In a message dated 97-07-26 20:50:02 EDT, up497@freenet.victoria.bc.ca (John
L. Walker) writes:
> I'd like
> to check the voltage box that leads to the gauges, but I don't know how.
>
> Has anyone dismantled one of these? By the sounds of it, it's a
> transistor (voltage regulator).
>
> The Haynes manual hasn't got a clue on how to test for electric problems,
> aside from replacing each part until you find a faulty one (something I
> don't have the money, or the fuses, to do)-- anyone conjured up a
> reliable method of testing?
Malcolm:
The regulator for the gauges, unless it is a modern replacement solid state
unit, works just like a turn signal flasher. It consists of a bi-metal strip,
a set of contacts, and a piece of heater element wrapped around the bi-metal
strip.
One of the contacts is fastened to the strip, and the other is fastened to
the case by an insulator. When cold, the contacts are closed. Current flows
through the closed contacts, through the heater element, and then to the
gauges. As current flows, the heater element heats up, bending the bi-metal
strip, which then opens the contacts. When this happens, current flow is shut
off, and the heat element cools, allowing the strip to straighten out,
closing the contacts, and starting the process all over again.
The end result of all this is an on-off aplication of voltage to the gauges.
By varying the ratio of on to off, the effective voltage to the gauges can be
controlled. (The gauges work on the same principle, btw. The sending unit
controls the current through the gauge, which has a heating element and
bi-metal strip also. The bending of the strip is a function of the heat
produced by the heating element, which is a function of the current flow. The
bending motion of the strip is mechanically converted to rotation of the
needle.)
To test the regulator, it must in the circuit, and the gauges (at least one)
must be functional. After the unit has had a couple of minutes to stabilize,
a voltmeter connected to the output should show the on-off action, that is,
the voltage should alternate between 12 volts and zero. Without specia
equipment, though, it is very difficult to determine if the average voltage
is correct or not. The input to the unit is a green wire, and the output is,
I think, a Light Green/Green wire.
Hope this helps!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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