Electrical Troubleshooting Problem

I have an issue with my Reverse Lights. I have a good fuse, and known good
lamps.

I pulled the lamps and found that I have 12v at the sockets. The circuit
design is a bit odd, the reverse switch feeds the left side lamp assembly,
then jumpers out on another pin over to the right side lamp assembly. I get
voltage at both lamp sockets when the lamps are both removed. But, when I
insert either lamp, the voltage drops to 0.

Voltage applied is a function of the load on the circuit, so I have decided
that I have a very dirty reverse switch on the transmission -- it's a manual
trans, by the way. The switch passes 12v, but has very high resistance,
which lowers the current available in the circuit. When the bulbs are
inserted, the load exceeds the available current, and the voltage drops to
0.

The tail lamp assemblies share a common ground, and none of the other lamps
are giving me trouble, and I can connect the reverse lamps directly to the
wires that give them 12v and ground, removing the bad ground scenario from
the equation, and the lights do not come on. I can plug the lamps into other
sockets, and they work fine.

If I connect my volt meter properly, it can tell me the current that is
available -- I haven't done this test yet. I would expect to see something
in the range of 10 amps (this is the rating of the fuse) to make the lamps
come on. If I only see 0.5 amps, this would seem to support my theory of a
bad switch. Is that right? Any other ideas?

Jeff Strickland wrote:[color=blue]
> If I connect my volt meter properly, it can tell me the current that
> is available -- I haven't done this test yet. I would expect to see
> something in the range of 10 amps (this is the rating of the fuse) to
> make the lamps come on. If I only see 0.5 amps, this would seem to
> support my theory of a bad switch. Is that right?[/color]

If you connect your "volt meter" to the ampere current range and connect
it to the socket, you will blow the fuse. And you might even fuse the
intermittent contact on the reverse switch, if indeed that is the
problem.

"Travis Jordan" <no.one@no.net> wrote:
[color=blue]
>Jeff Strickland wrote:[color=green]
>> If I connect my volt meter properly, it can tell me the current that
>> is available -- I haven't done this test yet. I would expect to see
>> something in the range of 10 amps (this is the rating of the fuse) to
>> make the lamps come on. If I only see 0.5 amps, this would seem to
>> support my theory of a bad switch. Is that right?[/color]
>
>If you connect your "volt meter" to the ampere current range and connect
>it to the socket, you will blow the fuse. And you might even fuse the
>intermittent contact on the reverse switch, if indeed that is the
>problem.
>[/color]

To measure current in a circuit you must 'open' the circuit
anywhere and place your meter set to 'current' 'across' this
point...in other words, the meter 'must' be placed 'in series'
with the rest of the circuit. BUT, you don't need to do this, the
bulb has already proven that there's a high resistance in series
with this circuit, you must get rid of this before you can go any
further...
--

-Gord.
(use gordon in email)

"Jeff Strickland" <crwlr@yahoo.com> wrote in message
news:ksCdnacRFOZD8N_ZRVn-pg@ez2.net...[color=blue]
>I have an issue with my Reverse Lights. I have a good fuse, and known good
>lamps.
>
> I pulled the lamps and found that I have 12v at the sockets. The circuit
> design is a bit odd, the reverse switch feeds the left side lamp assembly,
> then jumpers out on another pin over to the right side lamp assembly. I
> get voltage at both lamp sockets when the lamps are both removed. But,
> when I insert either lamp, the voltage drops to 0.[/color]

When you checked the voltage at the sockets, did you use your volt meter and
did you touch one meter lead to the positive terminal in the socket and the
other to a ground somewhere in the back of the car? Or did you touch the
negative lead to the negative terminal in the socket?

When you say that the voltage drops to 0 when either lamp is inserted, what
points are you measuring voltage between?
[color=blue]
>
> Voltage applied is a function of the load on the circuit, so I have
> decided that I have a very dirty reverse switch on the transmission --
> it's a manual trans, by the way. The switch passes 12v, but has very high
> resistance, which lowers the current available in the circuit. When the
> bulbs are inserted, the load exceeds the available current, and the
> voltage drops to 0.[/color]

When you say that the voltage drops to 0, what points are you measuring
voltage between?

When you say that the switch has very high resistance, did you unplug it and
actually measure resistance between the switch terminals with the switch
closed? If so, how much resistance did you measure? If you are getting
less than 1 ohm, then the switch is good.
[color=blue]
>
> The tail lamp assemblies share a common ground, and none of the other
> lamps are giving me trouble, and I can connect the reverse lamps directly
> to the wires that give them 12v and ground, removing the bad ground
> scenario from the equation, and the lights do not come on. I can plug the
> lamps into other sockets, and they work fine.
>
> If I connect my volt meter properly, it can tell me the current that is
> available -- I haven't done this test yet. I would expect to see something
> in the range of 10 amps (this is the rating of the fuse) to make the lamps
> come on. If I only see 0.5 amps, this would seem to support my theory of a
> bad switch. Is that right? Any other ideas?
>[/color]

Reverse lights should not be drawing 10 amps. You're more likely to see
somewhere under 3 amps. Also, check the amperage rating for your multi
meter. Many will not handle more than 3 amps unless you have a higher end
one or one with an inductive clamp.

The easiest way to check out your theory of a bad switch is to unplug the
switch and apply 12 volts to the circuit on the harness side of the switch
connector. If the bulbs light up, then the switch is bad. If the bulbs do
not light up, then you have another problem. I think that you have a poor
connection to ground.
--

Ray O
(correct punctuation to reply)

"Travis Jordan" <no.one@no.net> wrote in message
news:SPy0g.141403$nk.49361@fe04.news.easynews.com...[color=blue]
> Jeff Strickland wrote:[color=green]
>> If I connect my volt meter properly, it can tell me the current that
>> is available -- I haven't done this test yet. I would expect to see
>> something in the range of 10 amps (this is the rating of the fuse) to
>> make the lamps come on. If I only see 0.5 amps, this would seem to
>> support my theory of a bad switch. Is that right?[/color]
>
> If you connect your "volt meter" to the ampere current range and connect
> it to the socket, you will blow the fuse. And you might even fuse the
> intermittent contact on the reverse switch, if indeed that is the
> problem.
>
>[/color]

That's correct. But if I connect my volt meter properly, I'd be using the
ameter sockets and selection on the meter's dial ...

"Ray O" <rokigawa@tristarassociatesDOTcomn> wrote in message
news:1df56$44430e30$180fead6$32063@msgid.meganewsservers.com...[color=blue]
>
> "Jeff Strickland" <crwlr@yahoo.com> wrote in message
> news:ksCdnacRFOZD8N_ZRVn-pg@ez2.net...[color=green]
>>I have an issue with my Reverse Lights. I have a good fuse, and known good
>>lamps.
>>
>> I pulled the lamps and found that I have 12v at the sockets. The circuit
>> design is a bit odd, the reverse switch feeds the left side lamp
>> assembly, then jumpers out on another pin over to the right side lamp
>> assembly. I get voltage at both lamp sockets when the lamps are both
>> removed. But, when I insert either lamp, the voltage drops to 0.[/color]
>
> When you checked the voltage at the sockets, did you use your volt meter
> and did you touch one meter lead to the positive terminal in the socket
> and the other to a ground somewhere in the back of the car? Or did you
> touch the negative lead to the negative terminal in the socket?
>[/color]

Yes to both. I also used the appropriate pins on the connector after I
determined that the socket itself was getting its juice.

[color=blue]
> When you say that the voltage drops to 0 when either lamp is inserted,
> what points are you measuring voltage between?
>[/color]

The supply wire and the ground wire. Substitute any of the available options
for ground and source.



[color=blue][color=green]
>>
>> Voltage applied is a function of the load on the circuit, so I have
>> decided that I have a very dirty reverse switch on the transmission --
>> it's a manual trans, by the way. The switch passes 12v, but has very high
>> resistance, which lowers the current available in the circuit. When the
>> bulbs are inserted, the load exceeds the available current, and the
>> voltage drops to 0.[/color]
>
> When you say that the voltage drops to 0, what points are you measuring
> voltage between?
>[/color]

See above.


[color=blue]
> When you say that the switch has very high resistance, did you unplug it
> and actually measure resistance between the switch terminals with the
> switch closed? If so, how much resistance did you measure? If you are
> getting less than 1 ohm, then the switch is good.
>[/color]

No, I haven't found the switch yet -- admitting that I haven't spent a great
deal of effort looking for it. I am assuming that I have a great deal of
resistance on the switch because of the results I'm finding as I try to
figure out why the lights don't come on. I understand that less than 1 ohm
is good, my assumption is that I have several orders of magnitude more ohms
than that.




[color=blue][color=green]
>>
>> The tail lamp assemblies share a common ground, and none of the other
>> lamps are giving me trouble, and I can connect the reverse lamps directly
>> to the wires that give them 12v and ground, removing the bad ground
>> scenario from the equation, and the lights do not come on. I can plug the
>> lamps into other sockets, and they work fine.
>>
>> If I connect my volt meter properly, it can tell me the current that is
>> available -- I haven't done this test yet. I would expect to see
>> something in the range of 10 amps (this is the rating of the fuse) to
>> make the lamps come on. If I only see 0.5 amps, this would seem to
>> support my theory of a bad switch. Is that right? Any other ideas?
>>[/color]
>
> Reverse lights should not be drawing 10 amps. You're more likely to see
> somewhere under 3 amps. Also, check the amperage rating for your multi
> meter. Many will not handle more than 3 amps unless you have a higher end
> one or one with an inductive clamp.
>[/color]

You're right, they shouldn't be drawing 10 amps, but the fuse is rated that
high. I's guess the amperage of the lamps is closer to 3 amps. If I remember
right, the bulbs are rated to 4W. 12 / 4 = 3.

[color=blue]
> The easiest way to check out your theory of a bad switch is to unplug the
> switch and apply 12 volts to the circuit on the harness side of the switch
> connector. If the bulbs light up, then the switch is bad. If the bulbs
> do not light up, then you have another problem. I think that you have a
> poor connection to ground.
> --[/color]

That's the hard part. I happen to know that much of the driveline on the car
is covered by the exhaust system and a heat shield -- both of which I
hesitate to remove for such an insiginficant problem -- but it turns out the
tail end of the transmissiion is not covered.

I considered the poor ground connection, but have dismissed it for now. The
reason being that all of the tail lights (lights on the back of the car)
share the same ground, and they all work fine.

Just because "Jeff Strickland" <crwlr@yahoo.com> could, he/she/it
opin'd thus:
[color=blue]
>
>"Travis Jordan" <no.one@no.net> wrote in message
>news:SPy0g.141403$nk.49361@fe04.news.easynews.com...[color=green]
>> Jeff Strickland wrote:[color=darkred]
>>> If I connect my volt meter properly, it can tell me the current that
>>> is available -- I haven't done this test yet. I would expect to see
>>> something in the range of 10 amps (this is the rating of the fuse) to
>>> make the lamps come on. If I only see 0.5 amps, this would seem to
>>> support my theory of a bad switch. Is that right?[/color]
>>
>> If you connect your "volt meter" to the ampere current range and connect
>> it to the socket, you will blow the fuse. And you might even fuse the
>> intermittent contact on the reverse switch, if indeed that is the
>> problem.
>>
>>[/color]
>
>That's correct. But if I connect my volt meter properly, I'd be using the
>ameter sockets and selection on the meter's dial ...[/color]

I'm not entirely sure what you mean by that, but if your ammeter is
connected correctly (in series with the circuit under test) you can't
test how much current is available, only how much current is being
drawn by whatever is in the circuit . . . .

-Don
--
Nous avons assez de jeunesse; commencons a rechercher la fontaine du sens." -- Anonyme

"Jeff Strickland" <crwlr@yahoo.com> wrote in message
news:lZSdnVrYC7AjzNnZnZ2dnUVZ_sidnZ2d@ez2.net...[color=blue]
>
> "Ray O" <rokigawa@tristarassociatesDOTcomn> wrote in message
> news:1df56$44430e30$180fead6$32063@msgid.meganewsservers.com...[color=green]
>>
>> "Jeff Strickland" <crwlr@yahoo.com> wrote in message
>> news:ksCdnacRFOZD8N_ZRVn-pg@ez2.net...[color=darkred]
>>>I have an issue with my Reverse Lights. I have a good fuse, and known
>>>good lamps.
>>>
>>> I pulled the lamps and found that I have 12v at the sockets. The circuit
>>> design is a bit odd, the reverse switch feeds the left side lamp
>>> assembly, then jumpers out on another pin over to the right side lamp
>>> assembly. I get voltage at both lamp sockets when the lamps are both
>>> removed. But, when I insert either lamp, the voltage drops to 0.[/color]
>>
>> When you checked the voltage at the sockets, did you use your volt meter
>> and did you touch one meter lead to the positive terminal in the socket
>> and the other to a ground somewhere in the back of the car? Or did you
>> touch the negative lead to the negative terminal in the socket?
>>[/color]
>
> Yes to both. I also used the appropriate pins on the connector after I
> determined that the socket itself was getting its juice.
>
>[color=green]
>> When you say that the voltage drops to 0 when either lamp is inserted,
>> what points are you measuring voltage between?
>>[/color]
>
> The supply wire and the ground wire. Substitute any of the available
> options for ground and source.
>
>
>
>[color=green][color=darkred]
>>>
>>> Voltage applied is a function of the load on the circuit, so I have
>>> decided that I have a very dirty reverse switch on the transmission --
>>> it's a manual trans, by the way. The switch passes 12v, but has very
>>> high resistance, which lowers the current available in the circuit. When
>>> the bulbs are inserted, the load exceeds the available current, and the
>>> voltage drops to 0.[/color]
>>
>> When you say that the voltage drops to 0, what points are you measuring
>> voltage between?
>>[/color]
>
> See above.
>
>
>[color=green]
>> When you say that the switch has very high resistance, did you unplug it
>> and actually measure resistance between the switch terminals with the
>> switch closed? If so, how much resistance did you measure? If you are
>> getting less than 1 ohm, then the switch is good.
>>[/color]
>
> No, I haven't found the switch yet -- admitting that I haven't spent a
> great deal of effort looking for it. I am assuming that I have a great
> deal of resistance on the switch because of the results I'm finding as I
> try to figure out why the lights don't come on. I understand that less
> than 1 ohm is good, my assumption is that I have several orders of
> magnitude more ohms than that.
>
>
>
>
>[color=green][color=darkred]
>>>
>>> The tail lamp assemblies share a common ground, and none of the other
>>> lamps are giving me trouble, and I can connect the reverse lamps
>>> directly to the wires that give them 12v and ground, removing the bad
>>> ground scenario from the equation, and the lights do not come on. I can
>>> plug the lamps into other sockets, and they work fine.
>>>
>>> If I connect my volt meter properly, it can tell me the current that is
>>> available -- I haven't done this test yet. I would expect to see
>>> something in the range of 10 amps (this is the rating of the fuse) to
>>> make the lamps come on. If I only see 0.5 amps, this would seem to
>>> support my theory of a bad switch. Is that right? Any other ideas?
>>>[/color]
>>
>> Reverse lights should not be drawing 10 amps. You're more likely to see
>> somewhere under 3 amps. Also, check the amperage rating for your multi
>> meter. Many will not handle more than 3 amps unless you have a higher
>> end one or one with an inductive clamp.
>>[/color]
>
> You're right, they shouldn't be drawing 10 amps, but the fuse is rated
> that high. I's guess the amperage of the lamps is closer to 3 amps. If I
> remember right, the bulbs are rated to 4W. 12 / 4 = 3.
>
>[color=green]
>> The easiest way to check out your theory of a bad switch is to unplug the
>> switch and apply 12 volts to the circuit on the harness side of the
>> switch connector. If the bulbs light up, then the switch is bad. If
>> the bulbs do not light up, then you have another problem. I think that
>> you have a poor connection to ground.
>> --[/color]
>
> That's the hard part. I happen to know that much of the driveline on the
> car is covered by the exhaust system and a heat shield -- both of which I
> hesitate to remove for such an insiginficant problem -- but it turns out
> the tail end of the transmissiion is not covered.
>
> I considered the poor ground connection, but have dismissed it for now.
> The reason being that all of the tail lights (lights on the back of the
> car) share the same ground, and they all work fine.
>[/color]

You didn't mention what type of vehicle - is it the BMW?

BTW, the reverse switch may be in the shifter console instead of the shifter
itself. You might want to check it out by pulling back the shift boot and
looking from above.

Good luck!
--

Ray O
(correct punctuation to reply)

Jeff Strickland wrote:[color=blue]
> That's correct. But if I connect my volt meter properly, I'd be using
> the ameter sockets and selection on the meter's dial ...[/color]

One more time...

If you connect your amp meter to the lamp bulb socket and apply power
you will blow the fuse. Or worse. Ampere measurements are made in
series circuits only.

"Ray O" <rokigawa@tristarassociatesDOTcomn> wrote in message
news:d37c5$444485a3$180fead6$32334@msgid.meganewsservers.com...[color=blue]
>
> You didn't mention what type of vehicle - is it the BMW?
>
> BTW, the reverse switch may be in the shifter console instead of the
> shifter itself. You might want to check it out by pulling back the shift
> boot and looking from above.
>
> Good luck![/color]


Yeah, it's the BMW. The switch is in the tail of the trans. I also had
trouble with the Parking Brake indicator, and hoped that the parking brake
switch and the reverse switch shared a connection that wasn't made, and that
this lived inside the console somewhere. It turns out the parking brake
switch was missing a return spring -- a purely mechanical issue, not
electrical at all.

Thinking the reverse switch might be in the shifter itself, I asked on the
BMW newsgroup. Somebody posted a pic of it on the tail of the trans, and I
should be able to get at it with my floor jack.

"Jeff Strickland" <crwlr@yahoo.com> wrote in message
news:hqedncUX-ZQ7OdjZRVn-pA@ez2.net...[color=blue]
>
> "Ray O" <rokigawa@tristarassociatesDOTcomn> wrote in message
> news:d37c5$444485a3$180fead6$32334@msgid.meganewsservers.com...[color=green]
>>
>> You didn't mention what type of vehicle - is it the BMW?
>>
>> BTW, the reverse switch may be in the shifter console instead of the
>> shifter itself. You might want to check it out by pulling back the shift
>> boot and looking from above.
>>
>> Good luck![/color]
>
>
> Yeah, it's the BMW. The switch is in the tail of the trans. I also had
> trouble with the Parking Brake indicator, and hoped that the parking brake
> switch and the reverse switch shared a connection that wasn't made, and
> that this lived inside the console somewhere. It turns out the parking
> brake switch was missing a return spring -- a purely mechanical issue, not
> electrical at all.
>
> Thinking the reverse switch might be in the shifter itself, I asked on the
> BMW newsgroup. Somebody posted a pic of it on the tail of the trans, and I
> should be able to get at it with my floor jack.
>[/color]

Good luck with the repair. Next time, take an IS for a test drive ;-)
--

Ray O
(correct punctuation to reply)