Turning a light switch on turns off my bathroom exhaust fan

[Randyjp]

I've wired a single pole switch along with an exhaust fan timer in a bathroom, however I can't run the fan unless the light is off.
There is a single black and white wire coming into the box. The timer has a red white and black wire. I wired the red to the black coming from the fan. The blacks are tied together as well as the whites

 

[Guzzle]

Assuming no loose connections, at the moment I can’t draw a circuit that does what you say, but I’ll take your word on what you are seeing.

Series circuit #1 should be a SPST switch in series with a lamp.

Series Ckt #2 should be a timer acting as a SPST switch in series with a fan.

Ckts #1 & #2 should be in parallel & should be across a Hot lead & a Neutral lead.

There are many ways to wire this simple ckt.

You’ll need a VOM & another lamp like the first & some clip leads. The wire colors may be wrong.

 

[user 4883]

I see a switch leg on the light switch. Make sure the white wire on the switch leg is treated accordingly.

 

[Guzzle]

The fan is wired across the light switch, or vice versa. If it's true then neither the fan or the light are running at full voltage.

If you draw it out, there are four ways this can happen.
If you try each combination of light switch on or fan switch on or both on, you can zero in on which it is.

 

[JoeD]

If you only have a black and white wire coming into the switch box, you do not have a neutral.

 

[Guzzle]

With your symptoms & listing all cables into & out of the box & where each conductor [black, white, red] from each cable goes,
I'd think it will be clear exactly what the mistake is & if it can be fixed with the box wiring as it is now.

“When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.”

Wiring this on a benchtop would be easy, but using cables behind walls restricts what you can do & understanding what was done.

Keep your documentation for the next time you open this box.