If the fan will only work with the light and you are supposed to get power to the light when the fan is off, that means the wiring to the light is not working.
How many sets of romex are you running to the hood? Are there two romex wires, one for the fan, and one for the light?
Take a load tester to the power wire feeding the light, you should not get any load. Test the power wire feeding the fan too and you should get load.
If this is the case, you have a dead power wire to the light

Bee, I don't know. What did your electrician say? Can't he figure it out?

azlighting, what is a "load tester"? And what do you mean when you say "you should not get any load"???

This is Bee's husband. Our electrician is on vacation but the unit was pre-wired and I put the external fan unit on the house after and wired it in. So we didn't test beforehand and I have double-and triple checked the wiring at the fan unit...it's right.

So there is only one power circuit feeding the range hood. I will test the two different cables from the small electrical box into the hood with a voltmeter. One should have power from the panel - ~120V. The other should be connected out to the fan unit and therefore should have no power. Any other thoughts would be helpful.

here is the wiring diagram

{{!gwi}}

This diagram doesn't make sense or we don't have all of it. The diagram shows a plug, an outlet, 120v in and a blower motor. We don't see a light or any switches.

I assume that the plug that is wired for the blower motor is plugged into the outlet.

The blower is not wired in the range hood junction box correctly.

jake2007,
The diagram makes no sense because it is a diagram of how you should wire the unit, not of how the vent hood itself is wired. It neglects to show the fact that inside the switch panel of the vent hood, the power connections on the right side ("line in" and the three prong plug) actually supply power to some kind of power bus inside the hood. That power is then available to the two prong plug on the left side of the picture which feeds power through the switch out to the fan itself.

Thanks for all the responses so far. There is a lot to be checked tonight. Any other suggestions are welcome! Thanks again

Yeah, it's miswired, the light and fan are probably in series,or their switches. I'd bet. Is the problem no lights without fan AND vice versa, I know it sounds odd, but....
Meaning everything thing only comes on when you turn both switches on. But one switch will not turn on either device.
And that's a poor diagram.

The wiring diagram is so oversimplified it is useless.
By omitting the lights and switches it is very hard to try and figure out what they are really doing.

The remote blower Vikings I have installed have a single feed to the hood junction box that supplies power the hood.
The sped control for the motor has a separate pair of lines that are then fed to the blower motor.

Thanks everybody--Mr. Bee wasn't able to crawl back up into that very roomy hood last night and take a look. Hopefully he will take a look either tonight or tomorrow.

And no, the fan works fine without the lights. The lights just won't work without the fan.

He said his biggest problem is the size of the box up there is so small it's hard to get the wires back out. Wonder why Viking puts such a small box....

"Wonder why Viking puts such a small box...."

Because they can get away with it.
The NEMA code is not nearly as not as restrictive in may ways as the NEC since it covers equipment wiring.

The wiring used also has a higher temperature insulation than typical permanent wiring in a structure.
Between the well known loads present (lights and blower)using higher temperature insulation allows for a smaller enclosure and smaller conductors.

It is not designed to be worked on very much, so access and convenience take a back seat to saving even a small amount of $$ ($1 saved over 20,000 units is significant).

I want to be clear, when I said miswired, that absolutely includes the factory.

I bet this is one of those problems that one person could look at for an hour, and some other guy walks in with a donut in one hand and a medium dark in the other, and says LOOK, it's right there. These one's are hard by remote control...
Especially with that diagram...............

KEN

The hot wire goes to the light's you will need to plug in pigtail into the socket where the internal blower would have plug in its on the front inside of hood then the switch leg to blower gets hooked to it .fan should have power already to it in

David-

Hopefully, the OP resolved this 11 years ago... ;-)