1999 F-150 blower motor running erraticly for 30 minutes or so and then finally shutting down. Switching controls off and back on wood let it run maybe 10 seconds and then finally not at all. Have power to control switch and temperature control but nothing comes to blower switch control. Assume it is the resistor but unable to find it. Location help would be appreciated.....thanks

 

Not the resistor. The resistor can be removed from the truck and you'll still get high speed fan.

Probably worn out brushes in the motor. The blower switch is ont he "low" side of the motor. When the brushes loose connection, the circuit "upstream" of the switch opens and you loose voltage at the switch.

Steve

 

thanks for your reply Steve. Hooked up the blower motor to 12v direct using battery and it operates okay. Wiring to resistor was burned thru the connection but the resistor showed to be okay( changed out the plug in connector). Showed 12v coming to the hvac controls, light for the panel ok, shows voltage coming to the heat/cool switcher but doesn't show 12 coming into the fan switch. Fuse's(that I can find) are okay. Is their another fuse or relay that comes into the fan switch? Seems as tho the input 12v source should be the same for fan switch as well as for the other controls on the HVAC controller. Appreciate your help.............

 

The fan switch does not switch power to the motor, it switches the ground, either directly in high speed, or through varying resitances to control the speed. Power to the fan switch first must come through the motor and relay via the fuse.

You still have an open circuit in the blower motor circuit.

Here's the schematic to refer to: 1999 F150 HVAC Schematic.pdf

Again, the motor should still run on high speed regardless of whether the resistor is good or not.

If you have power on the brown/yellow motor lead but none on the motor's orange/black lead, the brushes are intermittent.

Steve

 

have a 92 e250 van no ac. blower motor was working all positions correctly.
then one day stoped opened hood moved resistor and blower motor wires nothing. then i smacked the motor it self and it turned on worked fine for 3 weeks then went out again. i dont have a working ohm meter (fluke 87 out for warrenty repair) hooked 12v test light up to blower motor wires. have light in all correct positions. ok so took blower out set it in bench vise took 10 amp battery charger on 2 amp mode. blower turns on pins amp gauge ok switch to 10 amp turns on runs faster pins gauge. ok hook blower up in truck again nothing all positions. ok take 2 amp 12v regulater power supply hook it to blower motor turns on but shorts out after 3 sec (protection) what should amp draw be on blower motor? have factory ford wiring and vacuume manual but says nothing on amp draw. took old chevy (79) blower motor hooked it to battery charger moves amp gauge to half way running with a bad bearing fast. could the amp draw on ford blower motor be to much and be messing up the blower relay (getting it hot)? dont want to go buy a new blower till i can get amp draw spec this is a used blower from junk yard out of 96 made buy siemens (no part number ) ford engineer number f6uh-19805-aa date 11-21-96c. was in truck when i bought it. going to hook head light up to blower wires that should put masive draw on blower relay and see if light flickers if not relay should be fine correct? and basically confirm 12volts at blower wires. if it powers a head light i should have relitively enough amps to run a blower motor in theory. suggestions please thanks for reading

 

Slapping the blower and then it runs tells me a bad brush or bearing. Should have replaced motor then likely and its pigtail if burned. When I test I unplug motor and look for 12v going into motor and ground on other side. Then with motor plugged in with resistor unplugged 12v coming out of blower motor on ground side. If no voltage coming out of motor bad motor. If voltage coming out ground wire check resistor and ground. If no voltage to motor check blower resistor and circuits at resistor connector.

 

sparkleg,
The burning at the resistor connector usually indicates that the blower motor is drawing too much current. This melts connector bodies and wire insulation at the weakest points. It also overheats the female blade terminals in the various connectors causing them to lose spring tension causing high resistance which, in turn, creates more heat and resistance, thus even more heat and resistance. I've seen female blade terminals act like a circuit breaker (on for a while, off for a while, then back on after cooldown).
The blower circuit draws pretty high current and is very suceptible to damage due to overcurrent conditions.
The root of your problem is the blower motor, the failure is at the weakest connection. You need to replace the motor and address any other ancillary damage to the circuit. When you replaced the resistor connector, you only treated the worst symptom. If you used crimp connectors on the new pigtail, you just added to the potential problems. Those connections should be soldered and covered with shrink-tube.

 

the problem was dash switch completely burned up this must be 2nd or third time it was replaced because there is no conector just wires with crimp on terminals. what a fire hazard switch was melted also. should not be made of plastic with that much current running through it thanks for your input

 

Do you wonder why it may have been repaired multiple times? Your new switch or connector will melt again if you don't replace the blower motor that is causing it to overheat in the first place. Treat the root cause, not just the symptom.

 

that was self explanatory never did recieve any draw spcs. must be classifed info. seimans motor was $40 switch was $12 resistors were $17 went to salvage yard got new connector for blower switch. so now have completely new heat system in a truck with 300,000 miles lol. truck runs like new still (351w) hoping it will see over 500000 miles like my old ranger pick up (515,000) and still ran when it went to crusher. just bought a 2000 e350 (van #5) it is getting a 557 injected big block and 4x4 dana 60 front axel 35 inch tires . building wiring harness for it at the moment. going to run a tweecer tuning system.

 

Cool.
You didn't mention anything but the switch and connector so it sounded like that was all you replaced.
I don't think I've ever seen a current draw spec for a blower motor. I just usually see the aftermath of excess draw anyway. Melted switches and connectors usually tell the tale pretty quickly.