully undid the loom up to where the wiring showed no signs of heat
damage
> and spliced new wiring in to replace all damaged wires. Still no change,
the
> darn light stayed on growing fainter when the engine was running. Hoping
that I
> hadn't suffered a short elsewhere I backtracked the whole days progress,
> rummaging through the burned and melted wiring that I'd cut & replaced
with new
> undamaged wires of the same gauge and size, I noticed what at first I'd
mistaken
> to be a partially burnt wire and sheathing. Looking closer revealed it to
be a
> small diode that was spliced into the alternator "tickler" lead wire. It
was
> a suppresser diode that must act like a one way gate or valve. Remember
> electricity flows in one direction like water in a tube. Evidently this
diode kept
> the current from going up and making the IGNITION light stay lit. Once I'd
> spliced it into the ticker wire the IGNITION light only went on when the
ignition
> key was turned on and the engine was started, once the engine started the
> light went off and it only reappeared if the motor went dead and
thankfully it
> immediately went off when I turned the key off. Only to reappear when the
> ignition was switched back on, as it should. But I've never seen this type
diode
> before on any alternator other than one on my PEUGEOT diesel. Is it a
normal thing
> to have this suppressor diode on the alternator "tickler" wire? It's a new
> one on me...
> -=Safety-Fast=-
> Albert Escalante, owner-1978 MGB
> Central Coast British Car Club (Ca.)
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