TURN SIGNALS vs. VOLTMETER

My latest wiring woe just cropped up. I ditched the ammeter and installed a voltmeter in its place. I pulled the + from an idle ign switch terminal and grounded it to the dash panel. Now when I use my Spartan turn signal switch I get a salsa dance from the voltmeter. It dips from 14+V to 12V in sync with the turn signal. Any thoughts?

 

Bad ground or high resistance connection to the +12V source. The turn signals are acting as a voltage divider somehow. Draw out the circuit on paper and measure the resistance of each leg.

 

Tim,

I have been diagnosed as mildly electron deficient......can you spell it out a little more? About all I know how to do is measure continuity without EXACT instructions. What exactly am I looking for? and where? are there "std" resistance values for these legs(what legs) or am I just looking for a noticable difference between the legs...wait....that sounds like a viagra commercial!

fwiw, the flasher circuit excites the voltmeter the same way.

 

Sounds like you have lost ground. Bet your fuel/temp gauges act funky also. Should run a new ground wire from one of your speedo mounting lugs to the dash, and then run ground wire from that connection down to frame/engine or both.

 

I had the same problem when I replaced my ammeter with a voltmeter in my 1971 CJ. I don't know why it occurred, and I don't know alot about electricity. However, I just ran another wire directly from the voltmeter to the alternator, and it worked fine.

 

Was the engine running when you tried it, or did you just have the key on? If you just had the key on, then sometimes it is common for the meter to dance when the alternator/gen set is not charging. Each time the turn signal sends juice to the bulbs it drops the voltage a little. Just a thought

 

Same condition with engine running or off..idle or highway speeds. The ground is the same as used for all the instrument lights and fuel gauge. They work great. What confuses me is I am pulling the power for the voltmeter off an extra ignition terminal at the keyed switch. Not sure how either blinker would draw this circuit down so much (2 volts+/-), especially with the alternator charging.

 

Re my previous reply, each "leg" of a circuit is a path from the voltage source (positive battery terminal) to the device/component (bulb, voltmeter, switch, whatever), or from the device to ground potential (here the negative terminal of the battery), or from one device to another.

Ideally these legs should contribute nothing to the circuit, ie be a zero resistance path connecting the two devices, or the sorce to a device, etc. In reality, the wires used for the legs have a small resistance - this means that some of the current conducted is lost as heat, and there is a small potential loss (voltage drop) through the leg. These drops are normally so small that they are negligible, and we ignore them. When the resistances get large, the drops get large, and they can cause symptoms like you describe.

The most common bad leg is the body ground. Cars use the body metal (or the frame) as a wire, forming a return path to the battery. The body is not an ideal conductor, but it's good enough for cars. Corrosion at ground connections (wires rom devices to the body or frame) is a very very very very common cause of these puzzling electrical behaviors, like you see with the voltmeter.

A voltage divider in this case is two circuits in parallel, the lights and the voltmeter. Each circuit has a resistance. If the resistance of the legs of one device or the other is not negligible, the potential (voltage) across the devices is changed by whether there is current flowing through one device or both.

I expect the voltage at the voltmeter is really changing when the indicators blink. For this to happen, the legs of the voltmeter circuit have to have a some large resistance somewhere. The most likely problem is a bad ground from the voltmeter to the body. Body grounds are always your first suspect. Wire connections on the plus side can also corrode.

You really need to learn a little about circuits and understand how to use a multimeter before you can approach this systematically. For now, I suggest you check all your grounds and all the wire connections in both the voltmeter and indicators circuit.

 

Thanx Tim!

Will dig into it this weekend.

 

Turn the key to "ON" and pull on the headlights and see what the Volt meter does..