Electrical advice please

[glend]

I have mounted a rear facing work light on my roof rack (55W Halogen). It operates in two fashiions: first it draws power off of my rear cargo area power point and through a switch (on, off, on). In the other on position it draws its power from the reversing light feed (which trips a relay to provide 12V) to the light - so in effect I have a bright high mounted reversing light as well. My problem is that while the light works fine when swiched directly, the relay side of the circuit is not working correctly. I have pulled it apart and checked all the connections and tested the relay circuit by manually applying 12V to the reversing feed wire - which does in fact trip the relay and the light operates. When I reconnect it to the vehicle reversing light wire from the loom it fails to operate. What am I doing wrong here? Do I need a diode in the reseversing light feed?

[sbadman]

It's likely that the reversing light is -ve switched, ie the switch at the transmission simply earths the light to enable it to work, in much the same way as the door pin switches for the interior light. Check this by putting your multimeter between your switching wire and an earthing point on the body in the 'Ohms' range and getting someone to activate reverse gear (you shouldn't even need the car on for this). If, when in reverse, the meter displays zero ohms, you have a negatively switched circuit. Run a positive feed to the other side of the relay coil (this can be from the reversing lights +ve feed if you wish) and you should be in business.

[glend]

Ok Scott I'll try that. The reason I have set it up to be +ve activated is that on the wiring diagram I have in the KJ Service Manual it shows the ground as a black wire on the other side of the reversing light globe, which is why I expected it to be +ve. Also when I put the meter on the reversing lead yes I do see earth - through the lamp globe to earth. I'll keep fiddling with it.

[glend]

I think I have found the problem. The +ve is only 8.8V when the reversing lights are on, and I have a 12V relay. Why 8.8V?

[glend]

I think I have it sorted now. Bosch relays have a lower 'pull in voltage', as low as 8 volts. So I picked up a Bosch relay at Ashdown's on Parramatta Road (the last one he had). They are more expensive but the Narva's just won't work. Anyway I connected ithe Bosch up and the circuit and light works! Very happy that is resolved because I was thinking of just going back to a second switch on the dash if it had not worked. I still don't have a diode in the circuit but I will pick one up at Jaycar later today.

Thanks for your help with that problem.

[sbadman]

Good to hear you got it going!

8.8V is very unusual, seems like the ECU is under-driving the reversing lamps on purpose, instead of utilising a lower wattage bulb....