[TR] gas gauge and voltage regulators

[Frank Fisher]

as alwaysthanks
      From: Randall <TR3driver at ca.rr.com>
 To: 'Frank Fisher' <yellowtr3 at yahoo.com>; 'Triumphs List' <triumphs at autox.team.net> 
 Sent: Thursday, July 2, 2015 3:25 PM
 Subject: RE: [TR] gas gauge and voltage regulators
   
>  the purpose of the voltage regulator is to stop the ups and 
> downs of the voltage caused by alternator as engine revs? 

Yes.  At idle, the stock Lucas alternator (or generator on the TR4/4A) puts out almost nothing, so the battery voltage can droop
quite a bit under heavy load (eg headlights on).  The gauges used on TR4-6 work on heat, and so are very sensitive to changes in
voltage (power and therefore heat is related to the square of the voltage).

> what then is usual output? ie 13/14 volts in----? volts out.

The stock "voltage stabilizer" doesn't actually change the voltage.  Instead, it interrupts the voltage output part of the time,
such that the average is 10 volts.  The technique is commonly known as Pulse Width Modulation (PWM) and is still quite common today.
But, modern devices use solid state switches (transistors) that can switch very quickly.  Transistors back then were expensive and
fragile, so Smiths (or whoever owned them by then) used a simple mechanical switch that only operated a few times per second.
Pretty ingenious really, but like so many other things on our cars, strange and poorly understood today.

Since the gauges are so slow to respond, they don't follow the constant up and down from the VS, but any voltmeter or DMM will.  So,
it's more or less impossible to check the original VS accuracy with a voltmeter or DMM.  Smiths' suggestion was to hook up a "known
good" gauge along with a dash light bulb in series to a regulated 10v source, note the resulting reading, then hook it up to the VS
to be tested and compare the reading (after waiting for it to settle each time).

But in my limited experience, the usual VS failure modes are either that the heater burns out (meaning it supplies input voltage all
the time), or the contacts go bad.  You can check the contact resistance with an ohmmeter.  Or just pull the wire off the
temperature sender and connect it to a test lamp (with the other side of the lamp grounded).  If you can see the light blink on and
off, the VS is working, and probably reasonably accurate.  Unless you are chasing a fairly subtle error in gauge reading, the
problem is not the VS.



> any one have a list of what smiths gauges require a voltage 
> stabilizer and those that don't. have not been to see the car 
> yet, but id like to be prepared.

Sorry.  I had a partial list, but can't seem to find it at the moment.  But all of the fuel and temperature gauges originally used
on TR4-TR6 (as well as Stag, 2000 and so on) require a VS.  

-- Randall  



  
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