Ge spacemaker xl 1800 microwave oven shows vent fan speen on but vent fan not coming on

I'm betting your base model number is JVM1860, JVM1861, JVM1870, JVM1871, JVM3660, or JVM3670.
That's the automatic louver you're having trouble with. There are micro switches in the louver assembly which tell the controller when the louver is open or closed.
If one of those switches or the louver motor fails, the louver will not work correctly.
Did your cooktop lights recently fail?
Sometimes the surge from them shorting will cause damage to the circuit board, which can be repaired.
There should be a "mini-manual" (tech sheet) hidden inside the unit behind the control panel or hidden on the left side behind the grille, which is very helpful when troubleshooting, testing, and locating components.
There is a video here showing how to remove a typical over the range control panel assembly in under 5 minutes.
You can find helpful exploded view diagrams and order parts by entering your full model number from the tag on the oven here.

https://www.manualslib.com/products/Ge-Jvm3670wf-Profile-Spacemaker-Xl-1800-36-Microwave-Oven-1842926.html

Click link to download manual

i have a ge 1800 model and it cooks fine but after awhile it shuts down and in about a hour it comes back on and ask you to set the clock.....

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If that doesn't work, leave it unplugged for about 5 minutes.

You likely need to replace the fan motor.

in the microwave there is a part which produses the heat which will be called as magnetron you check that part.

check fuse inside unit microwave has a fuse around 15 to 25 amps get at local hard ware store u should discharge capacitor before installing fuse not nessary in microwave

SOME BUT NOT ALL OVENS HAVE RELAYS TO TURN THINGS ON AND OFF CHECK THE OVEN TO SEE IF YOU HAVE ANY
RELAYS AND SEE WHAT THEY DO.

Maybe you and the person from the post below could get together and help each other? http://www.fixya.com/ThreadView.aspx?prdid=489218&thid=130453