Voltage drop when I hit the brakes Options

[PinetreePorsche]

What's wrong here? The voltage on my meter (stock- center console) falls from near 13 to 11-and-a-bit, daytime, and has a corresponding drop from a slightly lower starting (mid-12 area) with the lights on. Turning on the lights, without other loads, barely deflects the needle down, maybe .3 volts--that's for 2 fr. corner, two taillights and two markers. Why would the brake bulbs drain so much more? -is there a short somewhere--and no, I haven't been blowing any fuses. Just wondered--don't want to get stuck in an after-dark traffic jam, sit with the brakes on and drain the battery. -C

[davesprinkle]

Just for fun, I took my 914 voltmeter to work and verified its calibration with a laboratory power supply. Within its typical operating range (say, 10V to 14V), the meter's error was significantly less than 0.25V. Outside of that range, the error was less than 0.5V.

OK. You won't be making any precision measurements with this kind of accuracy, but it's good enough for use on the dash of a 914.

So if the meter is so decent on the bench, why is it so darned awful when hooked into the 914 wiring? The answer is this: the problem isn't with the meter -- it's with the wiring. The meter is accurately reporting the voltage at the FUSEBOX, not at the BATTERY. So if the voltmeter twitches by 0.5V in synch with the turnsignals, it's because the voltage at the fusebox is twitching by 0.5V.

So why does this happen? OK, just some rough numbers: 2 21W turnsignals will pull roughly 3.4 Amps. Given this current, a 0.5V twitch requires a wire resistance of only 0.15 Ohm. The circuits probably had that kind of resistance when they were fresh out of the factory -- 30 years of oxidation on the terminals has only made the resistance greater.

The fundamental problem is one of design -- the engineers neglected to ensure that the voltage sense lead carries little current.

So what's the solution? Run a new voltage sense lead all the way from the battery positive terminal. Connect it to the voltmeter by passing it through the contacts of a relay. Switch the coil of the relay with the previous voltmeter lead. There you go. The resistance of our new sense lead isn't significantly less, but because none of our lighting currents flow across that resistance, we'll no longer have the voltage error. The key here is to use the sense lead ONLY for sensing -- if you have any other load being fed by the lead, you'll corrupt the accuracy of the meter.

Or you could just tolerate the twitching needle...