Voltage drop not triggering reverse camera relay

[jcerecke 2]

I'm tapping into the reverse light wire in the C pillar of my e39 to trigger the camera input on my dynavin. But when I measure the voltage it's between 10-11VDC and doesn't trigger the 12v relay. I can't hook it straight up to the dynavin of course because the light check turns my reverse camera on every minute and I tried to change the light module to not check with navcoder but didn't have much luck. It mostly gives me errors when coding the light module... argh.

The cable isn't the thickest in the world but it's also not that thin either, I wouldn't say it's much thinner than the original reverse light wire.

Any ideas??

[hybrid 1039]

Test the relay first with a a separate 12 v

[blonky 2]

Most automotive relays I've come across don't actually need 12v to trigger - somewhere around 8v is enough, so might be worth buying a new relay with a lower pull on voltage rating.

If you ran a capacitor you could run the camera directly from the light power source as it should deal with the check spikes but you would be likely drawing too much current

[jcerecke 2]

Test the relay first with a a separate 12 v

Yep relay definitely works with the red/green stereo 12v wire. I've also tested with car on/off too. Voltage at the reverse light measured around 11v too when the car was off, 13v when it was on... I'll try see if the repco guys can let me test another one and return it if it doesn't work.

[hotwire 352]

As mentioned, the 12v relay will trigger at 9 + volts so the voltage you mention is not the issue.

Also, depending on battery voltage, the voltage at the rear with key on, no engine could well be down to 11v

What about your earth?

[jcerecke 2]

What about your earth?

The earth is the same whether I use the reverse light +ve wire or the stereo +ve wire so I would say the grounding shouldn't be an issue? I'll give it the once over to make sure though...

[blonky 2]

Just a thought...if the relay is wired inline (series) then maybe the voltage across the relay is too low since it has lower resistance than the bulb; wired in parallel with the bulb would fix that

EDIT: And also isn't it the ground of the bulbs that are switched, ie the hot wire (+12v) is always hot so if you tapped it with the relay and wired the -ve from the relay straight to ground, won't that short across the bulb?

Edited October 13, 2013 by SR E36

[hotwire 352]

Just a thought...if the relay is wired inline (series) then maybe the voltage across the relay is too low since it has lower resistance than the bulb; wired in parallel with the bulb would fix that

EDIT: And also isn't it the ground of the bulbs that are switched, ie the hot wire (+12v) is always hot so if you tapped it with the relay and wired the -ve from the relay straight to ground, won't that short across the bulb?

^^^^^ Definitely not. Supplies all come from LCM. Also he hasn't wired it in series - he has battery voltage (close to) across it

[jcerecke 2]

Tried a new relay from Jaycar. The guy tested it in store with 9VDC, hooked it up to the reverse light and still no go. There must be something weird with that wire. I'll try run some temporary cables from other lights and see what happens.

[hybrid 1039]

Load test it with a test light. The wire that is

[jcerecke 2]

Finally figured it out. Turns out the C-pillar has two Blue wires with yellow stripes! The Reverse light has two yellow stripes, the other has only one stripe - no idea what that one does though... All works correctly now!