Ambient Temperature Gauge Conversion

[handi_andi]

Hello All
A while back I asked about how to repair an ambient temperature gauge and whilst I got some very helpful responses and a link to a website for a repair method I really didn't feel like playing with the rather toxic substance that they are filled up with. Hence I went on a quest to convert mine over to electric.

The pictures below is the result and the basis of it is a volvo 340 ambient temperature gauge. The only change that I have really made is to open up and prise off the original front, remove the needle, trim down the end of the plastic tube, glue on the needle from the original Smiths gauge, and then put the Smiths gauge glass and chrome fitting onto the volvo gauge. The gauge is a tight fit back into the dashboard being about 1/2" longer than the Smiths gauge but fits nicely and looks the part. The only t that might annoy some people, although it could be removed, is that there is a red LED just below the needle that shows that the gauge is working.

Can someone tell me though where the original temperature gauge probe was originally located so that I can fit the little sensor in the same position please? I have wired the positive into the feed to the fuel gauge, the ground into a suitable existing ground, the built in light into the wiring for the original gauge light, which just leaves me to run the green wire to the sensor location and the black wire from the sensor back to a suitable ground location.

I brought the Volvo gauge & sensor off ebay for ?3.99 plus p&p as it was mislabelled as a water temperature gauge, but the picture told a different story. They seem to come up fairly often but tend to sell as a set for between ?20 and ?30.

Hope this is of help to everyone.

Cheers

Andy

[tdafforn]

Looks good...
However I may be wrong, but couldn't the new dial be replaced with the original dial. I looks as though it would be reasonably calibrated from -10 to +30 if you just reorientated the needle to match the zero position!
Cheers
Tim

[handi_andi]

That was my original plan, and there is a variable resistor on the back of the circuit board, which I assume, adjusts the zero point. However, the actual angular change required for each 10degree segment is very different between the two hence why I stuck with the volvo dial. No doubt someone with a better electronics background to myself could come up with something to adjust the angular swing. Probably just requires something simple as well. I'll leave that to someone else though to develop further, least give us all a starter to ten as to how to repair these gauges.
Andy

[msd1107]

Interesting and thoughtful conversion. And a very astute purchase. I have seen the chrome trim rings available separately on Ebay in case you want a consistant set of trim rings.

Which brings up the question, could the same conceptual conversion be performed for oil pressure and water temperature?

Single gauge analogue versions of oil peressure/temperature and water temperature are 270 deg sweep, providing good reading accuracy.

The electrical versions of these gauges are typically 90 deg sweep.

It would be nice to have a 270 deg sweep gauge that is electrically driven but still maintains that period Smith's look.

[handi_andi]

On paper yes, as the volvo gauge is just a a coil operating against a spring and I assume the sweep is proportional to the voltage applied. Interestingly since I posted that picture I have just replaced the modern oil pressure gauge with an original Lotus one, which was still in its box and the wiring for it was still in place in the car. The sender unit cost more and was a damn site harder to install than the gauge.

Most of the Smith's gauges seem easy enough to repair having taken apart the fuel gauge to fix it. The reason for the limited sweep on the smiths gauges is that they operate using a bimetallic strip with resitance wire wound around it which gets hot when electricity is applied to it. The bending of the strip then causes the pin on the needle to slide along it causing the needle to move. My fuel gauge had failed as the lacquer holding the heated wire to one of the terminal pegs had failed due to constant heating and cooling. A little of my ex wifes nail varnish solved the problem nicely!

Andy

[rgh0]

Original sensor was located in a loop of metal at the front of the air cleaner

cheers Rohan

[handi_andi]

Cheers Rohan. Was thinking about somewhere in the nose cone, prefably out of the air flow, so the original being there is excellent. Thanks
Andy