Hey all,
I posted this in the 7.3 fourm also but figured i might get some help here too.

A while back my blower went out. Orignal OEM Blower. Replaced with one from the auto parts store. Ever since it seemed to be a bit weak.
I have noticed the speed control will increase the fan speed from 1-2 and from 2-3 but from 3-4 it dose nothing !

So when switching from speed 3 to speed 4 the blower stays the same speed.

Troubleshooting so far .......

• I Checked the wiring on the fan to ensure its spinning the correct way.
• Replaced the switch
• replaced the resistor
• cleaned the evaporator core.
• all the blend doors are working correctly
• it dose it on any setting. MAX A/C , Heat, on your feet, defrost, in your face etc

It just will not go any higher than speed 3. Its like 4 is gone.

So i looked a little further and thought OH ! the blower relay, it could pass low voltage until switched to speed 4 then trip the relay supplying max voltage.
So today i stopped and got a 87a blower relay (direct replacement ) in hopes for a fix. NOPE same result.

I got the volt meter out and started looking at voltages at the blower. Heres what i got ........................

• RED and BLACK wire..... When the system is on the red wire has CONSTANT 12v.
• seting 1 the black wire had 8v
• setting 2 the black had 5v
• setting 3 the back had 3v
• setting 4 the black had 3v

So im assuming that when switching to setting 4 or speed 4 the voltage at the black wire should drop to 0v.
I took a jumper wire from the negative battery post and touched the connector on the black wire where it was exposed. The fan reved up to what it used to be.
So i was thinking i could just leave the jumper wire hooked up to the black wire on the fan. WRONG it keeps the voltage on the black wire at 0v all the time so if your in speed 1,2,or 3 its blowing wide open all the time.

The black wire to the fan must be coming from the resistor as it seems to be regulating the voltage in the fan somehow. Seeing as the fan speed increases the voltage in the black wire decreases. And with a fully functional system is should zero out at speed 4 running the fan at the full 12v.

MY QUESTION...... WHY AM I NOT GETTING SPEED 4 ?????

Seems ive gone through everything...

• resistor
• switch
• relay

Im missing something and need some help. Summer is approaching and in north florida its in the 90s already and the fan is not putting out enough.

Any help or advice is greatly appreciated.

 

Finally spotted MY. Attached is diagram, time to break out the old multi-meter.... Root cause possibly a bad connector. Or a self-inflicted wound caused by use of defective aftermarket electrical component (read chineese). Have seen a lot of folks parts swap out perfectly good Ford OEM parts with "new" but defective chineese parts, then ask what is wrong. The shame of using a bunch of overseas parts is that our older made in USA trucks evolve into made in CHINA trucks.

 

It is common when replacing the resistor the connector has to be replaced due to corrosion. Did a wire get swiitched ?

 

Thanks Jetty
The OEM fan started making a ton of noise. Grinding and not coming on unless hit with a hammer. It was summer and we were headed out of town so autozoo i went and got one.
BUT it worked on all 4 speeds. Just not quite as strong as the oem.

This issue came about a year or 2 later.

Thanks for the diagram. I will look it over and see what i can find.

 

The issue started before replacing the resistor.
I replaced the resistor thinking it may correct the issue.

I never changed the plug but will look at the connections and see what they look like.

 

Well ...... I GOT IT !!!!!

Chased wires all over and finally found the OG/BK wire at the back of the switch was pulled out of the connector ever so slightly. Pushed it back in and BAM got the 4th speed back.

Must have happened a few years back when i installed the new double din stereo. Needless to say thanks to all for the interest advice and help. And for providing the wiring diagrams. That was a huge help.