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Discussion Starter #1
Hi,
When I put the car in reverse, there's "yuge' voltage drop. Where do you guys suggest that I start to look? I'm thinking bad ground somewhere?

Thanks-
JOhn
 

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Richard Jemison
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Reverse switch

More likely a issue with the reverse switch in the bell housing shorting itself on oil or a positive line shorting to the chassis.
 

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Premium Member
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Well, before pulling the bell housing, please look at the wires & connector that go to the bell housing - statistically more likely than the switch and infinitely easier to rule out... the wire just sorta flop around down there, and can scrape their own insulation off.

DNAMHIK.
 

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When I put the car in reverse, there's "yuge' voltage drop. I'm thinking bad ground somewhere?
It isn't a bad ground. Bad grounds result in LESS current flow - your problem is too much current flow. The switch in the bellhousing may well be the culprit. It could also be a frayed wire, but usually something like that results in a dead short that passes enough current to blow the fuse.

Speaking of fuses, how did you detect that voltage drop? I would have expected the back-up light fuse to blow before the headlights went dim or "GEN" light came on.
 

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Discussion Starter #6
The gauge goes from it's normal charge, to around 12V or just below, especially if the engine isn't on turning the alt. It drops more than if I turn on the AC fan, even. That fan has been a common thread here, but I've not read about the reverse lights (or switch) causing a giant drop in volts. No blown fuses yet...
 

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The gauge goes from it's normal charge, to around 12V or just below, especially if the engine isn't on turning the alt. It drops more than if I turn on the AC fan, even. That fan has been a common thread here, but I've not read about the reverse lights (or switch) causing a giant drop in volts. No blown fuses yet...
Wow, that's strange. I wonder what else is powered by the fuse that handles the back-up light. The fuse must be fairly large capacity (10a? 15a?) to deliver that much current without blowing.

I would disconnect the wires to the BU light switch at the transmission and see what happens. Of course, it shouldn't draw any current with both wires unhooked. If you still see the large voltage drop, it's something between the light switch and the transmission switch. If the gauge shows no drop, then re-connect the hot lead to the switch (but not the wire going to the BU light) and try it again. If that produces a voltage drop, then it's the transmission switch. If not, it's something downstream of the transmission switch.
 

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But Mad North-Northwest
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If you have an ammeter, put it inline with the circuit and measure amps. Reverse lights are two 1156, I believe, and should pull about 4.2A total. If that's in spec then the circuit is fine.

The reverse lights are switched power and go through the ignition switch. If you have any voltage drop in this switch (very common) you'll see a big system voltage drop when you try to put amps through it.
 

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The reverse lights are switched power and go through the ignition switch. If you have any voltage drop in this switch (very common) you'll see a big system voltage drop when you try to put amps through it.
I think Ohm's law dictates otherwise. If there is a voltage drop across the ignition switch, then the back up light bulbs are going to receive less voltage relative to ground. Less voltage = less current for a fixed resistance load.

I'm not saying that John533i's ignition switch isn't bad; just that a bad ignition switch wouldn't produce excessive current draw in the back-up lights, or any other component.
 

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But Mad North-Northwest
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Bad ignition switch won't produce excessive current, you are correct. But it will cause system voltage to drop, which seems to be the issue here.

When you activate the reverse lights you're putting more current through the switch. If there's any resistance in the switch this'll show up as a voltage drop on the switched circuit (the dash voltage gauge is on the switched circuit).

You're correct that this voltage drop will cause the bulbs to pull less than their full 4.2 rated amps, but there will still be more amps going through the ignition switch than with the lights off. Thus you'll see a decrease in system voltage when the lights come on, because the switch is dropping more volts at the higher current.

This seems to be a common problem on Spiders: ignition switch contacts get dirty and you see switched voltage drop quite a bit as you add more switched load. You can directly measure the voltage drop across the switch to verify if this is an issue. Not saying this is the problem, but it's something to be aware of.
 

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I'd say your problem is most likely at one of the backup lights, the switch simply completes the circuit to send power to the lights and it seems that it is doing it's job. The bulbs are the actual load, or resistance in the circuit. It could be a short or partial short in the wire that goes from the switch to the lights. To test that remove both backup light bulbs and engage reverse to see if the voltage drop still occurs.
 

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I'd say your problem is most likely at one of the backup lights, the switch simply completes the circuit to send power to the lights and it seems that it is doing it's job. The bulbs are the actual load, or resistance in the circuit. It could be a short or partial short in the wire that goes from the switch to the lights. To test that remove both backup light bulbs and engage reverse to see if the voltage drop still occurs.
With both bulbs removed I no longer get the voltage drop.Does this mean it's the bulb receptacle?
 

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But Mad North-Northwest
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See my explanation above. It is the current draw through the ignition switch.

If it bugs you, get some LED bulbs for the reverse lights. This will greatly decrease the current draw and reduce the voltage drop.
 

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But Mad North-Northwest
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Heh. If the problem were dirty sockets that would decrease the current draw of the reverse light circuit, and as such cleaning them would make the system voltage drop problem worse.

The reverse lights have this problem because they're switched power: the current for them runs through the ignition switch, and it drops voltage on the entire switched power side of the wiring. The brake lights draw more current but you don't see this problem with them because they're unswitched power.

The real fix would be an ignition relay to bypass the ignition switch, but that's a bunch of work.
 

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Heh. If the problem were dirty sockets that would decrease the current draw of the reverse light circuit, and as such cleaning them would make the system voltage drop problem worse.

The reverse lights have this problem because they're switched power: the current for them runs through the ignition switch, and it drops voltage on the entire switched power side of the wiring. The brake lights draw more current but you don't see this problem with them because they're unswitched power.

The real fix would be an ignition relay to bypass the ignition switch, but that's a bunch of work.
Thanks, I'm a 69 year old newbie to this Alfa addiction. I think I can live with the voltage drop in reverse. What a great source of knowledge found on this sight.
 

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The real fix would be an ignition relay to bypass the ignition switch, but that's a bunch of work.
I wouldn't call it a bunch of work. Lower the fuse box, find the wires to & from the ignition switch, disconnect them from the fuse box and connect them to the relay. Then add a couple of wires to supply full system voltage to the switched side of the fuse box. I suppose it is easier said than done but not really a bunch of work. I did the above on our GTV6 - made a noticeable difference in wiper speeds and window speeds. Our Spider's battery light glows when it is on reverse - I suspect for the same reason as the OP's. I should probably add the relay to the Spider, too...

See: (scroll down to reply #11 or #12) https://www.alfabb.com/bb/forums/alfetta-gtv6-1972-1986/543281-proud-owner-82-gtv6.html
 

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Heh. If the problem were dirty sockets that would decrease the current draw of the reverse light circuit, and as such cleaning them would make the system voltage drop problem worse...
Tom, I'm just talking about an oil and grime mix that creates a conductive path to ground that bypasses the bulb. For cripes sake just clean it up and see if you get any results before re-engineering the whole circuit. Takes ten minutes and if it doesn't change anything there's no loss.
 
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