Help with old school car instruments - Page 1
Forum section: electronics / beginners

author: ourkid2000
author_rank: Newbie
timestamp: November 28, 2023, 04:31:18 pm
content: Good day! First post for me, thanks for entertaining my question. I figured this would be the place to discuss old analog panel meters!Just looking for a little help with the instruments in an old car of mine as I'm very curious as to how they work. I've attached a picture where I show the two gauges (one being the fuel gauge (lower in the diagram, the other being the temperature gauge - upper in the diagram). They appear to have very small resistors on the back of the instruments themselves.......what is this? A shunt resistor possibly? Something else?They both seem to operate the same way, Ithink. Voltage is applied and they have a variable resistor on the output (fuel tank sending unit or temperature sending unit) and the gauge deflects accordingly. I'm just not 100% sure how they work, how the fixed resistors factor into them, and am wondering if anyone can see exactly how these things do their job?I think what really has me scratching my head is that you have 12V applied to one terminal on the gauge right? Then if you go to the variable part of the circuit, say back to the fuel sending unit, which is on the ground side and remove the connection you won't measure 12V at that connector. It will be something much lower. Every time I measure them anyway, I always get a much lower voltage.So voltage drop means current is flowing somewhere which really confuses me in this case. Must have something to do with the fixed resistors? Thanks for any help you can offer! Cheers!

author: CaptDon
author_rank: Super Contributor
timestamp: November 28, 2023, 05:35:11 pm
content: Those look like a sort of balanced bridge style. Some of the balanced bridge gauges will remain at their last indicated position even when power is turned of. Some of the old fuel gauges would continue to show however much fuel was present when the key was turned off. You would probably measure around 6 volts or even 3 volts with the sending unit disconnected. There are also King-Seely gauges that use a hot wire movement. The sending unit has a wire wound bi-metal strip which is heated by the current flowing through the gauge (which also heats the gauge pointer mechanism) and as the bi-metal strip in the sending unit reaches a certain temperature the contact points on the strip go open circuit. Then as soon as cooling starts the contacts close again. This creates a P.W.M. type situation and the absolute position of the one contact point is movable and moved by fuel level, temperature, oil pressure, etc. Just google it up!!!

author: ourkid2000
author_rank: Newbie
timestamp: November 28, 2023, 05:43:53 pm
content: Interesting. I did notice that they do stay at their last indicated position when the power is removed. These would be wheatstone bridge circuits then?

author: CaptDon
author_rank: Super Contributor
timestamp: November 28, 2023, 06:05:51 pm
content: I can remember several of my Dad's cars from the early to mid 60's had those kind of gauges. My 1947 truck has King-Seely hotwire gauges and when I converted the truck from 6 volts to 12 volts I measured the total resistance of the gauge and sending unit in series and added a 5 watt ceramic resistor of equal value in series. It works very well indeed. One of the weird things about my truck gauges is that at power off the oil and fuel gauge settle to the low side but the temperature gauge goes to the high side. Of course if you disconnect the temp sender the gauge indicates off scale on the hot side. A safety feature maybe to induce you to shut the engine down? I remember Dad's 65 chevy impala had both a COLD and a HOT warning light!! 283 2BBL V8 engine.

author: ourkid2000
author_rank: Newbie
timestamp: November 28, 2023, 06:17:03 pm
content: Yes, this car is a 1966 Oldsmobile. I'm having a lot of fun trying to figure things out. The operation of these gauges were the first ones to catch my eye. I'm now enjoying learning about wheatstone bridges, LOL.