I have a 1991 5.2L in which the cab air blower slowly got more intermittent and finally stopped altogether. With the key on the other accessories work, but there is no power at the HVAC selector and no power at the resustor plug. There is full 12V to the 50A fuse in the engine compartment, but only about 1V across the fuse poles, so the wire may be hanging on by a thread somewhere I guess. Since the fuse is getting 12V when referenced to ground, I know that the problem is after the fuse. I tried hot wiring the blower fan directly and it spun fine.

Any ideas on this one? I thought maybe a fusible link, but I don't know where it would be and my Haynes manual is still in the mail.

 

Check for power at the blower motor itself. The resistor block is a known failure point.....

 

There is no power at the fan. There is also no power going TO the resistor block. I checked continuity across all the resistors, which were all fine, and then eventually replaced it anyway so the resistor block is new.

Just to recap:

• No power at the fan
• No power at the resistor, but there is continuity from the ground wire on the plug back to the battery.
• New Resistor
• No power to the fan at the HVAC selector plug in the cab. Only power on that plug is to the AC, which works.
• There IS power TO the fuse in the engine bay.
• There is 1V of potential across the fuse prongs, so there is a tiny amount of voltage there.

 

Pretty sure the A/C controls decide which output from the resistor block gets grounded, so, power goes batter->Fuse->blower motor->resistor block->a/c control head. If you aren't getting power TO the fan, then the issue should be further up stream. I might be tempted to swap the fuse for that just for giggles, and see if anything changes.

 

If you read across the fuse plugs, and it only shows a 1 volt load ? Is that the #1 fuse?
Pg AN 55 in the 94, 5.2m, FSM, top left of page, a fuzz, #1-30a, DG #12 wire to fan motor, then the fan motor plug has a #12 Bk/Tn goes to resistor, which is connected to control panel-- fan speed.
Do the wires from fan go thru the firewall below the motor resistors and return the same way. Almost where the inner fender meets the firwall? Actually the motor resistor and switches are in the ground circuit from the fan, it will show DC voltage ?
How to check the dash switches without removing them. Maybe able to check the disconnected resistor plug through the switch panel, for a ground ? The Blk/ Tn #12wire at that plugwill should show 12v through the fan. You could ground that wire, fan should run. Okay, it's not raining rite now I can check this.

 

Ha,ha, I went back and reread your post. I'm talking about the fuse panel over the drivers left foot. #1 is the bottom right corner, 30 amp. That's the one shown in the FSM. Which gives pwr to the fan.
The 50amp in the Power Distributor box, pg AN -1 - an2, fuze application charts. gives pwr throu the ignition switch, from the pwr dist box, a #12Rd wire to igy switch, then in run, it powers the #1- 30amp fuse, which pwrs the fan. THEN, it pwrs the motor resistor, then the control switch.
The 1 volt reading across the 50a terms, is likely showing everthing disconnected at the igy switch, showing a lite bulb, possibly. Or something small. If the ignition switch is on, then it should show the system. Or a bad connection at the firewall plug?
CHECK the 30amp fuze 1st.

30amp fuze 1st.

Bulkhead plug between 50a fuze and ignition switch. I bet its just like the pcm connection, it work till you look at ite

Way down past everything that little yellow thing is where the wires transfer from fan to resister and back to the contrl switch

 

I sincerely apologize for not getting back to this thread last week, I really appreciate all the input. You guys are very helpful, I should get some time to work on this more this week.

• Oddly enough, I do not have a 30A fuse for the blower in the cab. I have a 20A fuse labeled "HTR / AC" that looks fine, although it admittedly has not been changed. However, the AC does work, cycles on and off, and the button lights up.
• I used some jumper wires and started referencing power back to the battery. I noticed that at the blower plug, I do actually have 12V referenced back to the negative battery terminal. However, I do not have 12V across the plug at the blower.
• As I noticed the other day, there is ground wire continuity at the resister plug.

So, if there is apparently power getting to the blower fan plug, and ground continuity from the resister back to the battery, then it seems that I've narrowed my problem down to the ground wire between the fan plug and the resister. I'll have to do some more poking around.

Do I have that right? I need to find an FSM wiring diagram for this truck.

 

I figured this out last night. It turned out to be the resistor plug itself. I realized that if I tried just right I could get continuity through the black-with-brown wire from the resistor plug to the fan plug. With power feeding to fan plug, and continuity on the grey ground wire from the resistor plug back to the battery, I knew that it must be in the black-with-brown wire or in the plug.

Looking closely, I found that the connector on the plug that feeds power to the resistor block had warped from heat, and was slightly displaced. It turned out that when the plug plugged in, that connector was pushing out of the way. I applied some dielectric grease and used pliers to get it back into position. When I did that and carefully plugged it back in, the blower came on.

I wish I had figured that out before I tore the entire lower dash and panel bezel apart. But oh well, I got it fixed.

Thank you @TheRinky and @HeyYou for your responses, they were very helpful and I really appreciated the detail.