Rooftop light wiring help?

[Alaric Darconville]

I went out this morning to find which wire on the headlight plug is power and considering it's ground switched I expected to get 12 V on the multi meter when I found it. I hooked the ground lead to the negative battery post and probed the two connectors in the plug. I got no voltage in both. With the high beam switch on, I get the 12 V on the power lead for the connector and 24.3 mV on the ground for the connector. This was all unplugged from the bulb.

I believe on my Previa the H4's "neutral" pole is the one that gets the power, the "hots" for each individual filament are the ones that are the ground and their path to ground is switched. So try putting the a probe on the "neutral" and on the negative battery terminal. It's also possible that your headlamp circuit is supervised and if filament resistance isn't detected, it may cut the power.

 

[dylans]

I believe on my Previa the H4's "neutral" pole is the one that gets the power, the "hots" for each individual filament are the ones that are the ground and their path to ground is switched. So try putting the a probe on the "neutral" and on the negative battery terminal. It's also possible that your headlamp circuit is supervised and if filament resistance isn't detected, it may cut the power.

Alright, I tested as you suggested.

With no key in the ignition it reads .1 mV at both terminals on the plug.
With no key in the ignition and the driver door open (interior lights auto on) it reads 1.4 mV in one terminal and 1.3 mV at the other.
With key in ignition to ACC it reads 2.6 mV on one terminal and 2.4 mV at the other.
With key to ON it reads 4.9 mV on one terminal and 4.5 mV at the other.
With engine started it reads 12.6 mV on one terminal and 10.9 mV at the other.

There are two wires, one is thinner and is green with black stripe and the other is thicker and white with black stripe. The green/black wire is always higher in mV. With the driver door closed the voltages are always slightly lower.
My initial test this morning was after the truck had sat all night. These tests were done after driving around for a while.

Here are the first 3 pages from the FSM headlight wiring diagram section. I'll be tapping the left hand high beam.

 

[dylans]

Today I discovered that placing the negative probe on the positive battery post and then probing the bulb connector terminals gives -12.67 V. Both terminals give the same reading.

 

[dylans]

I finished the install. The Hella harness is great! I hooked it up by swapping the power and grounds on the battery and when I hooked the battery back up the lights stayed on when the headlights were off and they went off when I turned the headlights on.

I tapped the green and black wire on the high beam in the above diagram page 2. So I swapped the battery cables to 'normal'. Power to power ground to ground and they worked as intended. Off with lows, on with highs and cabin switch. Off with cabin switch off. They go off automatically when I switch from high to low and they come on when I flash the high (with cabin switch on).

Great! Thanks for all of the help . I guess my Toyota isn't ground switched?

 

[-Virgil-]

Weirdness and model-to-model inconsistency is the hallmark of Toyota lighting wiring. Glad you got it figured out!