The fan blows or not at will--I either have no heat or no air conditioning..it will remain off then come on at will later. What fuse or relay is responsible? Where is it located?

Most likely a 20 amp fuse. If the fuse is good, Find the coolant fan relay(under hood). Check to see if fan runs with A/C on. If the relay is good, the fan will run continuous with the A/C on. If the fan does not run with A/C on, The Relay is Bad. Check the fan by unplugging the relay and install a 14 Ga. jumper into the normally open terminal position(socket) If fan runs with jumper, check the remaining 2 positions in the socket for Voltage(13v.) If voltage is present, relay is bad. If voltage is not present, Temperature sensor(signal to relay) is Bad. Leave jumper in place to keep from over heating engine. Fan should stop running when Ignition switch is turned off. This engine will blow a head gasket if over heated. A service manual is recommended for component locations and troubleshooting.

cooling fan relay could be bad,check relay by making a fuse jumper wire.look at the relay control circuits on relay look for control circuits terminal number 85 and terminal 86 hook fuse jumper wire terminal 85 on relay to good ground and take terminal 86 on relay hook up a jumper wire touch battery positive terminal relay should click if not, relay bad.

Check the fuses, or the whole knob assembly went bad.

99 Deville - have you checked the large fan fuse(s) under and behind the main fuse/relay box ?? (I think this holds for 99 - 2000+ moved the maxi-fuses to the underhood box on the passenger side.)

Also, are your relays on a mount down near the fans ? Some have had trouble with the wiring/sockets there. 2000 or later moved the relays up to the fuse box itself. There are 3 - Fan 1, Fan 2, and a series/parallel (S/P) relay for slow and fast speeds.

Fans run in SLOW at 224 degrees F, FAST at 236 degrees F if there is no air conditioning or DEFROST function set. In the city or heavy traffic the gauge will go to the first tick mark past 12:00 and the fans come on. As the coolant lowers to 213 degrees F the fans turn off and the cycle repeats.

IF you set an air conditioning function, then fans run in SLOW all the time.

If your fans will not run under any conditions, check the large fuses in the underhood fuse/relay box, as well as the small fuses for the fan relays - there are 3 relays: Fan 1, Fan 2, and S/P (series/parallel): the fans are relay set to series operation (voltage drop) for Slow speed, parallel operation (full voltage) for Fast.

GO TO THIS SiTE TO CHECK THE FUSE BOX!
http://vwtech.no-ip.info/images/golf/post90.gif

This are the FUSE and RELAY number SLOT!
Fuse No. Description Ampere Rating 1 Headlight, low beam, left 10 2 Headlight, low beam, right 10 3 Instrument panel lights, license plate lights 10 4 Glove compartment light 15 5 Wiper/washer 15 6 Fresh air blower 16-valve only: Air conditioning 20 7 Taillights, side marker light, right 10 8 Taillights, side marker light, left 0 9 Rear window defogger 20 10 2.0 16-valve only: Fog lights 15 11 Headlight, high beam, left,High beam indicator light 10 12 Headlight, high beam, right 10 13 Horn 16-valve only: Radiator cooling fan 10 14 Back-up lights, washer nozzle heaters 10 15 2.0 16-valve only: Engine electronics, stabilizer valve, cold start valve 10 16 Instrument panel warning lights, 16-valve only: Multi function indicator 15 17 Emergency flasher switch, turn signals 10 18 Fuel pump, oxygen sensor heater 20 19 Radiator cooling fan, air conditioning 30 20 Brake lights, cruise control 10 21 Interior lights, digital clock 16-valve only: Luggage compartment light,multi function indicator, central locking 15 22 Radio, cigarette lighter 10
Relays Relay No. Description 1 Air conditioning 2 Open 3 Digifant control unit 4 Load reduction relay 5 Low coolant level control unit or Open 6 Emergency flashers 7 Rear Wiper 8 Intermittent wipers 9 Seat belt warning system control unit (possibly headlight warning too?) 10 Fog lights 11 Horn 12 Fuel pump (except diesel)Glow plugs (diesel) 13 Radiator cooling fan after run (except 16-valve)2.0 16-valve: Starter interlock relay 14 Automatic trans., coolant temperature indicator,starter interlock relay (except 2.0 16-valve)2.0 16-valve: Radiator cooling fan after run 15 ABS pump relay 16 ABS relay 17 Heated seat control unit, driver 18 Heated seat control unit, passenger, or power seat circuit breaker 19 Automatic transmission 20 Open 21 Power window relay, ABS hydraulic pump fuse 22 ABS valves fuse 23 Air conditioning, power seat fuse 24 Power window fuse

When the A/C is on one fan should be on ! P0480 Chevrolet Cooling Fan Relay Control Circuit

check the fuse
if it's good then your blower motor is bad

Generally speaking, any time you have issues with a relay or fuse blowing again and again, there is a short somewhere in the circuit. It could also be the fan motor going bad, but I believe that is less likely. Does the fan motor spin freely when the engine/key is off? Does it happen when turning the air conditioning on or when the fan starts from the engine coming up to temperature or both?

need to check ac fan or fans ,are they working? if not check fuses and fan relays,,,,,and fans (hot wire the fan to see if it/they work

Drive the vehicle around and warm it up,now go back home and park it with it running,set the e brake,pop the hood.Watch the fans with the engine running,they should come on and blow for awhile.If they do not come on,turn on your air conditioning on and go back and check the fans again.If they come on with the a/c,then the fans are capable,just not being commanded to come on.If the fans do not come on at all,then you need to check/replace fuses and relays.