ant auto sparkies out there

actually not quite ... that's what a sparky would say ..

A sender and a gauge are calibrated through the internal resistance of the sender and the gauge and the min current/max current is equivalent to the value range of the gauge .. lets say 0 -50ma for 0 - x degrees .. or 0-100ma .. depends on the sender/gauge .. now some manufacturers have a different wiring and don't wire against ground but against a temp compensated circuit or such .. but still the same principal applies ... you have to measure the milliamps flowing or measure resistance directly to derive the internal resistance of the sender as well as the gauge. You do the same with the second gauge ... now , in a simple case when the gauges are wired against minus and you hook up the second gauge, you will reduce the resitance of the circuit and the current will be hire and your original gauge will show more or probably will be against max .. you have to put a resistor in series with each gauge in the exact ratio of the internal resitance of the gauges in the way that the resulting paralleled resitance of the gauges is such that the current through the original gauge is the same as before but the current through the second gauge is as required by the second gauge ... you can use small trim potis to actually adjust them ... as a result the current through your sender will be higher, which is not really a problem because those currents are pretty low anyway. Sounds a bit confusing ? well .. it does work .. been there done that .. is a simple excercise for a year 11 student in physics .. measurement range adjustment of gauges it's called where I come from .. at least thats a direct translation .. don't know what they call it here ...

good luck
gmd

Hey mark Taylor after litttle bit of advice I have a vr series 2 commocore v6 in gq patrol cant get original tacho to work, is there different pulses on them and what can i do to get it to work. would one of those little things like a kpot I think it is called soldered in between wire and that can be adjusted with micro screw driverwork. I used them to to adjust tuning on kawasaki bikes wit hfuel injection years ago when people removed original exhaust and put on 4into ones on them itwas wired from the computer cost 90 cents from tandy and adjusted the fuel ect for the bike so it semed like it was running standard system.

Most gauges use a variable resistor to check the gauge itself. (Eg a VDO G100)

Some guages are "thermal" where a bi metal strip deflects and moves the pointer depending on the current flow through it which in turn is regulated by the varying resistance of the sender unit as the engine temperature changes.

Then there are "cross coil gauges" like the traditional VDO and some Veglia Borletti types. These have 2 coils of known resistance. One coil carries power straight to earth while the other coil "earths" through the varying resistance of a temperature sender unit. In the gauge above these coils is a disc magnet with a shaft and pointer connected. As the resistance of the temperature send unit changes, the current through the second coil changes causing a change in the magnetic flux around thie disc magnet and it moves and carries the pointer with it.

The point of all my ramblings is that these 2 different types of gauge constructions give hi and low temp readings with their resistance values at opposite ends of the scale.

A "thermal" or "hot wire" gauge has lo resistance value for full scale deflection (Eg 10 ohms cold and 70 ohms at hot) while a cross coil is the opposite., that is hi resistance for full scale deflection. (Eg 10 ohms at cold and 180 at hot FSD)

Good luck with your trim pots.

Been there, done that!

Cheers

MT

A "thermal" or "hot wire" gauge has lo resistance value for full scale deflection (Eg 10 ohms cold and 70 ohms at hot) while a cross coil is the opposite., that is hi resistance for full scale deflection. (Eg 10 ohms at cold and 180 at hot FSD)

Er got that wrong... try 70 ohms cold and 10 ohms hot for thermal gauges.

Too many black russians tonight.

Cheers

MT