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As part of the pre-flight inspection, one of the checks is to verify that the low voltage light is illuminated when switching just the battery on. This makes sense; there's a low voltage and the battery is being discharged because we're running everything off the battery, and the alternator is not providing any power.

However, as part of the startup - just before ignition - we switch both the battery and alternator switches on. At this point, the low voltage light is not illuminated, despite the alternator not providing power as the engine is not running. This doesn't make sense to me - though we've turned the ACU on, the alternator is not providing power. This seems like something that should trigger the warning light.

I've examined the C152 POH and it shows a basic electrical layout. I'm not sure as to the copyright of the handbook, so I won't post it here. It is available online for those interested, figure 7.7. Unfortunately, this did not help explain the behaviour I'm seeing.

I've reproduced this on two different 152s. Why does the warning light not detect the fact that the alternator is providing no power, or am I completely misunderstanding the function of this light?

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C152 models until 1978 had a voltage regulator, an over-voltage sensor, and an over-voltage indicator light. See Carlo's answer for the wiring diagram.

Starting from 1979, Cessna used an Alternator Control Unit (ACU) (Cessna Service News, 1978). This ACU integrates the regulator, as well as over- and low-voltage sensors. There is no over-voltage indicator any more. Instead, the ACU automatically reduces the alternator field voltage when over-voltage is sensed. This is the new (as of 1979) wiring diagram:

C152 wiring diagram Image source: Flight Literacy

The low-voltage warning light is directly connected to the ACU. Unfortunately it is not easy to find a schematic of what is inside the ACU, and it appears as black box to me.

The expected behavior of the low-voltage warning light is to indicate whenever the alternator does not provide power, but the battery does. It is normal behavior if the low-voltage warning light comes on when the engine is idling, since the alternator does not provide sufficient power at low rpm.

I read somewhere that the low-voltage sensor in this ACU simply compares the voltage (on the primary bus) with a fixed threshold. [will add the source if I can find it again]. It is not able to directly sense if the power comes from the alternator or the battery. Instead, it relies on the fact the alternator needs to supply about 4 V above the battery voltage to charge the batteries.

You tried but could not see the low-voltage light when both sides of the master switch are on, and the engine is not started. You tried on several C152 to make sure your first plane is not behaving abnormal.

My best guess is that all the planes you have tried had healthy and fully charged batteries. The voltage levels were always above the trigger threshold of the low-voltage sensor.

You could try to switch on as much electric load as possible and see if the warning light comes on, immediately or after some time of discharge. If you have the possibility, you could measure the bus voltage.

A full 24 V battery without load would show about 25.4 V (or higher for AGM or gel types), and I believe that is above the threshold for the low-voltage sensor.

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    $\begingroup$ This definitely seems to be a likely contender for the explanation - I'll give it a go on my next flight. Hopefully I'll be in a plane that's been sitting around for a bit and isn't fully charged. Thanks! $\endgroup$ – Rob Dec 12 '19 at 0:59
  • $\begingroup$ Thinking about this for a bit - if the batteries were fully charged and healthy and were providing voltage higher than the threshold, then I'd expect the light not to turn on even when just the battery switch was on. So there's clearly something going on with the ACU, but I'll test leaving both switches on and applying a load for a while next time anyway $\endgroup$ – Rob Dec 12 '19 at 13:14
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    $\begingroup$ In the case of BAT=on and ALT=off, the ACU is not powered and the ACU somehow pulls the low-voltage light is pulled to ground. The other end of the low-voltage light is connected to the primary bus, that is connected to the battery. This is an intended feature to allow testing the light. $\endgroup$ – bogl Dec 12 '19 at 13:30
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    $\begingroup$ I've ended up doing a bunch of research around alternators and regulators (the majority being in cars, but the idea should be approximately the same). The flaw in my thinking was assuming that the sense wires were responsible for controlling the light. Turns out, those inputs are used to regulate the field over the alternator. With that in mind, it's highly likely that the power in line is used to control the light. When the ACU is unpowered, you rightly stated the light is pulled to ground (and thus turns on). When the ACU is powered on it's supplying the battery level via 'Power in' (cont) $\endgroup$ – Rob Dec 12 '19 at 14:38
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    $\begingroup$ And that is the voltage used for the measurement to either balance the voltage across the light, or to drop it enough to illuminate. Again, you're right that draining the battery should switch the light on with the ACU unpowered. Also, it seems that the alternator is only being indirectly checked for proper function - if the load is low enough, a faulty alternator may not trigger the light until the battery discharges enough. I suppose that should have been obvious to me given it's a low voltage warning, and not a faulty alternator warning :) Thank you for pointing me in the right direction! $\endgroup$ – Rob Dec 12 '19 at 14:38
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If the ALT switch is not closed, but the BATT switch is closed, battery power is allowed to flow through the ALT FIELD breaker, through the Over Voltage warning light, and grounds through the Over Voltage Sensor. If both the BATT and ALT switches are closed, current flows through both the ALT switch and maintains battery voltage at both terminals of the Over voltage warning light which prevents it from illuminating. This design feature allows the Over voltage warning light to illuminate when either the alternator is not connected to the main electrical bus or the alternator is malfunctioning.

enter image description here

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  • $\begingroup$ The electrical layout here is a little different from what I've seen. Here's what I saw at my training and found online: i.stack.imgur.com/5gI34.png . Your picture makes it a bit clearer as to what's happening. However, looking at your diagram, it looks as though if the wire between the alternator and the main bus (joining just next to the ALT CB) was severed, the light still would not illuminate as the voltages are balanced. In fact, that appears to be the case even if the entire alternator was removed, so long as the regulator was still grounded. Could you expand on this? $\endgroup$ – Rob Dec 9 '19 at 1:51
  • $\begingroup$ Well this is the schematic versus the actual wiring of the electrical system on the airplane. However your point does raise some merits. What would be required is the severing of the wire between the battery and the alternator breaker. This, or, effectively, simply switching off the BATT switch, would extinguish the Over voltage warning light, regardless of the position of the ALT switch. $\endgroup$ – Carlo Felicione Dec 9 '19 at 3:33
  • $\begingroup$ That’s going to depend on the year that that airplane was built, is that varies. I would also have to see the specific POH for the airplane Rob is flying to see exactly what system is installed on it. The principal remains about the same. $\endgroup$ – Carlo Felicione Dec 9 '19 at 14:23

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