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TR2/3/3A Voltage regulators

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M

marting tr3&5

Freshman Member
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Hi all

i have had a tr5 for a number of years and I have recently bought a tr3a that has been off the road since 1975. I have replaced the internals of the mechanical temperature gauge with an electronic one and fitted a sensor. in the tr5 there is a voltage regulator for the gauges. I cannot see one on the tr3. I assume it has one otherwise the fuel gauge reading would vary depending on the the engine revs and therefore the output of the Dynamo. Is there one somewhere that I have not found. Does the fuel gauge have an internal regulator. Or do I need to fit one for my electronic fuel guage?
 
The box on the RH side of the firewall regulates the dynamo. There is no other voltage regulator. I've never noticed my fuel gage reading changing as I rev the engine, so I don't quite follow...

Inside the fuel gage there is only a magnetized needle and 3 coils. No regulator there either.
 
The TR3 fuel gauge uses what is called a "balanced" movement; essentially it compares the voltage across two voltage dividers, one leg of which is the sending unit in the tank. Thus it is relatively unaffected by changes in system voltage and no external "voltage stabilizer" is required. TR4 and later use a much simpler (aka cheaper) gauge movement for both fuel and temperature, which is affected directly by supply voltage and hence need the external "voltage stabilizer".

You'll need to examine the instructions for your electronic fuel gauge, but I've never heard of one that did not include it's own internal regulation. Using a Triumph "voltage stabilizer" would clearly be a mistake, since they actually switch the power on and off relatively slowly to achieve an average of 10 volts. The gauges respond too slowly to see the switching, but if you hook up an ordinary test light, you'll see it flash on and off. Your electronic gauge would flash on and off as well :smile:
 
Aloha Martin,

The fuel gauge on the TR2/3 (and MGA) operates on a balance circuit. Page P.3 of the factory manual shows the set-up, with three fixed resistances in the gauge and a variable resistance at the tank. Barney Gaylord (aka MGA Guru) has a great tech article at https://mgaguru.com/mgtech/electric/fg_05.htm on it. Electrically it seems similar to a Whetstone Bridge. As John said, the needle is between two coils and is pulled one way or the other based on current flow/magnetism through the 'sides'. Since both 'sides' of the balance circuit share the same power source, voltage changes from RPM, load, etc are negated.

Hopefully yours works, because the coils in the gauge are tiny, with hairs for wires. Good luck!

Jeff

Ha - I see Randall came in with the explanation as I was typing!
 
Last edited:
Thanks all. That explains why I cant find it. The capillary tube to the original temperature gauge was broken. As such I have transferred the face, bezel and glass on a more recent electrical temperature gauge that uses a sender unit. Basically the same set up a later TR's. As such I think I will need to use a voltage stabiliser just for this one gauge. The finished result looks quite good though. The only give away is the needle is too white.

View attachment 35724
 
If it is the same as later TRs, then you will definitely need the VS.
 
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