Headlight switch is on dash, left of the steering wheel, Ford P/N (for later model years, early model years is different P/N) F81Z-11654-AA (check eBay- $60 to $125), with gray & black connectors. Pin6 on black connector is "HDL" white 18ga. This runs to Pin6 on the 12pin VIP Smart Wheel module, and comes out as headlight signal "HDLTS" on pin2 of the larger 4 pin VIP Smart Wheel connector (there is a smaller 4 pin connector also).
HDLTS white 18ga runs to pin9 of the 23 pin connector on front Vansco VMM ahead of passenger's feet behind plastic kick panel. Circuits up to this point are (I believe) digital seeking ground as switch signal. Starting @ output of the VMM and lighting Output LEDs 1 & 3, or 2 & 4 (low or hi beams respectively based on other input), circuit will be simple 12v for high or less for lo beam, with ~6A auto-resetting breaker feature w/in the VMM.
Headlight signal coming out of the VMM is labeled 1007R or 1007L for Right or Left Hi Beams at VMM 35 pin connector pins5 & 3 respectively, or 1006A for Lo Beams at pin 24 (a splice in the loom takes 1006A to 1006R & L for Right & Left Lo Beam). All the above all takes place on the passenger side of front firewall. The 1007's & 6's run thru inside loom, and somewhere exit the cab to the front side of the firewall, location unknown to me at this point. However, 1007's come off the front (outside) of firewall within one of the round black plug connectors that connect dash stuff to bumper stuff, and there should be enough exposed white wire to find the number markings. On the bumper side of the black cylindrical connector, these go the left & right headlights.
1) probe the black 18ga "4435" wire at headlight switch for ground. If none, establish ground & see if that cures the issue. Otherwise
2) If yes to ground, then turn headlight switch to headlights & probe the white 18ga HDL (pin6 black conn) for ground- yes go to step3, no its probably headlight switch.
3) probe HDL at VIP Smart Wheel module for ground; if yes go to step4; if no problem is in wire between HDL Switch & VIP module
4) probe HDLTS wire out of VIP module for signal (I confess I don't know if that's Bat+, Gnd, computer signal, or what) & HDLTS into VMM for same. Input LED 21 on the VMM should light for receiving Headlight signal; per the coach owner that doesn't happen; just guessing cuz the other VMM inputs work like this- circuit for LED21 puts out ~11.9v on the "input" wire HDLTS looking for Gnd as signal to light headlights.
Could be the Smart Wheel module circuit is burned out inside the module, could be the VMM circuit inside VMM is burned out. Could be wiring problem between components. Could be bad ground in '4435' black wire at headlight switch or headlight switch itself.
Could also be voodoo I haven't figured out.
Baja-tested '08 2-slide 36'
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