QuestionI have a 61 chevy impala
I have replaced the fule sending unit with a new unit for a 61 impala. I also sent the guage off to be fixed if needed. ($$ later) Yesterday I installed the sending unit and the guage. As soon as i turned the key on smoke started coming from the wiring harness at the guage. i took the guage out and let it hang and then turned the key on again. Nothing happened .
i have my own ideas what may be wrong but I'm not an expert.
can you help?
AnswerSome part of the gauge unit must have been touching ground in such a way that the power feed to it got shorted to ground.
These gauge systems are somewhat complicated to work on, and could be dangerous.
I think the best thing for me to do is assume you need a complete course on how they work - I hope this isn't a waste of our time!
The oil, temp and fuel gauges work on a voltage produced by a "gauge regulator", which is sometimes also called a "5 volt" regulator. Actually, what it produces is a pulsed 12 volt which is "on" 42% of the time, so that an averaging type meter will indicate approximately 5 volts. A test light will show a pulsing 12 volts, as will a rapidly responding voltmeter. the pulse rate is about once per second. The gauge regulator has 3 connections: 12 volts from the ignition circuit, ground (which may be provided by the mounting screw) and "5 volts".
This so called "5 volts" is supplied to one terminal of each gauge. The second terminal on the gauge goes to the sender associated with that gauge. In the case of the fuel gauge, the sender is a variable resistor which ranges from under 10 Ohms for a full tank to over 50 Ohms for an empty tank.
Some cars incorporate the gauge regulator within one of the dash gauge units. That particular gauge unit will have more than 2 terminals on it. Other cars mount the gauge regulator externally to the gauges, usually on a mounting screw on the backside of the speedometer head. In that case each gauge only has 2 terminals on it.
The gauge units consist of a bi-metallic strip which is heated by the current flowing through the gauge unit and out to ground via the sending unit. As the sending unit in the fuel tank's resistance changes, it varies the amount of this current, and thus the heat generated within the gauge indicator. The bi-metallic strip is connected to the indicator needle in such a way that full scale on the gauge corresponds to a full tank (minimum resistance).
Your experience with smoke means that something got grounded directly, rather than through the current limiting resistance of the sender. This could be because the sender wire is touching ground somewhere, or because there is a problem with the sender itself.
You will need a multi-meter to sort this out. I recommend that anyone who is working on their own car have one - Radio Shack can provide you with a multi-meter with AMP, DC and AC VOlTS, and OHMs scales. I prefer the analog readout meters for automotive work, not the digital readout meters.
My advice is to get a multi-meter and check the resistance of the wire back to your sender to see if it is within the range of about 5 to 100 ohms, with a full tank producing the lower numbers. If the resistance is zero or very low, there is a short to ground somewhere on that wire.
Then, set your multi-meter on the DC volts scale of at least 15 volts, and identify the source of 12 volts to the gauge regulator (it comes from the ignition circuit so the key must be on) and the pulsing "5 volt" signal, which goes to each gauge (except the ammeter). Make sure you have the wires on the right terminals - they are labeled, but the labels are very hard to see.
The danger to be avoided is that should the full 12 volts get connected directly to the tank unit, it will bring the resistance element inside the gas tank up to ignition temperature very quickly, which could possibly cause a nasty fire or even an explosion.
Good luck - and let me know if I can help further.
Dick
