Not sure where to go with this, or if I really even have an issue at all. This has come about because I started researching into why my voltmeter (old one and now a new one) drops in volts when I apply the brake lights or when I turn on the reverse lights. If the engine is not running but the ignition is on I get 12 volts on the guage. Applying the brake lights or reverse lights drops the guage to below 12. Like to 10. New battery. With engine running at idle of course I have higher than 12 volts, but still experience the voltage drop (just not as much).

All the guages, the reverse lights and the brake lights are connected to fuse 9. Fuse 9 gets its juice from terminal 15 of the ignition switch, which also feeds directly (fuse-less) to terminal 15 of the coil. So I started to wonder if terminal 15 on my ignition switch is feeding enough juice to this circuit.

To test this idea I jumpered from the feed end of fuse 10 (which is directly connected to the battery) over to the feed end of fuse 9 and presto, the voltmeter does not drop hardly any when I apply brake lights or reverse lights.

I checked the lead from the battery to my ignition switch that feeds terminal 15 of the switch, and it seems fine. It's as if something is going on inside the ignition switch and a full connection is not being made from the lead to terminal 15.

Has anyone researched into anything like this? Since circuit 15 is such an important one, feeding the coil and all, I am a bit curious why the voltage is so unstable within.

Thanks!

I believe that is the correct operation for your 914 volt meter... I will check, but if I recall correctly, mine does exactly the same thing under electrical load. Any of you EE's out there care to explain it to us non engineers?

IIRC you have 35w bulbs in the brake and revers light positions? If so you have a total of 70w being drawn from your battery that is not being put back by the alternator.

And now for the math:

Wattage = current (amps) * Volts
70w = X amps * 12 volts
X amps = 70w/12v

amps = 5.8 amps of draw down from your battery. This is why LEDs kick so much ass! Low voltage and current draw.

So in simple terms, it is normal for your battery to feel a significant draw from these bulbs, particularly when the engine is not running.

Compare this to your headlights coming on...how low does the voltmeter go then?

Just to make sure I mention this, yes Amps and volts are not the same thing, but when you draw current form you battery you will experience a voltage drop as well for reasons that are beyond the scope of this quesiton.

here is something you might want to try. Measure the voltage at the battery. If it is still showing 12 volts when you hit the brakes then you know that the battery is capable of the current required.

If the battery is good then you have a 2 volt drop in the cct somewhere between the battery and where you are measuring 10v. Likely cause is contact resistance. Lets see if I can remember back a few decades. The other formula to match Mikes is V=IR Voltage=current times load. Mike estimates 5.8Amps on that cct. So moving the formula around a bit V/I=R 2volt drop / 5.8 Amps = .33 ohms

That is not an awful lot resistance but you should be able to find it. Just do your voltage measurements at each terminal point working back to the battery. At some point you should see a jump (or perhaps a series of jumps) back to 12 volts. My bet is that you see that 2 volt jump across your ignitions switch or fuse block - contact resistance.

the curcut for the volt meter sucks by design. it's measuring a voltage drop at the fuse panel. not at the battery. when you step on the brake or any thing else that draws a good amount of current there is a drop in voltage because of the restistance of the wire at that high of current draw. the lower the voltage the higher the amperage draw. these cars have a wimpy gage wire for most curcuts, imo only good enoough for runnig to relay.