Smith Temp Guage Lighting Problem

[Guest david_l_perry]

I have a Smiths Classic gauge temperature gauge fitted to my hood but cant get the lamp to work when its inside

 

 

After replacing the original broken Sender unit with a direct smiths replacement, the temperature gauge now works fine and reads typically around the Neutral mark when the engine is running, so I figure all is well.

 

Only connections are the 10v + feed from the voltage regulator and the Negative directly from the sender unit.

 

 

The bulb is metal cased and must get its earth from the gauge casing.

 

 

As soon as I place the lamp in the housing the gauge dial appears to swap polarity and bangs down to zero. Remove the lamp and the gauge works fine again.

 

The same happens if I earth the case with a separate cable...the dial bangs down to zero

 

 

The lamp will only light when I connect the separate earth feed to the gauge chassis, but then the dial stops working as described......

 

 

The gauge is mounted on a ply bulkhead so it does not get any earth contact other than from the sender unit......

 

Any body any suggestion ??

[ibrooks]

You sure you've got the sender and power connected the right way round? The casing of the gauge should be earthed so if that's making it drop to zero then something is wrong.

 

It's not from a positive earthed car is it?

 

Iain

[knights_templar]

you mention +10v feed to sender and -0v through sender

 

you do not mention the feed for the bulb which should be a separate +12v from the lighting circuit, earth through the case, which should have its own earth with your dash

[Guest david_l_perry]

You sure you've got the sender and power connected the right way round? The casing of the gauge should be earthed so if that's making it drop to zero then something is wrong.

 

It's not from a positive earthed car is it?

 

Iain

 

Its negative earthed car

 

I have just gone and checked it out again. Swapped the two inputs to the gauge, attached the earth and connected the lamp.

 

The Lamp works but bugger all from the gauge. I then remove the Lamp, no change, then removed the earth, no change.

 

I then connected the two inputs back to how I had them and the gauge is back working, but again stops as soon as I connect the earth to the case or the lamp.....

 

 

And here she is working with just the input +10v feed from the voltage regulator and sender input connected.....

 

 

The easy way round this is to replace the metal lamp holder with a plastic one that has its own earth feed....but I am pretty sure its got to be a simple fix.....trouble is I appear to be simpler than the fix is.

 

I am more interested in the gauge working than the lamp....but it would be nice to have both

[Guest david_l_perry]

you mention +10v feed to sender and -0v through sender

 

you do not mention the feed for the bulb which should be a separate +12v from the lighting circuit, earth through the case, which should have its own earth with your dash

 

Yes the Lamp has its own separate +12v feed from the lighting

[Guest david_l_perry]

Just found this wiring diagram and this does not indicate a separate earth for the gauge, and separate feeds for the lamp....which would make sense for my situation....but does not fit with the metal lamp holder....need to ask Mitch if it ever worked......

 

[biksz]

use some heat shrink over the metal lamp input then use seperate earth for bulb if that works you no it down to metal bublb holder interfering with it

[Guest david_l_perry]

As its push fit into the back of the case with the metal one, it would not fit adding heat shrink, I would need to get a new plastic lamp holder to fit it

[fry61]

Think there must be an internal fault with the gauge if connecting casing to direct earth makes reading go to FSD; as a normal dash the casing would be earthed through the metal of the dash. It also throws a wobbly when lamp is in circuit cause lamp resistance is much lower than temp.sender resistance. Needs a replacement gauge IMO.

[knights_templar]

I would go with florin metal work, internal fault

[speedtripledan]

Take gauge out and do resistance check from the sender input to the case it should at a guess be open circuit I have a spare smiths gauge in the garage I will test one in a bit

[Guest david_l_perry]

Thanks for that....

[speedtripledan]

Tested mine and they are open circuit from all inputs to case

[Guest david_l_perry]

Just tested my temp Gauge and there is a closed circuit between the case and the input terminals....

 

I will open it up over the weekend and see if its anything obvious.....

 

 

Dave

[ibrooks]

I didn't mean was your car a positive earth but had the gauge come from a car that was. Smiths gauges are not all the same and have been fitted to lord only knows how many different cars over the years and it's quite possible to have a gauge made for a positive earth car that looks just like a gauge for a negative earth car.

 

Assuming it's not......

 

Check the rear of the gauge first - some of the Smiths ones had screw terminals that were insulated from the case with fibre washers. If the screw terminal has been off and replaced without the washers it could be contacting the case rather than there being a problem inside the gauge.

 

Iain